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See Supplemental Material for detailed sample structure, data on thickness-dependent YIG|Pt, a back-to-back comparison of the THz signals, data on magnetic dipole radiation, reproducibility, growth conditions and fluence dependence of magnetite.

Chemical substitution is one of the most efficient tools to tune and optimize magnetic and magnetocaloric properties of the giant magnetocaloric materials. In particular, Indium substitutions could be useful both for tuning properties of these interesting intermetallic materials and to unveil their local-scale behavior across the magnetostructural transition via hyperfine techniques. Hence, in order to investigate the effect of Indium additions on the crystal structure, micro-structure, magnetic and magnetocaloric properties, a series of In-containing samples derived from the base Gd5Si1.2Ge2.8 stoichiometry were prepared. The major findings are that while In is insoluble in the 5: 4 phase, it will instead promote the emergence of the impurity 5: 3 phase and segregates into this phase. Hence, In leads to major crystallographic changes, which enhance atomic disorder and disrupt the Si to Ge ratio in the 5: 4 phase. Subsequently, a higher 5: 4 unit cell volume and a lower magnetic ordering temperature are found in the In-substituted samples. Finally, the magnetocaloric properties of the In-substituted samples reveal a detrimental effect on the maximum magnetic entropy change.

We have studied the anomalous Nernst effect (ANE) in [Fe3O4/Pt]-based heterostructures grown at high temperature, by measuring the ANE-induced electric field with a magnetic field applied normal to the sample surface, in the perpendicular magnetized configuration, where only the ANEs from the ferromagnetic layers or magnetic proximity effects can be detected. An ANE voltage is observed for [Fe3O4/Pt]n multilayers, and we further investigated its origin by performing measurements in [Fe3O4/Pt/Fe3O4] trilayers as a function of the Pt thickness. Our results suggest the presence of an interface-induced ANE in the metallic layer, possibly driven by a heat induced subnanometer interdiffusion which affects the nature of the Fe3O4/Pt interface. Despite this ANE, the spin Seebeck effect is the dominant mechanism for the transverse thermoelectric voltage in the in-plane magnetized configuration, accounting for about 70% of the voltage in the multilayers.

© 2021The Author(s). Artículo firmado por más de diez autores. We thank V. P. Amin, S. Sangiao, A. Fert, and F. Casanova for valuable discussions. This research was supported by the Regional Government of Madrid through Project No. P2018/NMT-4321 (NANOMAGCOST-CM) and the Spanish Ministry of Economy and Competitiveness (MINECO) through Project Nos. RTI2018-097895-B-C42, RTI2018-097895-B-C43 (FUN-SOC), PGC2018-098613-B-C21 (SpOrQuMat), PGC2018-098265-B-C31, and PCI2019-111867-2 (FLAG ERA 3 grant SOgraphMEM). J.M.D.T. and A.G. acknowledge support from MINECO and CM through Grant Nos. BES-2017-080617 and PEJD-2017-PREIND-4690, respectively. I.A. acknowledges financial support from the Regional Government of Madrid through Contract No. PEJD-2019-POST/IND-15343. IMDEA Nanoscience is supported by the "Severo Ochoa" Program for Centres of Excellence in R&D, MINECO (Grant No. SEV-2016-0686). A.A., S.P.-W., and J.-C.R.-S. acknowledge support from Toptronic ANR through Project No. ANR-19-CE24-0016-01. P.J.-C., I.L., L.M., P.A.A., and M.R.I. acknowledge support from Project No. MAT2017-82970-C2-R. Electron microscopy observations were carried out at the Centro Nacional de Microscopia Electronica at the Universidad Complutense de Madrid., Spin Hall and Rashba-Edelstein effects, which are spin-to-charge conversion phenomena due to spin-orbit coupling (SOC), are attracting increasing interest as pathways to manage rapidly and at low consumption cost the storage and processing of a large amount of data in spintronic devices as well as more efficient energy harvesting by spin-caloritronics devices. Materials with large SOC, such as heavy metals (HMs), are traditionally employed to get large spin-to-charge conversion. More recently, the use of graphene (gr) in proximity with large SOC layers has been proposed as an efficient and tunable spin transport channel. Here, we explore the role of a graphene monolayer between Co and a HM and its interfacial spin transport properties by means of thermo-spin measurements. The gr/HM (Pt and Ta) stacks have been prepared on epitaxial Ir(111)/Co(111) structures grown on sapphire crystals, in which the spin detector (i.e., top HM) and the spin injector (i.e., Co) are all grown in situ under controlled conditions and present clean and sharp interfaces. We find that a gr monolayer retains the spin current injected into the HM from the bottom Co layer. This has been observed by detecting a net reduction in the sum of the spin Seebeck and interfacial contributions due to the presence of gr and independent from the spin Hall angle sign of the HM used., Ministerio de Economía y Competitividad (MINECO), Ministerio de Ciencia e Innovación (MICINN), Agence Nationale de la Recherche (ANR), Comunidad de Madrid, Centros de Excelencia Severo Ochoa (MINECO), Depto. de Física de Materiales, Fac. de Ciencias Físicas, TRUE, pub

© 2021The Author(s). Artículo firmado por más de diez autores. We thank V. P. Amin, S. Sangiao, A. Fert, and F. Casanova for valuable discussions. This research was supported by the Regional Government of Madrid through Project No. P2018/NMT-4321 (NANOMAGCOST-CM) and the Spanish Ministry of Economy and Competitiveness (MINECO) through Project Nos. RTI2018-097895-B-C42, RTI2018-097895-B-C43 (FUN-SOC), PGC2018-098613-B-C21 (SpOrQuMat), PGC2018-098265-B-C31, and PCI2019-111867-2 (FLAG ERA 3 grant SOgraphMEM). J.M.D.T. and A.G. acknowledge support from MINECO and CM through Grant Nos. BES-2017-080617 and PEJD-2017-PREIND-4690, respectively. I.A. acknowledges financial support from the Regional Government of Madrid through Contract No. PEJD-2019-POST/IND-15343. IMDEA Nanoscience is supported by the "Severo Ochoa" Program for Centres of Excellence in R&D, MINECO (Grant No. SEV-2016-0686). A.A., S.P.-W., and J.-C.R.-S. acknowledge support from Toptronic ANR through Project No. ANR-19-CE24-0016-01. P.J.-C., I.L., L.M., P.A.A., and M.R.I. acknowledge support from Project No. MAT2017-82970-C2-R. Electron microscopy observations were carried out at the Centro Nacional de Microscopia Electronica at the Universidad Complutense de Madrid., Spin Hall and Rashba-Edelstein effects, which are spin-to-charge conversion phenomena due to spin-orbit coupling (SOC), are attracting increasing interest as pathways to manage rapidly and at low consumption cost the storage and processing of a large amount of data in spintronic devices as well as more efficient energy harvesting by spin-caloritronics devices. Materials with large SOC, such as heavy metals (HMs), are traditionally employed to get large spin-to-charge conversion. More recently, the use of graphene (gr) in proximity with large SOC layers has been proposed as an efficient and tunable spin transport channel. Here, we explore the role of a graphene monolayer between Co and a HM and its interfacial spin transport properties by means of thermo-spin measurements. The gr/HM (Pt and Ta) stacks have been prepared on epitaxial Ir(111)/Co(111) structures grown on sapphire crystals, in which the spin detector (i.e., top HM) and the spin injector (i.e., Co) are all grown in situ under controlled conditions and present clean and sharp interfaces. We find that a gr monolayer retains the spin current injected into the HM from the bottom Co layer. This has been observed by detecting a net reduction in the sum of the spin Seebeck and interfacial contributions due to the presence of gr and independent from the spin Hall angle sign of the HM used., Ministerio de Economía y Competitividad (MINECO), Ministerio de Ciencia e Innovación (MICINN), Agence Nationale de la Recherche (ANR), Comunidad de Madrid, Centros de Excelencia Severo Ochoa (MINECO), Depto. de Física de Materiales, Fac. de Ciencias Físicas, TRUE, pub

We report the disentanglement of bulk and interfacial contributions to the thermally excited magnon spin current in the spin Seebeck effect under static heating. For this purpose, we have studied the dependence of the inverse spin Hall voltage and the thermal conductivity on the magnetic layer thickness. Knowledge of these quantities allows us to take into account the influence of both sources of thermal spin current in the analysis of the voltage dependence. The magnetic layer thickness modulates the relative magnitude of the involved thermal drops for a fixed total thermal difference throughout the sample. In the end, we attain the separate contributions of both sources of thermal spin current—bulk and interfacial—and obtain the value of the thermal magnon accumulation length scale in maghemite, which we find to be 29(1) nm. According to our results, bulk magnon accumulation dominates the spin Seebeck effect in our studied range of thicknesses, but the interfacial component is by no means negligible

Accessing the regime of coherent phonon propagation in nanostructures opens enormous possibilities to control the thermal conductivity in energy harvesting devices, phononic circuits, etc. In this paper we show that coherent phonons contribute substantially to the thermal conductivity of LaCoO3/SrTiO3 oxide superlattices, up to room temperature. We show that their contribution can be tuned through small variations of the superlattice periodicity, without changing the total superlattice thickness. Using this strategy, we tuned the thermal conductivity by 20% at room temperature. We also discuss the role of interface mixing and epitaxial relaxation as an extrinsic, material dependent key parameter for understanding the thermal conductivity of oxide superlattices.

We report the disentanglement of bulk and interfacial contributions to the thermally excited magnon spin current in the spin Seebeck effect under static heating. For this purpose, we have studied the dependence of the inverse spin Hall voltage and the thermal conductivity on the magnetic layer thickness. Knowledge of these quantities allows us to take into account the influence of both sources of thermal spin current in the analysis of the voltage dependence. The magnetic layer thickness modulates the relative magnitude of the involved thermal drops for a fixed total thermal difference throughout the sample. In the end, we attain the separate contributions of both sources of thermal spin current—bulk and interfacial—and obtain the value of the thermal magnon accumulation length scale in maghemite, which we find to be 29(1) nm. According to our results, bulk magnon accumulation dominates the spin Seebeck effect in our studied range of thicknesses, but the interfacial component is by no means negligible.

We show that an optimized growth of magnetic layer/non-magnetic layer stacks allows for the improvement of the spin-to-charge conversion efficiency. From the analysis of the voltage signal generated in spin pumping experiments due to the inverse spin Hall effect (ISHE) on Y3Fe5O12 (YIG)/Bi stacks, we have determined values for the spin Hall angle and the spin-diffusion length in Bi of 0.0068(8) and 17.8(9) nm, respectively. Based on these results, we have also studied the influence of aging on the spin-to-charge conversion efficiency by performing spin pumping experiments on YIG/Bi stacks after exposing the samples to ambient conditions for several days and up to 150 days. We have found that in YIG/Bi samples with Bi thicknesses around or below the spin-diffusion length, the ISHE voltage signal is still above 80% of its initial value after 100 days.

A metal–insulator transition is observed in spin–orbit-coupled IrO2 thin films upon reduction of the film thickness. In the epitaxially grown samples, the critical thickness (t ∼ 1.5–2.2 nm) is found to depend on growth orientation (001), (100) or (110). Interestingly from the applied point of view, the insulating behavior is found even in polycrystalline ultrathin films. By analyzing the experimental electrical response with various theoretical models, we find good fits to the Efros–Shklovskii-VRH and the Arrhenius-type behaviors, which suggests an important role of electron correlations in determining the electrical properties of IrO2. Our magnetic measurements also point to a significant role of magnetic order. Altogether, our results would point to a mixed Slater- and Mott-type of insulator.

We studied in detail the in-plane magnetic properties of heterostructures based on a ferroelectric BaTiO3 overlayer deposited on a ferromagnetic La2/3Sr1/3MnO3 film grown epitaxially on pseudocubic (001)-oriented SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7 and LaAlO3 substrates. In this configuration, the combination of both functional perovskites constitutes an artificial multiferroic system with potential applications in spintronic devices based on the magnetoelectric effect. La2/3Sr1/3MnO3 single layers and BaTiO3/La2/3Sr1/3MnO3 bilayers using the pulsed-laser deposition technique. We analyzed the films structurally through X-ray reciprocal space maps and high-angle annular dark field microscopy, and magnetically via thermal demagnetization curves and in-plane magnetization versus applied magnetic field loops at room temperature. Our results indicate that the BaTiO3 layer induces an additional strain in the La2/3Sr1/3MnO3 layers close to their common interface. The presence of BaTiO3 on the surface of tensile-strained La2/3Sr1/3MnO3 films transforms the in-plane biaxial magnetic anisotropy present in the single layer into an in-plane uniaxial magnetic anisotropy. Our experimental evidence suggests that this change in the magnetic anisotropy only occurs in tensile-strained La2/3Sr1/3MnO3 film and is favored by an additional strain on the La2/3Sr1/3MnO3 layer promoted by the BaTiO3 film. These findings reveal an additional mechanism that alters the magnetic behavior of the ferromagnetic layer, and consequently, deserves further in-depth research to determine how it can modify the magnetoelectric coupling of this hybrid multiferroic system.

Spin Pumping technics have been used here in a YIG//Bi bilayer structure to generate a pure spin current flowing within the Bi metallic layer, in order to measure spin to charge conversion parameters. This spin to charge conversion, is observed in the metallic (Bi) layer as a result of the Inverse Spin Hall Effect (ISHE), widely documented in literature. After setting up an external static magnetic field, Ferromagnetic Resonance (FMR) h as been excited within the magnetic layer (YIG) of the samples, using a broadband shorted microstrip RF setup. A pure spin current is obtained in the Bi metallic layer, where Spin Orbit Interaction (SOI) gives rise to a selective scattering, resulting in perpendicular charge current, which has been measured, directly, or via the voltage induced by this current on the sample edges. YIG magnetic insulator layer in place of a conducting one has been chosen to avoid superposition of unwanted electric signals under resonance condition as done in ref [3]. The dependences of the measured ISHE Voltage with frequency, metallic layer thickness, static magnetic field angle and radiofrequency power have been measured and analysed. Results were compared with those obtained in references (where a conducting magnetic layer was used), (where Platinum was used as metallic layer in place of Bismuth) and (where the same bilayer nanostructure YIG//Bi was analysed in a RF monofrequency cavity setup) Finally, the evolution of the ISHE voltage with ageing of the samples has been observed and systematically investigated.

In this work we show the fabrication of epitaxial Y3Fe5O12 (YIG) thin films on Gd3Ga5O12 (111) (GGG) substrates by means of Polymer Assisted Deposition (PAD). Cubic YIG is a well known and characterized ferrimagnetic material at room temperature, with excellent magneto-optical properties, high electrical resistivity, and a very narrow ferromagnetic resonance, which makes it particularly suitable for applications in filters and resonators at microwave frequencies such us in potential devices based on spintronics, magnonics and spin caloritronics. In the case of YIG, the Fe3+ sublattices order antiferromagnetically, with nonmagneticY3+ ions located at the dodecahedral sites. Therefore, it is the uncompensated Fe3+ at a tetrahedral site which is responsible for the ferrimagnetic state at room temperature, up to a Curie temperature ∼560K. Thus, it is upon the precise stoichiometry and distribution of Fe3+ ions among the octahedral/tetrahedral sites of this complex structure, where relies the emergence of all these interesting properties, which to date has normally hampered the production of high-quality YIG thin films by affordable chemical methods. The standard method for YIG single-crystal fabrication, liquid phase epitaxy, and other bulk synthetic are not suitable methods to fabricate samples down to the nanometer range. Even when this problem has been overcome by using physical techniques such as pulsed laser deposition or sputtering, chemical solution deposition offers important advantages regarding affordability and scalability for thin-film fabrication. Here we report the chemical solution synthesis of YIG thin films, with excellent chemical, crystalline, and magnetic homogeneity. The films show a very narrow ferromagnetic resonance (long spin relaxation time), comparable to that obtained from high vacuum physical deposition methods. These results demonstrate that chemical methods can compete to develop nanometer-thick YIG films with the quality required for spin

Since the discovery of the spin Seebeck effect (SSE), much attention has been devoted to the study of the interaction between heat, spin, and charge in magnetic systems. The SSE refers to the generation of a spin current upon the application of a thermal gradient and detected by means of the inverse spin Hall effect. Conversely, the spin Peltier effect (SPE) refers to the generation of a heat current as a result of a spin current induced by the spin Hall effect. Here we report a strong enhancement of both the SSE and SPE in Fe3O4/Pt multilayered thin films at room temperature as a result of an increased thermo-spin conversion efficiency in the multilayers. These results open the possibility to design thin film heterostructures that may boost the application of thermal spin currents in spintronics.

We report the fabrication of epitaxial Y 3 F e 5 O 12 (YIG) thin films on G d 3 G a 5 O 12 (111) using a chemical solution method. Cubic YIG is a ferrimagnetic material at room temperature, with excellent magneto-optical properties, high electrical resistivity, and a very narrow ferromagnetic resonance, which makes it particularly suitable for applications in filters and resonators at microwave frequencies. But these properties depend on the precise stoichiometry and distribution of F e 3 + ions among the octahedral/tetrahedral sites of a complex structure, which hampered the production of high-quality YIG thin films by affordable chemical methods. Here we report the chemical solution synthesis of YIG thin films, with excellent chemical, crystalline, and magnetic homogeneity. The films show a very narrow ferromagnetic resonance (long spin relaxation time), comparable to that obtained from high-vacuum physical deposition methods. These results demonstrate that chemical methods can compete to develop nanometer-thick YIG films with the quality required for spintronic devices and other high-frequency applications

We show here that superconductivity can be induced in bismuth nanostripes through electrical contact with an s-wave superconductor. By studying the electrical transport properties of devices containing a bismuth nanostripe with superconducting contacts, we have found that bismuth nanostripes with inner probe distance shorter than 125 nm become superconductor at temperatures above 2 K due to proximity-induced superconductivity. This finding, along with previously demonstrated large spin–orbit coupling in bismuth, opens the way to the design of bismuth-based nanostructures bearing Majorana bound states.

Terahertz emission spectroscopy (TES) of ultrathin multilayers of magnetic and heavy metals has recently attracted much interest. This method not only provides fundamental insights into photoinduced spin transport and spin–orbit interaction at highest frequencies, but has also paved the way for applications such as efficient and ultrabroadband emitters of terahertz (THz) electromagnetic radiation. So far, predominantly standard ferromagnetic materials have been exploited. Here, by introducing a suitable figure of merit, we systematically compare the strength of THz emission from X/Pt bilayers with X being a complex ferro-, ferri- and antiferromagnetic metal, that is, dysprosium cobalt (DyCo5), gadolinium iron (Gd24Fe76), magnetite (Fe3O4) and iron rhodium (FeRh). We find that the performance in terms of spin-current generation not only depends on the spin polarization of the magnet's conduction electrons, but also on the specific interface conditions, thereby suggesting TES to be a highly interface-sensitive technique. In general, our results are relevant for all applications that rely on the optical generation of ultrafast spin currents in spintronic metallic multilayers.

We report the fabrication of epitaxial Y3Fe5O12 (YIG) thin films on Gd3Ga5O12 (111) using a chemical solution method. Cubic YIG is a ferrimagnetic material at room temperature, with excellent magneto-optical properties, high electrical resistivity, and a very narrow ferromagnetic resonance, which makes it particularly suitable for applications in filters and resonators at microwave frequencies. But these properties depend on the precise stoichiometry and distribution of Fe3+ ions among the octahedral/tetrahedral sites of a complex structure, which hampered the production of high-quality YIG thin films by affordable chemical methods. Here we report the chemical solution synthesis of YIG thin films, with excellent chemical, crystalline, and magnetic homogeneity. The films show a very narrow ferromagnetic resonance (long spin relaxation time), comparable to that obtained from high-vacuum physical deposition methods. These results demonstrate that chemical methods can compete to develop nanometer-thick YIG films with the quality required for spintronic devices and other high-frequency applications.

The spin Peltier effect (SPE), heat-current generation as a result of spin-current injection, has been investigated in alternately stacked Pt/Fe3O4 multilayer films. The temperature modulation induced by the SPE in the [Pt/Fe3O4]×n films was found to be significantly enhanced with increasing the number of Pt/Fe3O4 bilayers n. This SPE enhancement is much greater than that expected for a simple stack of independent Pt/Fe3O4 bilayers. The observed n dependence of the SPE can be explained by introducing spin-current redistribution in the multilayer films in the thickness direction, in a manner similar to the enhancement of the spin Seebeck effect in multilayers.

We report temperature dependent measurements of the spin Seebeck effect (SSE) in multilayers formed by repeated growth of a FeO/Pt bilayer junction. The magnitude of the observed enhancement of the SSE, relative to the SSE in the single bilayer, shows a monotonic increase with decreasing the temperature. This result can be understood by an increase of the characteristic length for spin current transport in the system, in qualitative agreement with the recently observed increase in the magnon diffusion length in FeO at lower temperatures. Our result suggests that the thermoelectric performance of the SSE in multilayer structures can be further improved by careful choice of materials with suitable spin transport properties.

We report a systematic study on the thermoelectric performance of spin Seebeck devices based on Fe3O4/Pt junction systems. We explore two types of device geometries: a spin Hall thermopile and spin Seebeck multilayer structures. The spin Hall thermopile increases the sensitivity of the spin Seebeck effect, while the increase in the sample internal resistance has a detrimental effect on the output power. We found that the spin Seebeck multilayers can overcome this limitation since the multilayers exhibit the enhancement of the thermoelectric voltage and the reduction of the internal resistance simultaneously, therefore resulting in significant power enhancement. This result demonstrates that the multilayer structures are useful for improving the thermoelectric performance of the spin Seebeck effect.

The spin transport and spin-to-charge current conversion properties of bismuth are investigated using permalloy/copper/bismuth (Py/Cu/Bi) lateral spin valve structures. The spin current is strongly absorbed at the surface of Bi, leading to ultrashort spin-diffusion lengths. A spin-to-charge current conversion is measured, which is attributed to the inverse Rashba-Edelstein effect at the Cu/Bi interface. The spin-current-induced charge current is found to change direction with increasing temperature. A theoretical analysis relates this behavior to the complex spin structure and dispersion of the surface states at the Fermi energy. The understanding of this phenomenon opens novel possibilities to exploit spin-orbit coupling to create, manipulate, and detect spin currents in two-dimensional systems.

The present invention relates to a thermoelectric device which comprises: a layer of a non-magnetic material (NM) disposed over a layer of a magnetic material (F), wherein said layers form a first bi-layer junction (NM1/F1) of materials having spin Seebeck effect properties; and at least one second bi-layer junction of non-magnetic and magnetic materials (NM2/F2) having spin current transmission properties; wherein the second bi-layer junction (NM2/F2) is arranged to form, together with the first bi-layer junction (NM1/F1), a multilayer structure of materials having an amplified spin Seebeck effect compared to that of the first bi-layer junction (NM1/F1) alone. An optimized device can be obtained by stacking sequences of these bi-layers in a multilayered structure n×(NM/F). The invention provides improved spin Seebeck thermoelectric devices, through a novel arrangement of materials which provide a substantial amplification of the spin pumped currents within the multilayer structure, thus generating enhanced voltage signals compared to those present in the prior art.

We show here that using spin orbit coupling interactions at a metallic interface it is possible to control the sign of the spin to charge conversion in a spin pumping experiment. Using the intrinsic symmetry of the “Inverse Rashba Edelstein Effect” (IREE) in a Bi/Ag interface, the charge current changes sign when reversing the order of the Ag and Bi stacking. This confirms the IREE nature of the conversion of spin into charge in these interfaces and opens the way to tailoring the spin sensing voltage by an appropriate trilayer sequence.

Epitaxial films of SrMnO3 and bilayers of SrMnO3/La0.67Sr0.33MnO3 have been deposited by pulsed laser deposition on different substrates, namely, LaAlO3 (001), (LaAlO3)0.3(Sr2AlTaO6)0.7 (001), and SrTiO3 (001), allowing us to perform an exhaustive study of the dependence of antiferromagnetic order and exchange bias field on epitaxial strain. The Néel temperatures (TN) of the SrMnO3 films have been determined by low-energy muon spin spectroscopy. In agreement with theoretical predictions, TN is reduced as the epitaxial strain increases. From the comparison with first-principles calculations, a crossover from G-type to C-type antiferromagnetic orders is proposed at a critical tensile strain of around 1.6±0.1%. The exchange bias (coercive) field, obtained for the bilayers, increases (decreases) by increasing the epitaxial strain in the SrMnO3 layer, following an exponential dependence with temperature. Our experimental results can be explained by the existence of a spin-glass (SG) state at the interface between the SrMnO3 and La0.67Sr0.33MnO3 films. This SG state is due to the competition between the different exchange interactions present in the bilayer and favored by increasing the strain in the SrMnO3 layer.

A novel mechanism of ferroelectricity driven by off-centering magnetic Mn4+ ions was proposed in (Sr1–xBax)MnO3, in its ideal perovskite phase, which yields enormous expectations in the search for strong magnetoelectric materials. Still, the desired perovskite phase has never been stabilized in thin films due to its extremely metastable character. Here, we report on a thorough study of the perovskite phase stabilization of (Sr1–xBax)MnO3 thin films, 0.2 ≤ x ≤ 0.5, grown by pulsed laser deposition onto (001)-oriented perovskite substrates. X-ray diffraction measurements and scanning transmission electron microscopy reveal that, under appropriate deposition conditions, the perovskite phase is fully stabilized over the nonferroelectric hexagonal phase, despite the latter being increasingly favored on increasing Ba-content. Moreover, we have managed to grow epitaxial coherent cube-on-cube (Sr1–xBax)MnO3 films upon strains ranging from 0% to 4%. Our results become a milestone in further studying perovskite (Sr1–xBax)MnO3 thin films and pave the way for tailoring ferroic and magnetoelectric properties either by strain engineering or Ba-doping.

Los óxidos de metales de transición exhiben una rica variedad de fenómenos físicos que incluyen la superconductividad de alta temperatura, magnetorresistencia colosal (CMR), efectos espintrónicos y multiferroicidad. En los últimos años, esta exótica fenomenología ha estado acompañada por el avance en el crecimiento de láminas delgadas epitaxiales de alta calidad de estos materiales, conduciendo al diseño de nuevas funcionalidades y al descubrimiento de fenómenos emergentes en las interfaces. Además, tras la reducción de la dimensionalidad, se esperan que efectos de confinamiento den lugar a nuevas propiedades en la nanoescala. Los resultados más importantes de esta tesis son: 1) Hemos combinado la litografía óptica con la litografía por hace de iones para producir nanohilos monocristalinos de La2/3Ca1/3MnO3 (LCMO) y La2/3Sr1/3MnO3 (LSMO) con una alta relación longitud-ancho superior a 300 y 700, respectivamente, que preservan, o incluso mejoran las propiedades del sistema en comparación con el material masivo y en la lámina delgada. Cabe destacar que en el nanohilo más estrecho de LCMO (w = 150 nm), hemos reportado un valor de magnetorresistencia de 34% a un campo magnético aplicado de 0.1 T. Hemos asociamos dicho comportamiento al relajamiento de la estructura cristalina del LCMO en los bordes del nanohilo junto con la desestabilización de las regiones aislantes. Tanto el método de fabricación de nanohilos como la obtención de altos valores de magnetorresistencia a bajos campos magnéticos aplicados abrirían nuevas rutas para implementar dichas estructuras en dispositivos funcionales para espintrónica., 2) En el caso de láminas delgadas de LCMO y heteroestructuras tipo unión túnel (LSMO/SrTiO3/LCMO), experimentos de holografía electrónica mostraron que el estrés epitaxial inducido por el sustrato promueve la segregación de una capa uniforme no ferromagnética en la parte superior de una capa ferromagnética (FM) en la manganita de LCMO. Análisis químicos microscópicos realizados mediante STEM-EELS demostraron que toda la capa de LCMO es química y estructuralmente homogénea descartando que dicha capa no ferromagnética tuviese un origen de naturaleza química. La observación de un efecto de ¿Exchange Bias¿ en medidas macroscópicas de la imanación revelaron que dicha capa no ferromagnética debería tener un carácter antiferromagnético (AFM). Para diferentes sustratos y condiciones de crecimiento la tetragonalidad de LCMO a temperatura ambiente, definida como ¿ = |c-a|/a, es la que conduce a la coexistencia de fases por encima de un valor critico ¿C ~ 0.024. Cálculos teóricos demuestran que el aumento de la tetragonalidad cambia el balance de energía de los estados fundamentales FM y AFM en láminas de LCMO tensionadas, lo que permite la formación de estados magnéticamente no homogéneos. Este trabajo proporciona evidencia clave que abre una nueva ruta para sintetizar bicapas FM/AFM inducidas por estrés en una única lámina delgada, que presentan el fenómeno de Exchange Bias lo cual podría servir en futuros dispositivos para espintrónica.

We have combined optical and focused ion beam lithographies to produce large aspect-ratio (length-to-width >300) single-crystal nanowires of La 2/3Ca1/3MnO3 that preserve their functional properties. Remarkably, an enhanced magnetoresistance value of 34% in an applied magnetic field of 0.1 T in the narrowest 150 nm nanowire is obtained. The strain release at the edges together with a destabilization of the insulating regions is proposed to account for this behavior. This opens new strategies to implement these structures in functional spintronic devices. © 2014 American Chemical Society.

En los últimos años ha resurgido la búsqueda de materiales multiferroicos que muestren simultáneamente orden ferroeléctrico y magnético. Tanto la escasez de estos materiales, debido a la competencia entre ambas propiedades, como el débil acoplamiento magnetoeléctrico en la mayoría de ellos obliga al desarrollo de nuevos métodos que eviten estas restricciones y posibiliten la creación de dispositivos con diversas aplicaciones. Recientes predicciones teóricas muestran como el sistema SrMnO3 puede volverse multiferroico gracias a la expansión de la celda unidad debida a la tensión epitaxial. En este sistema, la ferroelectricidad se debería al desplazamiento de su posición central del ión magnético Mn4+, por lo que se espera un fuerte acoplamiento magnetoeléctrico con temperaturas de orden similares. En este trabajo se presenta un estudio de las propiedades estructurales, eléctricas y magnéticas de películas delgadas epitaxiales de SrMnO3, crecidas por la técnica de deposición por láser pulsado, en función de la tensión epitaxial experimentada. Se ha comprobado la estabilización de la fase pseudo-cúbica por difracción de rayos X y se ha monitorizado el crecimiento in-situ gracias a medidas de RHEED. Las películas con mejor calidad cristalina fueron estudiadas por STEM (scanning transmission electron microscopy) mostrando excelente homogeneidad y ausencia de defectos. Además, se realizó un estudio de la estequiometría y del estado de oxidación del Mn por EELS (electron energy loss spectroscopy). Por último, se mostrarán los resultados tanto de las propiedades ferroeléctricas de las películas a partir de medidas eléctricas y de PFM, como de sus propiedades magnéticas, lo que demuestra su carácter multiferróico.

In this work, well-ordered epitaxial FePt thin films have been grown by RF sputtering on two different substrates (MgO (100) and SrTiO3 (100)) and the effect of different lattice parameters between the substrate and FePt film on morphology and magnetic behavior has been considered. Growth conditions have been optimized to obtain different morphologies and magnetic behaviors

Multiferroic materials showing simultaneous magnetic and ferroelectric ordering have become the subject of intensive research in recent years. The scarcity of these materials due to the different phenomena responsible for both properties [1], and also, the weak magnetoelectric coupling in most of them force to develop new methods to avoid the excluding mechanisms of ferroelectricity and magnetism. Recent theoretical predictions suggested that epitaxially strained SrMnO3 should become multiferroic [2] by means of the interplay of spins, lattice phonons and strain of the perovskite unit cell. In this system, Mn4+ (d3 ion) is expected to drive both the magnetic order (Mn-O-Mn magnetic superexchange interaction) and the required non-centrosymmetric distortion for ferroelectric order (Mn4+ off-centers from MnO6 octahedra), and therefore a strong magnetoelectric coupling with similar ordering temperatures is expected. Here, we have investigated the effect of epitaxial strain on the structure, electrical and magnetic properties of SrMnO3 (SMO) thin films. Pulsed laser deposition technique was used to grow strained SMO thin films onto different substrates ranging from -1% compressive to 4% tensile mismatch. Coherent cube-on-cube growth is borne out by X-ray diffraction measurements, which also show that under appropriate deposition conditions and substrate selection the perovskite pseudo-cubic phase is stabilized in thin films. The growth of the samples has been monitored by in situ RHEED measurements. Selected films were observed by scanning transmission electron microscopy, which prove the homogeneity and defect-free crystal quality of films. Chemical composition mapping, stoichiometry and 4+ oxidation state of Mn were corroborated by electron energy loss spectroscopy. Nonlinear optics measurements reveal that SMO films, under epitaxial stress, emitt optical second harmonic generation signal, pointing to inversion symmetry being broken (necessary condition for ferroelectr

A generalized procedure for the in situ application of magnetic fields by means of the excitation of the objective lens for magnetic imaging experiments in Lorentz microscopy and electron holography is quantitatively described. A protocol for applying magnetic fields with arbitrary in-plane magnitude and orientation is presented, and a freeware script for Digital Micrograph¿ is provided to assist the operation of the microscope. Moreover, a method to accurately reconstruct hysteresis loops is detailed. We show that the out-of-plane component of the magnetic field cannot be always neglected when performing quantitative measurements of the local magnetization. Several examples are shown to demonstrate the accuracy and functionality of the methods.

We report the experimental observation of the spin Seebeck effect in magnetite thin films. The signal observed at temperatures above the Verwey transition is a contribution from both the anomalous Nernst (ANE) and spin Seebeck (SSE) effects. The contribution from the ANE of the Fe3O 4 layer to the SSE is found to be negligible due to the resistivity difference between Fe3O4 and Pt layers. Below the Verwey transition, the SSE is free from the ANE of the ferromagnetic layer and it is also found to dominate over the ANE due to magnetic proximity effect on the Pt layer. © 2013 American Institute of Physics.

Magnetization studies have been performed on a polycrystalline sample of Dy5Si3Ge as a function of an applied magnetic field (up to 50 kOe) and hydrostatic pressure (up to 10 kbar) in the 5-300 K temperature range. The anomalous behavior of the magnetic susceptibility indicates that a Griffiths-like phase exists at low magnetic fields and pressures up to 10 kbar. We present evidence that the high-temperature second-order ferromagnetic transition can be coupled with the low-temperature first-order crystallographic transformation into a single first-order magnetic-crystallographic transformation using a magnetic field and hydrostatic pressure as tuning parameters. The effect of pressure on the Griffiths-like phase is reported and analyzed in the framework of the complex competition between the interslab and intraslab magnetic interactions. © 2013 American Physical Society.

Bismuth nanostructures of initial lateral size of about 150 nm were successfully electromigrated at room temperature under high vacuum conditions through the application of voltage ramps and accurate control of their conductance. The imaging of the nanogap formation was followed by scanning electron microscopy. An appropriate design of the initial Bi nanostructures has made the electromigration process of semimetallic Bi feasible. Beyond the intrinsic interest in the generation of Bi structures with size tailored at the nanoscale, remarkable features have been observed in the time-dependent conductance curves of the Bi nanoconstrictions. In particular, sub-quantum conductance plateaus can be detected before the rupture of the constriction. An alternative procedure to study the transport through Bi nanoconstrictions has been explored using a focused-Ga-ion etching process with simultaneous control of the conductance. This second approach confirms the transport behavior observed in electromigrated Bi nanoconstrictions. © 2013 The Owner Societies.

Advances in nanofabrication of magnetic nanomaterials leads to search for new tools to measure physical properties at the nanoscale, such as magnetization. Accurate characterization of the magnetization states in nanostructures is of the utmost importance in the development new devices such as magnetic tunnel junctions (MTJs). Transmission electron microscopy (TEM) techniques such as Electron Holography (EH) allows the quantitative imaging of the magnetization states of ferromagnetic materials with unprecedented spatial resolution at the nanometer level. Furthermore, EH can be combined with the in situ variation of external parameters such as magnetic and electric fields, temperature, etc. In particular, we use a TEM cryo-holder to image magnetization states of ferromagnets with low TC (down to 100 K) while varying in situ the magnetic field applied by the objective lens of the TEM. In this work, we present the EH study of the magnetization switching of La-Sr and La-Ca manganite thin films (TC = 180 and 300 K, respectively) and manganite-based MTJs. The (de)coupling of magnetic electrodes in MTJs is analyzed by performing hysteresis loops at 100 K to determine the switching fields from the direct observation of the magnetization orientation o fthe electrodes upon magnetic field.

Progress in nanofabrication of nanomaterials is pushing the scientific community to search for and develop new tools to measure physical properties at the nanoscale. In the case of magnetic nanomaterials, the measurement of the magnetic properties of individual nanomagnets is of the utmost importance to understand the magnetization processes underway and to correlate the local magnetic behavior with the macroscopic properties expected to lead to new technological applications. Spintronics is a very active area in magnetism for the development of magnetic nanostructures for devices, such as magnetic tunnel junctions (MTJs). Though great advances have been done on the magnetic and magnetotransport macroscopic behavior of MTJs, a great physical insight would be gained by direct visualization of magnetization switching processes with nanometer resolution. High spatial resolution techniques are required for local characterization of magnetization processes in nanostructures. Transmission Electron Microscopy (TEM) techniques such as Electron Holography (EH) allows the quantitative imaging of the magnetization configurations of ferromagnetic (FM) materials with unprecedented nanometer spatial resolution. Furthermore, EH can be combined with in situ TEM experiments applying external constraints such as magnetic and electric fields, temperature, etc. In this work, we use a TEM cryo-holder to image by EH the magnetization states of FM materials whose Curie temperature (TC) is below room temperature while varying in situ an external magnetic field applied on the thin sample We will present the EH study of the magnetization switching of LaxCa1-xMnO3 (LCMO) and LaxSr1-xMnO3 (LSMO) based thin film MTJs (whose TC = 180 and 300 K, respectively) as a function of the temperature and magnetic field applied thanks to the objective lens. The interlayer coupling/decoupling of magnetic electrodes in MTJs will be analyzed by performing hysteresis loops at 100 K, mapping the magnetic config

The discovery of giant magnetoresistance (GMR) supposed a major breakthrough in Electronics that opened the way to new devices. At this time there was a considerable research effort devoted to the implementation of the phenomenon of anisotropic magnetoresistance (AMR) [1] in read-back heads used to read out the information in hard disk drives, but the AMR effect is small, of the order of a few percent, thus, the GMR effect reported by A. Fert of 50% change in resistance under magnetic field in Fe/Cr multilayers [2] had a very strong impact in recording industry. By December 1997, IBM introduced the first hard disk product using GMR heads and this substitution supposed an immediate growth rate up to 100% per year. GMR opened the door to a new field of science, spin electronics, where both properties of the electron, its charge and its spin are manipulated simultaneously, not only its charge as traditional Electronics does. It is likely that development of spin electronics will trigger a revolution in modern electronics.

Multiferroic materials exhibiting simultaneous ferroelectricity and ferromagnetism have potential applications in information storage and in the emerging field of spintronics. Ferromagnetic / ferroelectric multilayers could be a way to obtain a multiferroic heterostructure. We addressed to deposit the ferromagnetic phase of the La1-xCaxMnO3 and the ferroelectric BaTiO3 seeking a multiferroic properties in these hetero structures. We have optimized the growth parameters for depositing BaTiO3 (BTO) / La2/3Ca1/3MnO3 (LCMO) / (001)SrTiO3 by pulsed laser deposition (PLD) at pure oxygen atmosphere and a substrate temperature of 820 C. The bilayer structure and microstructure were studied by x-ray diffraction (XRD) and atomic force microscopy (AFM). For individual layers, lattice parameter is aBTO=4.068 å, and aLCMO=3.804 å, whereas in the bilayer, Bragg peaks for LCMO maintain its position but BTO peak shift to lower Bragg angle indicating a strained BTO film. Magnetization and polarization measurements indicate a possible multiferroic heterostructures.

In order to study the crystal structure, phase relationships, and magnetic properties of the system Tb 5-xLa xSi 2Ge 2, a series of polycrystalline samples with compositions ranging from x=0 to x=5 have been synthesized and characterized in detail. At room temperature, two structures have been detected: Compounds in the concentration range 0≤x<1 present a monoclinic Gd 5Si 2Ge 2-type structure, while for x1 a tetragonal Zr 5Si 4-type structure is observed. The unit cell volume increases linearly with La concentration but with two different slopes: ∼18 Å3/x and ∼33 Å3/x for 0≤x<1 (monoclinic) and x1 (tetragonal), respectively. In the monoclinic region, an increase of T C was observed, reaching a maximum value of T C ∼154 K, at the x=0.75 composition. This feature is explained based on specific La occupancy at the R2 site which was supported by density functional calculations for low La concentration. The samples that crystallized in the tetragonal structure exhibit a linear decrease of T C(x) with a slope of ∂T C/∂x ∼-38 K/x, reaching 0 K for the x=5 composition. A magnetic and structural x-T phase diagram of the Tb 5-xLa xSi 2Ge 2 system in the temperature range 4-300 K is proposed. © 2012 American Physical Society.

A systematic set of annealings on arc-melted synthesized Gd 5Si 2Ge 2 sample was performed. Through powder X-ray diffraction (XRD) and magnetometry measurements we monitored the effect of varying the annealing time with constant temperature (T = 1473 K) on the formation of the monoclinic (M) crystallographic phase fraction, which is the one responsible for the giant magnetocaloric effect (GMCE) in this compound. The conversion of the orthorhombic O(I) crystallographic phase into M was achieved, resulting in a significant increase of the M mass fraction. Such conversion led to a change in the magnetic transition nature, evolving from a second to a first order transition for the as-cast and annealed samples, respectively. An optimal annealing time range for the M phase conversion was identified to be within 80-120 min at T = 1473 K followed by a rapid quenching to liquid N 2. Furthermore, an increase up to ∼50 of the magnetocaloric effect was obtained for the sample annealed during 120 min. © 2012 Elsevier B.V. All rights reserved.

We report magnetic, dynamic and transport properties of discontinuous metal-insulator multilayers Fe/MgO grown on amorphous Corning® glass and single-crystalline MgO (001) substrates. The films of structure Substrate/MgO (3 nm)/[Fe (0.6 nm)/MgO (3.0 nm)]×10 were prepared in ultra-high vacuum conditions using Pulsed Laser Deposition. It was shown that conditions of epitaxial growth are favorable for MgO substrates. As a result a substantial increase of tunneling magnetoresistance caused by spin-filtering effect was observed and reasonably theoretically explained. The value of TMR ̃ 9.2% at room temperature in 18 kOe magnetic field is three times higher comparing to that for the samples grown on Corning® glass substrates. Copyright © 2012 American Scientific Publishers All rights reserved.

Combinatorial pulsed laser deposition (PLD) makes use of the angular spread of laser-ablated material to prepare thin films with lateral compositional gradient. In this paper we have used combinatorial PLD to grow discontinuous Fe/MgO multilayers by alternate ablation from two separate Fe and MgO targets. Films of composition [Fe(tFe)/MgO(tMgO)] 15 were deposited on glass substrates. The thickness of Fe and MgO were varied in the vicinity of critical values determined in previous studies to maximize the tunneling magnetoresistance (TMR) in the current-inplane configuration. Optimized multilayers show a substantial improvement of both TMR and field sensitivity at room temperature. © 2012 Springer-Verlag.

Magnetic and magnetocaloric properties of single-crystalline Er 5Si 4 have been investigated as a function of the applied magnetic field (up to 50 kOe) and the hydrostatic pressure (up to 10 kbar) in the 5-300 K temperature range along the three main crystallographic directions. The magnetization isotherms show a highly anisotropic behavior with the easy-magnetization direction along the b axis for the low-pressure monoclinic and high-pressure orthorhombic structures, in good agreement with previous neutron scattering experiments. Below T C, the approach to the saturation shows a steplike behavior when the magnetic field is applied along the hard directions. The steps are sharper as the pressure increases. At constant magnetic field change, increasing the pressure induces a highly anisotropic enhancement of the magnetic entropy change. An enhancement of 20% is observed along the easy axis b, where the magnetic entropy change is maximum. The different evolution of the magnitude and temperature dependence of the magnetocaloric effect along the three crystallographic directions with pressure is discussed. © 2012 American Physical Society.

The pattern of charge modulations and local anisotropies below the Verwey transition has been determined and quantified in high-quality Fe3O4 single crystals and thin films grown on MgO by using resonant X-ray scattering at the Fe K-edge. The energy, polarization and azimuthal angle dependencies of an extensive set of reflections with potential sensitivity to charge or local anisotropy orderings have been analyzed to explore their origins. A charge disproportion on octahedral B sites of 0.20 ± 0.05 e- with [0 0 1] and cubic periodicities has been confirmed, while no significant charge disproportion has been obtained with [0 0 1/2] cubic periodicity. Additional charge modulations in the monoclinic a-b plane are also present. In addition, the occurrence of new forbidden (1, 1, 0) and (0, 0, 2n + 1/2) cubic reflections that arise from the anisotropy of the local structure around different tetrahedral and octahedral Fe atoms is shown. This complex pattern of weak charge modulations and local anisotropies is fully compatible with the low-temperature crystal structure refined in the non-polar C2/c space group and disproves any bimodal charge disproportion of the octahedral Fe atoms.

Magnetic-field dependence of conductivity in ultrathin Bi films is measured in applied magnetic fields up to 9 T, in both directions, perpendicular and parallel to the film plane, at temperatures down to 0.4 K, and analyzed in terms of the weak anti-localization theory in two-dimensional systems. With the reduction of film thickness, the classical magnetoresistance effect is completely suppressed, and only the weak anti-localization effect is observed. The parameters extracted from the analysis allow the study of the contribution of the different scattering mechanisms to the electronic transport properties in ultrathin Bi films. In particular, the thickness-independent spin-orbit scattering length indicates that the spin-orbit split surface states dominate the transport in the ultrathin-film limit.

Superconductor-ferromagnetic in-planenanocontacts have been created with focused-electron/ion-beam-induced deposition techniques for studies of AndreevReflection. The final resistance of the nanocontact is tuned during the growth by in situ resistance measurements. The results show that Co nanodeposits grown with focused electron beam have large spin polarizatiomn (-35%), making this nanomaterial of great potential for use in Spin Electronics applications. The experiments have also allowed the determination of the superconducting gap of the W-based nanodeposits grown with focused ion beam.