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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.

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

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.

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.

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.

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.

We study the magnetic-field dependence of the conductance in planar ferromagnet-superconductor nanocontacts created with focused-electron/ion-beam techniques. From the fits of the differential conductance curves in high magnetic fields, we obtain the magnetic field dependences of the superconducting gap and the broadening parameter. Orbital depairing is found to be linear with magnetic field. We evaluate the magnetic field dependence of the quasiparticle density of states, and we compare it with the value obtained by scanning tunneling spectroscopy experiments. © 2011 American Physical Society.

We present a detailed study on the magnetization under high magnetic fields of Tb5Si2Ge2 and Gd5Si2Ge2 compounds. From the Arrott plot construction (A. Arrott, Phys. Rev. 108, 1394 (1957)), we were able to estimate the TC of each structure (M and O(I)) experimentally and found that the TC of the O(I) phase can be directly obtained by extrapolating the TC curve of this phase in the respective phase diagram. Using a physical model based on free energy considerations, one explains the (de)coupling of the magnetic and structural transitions in R5(SixGe1−x)4 (R = Tb, Gd) compounds.

In this letter, it is shown that when replacing Ge by Fe in Tb5Si2Ge2 the structural transition still occurs and enhances the magnetocaloric effect (MCE) (up to 66%) with maximum of MCE at a critical Fe amount where the magnetic and structural transitions become fully coupled. It is observed that Fe concentration is able to mimic the effect of external pressure as it induces a complex microstructure, that tunes long range strain fields. This knowledge is crucial for the development of strategies toward materials with improved performance for efficient magnetic refrigeration applications.

Epitaxial discontinuous Fe/MgO multilayers have been grown by pulsed laser deposition on MgO(001) single-crystal substrates. The multilayers with 0.6 nm nominal Fe layers thickness are superparamagnetic and demonstrate tunneling magnetoresistance (TMR) in the current-in-plane configuration. Increasing deposition temperature causes an improvement in crystal quality and is accompanied by higher TMR ratios. The maximum value (9.2% TMR at room temperature and 18 kOe magnetic field) trebles that of polycrystalline samples deposited simultaneously on glass substrates. A model formula for TMR ratio that includes both spin-dependent tunneling and spin-filtering effect is proposed to explain these results.

Great efforts have been directed towards the research field of Spintronics in the last years after the discovery of the Giant Magnetoresistance effect (GMR). An essential issue in this field is the obtention of magnetoresistive devices such as Magnetic Tunnel Junctions (MTJs) with half-metallic ferromagnetic materials as active electrodes at room temperature. Such materials have an energy gap at the Fermi level (EF ) for one spin direction, therefore, showing a spin polarization of 100%, which optimizes the output signal of the spintronic devices. In order to obtain devices fully operative at room temperature, materials with a high Curie temperature, TC, are required. However, as only a few oxides, semiconductors and metallic alloys are candidates to exhibit half-metallicity, finding new half-metallic materials is nowadays of great interest. Promising materials are, for example, magnetite and double perovskite oxides. In addition, the problem of the magnetization thermal fluctuation in high-density magnetic storage systems is of recent concern, and could be solved with materials with high magnetocrystalline anisotropy, such as the L10 - ordered FePt. The growth of thin films and heterostructures based on these materials is an issue of great interest. The epitaxial growth can be achieved by using suitable monocrystalline substrates with similar lattice parameters, by means of different deposition techniques such as laser ablation , sputtering, molecular beam epitaxy, or chemical deposition. The properties of the materials grown by means of these techniques can be tuned depending on different parameters such as the film-substrate lattice mismatch or the epitaxial growth coherence along the film thickness, which offers interesting possibilities. The objective of this Thesis has been the growth and characterization of thin films and heterostructures of half-metallic magnetic materials, in which the nanometric-scale phenomena are relevant, with potential applications in

The evolution of the morphology, magnetic and transport properties of Fe(t nm)/MgO(3.0 nm) multilayers with respect to the nominal metallic layer thickness was investigated. A comparison with existing experimental data on discontinuous metal–insulator multilayers, ultrathin epitaxial Fe films on MgO substrates and granular cermet films is made. It is confirmed that the deposition conditions and the material composition play a crucial role in the percolation process. Nominal thicknesses of Fe layers at which an infinite metallic cluster is formed and the conditions for continuous Fe coverage were determined. Different methods of percolation threshold detection were analysed. We show that investigation of the temperature dependence of resistance in nanostructures could lead to an overestimation of the percolation threshold value, while magnetic measurements alone could lead to its underestimation.

We report the structural and magnetotransportproperties of three 300-nm-thick Bithinfilmsgrown on Si (0 0 1) substrates by means of thermalevaporation using different buffer layer (175-nm-thick Si3N4, highly resistive Si and 800-nm-thick SiO2, respectively). X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate that the Bifilms are polycrystalline with the grains preferentially oriented along the trigonal-axis [0 0 1]. The Bifilm evaporated on SiO2 presents the highest crystallinity which reflects on a higher magnetoresistance value (120% at room temperature and 160% at 2 K and 90 kOe), whereas the lowest magnetoresistance value is found for the Bifilm evaporated on Si3N4 (90% at room temperature and 140% at 2 K and 90 kOe). The magnetic field dependence of the Hall resistivity indicates the presence of both electrons and holes whose contributions strongly depend on the temperature. As a function of temperature, a significant enhancement of the hall resistivity is found below 150 K. Interestingly, the MR values show an enhancement at the same temperature, which indicates changes in the carrier densities and mobilities below 150 K.