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Publisher Copyright: © 2022 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH. The critical-element-free ε-Fe2O3 ferrimagnet exhibits giant magnetic coercivity even at room temperature. It is thus highly attractive material for advanced applications in fields such as spintronics, high-density data storage, and wireless communication. However, a serious obstacle to overcome is the notoriously challenging synthesis of ε-Fe2O3 due to its metastable nature. Atomic layer deposition (ALD) is the state-of-the-art thin-film technology in microelectronics. Herein, it is demonstrated that it has also true potential for the fabrication of amazingly stable in situ crystalline and high-performance ε-Fe2O3 thin films from simple (FeCl3 and H2O) chemical precursors at a moderately low deposition temperature (280 °C). Standard X-ray diffraction and Fourier transfer infrared spectroscopy characterization indicates that the films are of high level of phase purity. Most importantly, precise temperature-dependent 57Fe Mössbauer spectroscopy measurements verify that the hematite (α-Fe2O3) trace in the filmsis below 2.5%, and reveal the characteristic low- and high-temperature transitions at 208–228 K and ≈480 K, respectively, while magnetization measurements confirm the symmetric hysteresis loops expected for essentially phase-pure ε-Fe2O3 films. Excitingly, the highly c-axis oriented film growth, the overall film quality, and the unique magnetic properties remain the same, independently of the substrate material used.

The examination of local magnetic properties of thin films requires a sensible measurement device and small chip-like specimens, which fit in the machine’s magnetic field. Therefore, the carrier wafer has to be diced, which, however, renders it unusable for further processing. If more than a single spot on the wafer is of interest, multiple of these chips have to be examined one piece at a time. In contrast, the device developed within the scope of this thesis allows the measurement of local magnetic properties of continuous layers on whole wafers. For this purpose, it employs techniques known and used in the field of transformer sheet characterization. The principle of placing a magnetizing yoke on top of the specimen has been adapted in order to allow the investigation of layers with a thickness as low as 100 nm. The resulting measurement head generates the necessary field and contains all the sensors for the detection of the hysteresis loop of the specimen. Using algorithms, the flux density in the layer is calculated on the basis of the flux in the magnetizing yoke, which eliminates the need of placing a coil around the specimen. Due to the close matching of the developed hardware, electronics, and software, magnetic measurements with excitation fields of up to 2.5 kA/m in the frequency range from 50 Hz to 50 kHz are feasible. In order to obtain a high reproducibility and enable spatially resolved measurements, a motorized test stand was constructed, which is controlled by a software solution implemented with LabVIEW. The characterization of the device revealed RMS noise levels of 0.1 A/m in terms of field strength and 2·10−11 Wb in terms of flux, which corresponds to a flux density of 25 mT in a 100 nm layer. The acquired hysteresis loops of different thin film samples are compared to reference measurements obtained both with a vibrating sample magnetometer and a hysteresis loop tracer. The results demonstrate the functionality of the device but also show that there is a deviation of the measured coercivity, while the flux density matches the references well. In summary, this novel measurement method represents a fast and convenient way for the examination of the magnetic properties of thin films. Employed in a suitable test stand, it could enable fully automated test procedures like a mapping of the layer on a wafer within minutes. Since it is a non-destructive measurement method, it could be used in the clean room in between different process steps and therefore reduce development times and facilitate quality assurance.

All-optical switching of magnetization has great potential for use in future ultrafast and energy efficient nanoscale magnetic storage devices. So far, research has been almost exclusively focused on rare-earth based materials, which limits device tunability and scalability. Here, we show that a perpendicularly magnetized synthetic ferrimagnet composed of two distinct transition metal ferromagnetic layers, Ni3Pt and Co, can exhibit helicity independent magnetization switching. Switching occurs between two equivalent remanent states with antiparallel alignment of the Ni3Pt and Co magnetic moments and is observable over a broad temperature range. Time-resolved measurements indicate that the switching is driven by a spin-polarized current passing through the subnanometer Ir interlayer. The magnetic properties of this model system may be tuned continuously via subnanoscale changes in the constituent layer thicknesses as well as growth conditions, allowing the underlying mechanisms to be elucidated and paving the way to a new class of data storage devices.

FAPEMIG - Fundação de Amparo a Pesquisa do Estado de Minas Gerais Neste trabalho utilizamos técnicas de espectroscopia de absorção de raios X (XAS) para investigar aspectos estruturais e eletrônicos de dois sistemas com fortes correlações entre os elétrons. Estudamos a perovskita dupla desordenada BaTi(1/2)Mn(1/2)O(3) (BTMO) e o sistema intermetálico de férmions pesados Ce(2)Rh(1-x)Ir(x)In(8) (x=0.00, 0.25, 0.50, 0.75, 1.00) - Ce218. O BTMO foi recentemente investigado em nosso grupo de pesquisa em amostras volumétricas (estrutura hexagonal e grupo espacial R-3m), e revelou-se um sistema frustrado magneticamente em todo o intervalo de temperaturas medido. Nossos estudos também permitiram sugerir que o BTMO seja um candidato a líquido de spin. Neste trabalho, apresentamos os resultados quantitativos de XAS em amostras volumétricas e em filmes finos epitaxiais de BTMO. Nestes, que têm estrutura de perovskita simples com defeitos, observamos o aparecimento de magnetismo de longo alcance (ferro/ferri-magnético) nas temperaturas medidas, 5, 100 e 300 K, aparentemente associado com a mudança de dimensionalidade das interações magnéticas Mn-O-Mn. O sistema Ce218, por outro lado, foi estudado previamente por grupos colaboradores que mostraram o aparecimento de duas fases supercondutoras nesse composto: a fase SC1, observada na região rica em Rh e favorecida pela aplicação de pressão, e a fase SC2, observada em torno da concentração de Ir x ~ 0.6 e suprimida pela aplicação de pressão. Através de técnicas microscópicas nós mostramos que o sítio do Ir não apresenta desordem. Devido ao limite do fluxo do anel de luz síncrotron em energias da ordem de 24 keV, as medidas de absorção na borda K do Rh foram realizadas com baixa relação sinal-ruído, impossibilitando a análise confiável dos dados de EXAFS nessa borda. A análise dos dados experimentais foi realizada com simulações computacionais de espalhamentos múltiplos e cálculos de multipletos atômicos. No caso dos filmes finos de BTMO, crescidos pela técnica de Deposição por Laser Pulsado (PLD) sobre substratos de SrTiO(3) (STO 001), a caracterização física e química foi feita através de difração de raios X, mapeamento de espaço recíproco, medidas de magnetização, microscopia eletrônica de transmissão e microscopia de força atômica. In this work, we use X-ray absorption spectroscopy (XAS) techniques to investigate structural and electronic aspects of two systems with strong correlations amongst electrons. We studied the double disordered perovskite BaTi(1/2)Mn(1/2)O(3) (BTMO) and the heavy-fermion intermetallic system Ce(2)Rh(1-x) Ir(x)In(8) (x=0.00, 0.25, 0.50, 0.75, 1.00) - Ce218. BTMO was recently investigated in our research group on volumetric samples (hexagonal structure and space group R-3m, and it was characterized as magnetically frustrated throughout the measured temperature range. Studies also allowed us to suggest the BTMO as a candidate for the spin liquid. In this work, we present the quantitative results of XAS in BTMO bulk samples and epitaxial thin films. For the latter, we observed long-range magnetic order (ferro/ferri-magnetic) at 5, 100 and 300 K, apparently associated with the change in the dimensionality of the Mn-O-Mn magnetic interactions. The Ce218 system, on the other hand, was previously studied by collaborators. It was shown the appearance of two superconducting phases: the SC1 phase in the Rh-rich region, which is favored by applying pressure, and the SC2 phase, observed around the Ir concentration of x ~ 0.6, which disappears with applying pressure. With spectroscopic techniques we show that the Ir site has little structural disorder. Due to the limit of photon flux at energies of 24 keV, the XAS experiments at Rh K edge had low signal/noise ratio, making the analysis at the edge unreliable. We used EXAFS and XANES techniques together with computational simulations of multiple scatterings and atomic multiplet calculations. The BTMO thin films were epitaxially grown by Pulsed Laser Deposition (PLD) over SrTiO(3) (STO 001) substrates. We followed their chemical and physical details through X-ray diffraction, reciprocal space mapping, magnetization data, transmission electron microscopy, and atomic force microscopy. Tese (Doutorado)

This work addresses the growth of ultrathin magnetite (Fe3O4) and cobalt ferrite (CoFe2O4) films and their thorough structural, electronic, and magnetic characterization. In a first step, ultrathin Fe3O4 films are grown on SrTiO3(001) substrates by reactive molecular beam epitaxy (RMBE) and the substrate-induced anomalous strain behavior of the films is investigated by complementary high-resolution transmission electron microscopy (HRTEM) and (grazing incidence) X-ray diffraction [(GI)XRD] measurements. Next, an additional CoO film is deposited on similar Fe3O4/SrTiO3(001) heterostructures to demonstrate an alternative route for the synthesis of cobalt ferrite films through the thermally mediated interdiffusion of both oxide films. The evolution from the initial bilayer stacks to completely reacted cobalt ferrite films is extensively monitored by soft and hard X-ray photoelectron spectroscopy (soft XPS and HAXPES) and (GI)XRD. Complete intermixing and formation of single cobalt ferrite films is confirmed by angular-resolved HAXPES (AR-HAXPES) and X-ray reflectivity (XRR). The study of the cationic distribution resulting from this novel synthesis technique and its effects on the magnetic properties of the cobalt ferrite films is the subject of the subsequent part. Here, X-ray magnetic circular dichroism (XMCD) and superconducting quantum interference device (SQUID) magnetometry serve as key investigation techniques, which are further complemented by AR-HAXPES and atomic force microscopy (AFM) measurements. In a final step, highly crystalline cobalt ferrite films with different cationic stoichiometries are grown on MgO substrates using RMBE while their growth behavior is captured in real-time using operando XRD. Further structural characterization of the films is carried out by low-energy electron diffraction and XRR, whereas HAXPES and SQUID provide fundamental information on the electronic, chemical, and magnetic properties of the films.

Industrial wastewater can be properly treated using nanotechnologies and nanomaterials. This paper presents the synthesis and characterization of three series of magnetic nanoparticles (MNPs) and corresponding thin films, used for the degradation of organic compounds and removal of heavy metals from industrial wastewater. The samples were obtained by co-precipitation from a ferric (Fe3+) and ferrous (Fe2+) ions solution in a molar ratio of 2:1, at temperatures between 80-95 °C. The characterization of the samples was performed by scanning electron microscopy (SEM), and X-ray diffraction (XRD) methods. The magnetic nanoparticles were deposited on glass substrates by the centrifugal coating technique and the optical and magneto-optical activity was investigated by UV-Vis spectroscopy and magnetic circular dichroism technique (MCD). The effect of the investigated samples on the decomposition under UV irradiation of organic dyes was monitored by UV-Vis spectroscopy. Our preliminary results have shown that the magnetite and maghemite MNPs can be effective in UV degradation of methylene blue (MB) dye.

Phase transitions occurring within spatially confined regions can be useful for generating nanoscale material property modulations. Here we describe a magneto-structural phase transition in a binary alloy, where a structural transition from short range order (SRO) to body centered cubic (bcc) results in the formation of depth-adjustable ferromagnetic layers, which reveal application-relevant magnetic properties of high saturation magnetitzation (Ms) and low Gilbert damping (α). Here we use Fe₆₀V₄₀ binary alloy films which transform from initially Ms = 17 kA/m (SRO structure) to 747 kA/m (bcc structure) driven by atomic displacements caused by penetrating ions. Simulations show that estimated ~1 displacement per atom triggers a structural transition, forming homogeneous ferromagnetic layers. The thickness of ferromagnetic layer increases as a step-like function of the ion-fluence. Microwave excitations of the ferromagnetic/non-ferromagnetic layered system reveals an α = 0.0027 ± 0.0001. The combination of nanoscale spatial confinement, low α and high Ms provide a pathway for the rapid patterning of magnetic and microwave device elements.

Ferromagnetic thin films were synthesized by implantation of Fe ions with energies between 40 keV and 120 keV in 60 nm of polycrystalline Pd. The possibility to engineer magnetic properties in such films through the Fe concentration depth profile was demonstrated. In-plane magnetic moments were measured both while varying the sample temperature at constant applied magnetic field, and varying the applied field at constant temperature. All samples were explicitly shown to exhibit ferromagnetism at sufficiently low temperature, with threshold temperatures in the range from 35 K to 140 K. Saturation moments at 5 K were between 3.5 and 4.6 Bohr magnetons per implanted Fe atom for implantation fluences between 5 × 1015 and 2.3 × 1016 ions/cm2. Coercivities were between 1 and 3 mT. The samples included in the study were carefully characterized with Rutherford backscattering spectrometry, and selected samples with elastic recoil detection analysis and cross-sectional transmission electron microscopy, providing information on Fe concentration profiles, impurity fractions, structure as well as sample modification during implantation. A new model was proposed to calculate the scaling of the films’ magnetic moments with temperature from the Fe concentration profiles. The model qualitatively reproduced the data and quantitative differences were explained. It can thus be applied to predict the modification of the magnetic moment at a given temperature for a similar film when the implantation parameters are varied.

The magnetic and transport properties of pulsed laser-deposited Co100−x–Cux thin films were tailored through their nano-morphology and composition by controlling for the deposition geometry, namely normal or oblique deposition, and their Cu content. All films were composed of an amorphous Co matrix and a textured growth of Cu nanocrystals, whose presence and size d increased as x increased. For x = 50, all films were superparamagnetic, regardless of deposition geometry. The normally deposited films showed no in-plane magnetic anisotropy. On the contrary, controllable in-plane uniaxial magnetic anisotropy in both direction and magnitude was generated in the obliquely deposited films. The magnetic anisotropy field Hk remained constant for x = 0, 5 and 10, Hk ≈ 35 kAm−1, and decreased to 28 and 26 kAm−1 for x = 20 and 30, respectively. This anisotropy had a magnetostatic origin due to a tilted nano-sheet morphology. In the normally deposited films, the coercive field Hc increased when x increased, from 200 (x = 0) to 1100 Am−1 (x = 30). In contrast, in obliquely deposited films, Hc decreased from 1500 (x = 0) to 100 Am−1 (x = 30) as x increased. Activation energy spectra corresponding to structural relaxation phenomena in obliquely deposited films were obtained from transport property measurements. They revealed two peaks, which also depended on their nano-morphology and composition.

Relaxation fundamentally determines the operation speed and energy efficiency of spintronic and spinorbitronic devices. We develop a theory of the longitudinal contribution to the relaxation of domain walls in ferromagnetic films of any thickness with the Dzyaloshinskii-Moriya interaction, which allows quantitative comparison with experiments. We show that the longitudinal contribution increases with a decrease of the transversal relaxation (e.g., the Gilbert constant). We predict a substantial enhancement of the contribution of the longitudinal relaxation to the damping of magnetic solitons with a decrease of the film thickness. We demonstrate that for ultrathin ferromagnetic films, the contribution of the longitudinal relaxation to the damping of domain walls is comparable to or stronger than any other traditional transversal mechanisms, including spin pumping. Although in this work we focus on the analysis of longitudinal relaxation for domain walls, in ultrathin samples it should be taken into account also for other magnetic solitons including skyrmions. This work adds to the fundamental understanding of the design and optimization of spintronic and spinorbitronic devices based on moving solitons in ultrathin films.

Previously, we reported the aqueous electrodeposition of rare earth - iron group alloys. A key factor was the complexation of the metal ions with various coordination compounds (e.g., aminoacetic acids), without which only the ferrous metal and rare earth hydroxides/oxides are deposited. In this work, samarium cobalt (SmCo) alloys were synthesized using direct current (DC) aqueous electrodeposition. The basic electrolyte solution consisted of 1 M samarium sulfamate, 0.05 M cobalt sulfate, and 0.15 M glycine, resulting in deposits containing >30 at% Sm at 60°C with current density of 500 mA/cm2. Supporting electrolytes (i.e., ammonium salts) decreased the Sm content in the deposit. Crystallinity of deposited films altered from nanocrystalline to amorphous as the Sm content increased. Deposits with high Sm content (32 at%) became isotropic with reduction in magnetic saturation (Ms) and coercivity (Hc). A deposition mechanism involving stepwise reduction of the complexed Sm-Co ions by depositing hydrogen atoms was proposed.

Magneto-ionics, by which the magnetic properties of a thin layer can be modified through the migration of ions within a liquid or solid electrolyte, is a fast developing research field. This is mainly due to the perspective of energy efficient magnetic devices, in which the magnetization direction is controlled not by a magnetic field or an electrical current, as done in traditional devices, but by an electric field, leading to a considerable reduction of energy consumption. In this work, the interfacial perpendicular magnetic anisotropy (PMA) of a series of Pt/Co/oxide trilayers covered by a ZrO$_2$ layer, acting as a ionic conductor, was finely tuned by a gate voltage at room temperature. The non-volatility and the time evolution of the effect point at oxygen ion migration across the ZrO$_2$ layer as the driving mechanism. A large variation of the PMA is obtained by modifying the degree of oxidation of the cobalt layer with the flux of oxygen ions: the easy magnetization axis can be switched reversibly from in-plane, with an under-oxidized Co, to in-plane, with an over-oxidized Co, passing through an out-of-plane magnetization with maximum PMA. The switching time between the different magnetic states is limited by the oxygen ion drift velocity through the multilayer structure. This was shown to depend exponentially on the applied bias voltage, and could be varied by over 5 orders of magnitude, from several minutes to a few ms. On the other hand, for a fixed gate-voltage, the oxidation of the cobalt layer decreases exponentially as a function of time. This behavior is in agreement with the theoretical model developed by Cabrera and Mott (1949) to explain the growth of very thin oxides at low temperatures. The possibility to explain the observed effect with a relatively simple theoretical model opens the possibility to engineers materials with optimized properties., 8 pages, 6 figures

Three-dimensional (3D) spintronic devices are attracting significant research interest due to their potential for both fundamental studies and computing applications. However, their implementations face great challenges regarding not only the fabrication of 3D nanomagnets with high quality materials, but also their integration into 2D microelectronic circuits. In this study, we developed a new fabrication process to facilitate the efficient integration of both non-planar 3D geometries and high-quality multi-layered magnetic materials to prototype 3D spintronic devices, as a first step to investigate new physical effects in such systems. Specifically, we exploited 3D nanoprinting, physical vapour deposition and lithographic techniques to realise a 3D nanomagnetic circuit based on a nanobridge geometry, coated with high quality Ta/CoFeB/Ta layers. The successful establishment of this 3D circuit was verified through magnetotransport measurements in combination with micromagnetic simulations and finite element modelling. This fabrication process provides new capabilities for the realisation of a greater variety of 3D nanomagnetic circuits, which will facilitate the understanding and exploitation of 3D spintronic systems.

Magnonics is an emerging field in spintronics, aiming at the development of new-concept magnetic devices processing information via the manipulation of spin waves (SWs) in magnetic nanostructures. One of the most popular SW waveguides exploited currently is ferromagnetic nanostrips. Due to quantization caused by the lateral confinements, the dispersion of SWs propagating in a strip is characterized by a multi-branched structure. Consequently, SWs excited in the system involve superpositions of degenerate modes from different branches of the dispersion curves. In this work, we theoretically study the SW branch hybridization effect for two types of excitation methods, either by using a local oscillating magnetic field or a fast-moving field pulse. The former is based on the resonance effect and the latter on the Cherenkov-like emission mechanism. Micromagnetic simulations yield a variety of SW profiles with rather complex structures, which can be well explained by mode superpositions. These results draw attention to the significance of the SW branch hybridization effect when dealing with SWs in nanostrips and provide new aspects for the manipulation of SWs.

Binary magnetic alloys like Co–Pt are relevant for applications as components of magnetic exchange coupled composites. Numerous approaches exist to tune the coercive field of Co–Pt alloys primarily relying on high-temperature processing aiming to realize chemically long-range ordered phases. The peculiarity of Co–Pt is that large coercive field and magnetic anisotropy can be achieved even in chemically disordered alloys relying on short-range order. Here, we study alloying of Co–Pt from bilayers of Pt(14 nm)/Co(13 nm) at temperatures up to 550 °С, where bulk diffusion processes are suppressed and the dominant diffusion mechanism is grain boundary migration. We demonstrate that grain boundary diffusion mechanism can lead to the realization of a homogeneous yet chemically disordered Co56Pt44 alloy at temperatures of 500 °С and higher. A pronounced increase of the coercive field for samples processed at temperatures higher than 400 °С is attributed to short-range ordering. With this work, we pinpoint the grain boundary diffusion as the mechanism responsible not only for the homogenization of binary alloy films but also as a driving force for the realization of short-range order in Co–Pt. Our results motivate further research on grain boundary diffusion as a mechanism to realize chemically long-range ordered phases in Co–Pt alloys.

Properties of nanolayers can substantially differ from those of bulky materials, in part due to pronounced interface effects. It is known that combinations of layers of heavy and ferromagnetic metals leads to the appearance of specific spin textures induced by interface-induced Dzyaloshinskyi–Moria interaction (DMI), which attracts much interest and requires further studies. In this paper, we study magneto-optical effects in two- and three-layer films composed of a few nanometer thick Co layer adjacent to nanofilms of non-magnetic materials (Pt, W, Cu, Ta, MgO). For experimental studies of the interface magnetization-induced effects, we used the optical second harmonic generation (SHG) technique known for its high sensitivity to the symmetry breaking. We found that the structural asymmetry leads to the increase of the averaged SHG intensity, as well as to the magnetic field-induced effects in SHG. Moreover, by choosing the proper geometry of the experiment, we excluded the most studied linear in magnetization SHG contributions and, thus, succeeded in studying higher order in magnetization and non-local magnetic effects. We revealed odd in magnetization SHG effects consistent with the phenomenological description involving inhomogeneous (gradient) magnetization distribution at interfaces and found them quite pronounced, so that they should be necessarily taken into account when analyzing the non-linear magneto-optical response of nanostructures.

The ability to induce and control the perpendicular magnetic anisotropy (PMA) of ferromagnetic layers has been widely investigated, especially those that offer additional functionalities (e.g., skyrmion stabilization, voltage-based magnetization switching, rapid propagation of domain walls). Out-of-plane magnetized ferromagnetic layers in direct contact with an oxide belong to this class. Nowadays, investigation of this type of system includes antiferromagnetic oxides (AFOs) because of their potential for new approaches to applied spintronics that exploit the exchange bias (EB) coupling between the ferromagnetic and the AFO layer. Here, we investigate PMA and EB effect in NiO/Co/Au and NiO/Co/NiO layered systems. We show that the coercive and EB fields increase significantly when the Co layer is coupled with two NiO layers, instead of one. Surrounding the Co layer only with NiO layers induces a strong PMA resulting in an out-of-plane magnetized system can be obtained without a heavy metal/ferromagnetic interface. The PMA arises from a significant surface contribution (0.74 mJ/m2) that can be enhanced up to 0.99 mJ/m2 by annealing at moderate temperatures (~450 K). Using field cooling processes for both systems, we demonstrate a wide-ranging control of the exchange bias field without perturbing other magnetic properties of importance.

In this review, the technique of resonant soft X-ray reflectivity in the study of magnetic low-dimensional systems is discussed. This technique is particularly appealing in the study of magnetization at buried interfaces and to discriminate single elemental contributions to magnetism, even when this is ascribed to few atoms. The major fields of application are described, including magnetic proximity effects, thin films of transition metals and related oxides, and exchange-bias systems. The fundamental theoretical background leading to dichroism effects in reflectivity is also briefly outlined.

The Ni(2)Mn(1+x)Z(1-x) Heusler alloys, where Z = In, Sn, Sb are attractive candidates for shape memory applications. The effect is directly related to their magnetic properties. We have studied these systems by ab-initio calculations and experiments, focusing on magnetic moments and exchange-bias fields. The calculations indicate that the composition dependence of the total magnetic moment is due to competing ferro-magnetic and antiferromagnetic alignment between Mn atoms on different sublattices and with different local environments. This phenomenon can also explain the composition dependence of the exchange-bias effect. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Funding Agencies|Polish National Science Centre [DEC-2014/15/B/ST3/02927]; Strategic Research Areas the Swedish eScience Research Centre (SeRC); Advanced Functional Materials at Linkoping University (Faculty Grant SFOMatLiU) [2009 00971]; Ministry of Science and Higher Education of the Russian Federation of NUST "MISIS" [K2-2019-001, 211]

Novel Ir1–xCrxO2 thin films have been prepared by reactive co–sputtering deposition. Composition, structure and electric and magnetic behavior have been analyzed by different techniques including EDX, XRR, XRD, SQUID magnetometry, electrical resistivity and XANES and XMCD spectroscopies. Despite the difficulty in growing CrO2 by physical deposition techniques, an Ir1–xCrxO2 solid solution phase could be achieved for 0 ≤ x ≤ 0.8, where the oxidation state of Cr is found to remain as 4+. Both the electrical and the magnetic behavior are shown to starkly depart from those of the parent IrO2 (paramagnetic metal) and CrO2 (half–metal ferromagnet) compounds. In particular, they show a semiconducting behavior, dρ/dT < 0 and giant magnetic coercivity at low temperatures. XMCD reveals a significant contribution of Ir to the magnetic response of the Ir1–xCrxO2 films. In addition, the nature of the magnetic moment of the Ir4+ ion (〈ml〉/〈ms〉 = 0.09) is completely different from the large orbital moment that is a hallmark of insulating Ir4+ oxides. This suggests a Cr–induced magnetic moment, which is a remarkably surprising result for an oxide., This work was partially supported by the Spanish Ministry of Science and Innovation under contract projects MAT2014–54425–R and MAT2017–83468–R. The authors would like to acknowledge the use of Servicio General de Apoyo a la Investigación–SAI, Universidad de Zaragoza. This research used resources of the APS, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE–AC02–06CH11357. E.A.–E. acknowledges the Spanish MINECO and the European Social Fund for a FPI (Formación de Personal Investigador, 2015) grant.

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2020, Tutor: Òscar Iglesias Clotas, In this work, the ground state configurations of magnetic thin films and some finite size magnetic elements are studied by means of Monte Carlo simulations. For this purpose, we have set up a code to simulate Ising and Heisenberg spin classical models on squared and triangular lattices including exchange (J), dipolar (g) interactions and magnetocrystalline anisotropy (K). We show how the interplay between short and long-ranged interactions stabilizes stripe-like patterns similar to those observed experimentally. Phase diagrams in the (J=g, T) and (K=g, T) planes are presented and the corresponding reorientation transitions are characterized, stressing the differences when comparing different lattices. Finally, the magnetic configurations obtained for thin films are compared to those for systems of finite-size, demonstrating that our code can be used also to study the properties of nanoscale elements.

Thermally-activated phase transitions in Pt/Mn/Fe thin films were investigated by a combination of x-ray diffraction, transmission electron microscopy, secondary neutral mass spectrometry depth profiling, atomic force microscopy, and magnetic properties measurements. Post-annealing was carried out in vacuum to different temperatures up to 620 °C. Initially, at temperatures between 280 °C-450 °C first L10-MnPt is formed at the Mn/Pt interface followed by the most likely formation of metastable bcc Fe3Pt, which gets transformed by further annealing to fcc Fe3Pt and eventually to chemically ordered L12-Fe3Pt. The final product after annealing at 620 °C consists of two interesting phases, which are relevant for spintronic applications, antiferromagnetic L10-MnPt with addition of Fe and ferromagnetic L12-Fe3Pt, consistent with the initial element composition.

This study investigates the magnetisation process of a martensitic Ni-Mn-Ga thin film with microstructure optimized to obtain a unidirectional and reversible magnetisation jump. The study has been realised by a thorough vector magnetometry characterisation and supported by micromagnetic modelling, considering different orientations of the applied field with respect to the symmetry directions of the sample. The model has been built on the film microstructure and experimental characteristics. The main features of the magnetisation curves measured along the film symmetry directions can be well reproduced by the micromagnetic model, that is, neglecting structural and magnetostructural contributions to the free energy. The model also well reproduces the field-dependent behaviour of the transverse magnetisation components. The agreement demonstrates that the spatial organisation of magnetocrystalline anisotropy axes due to martensitic twinning has a dominant effect on the magnetisation process, giving rise to magnetisation jumps when the magnetic field is applied along the alignment direction of the twin boundaries. When a reverse field is applied along this direction, simulations show that magnetisation reversal proceeds through the formation and three-dimensional expansion of magnetic domains, passing around the zero field through a closed-flux domain configuration, with the domain walls showing perpendicular orientation of the magnetic moments.

In the present information age, people across the globe generate an enormous digital footprint. Therefore, it is necessary to store and process the information in an elegant way. Spin-logic circuits are being considered potential candidates beyond CMOS integrated computing due to high clock speed, non-volatility, and higher logical efficiency. A basic building block for spin-logic circuits is ferromagnetic (FM) and normal-metal (NM) heterostructures. An example of a spin-logic data processing circuit is the spin-orbit torque (SOT) magnetic random access memory (MRAM). Energy efficient operation of such circuits depends on the interfacial electronic and magnetic properties of the heterostructures. These properties are known as Gilbert damping of the FM layer, spin Hall angle of the NM layer, spin-mixing conductance and spin-memory loss at the interface. To reduce the power consumption of such circuits, we propose two methods; doping and interface engineering. In paper I, we use ultra-low damping Fe65Co35 thin films as the FM layer and Ru as the NM layer. The interfacial parameter, the spin-mixing conductance, of the FeCo/Ru heterostructure is enhanced by 98% with Re doping in the FeCo. In paper II, we use Co2FeAl as the FM layer and β-Ta as the NM layer, and inserting a Cu continuous layer at the interface of Co2FeAl/β-Ta, yielding an enhancement of the spin-mixing conductance. In both cases, the enhancement in spin-mixing conductance can be linked to the increase in number of quantum conductance channels on the NM side, which helps to reduce the power consumption, and may provide a direction towards ultra-fast efficient operation of spin-logic circuits.

The spin&ndash, spin interactions between chiral molecules and ferromagnetic metals were found to be strongly affected by the chiral induced spin selectivity effect. Previous works unraveled two complementary phenomena: magnetization reorientation of ferromagnetic thin film upon adsorption of chiral molecules and different interaction rate of opposite enantiomers with a magnetic substrate. These phenomena were all observed when the easy axis of the ferromagnet was out of plane. In this work, the effects of the ferromagnetic easy axis direction, on both the chiral molecular monolayer tilt angle and the magnetization reorientation of the magnetic substrate, are studied using magnetic force microscopy. We have also studied the effect of an applied external magnetic field during the adsorption process. Our results show a clear correlation between the ferromagnetic layer easy axis direction and the tilt angle of the bonded molecules. This tilt angle was found to be larger for an in plane easy axis as compared to an out of plane easy axis. Adsorption under external magnetic field shows that magnetization reorientation occurs also after the adsorption event. These findings show that the interaction between chiral molecules and ferromagnetic layers stabilizes the magnetic reorientation, even after the adsorption, and strongly depends on the anisotropy of the magnetic substrate. This unique behavior is important for developing enantiomer separation techniques using magnetic substrates.

Magnetic shape memory Heuslers have a great potential for being exploited into the next generation of cooling devices and actuating systems due to their "giant" caloric and thermo/magnetomechanical effects, arising from the combination of magnetic order and a martensitic transition. Thermal hysteresis, broad transition range, and twinning stress are among the major obstacles preventing the full exploitation of these materials. In order to find possible solutions to overcome these unfavorable obstacles, it is necessary to gain a comprehensive view of the configuration of the twin variants at the different length scales and its evolution upon martensitic phase transition. In the literature, there are a few works focused on the crystallographic structures and the martensitic configurations of epitaxial Ni-Mn-Ga thin films through experiments and models [1-4]. However, the knowledge about the multiscale hierarchical self-accommodation of the twin variants in the martensitic phase and its possible links to the transition route is still limited, mainly due to lack of direct multiscale observations. In the present study, we directly visualize the crystallography of seven-fold modulated (7M) Ni-Mn-Ga epitaxial films, the symmetry relations between the twin boundaries [5], and the interfaces [6] between colonies of different twin boundaries in the martensitic phase by means of different transmission electron microscopy (TEM) techniques. We combine our direct observations through TEM techniques to atomic force microscopy (AFM) topography imaging vs. temperature [7]. We propose a route for the martensitic forward and reverse transitions, highlighting the major role played by the different martensitic interfaces [7]. The present results represent a step forward in the understanding of the transition processes, and pave the way to the possibility of tuning the characteristics of the transition, e.g. hysteresis and transition width, by microstructure engineering aimed at the full exploitation of martensitic Heuslers for applications requiring cyclic phase transition.

This work presents a magnetic field imaging method based on color centers in diamond crystal applied to a thin film of a nanolaminated Mn$_2$GaC MAX phase. Magnetic properties of the surface related structures have been described around the first order transition at 214 K by performing measurements in the temperature range between 200 K and 235 K with the surface features fading out by increasing temperature above the transition temperature. The results presented here demonstrate how Nitrogen-Vacancy center based magnetic microscopy can supplement the traditionally used set of experimental techniques, giving additional information of microscopic scale magnetic field features, and allowing to investigate the temperature dependent magnetic behavior. The additional information acquired in this way is relevant to applications where surface magnetic properties are of essence., Comment: 8 pages, 5 figures

Charged magnetic domain walls have been visualized in soft magnetic nanostructured Fe thin films under both static and dynamic conditions. A transition in the core of these zigzagged magnetic walls from N&eacute, el-type to Bloch-type through the formation of crosstie walls has been observed. This transition in charged zigzagged walls was not previously shown experimentally in Fe thin films. For film thicknesses t &lt, 30 nm, N&eacute, el-type cores are present, while at t &asymp, 33 nm, walls with crosstie cores are observed. At t &gt, 60 nm, Bloch-type cores are observed. Along with the visualization of these critical parameters, the dependence on the film thickness of the characteristic angle and length of the segments of the zigzagged walls has been observed and analyzed. After measuring the bistable magneto-optical behavior, the values of the wall nucleation magnetic field and the surface roughness of the films, an energetic fit to these nucleation values is presented.

Magnetic-shape-memory Heuslers are an important class of ferroic materials, constantly opening new fields of research and application (e.g. magnetic actuation, energy harvesting, ferroic cooling) arising from the strong interplay between magnetism and structure. The exploitation of thin films opens up barely new perspectives in the realization of micro/nanomachines, memories and smart devices, taking advantage also of their possible integration in microsystems. The control of microstructure is a crucial goal for the full exploitation of their functional properties (e.g. shape memory, magneto-mechanical) since the twin variants configuration plays a major role. Few recent works have been addressed to microstructure engineering, mainly exploiting the epitaxial growth on different substrates and varying film thickness and growth parameters. In previous papers we have shown the correlation between structure, microstructure and magnetic properties at the different legth-scales of substrate-constrained Ni-Mn-Ga films grown on Cr/MgO(100) [1]. More recently, we have studied the effects of size confinement and reported the possible thermo-magnetic actuation of free-standing Ni-Mn-Ga nano-disks, enabled by the peculiar martensitic microstructure [2] In the present talk we present an important step forward [3]. By means of a thorough multiscale magnetic and structural study, we demonstrate that a variety of simple post-growth treatments, i.e. post-annealing at low T (350°C), magnetic field cooling and mechanical stress, open up the possibility to tailor the twin variant configuration of epitaxial thin films. In particular, depending on the selected treatment, it is possible to tune the type of variant, i.e. X-type with out-of-plane magnetic easy axis or Y type with in-plane magnetic easy-axis, and their geometrical distribution. The mechanisms underlying the overall and local manipulation of microstructure are discussed by taking into account the role of microstructural defects and disorder together with the influence of external fields on the martensitic transformation path. Our findings definitely provide a platform to easily control and manipulate microstructural and magnetic patterns in magnetic shape memory thin films, paving the way to their full exploitation in smart applications. In a broader context, this remarkable "magnetic flexibility", enabled by the peculiarity of martensitic microstructure, makes magnetic shape memory Heusler thin films, a unique class, among magnetic materials, for the easy manipulation of their magnetic configuration by simple after-growth treatments.

We here examine the correlation between martensitic microstructure and magnetization process in Ni-Mn-Ga films epitaxially grown on MgO(100) or Cr/MgO(100) by r.f. sputtering. We have obtained a variety of martensitic patterns by modifying the growth conditions, applying a stress during or after growth and annealing the films. The patterns are different in orientation and spatial organization of the martensitic twin variants and give rise to distinctive magnetization processes, with magnetization jumps of variable intensity along different crystallographic directions. The occurrence of magnetization jumps is typically attributed to the magnetically induced reorientation of twin variants, similarly to what occurs in bulk materials. We have instead simulated magnetization processes of purely magnetic origin in films with different martensitic patterns by the OOMMF code. The micromagnetic simulations demonstrate that magnetization jumps of purely magnetic origin and with variable intensity can occur in the first quadrant of the (M,H) diagram, showing a good agreement with the experimental results.

Nanoporous Co40Fe40B20 (CoFeB) and sandwich tungsten (W)/CoFeB/W thin films were fabricated via an anodic aluminum oxide (AAO) template-assisted magneto sputtering process. Their thickness-dependent magneto-optical Kerr effect (MOKE) hysteresis loops were investigated for enhanced Kerr rotation. Control of the Kerr null points of the polarized reflected light can be realized via the thicknesses of the CoFeB layers and W layers. Simulation of the thickness-dependent phase difference change by the finite element method reveals the existence of the two Kerr null points for W/CoFeB/W thin films, matching the experimental result very well. However, there are two additional Kerr null points for pure CoFeB thin films according to the simulation by comparing with the experimental result (only one). Theoretical analysis indicates that the different Kerr null points between the experimental result and the simulation are mainly due to the enhanced inner magnetization in the ferromagnetic CoFeB layer with the increased thickness, which is usually omitted in the simulation. Clearly, the introduction of non-ferromagnetic W layers can experimentally regulate the Kerr null points of ferromagnetic thin films. Moreover, construction of W/CoFeB/W sandwich thin films can greatly increase the highest magneto-optical susceptibility and the saturated Kerr rotation angle when compared with CoFeB thin films of the same thickness.

The static and dynamic magnetization response of the CoFeB/IrMn/CoFeB trilayer system with varying thickness of the antiferromagnetic (AF) IrMn layer is investigated using magnetization hysteresis (M–H) and ferromagnetic resonance (FMR) measurements. The study shows that the two CoFeB layers are coupled via a long-range dynamic exchange effect through the IrMn layer up to a thickness of 6 nm. It is found that with the increase in IrMn layer thickness a nearly linear enhancement of the effective magnetic damping constant occurs, which is associated with the simultaneous influence of spin pumping and interlayer exchange coupling effects. An extrinsic contribution to the linewidth originating from the two-magnon scattering is also discussed. The AF-induced interfacial damping parameter is derived by studying the evolution of damping with inverse CoFeB thickness. The static magnetic measurements also reveal the interlayer exchange coupling across the IrMn layer both at room temperature and low temperature. The asymmetric hysteresis loop and training effect observed at low temperature is related to the presence of a metastable AF domain state. We show that both the static and dynamic magnetic properties of trilayer films can be adjusted over a wide range by changing the thickness of the IrMn spacer layer.

Soft magnetic materials are used in integrated power magnetic devices (such as micro-inductors and micro-transformers) to achieve reasonable inductances at frequencies over a few megahertz. However, the out-of-plane (perpendicular) anisotropy, in thick films produced by vapor deposition, worsens the in-plane soft magnetic properties. This paper reports on the nano-lamination of Permalloy (Ni $_{81}$ Fe $_{19}$ ) thin films with the introduction of nitrogen-doped tantalum in between to eliminate the out-of-plane anisotropy and improve the in-plane anisotropy. This significantly improves the in-plane soft magnetic properties, reducing the coercivity from 1352 A/m to 25.5 A/m and increasing the anisotropy field from 180 A/m to 660 A/m. The high-frequency permeability was uniform up to 500 MHz, and the ferromagnetic resonance (FMR) frequency was increased to 1 GHz. These properties are ideal for high efficiency magnetic applications at high frequencies and an extended FMR frequency is imperative for gigahertz-range devices.

Amorphous rare-earth (RE) transition-metal (TM) alloys are ferrimagnetic (FiM) materials. In RE-TM, magnetic moments in RE and TM couples antiferromagnetically and form two magnetic sublattices. With the two sublattices, the magnetic properties of RE-TM thin films can vary significantly with temperature, composition, and thickness. For example, the magnetization of RE-TM alloys vanishes at the compensation temperature (Tcomp). Such flexibility has both advantages and disadvantages. On the one hand, their properties can be tuned for specific applications, such as magneto-optical recording devices and skyrmion based devices. On the other hand, with so many tunable parameters, it could be very time consuming to optimize their properties experimentally. Guidance from simulations can provide crucial knowledge to reduce the time and the cost of experiments. This thesis focuses on the tuning of magnetic properties in RE-TM thin films through simulations and experiments. Based on experimental results, computational models are developed. Then, simulations results can be used to guide experiments. In this study, RE- TM thin films were deposited on various substrates by radio frequency (RF) magnetron sputtering at room temperature. In TbFeCo on thermally oxidized Si substrates, the exchange bias effect was revealed near Tcomp. Magnetic modeling was needed to investigate the origin of the exchange bias effect. To study this effect, the micromagnetic model is adopted into the two-sublattice model, where each sublattice evolves under its own Landau-Lifshitz-Gilbert (LLG) equation. Using the two-sublattice model, exchange interactions between two nanoscale amorphous phases were found to be the origin of the exchange bias. Using this model, one can explore different FiM heterostructure to develop desirable exchange bias properties for applications at room temperature. In addition to exchange bias, the ability to control magnetic anisotropy can provide great flexibility in writing and storing information. Using TbFeCo deposited on Kapton, the magnetic properties of TbFeCo are varied through the bending of Kapton. Furthermore, in VO2/TbFeCo heterostructure, a decrease in magnetic anisotropy in TbFeCo was found to correspond to the tensile strain from the structural transition of the underlying VO2 films. These results provide guidance on tuning magnetic anisotropy through interfacial strain. Amorphous RE-TM thin films are also promising materials for hosting magnetic skyrmions, which are proposed for magnetic memory devices to improve density and efficiency. In RE-TM thin films, skyrmions are stabilized through the interfacial Dzyaloshinskii-Moriya Interaction (DMI), originates from an adjacent heavy metal layer. In this study, computational models are employed to investigate room-temperature skyrmions in GdCo thin films. Using atomistic simulations, various parameters, including thickness, composition, anisotropy, and interfacial DMI, are varied to explore their effects on skyrmions. Results show that increasing thickness and reducing interfacial DMI in GdCo can further reduce the size of skyrmion at room temperature, which is crucial for applications.

In this work we present a careful study on the relationship between the magnetic and structural properties of a highly magnetostrictive Fe0.89Ga0.11 (Fe-Ga) alloy deposited onto glass, Si and MgO substrates. When grown on glass, the films are polycrystalline with randomly oriented grains without any texture, while the ones on Si and MgO present preferred growth directions. Fe-Ga/Si films show a [1 1 3] fibre-like texture, and Fe-Ga/MgO presents a quasi monocrystalline behavior with the (1 0 0) film plane direction parallel to the substrate surface. When Fe-Ga/MgO films are annealed an additional (1 1 0) texture is also observed. Magnetometry and ferromagnetic resonance (FMR) show that the magnetic behavior is closely related to the structural observed textures. Furthermore, the structural analysis allowed us to get a deeper understanding of the magnetic behavior. This point is very important to get the ability of controlling the crystalline texture by means of growing onto different substrates and/or thermal treatments, which in turns opens the possibility of handling the magnetic texture which is particularly important in magnetostrictive materials for electronic devices. Fil: Ramírez Chamorro, Gerardo Alexis. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; Argentina Fil: Malamud, Florencia. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina Fil: Gomez, Javier Enrique. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; Argentina Fil: Rodríguez, Luis M.. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Fregenal, Daniel Eduardo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina Fil: Butera, Alejandro Ricardo. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; Argentina Fil: Milano, Julian. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología; Argentina

Important applications of spin polarized low energy electron microscopy (SPLEEM) employ this technique's vector imaging capability to resolve domain wall (DW) spin textures. Studying several thin film systems including Co/W(110), Co/Cu(001) and (Co/Ni)n/W(110), we show that an additional contrast can appear at magnetic DWs. By imaging the magnetization as a function of electron landing energy, electron energies are selected at which the magnetic domain contrast vanishes. Surprisingly, under such conditions of zero contrast between magnetic domains, we observe the appearance of magnetic contrast outlining the DWs. This DW contrast does not depend on the DW spin texture. Instead, our measurements show that this DW contrast results from a combination of the energy-dependence of the spin reflectivity asymmetry of the magnetic film, the finite energy width of the spin polarized electron source, and the dispersion of the magnetic prism array that separates the illumination and imaging columns of the instrument. Awareness of this DW contrast mechanism is useful to aid correct interpretation of SPLEEM images.

Experimental and computational evidence of a surface roughness induced magnetic anisotropy in NiFe thin films coated onto substrates of various surface roughnesses is reported. Magnetic coercive fields of 15 nm NiFe thin films coated on substrates with approximately 7 nm average roughness were remarkably 233% larger than identical thin films coated onto smooth substrates with < 1 nm average roughness. The NiFe films coated onto rough substrates developed hard and easy axes, normally non-existent in NiFe Permalloy. A linear correlation of the incline angles of the hard axis hysteresis loops to the average roughness values of the individual substrates was observed, with 99% correlation level. Using a modified micromagnetics theory that incorporates the effects of surface roughness, it is shown the observed magnetic anisotropy arises due to the spatial anisotropy of the surface roughness, resulting in an effective in-plane uniaxial magnetic anisotropy with energy density up to 15 kJ/m3.

Magnetic shape memory materials show outstanding and multifunctional properties (e.g. "giant" magnetomechanical, magnetocaloric, barocaloric), originating from the occurrence of both a martensitic transformation and magnetic order. Thin films and nanostructures of these materials have a great potential for different applications, such as micro- or nano-actuators, energy harvesters, valves and solid-state microrefrigerators [1]. We have demonstrated that the microstructure and magnetic properties of Ni-Mn-Ga thin films can be engineered by properly choosing substrate, growth conditions [2] and post- growth treatments. The films have been epitaxially grown on MgO(100) or on Cr/MgO(100) by RF sputtering, with thicknesses up to 200 nm. We have examined the relation between microstructure and magnetization process, simulating magnetization processes in films with different orientation and spatial organization of the martensitic twin variants. The micromagnetic simulations show a good agreement with the experimental results (Figure 1). Starting from the films grown on Cr/MgO, we have also realized Ni-Mn-Ga nanodots (d=160, 650 nm) by polystyrene-nanosphere lithography, and freestanding nanodisks, by subsequently removing the Cr underlayer via chemical etching. The microstructure and magnetic configuration of the nanostructures are influenced by the lateral confinement and release from the substrate. Furthermore, by varying temperature and applying a magnetic field to the free-standing nanodisks, we have obtained important microstructural changes, enabling different actuation modes [3]. [1] A. Backen et al., Adv. Eng. Mater. 14, 696-709 (2012) [2] P. Ranzieri et al., Adv. Mater. 27, 4760 (2015) [3] M. Campanini et al., Small 2018, 1803027

Ferromagnetic shape memory alloys (FSMA) such as NiMnGa thin films show a strong coupling between magnetic and structural degrees of freedom, which makes it interesting for promising application in smart micro and nano-devices [1]. The ability to control the microstructure at different length scales is of particular interest for the magnetic field induced strain applications. In low-temperature ferromagnetic phase, NiMnGa film consists of differently oriented twin microstructures [2,3]. Magnetic properties can be tuned by engineering these microstructures [4]. In the present study, NiMnGa films (75-200nm) were epitaxially grown on MgO (100) at 200-380°C using RF sputtering technique. The deposition rate was 38.3 to 60.3Å/min. Morphology, composition, and microstructural characterizations were performed using AFM, SEM, EDS, XRD and TEM. Magnetic configuration and behavior were studied by MFM, AGFM, and SQUID. Samples were post-treated by annealing, mechanical stress, and magnetic field cooling. We found that hierarchical twin structure and magnetic properties of the substrate-constrained films can be manipulated by growth temperature, post-heating, mechanical stress, and field cooling.

Ferromagnetic shape memory Heuslers, such as NiMnGa, show multifunctional properties arising from a strong coupling between magnetic, thermal and mechanical degrees of freedom. Thin films have recently attracted much interest for their possible integration in microsystems for the realization of new-concept devices (e.g. microactuators, energy harvesters, solid-state microrefrigerators). Ni2MnGa shows a martensitic phase transformation from a cubic L21 phase (austenite) to a lower symmetry phase (martensite), by decreasing T. In the presently investigated composition (i.e. Ni54Mn22Ga24) the martensitic phase is ferromagnetic with a monoclinic 7M incommensurated structure. The martensitic microstructure is made of complex arrays of differently oriented hierarchical twin microstructures, i.e. X-type and Y-type, where the easy-magnetization directions are respectively out-of-plane and in-plane. In a previous paper we showed that growth conditions (including stress applied during growth) strongly influence the martensitic microstructure of films grown on a Cr underlayer [1]. In the present study we focus on microstructure engineering by post-growth treatments of films directly grown on MgO. Ni54Mn22Ga24 thin films of thickness 200 nm were epitaxially grown on MgO (001) in the range T=200-400 °C by rf sputtering. A multiscale thorough magnetic and structural study was performed be means of several techniques. AFM, SEM, TEM, XRD were used for characterizing microstructure, while the magnetic properties were studied by MFM, Lorentz microscopy, AGFM and SQUID magnetometry. By applying T, magnetic fields and stress to the substrate after growth, a variety of martensitic patterns was obtained, demonstrating that it is possible to engineer the magnetization patterns and strongly influence the magnetization processes. The intimate link between magnetic and structural degrees of freedom and the flexible twin-within-twin martensitic structure makes epitaxial NiMnGa films a unique platform for the precise control of the magnetic configuration from the atomic to the macro-scale also by means of easy and suitable post-growth treatments.

Ferromagnetic shape memory Heuslers show multifunctional properties arising from a strong coupling between magnetic, thermal and mechanical degrees of freedom. Within this class of compounds, Ni2MnGa is a model system, which shows a martensitic phase transformation from a cubic L21phase (austenite) to a lower symmetry phase (martensite) by decreasing temperature. We have investigated the possibility of modifying the martensitic microstructure in Ni-Mn-Ga films with thickness between 75 and 200 nm, grown by r.f. sputtering on MgO(100) or Cr/MgO(100). Films were grown in the temperature range 200 - 400 °C, with different growth parameter (i.e., sputtering rate, sputtering power) and composition Ni54Mn22Ga24. A multiscale magnetic and structural study was performed by means of several techniques: AFM, SEM, TEM, XRD were used for characterizing microstructure, while the magnetic properties were studied by MFM, Lorentz microscopy, AGFM and SQUID magnetometry. The L21austenitic phase grows epitaxial at high temperature both on MgO(100) and Cr/MgO(100). The martensitic phase, which is stable at room temperature, has a monoclinic 7M incommensurated structure. Both the phases are ferromagnetic, but the martensitic phase shows higher magnetocrystalline anisotropy. The martensitic microstructure is made of complex arrays of differently oriented hierarchical twin microstructures, i.e., X-type and Y-type, where the easy-magnetization directions are respectively out-of-plane and in-plane [1]. Controlling the orientation and organization of X- and Y-type twins in epitaxial films and nanostructures could pave the way towards multifunctional applications such as thermomagnetic actuation [2], magnetic storage [3] and magnetic anisotropy dependent properties in fluids [4]. We have focused on microstructure/magnetic pattern engineering by post-growth treatments such as post-annealing, stress, annealing in magnetic field. Through these post-growth treatments, a variety of martensitic patterns (i.e. orientation and spatial organization of the martensitic twin variants) were obtained, demonstrating that it is possible to engineer the magnetic patterns and strongly influence the magnetization processes (Figure 1). The intimate link between magnetic and structural degrees of freedom and the flexible twin-within-twin martensitic structure makes epitaxial Ni-Mn-Ga films a unique platform for the precise control of the magnetic configuration from the atomic to the macro-scale also by means of easy and suitable post-growth treatments.

Balıkesir Üniversitesi Fen Bilimleri Enstitüsü Fizik Ana Bilim Dalı, Bu çalışmada, FeNiCrCd ince filmler DC magnetron püskürtme tekniği ile esnek polimer asetat alt tabaka üzerine biriktirildi. İnce filmler farklı biriktirme oranlarında (0.02, 0.04, 0.06 ve 0.08 nm/s) ve alt tabakanın farklı dönüş hızlarında (0, 15, 30 ve 45 rpm) ayrı ayrı üretildiler. Biriktirme oranının ve alt tabaka dönüş hızının ince filmlerin yapısal ve manyetik özellikleri üzerinde etkileri incelendi. Biriktirme oranının arttırılması (0.02, 0.04, 0.06 ve 0.08 nm/s) ile ince filmlerin atomik içeriklerinde Fe %2.1 kadar azaldı ve Cd %4.6 kadar arttı. Ni ve Cr içerikleri de ~%2 kadar değişti. İnce filmlerin yapısal analizlerinde hcp ve bcc tespit edildi. Biriktirme oranının artışı ile bcc yapıdaki (110) ve (200) piklerinin şiddetleri azalırken hcp (101) pikinin şiddeti arttı. İnce filmlerin tanecik boyutları biriktirme oranının artıması ile birlikte 40.1 nm’den 23.4 nm’e kadar azalış gösterdi. Depozisyon oranının artması ve tanecik boyutunun azalması ile daha parlak filmler elde edildi. Film düzlemine paralel yapılan manyetik ölçümlere göre, depozisyon oranının artmasıyla ince filmlerin doyum Manyetizasyonları, Ms 1030 emu/cm3’den 217 emu/cm3’e kadar ve koersivite, Hc değerleri 11 Oe’den 2 Oe’e kadar azaldı. Elementel analizlere göre alt tabaka dönüş hızlarının arttırılmasıyla (0, 15, 30 ve 45 rpm) ince filmlerin atomik içeriklerinde Fe miktarı %26.8’den %22.9 değerine kadar azalırken Cd içeriği %50.0’den %57.5’ye kadar arttı. Ni ve Cr içeriklerinin de ~%2 kadar değiştikleri bulundu. İnce filmlerin kristal yapılarının bcc olduğu gözlendi ve dönüş hızının artışıyla (110) ve (200) piklerinin şiddetleri azaldı. Film yüzeyine paralel yapılan manyetik ölçümler ince filmlerin Ms değerlerinin alt tabaka dönüş hızının artması ile 780 emu/cm3’den 332 emu/cm3’e azaldığını ve Hc değerlerinin 6 Oe’den 10 Oe’e arttığını gösterdi. Tüm ince filmlerin manyetik kolay eksen yönelimlerinin şekil anizotropisinden dolayı film yüzeyine paralel olduğu ve ince filmlerin yumuşak manyetik malzeme özellikleri gösterdiği belirlendi., In this study, the FeNiCrCd thin films were deposited on a flexible polymer acetate substrate using a DC magnetron sputtering technique. Thin films were produced at different deposition rates (0.02, 0.04, 0.06 and 0.08 nm/s) and the different rotation speeds of the substrate (0, 15, 30 and 45 rpm), respectively. The effects of deposition rate and substrate rotation speed on the structural and magnetic properties of thin films were investigated. It was found that Fe content decreased by 2.1% and Cd Content increased by 4.6% with increasing deposition rate (0.02, 0.04, 0.06 ve 0.08 nm/s). Ni and Cr contents were also changed by ~2%. A combination of hcp and bcc were observed in crystal structural analysis. The intensity of the bcc (110) and (200) peaks decreased while the intensity of hcp (101) peak increased with the increase of deposition rate. Also, the brighter films were obtained when the grain size decreased and the deposition rate increased. To the magnetic measurements parallel to the film plane, the saturation magnetization, Ms decreased from 1030 emu/cm3 to 217 emu/cm3 and coercivity, Hc decreased from 11 Oe to 2 Oe with increasing deposition rate. According to compositional analysis, Fe atomic contents decreased from 26.8% to 22.9% while Cd atomic content increased from 50.0% to 57.5% with the increase of rotation speed (0, 15, 30 ve 45 rpm). Ni and Cr atomic contents in the thin films changed almost by ~2%. The crystal structure of all thin films was observed to be bcc and the intensity of (110) and (200) peaks decreased with the increase of rotation speed. And, the grain size of thin films increased from 25 nm to 51 nm as the substrate rotation speed increased. The surface roughness of the thin films decreased when the substrate rotation speed increased. Magnetic analysis performed parallel to the film plane displayed that the MS values decreased from 780 emu/cm3 to 322 emu/cm3 by the substrate rotation speed increased and the HC values increased from 6 Oe to 10 Oe. For all thin films, it is also seen that the magnetic easy axes were in the film plane due to the shape anisotropy and all thin films show the characteristics of soft magnetic material., Bu tez çalışması Balıkesir Üniversitesi Bilimsel Araştırma Projeleri Birimi tarafından 2018/107 nolu proje ile desteklenmiştir.

Ferromagnetic shape memory materials (FSM) such as Ni-Mn-Ga belong to a class of shape memory alloys, showing a strong coupling between magnetic and structural degrees of freedom, thus giving rise to an evident correlation between magnetic, thermal and mechanical characteristics [1]. In particular, FSM thin films are of special interest due to possible integration into micro- and nanoscale thermomagnetomechanical systems with magnetocaloric, energy harvesting and actuation properties [2-5]. By varying the temperature, Ni-Mn-Ga encounters reversible thermodynamically governed phase transformation between cubic austenite and low symmetry martensite. In order to maintain the compatibility of the two phases and to accommodate the stress caused by martensitic phase transformation, martensite cells form arrays of hierarchical 3D zigzag patterns, which alternate their orientation periodically. The symmetry relation for these patterns, which are called twin variants, is determined as rotation or (and) mirror at the twin boundaries. Upon martensitic transformation, a self-accommodation of different twin variants takes place in nanometric, mesoscopic, and macroscale. In epitaxial Ni-Mn-Ga thin films, the orientation of the twin variants is determined with respect to the substrate. From the total six orientations of the possible twin boundaries along {101} cubic austenitic cell, the so-called Y-type is determined by the two equivalent twin boundaries perpendicular to the substrate, whereas in the so-called X-type, the four equivalent twin boundaries are 45? inclined to the substrate plane. Remarkably, the magnetization easy axis of the martensitic cells alternates in the plane of the film for Y-type across the boundary while the easy magnetization axis alternates in and out of plane of the film for X-type [6]. The ability to control these microstructures at different length scales will pave way towards controlling thermomagnetomechanical properties in magnetic shape memory thin films [7].

The paper is focused on the development and optimization of strontium ferrite nanomaterial and photosintered flexible thin films. These magnetic thin films are characterized with direct current (DC) and high frequency measurements. For photosintered strontium ferrite samples, we achieved relative complex permeability of about 29.5-j1.8 and relative complex permittivity of about 12.9-j0.3 at a frequency of 5.9 GHz.

We investigate the structural and magnetic properties of Ni 78 Fe 22 thin films sandwiched between low-softening-point (LSP) glasses, which can be used in spin quantum cross (SQC) devices utilizing stray magnetic fields generated from magnetic thin-film edges. We also calculate the stray magnetic field generated between the two edges of Ni 78 Fe 22 thin-film electrodes in SQC devices and discuss the applicability to spin-filter devices. Using the established fabrication technique, we successfully demonstrate the formation of LSP-glass/Ni 78 Fe 22 /LSP-glass structures with smooth and clear interfaces. The coercivity of the Ni 78 Fe 22 thin films is enhanced from 0.9 to 103 Oe by increasing the applied pressure from 0 to 1.0 MPa in the thermal pressing process. According to the random anisotropy model, the enhancement of the coercivity is attributed to the increase in the crystal grain size. The stray magnetic field is also uniformly generated from the Ni 78 Fe 22 thin-film edge in the direction perpendicular to the cross section of the LSP-glass/Ni 78 Fe 22 /LSP-glass structures. Theoretical calculation reveals that a high stray field of approximately 5 kOe is generated when the distance between two edges of the Ni 78 Fe 22 thin-film electrodes is less than 5 nm and the thickness of Ni 78 Fe 22 is greater than 20 nm. These experimental and calculation results indicate that Ni 78 Fe 22 thin films sandwiched between LSP glasses are useful as electrodes for SQC devices, serving as spin-filter devices.

A MnxGa1-x (x=0.70) epitaxial thin film with perpendicular magnetic anisotropy and a large coercivity (mu H-0(c) =1 T) was patterned into nanodots using a combined self-assembly nanolithography and plasma etching procedure. Nanostructuring is achieved by self-assembly of polystyrene nanospheres acting as a mask on the magnetic film. This procedure allows easy patterning of a large area although introduced some chemical disorder, which resulted in a soft magnetic component in the magnetic hysteresis loops. However, chemical order was recovered after vacuum annealing at low temperature. The resulting nanodots retain the properties of the original film, i.e. magnetization oriented perpendicular to the particle and large coercivity. Our results suggest this lithography procedure could be a promising direction for nanostructuring tetragonal Heusler alloys. (C) 2016 Elsevier B.V. All rights reserved.

Materials science has been intensively developed during the last 30 years. This is due, on the one hand, to an increasing demand of new materials for new applications and, on the other hand, to technological progress which allows for the synthesis of materials of desired characteristics and to investigate their properties with sophisticated experimental apparatus. Among these advanced materials, magnetic materials at nanometric scale such as ultra thin films or ultra fine aggregates are no doubt among the most important for electronic devices.In this review, we show advanced theoretical methods and solved examples that help understand microscopic mechanisms leading to experimental observations in magnetic thin films. Attention is paid to the case of magnetically frustrated systems in which two or more magnetic interactions are present and competing. The interplay between spin frustration and surface effects is the origin of spectacular phenomena which often occur at boundaries of phases with different symmetries: reentrance, disorder lines, coexistence of order and disorder at equilibrium. These phenomena are shown and explained using of some exact methods, the Green's function and Monte Carlo simulation. We show in particular how to calculate surface spin-wave modes, surface magnetization, surface reorientation transition and spin transport., To appear in JSAMD

A general equation describing the motion of domain walls in a magnetic thin film in the presence of an external magnetic field has been reported in this paper. The equation includes all the contributions from the effects of domain wall inertia, damping and stiffness. The effective mass of the domain wall, the effects of both the interaction of the DW with the imperfections in the material and damping have been calculated.

Co/Ni multilayers ordered according to a periodic, Fibonnacci and Thue Morse sequence have been obtained by Atomic Layer Deposition and a subsequent process of thermal reduction. The morphology of the multilayers was investigated by scanning electron microscopy, while longitudinal hysteresis curves were obtained by magneto-optical magnetometry of Kerr effect. The morphology of the films varies as a function of their sequence and thickness. Multilayers exhibit coercivities much higher than expected from samples synthesized with other methods. The control of the magnetic properties of multilayers, as a function of their sequence, may allow their use in spintronic devices., This work was supported by Fondequip (refs. EQM120045, EQM140092 and EQM160155), Fondecyt (refs. 1200302, 3180042 and 1201491), Basal Project (ref. AFB180001) and Universidad Central (ref. CIP2018006).