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The static magnetization and dynamic susceptibility responses of the cluster system within a Griffiths phase of the magnetocaloric compound Tb5−xLaxSi2Ge2 (x=0.075) have been investigated. A novel cluster-glass state within the Griffiths phase is formed at a characteristic freezing temperature where short-range ferromagnetic correlations set in the paramagnetic regime. Ferromagneticlike correlations are built up at around 155 K, which suddenly become frozen at a lower temperature ∼140K, thus in analogy with a reentrant spin glass behavior. The ac susceptibility near the freezing temperature follows a critical slowing down process characterized by τ0=10−13s and dynamic exponents zν∼6 and β∼0.4, similar to well-known spin glass systems. The nonlinear ac susceptibility analysis shows clearly the existence of a transition associated to the reentrant behavior. The origin of the intermediate cluster-glass phase inside the Griffiths phase is proposed to be the result of a combination of short-ranged RKKY intralayer positive exchange interactions between rare-earth Tb3+ ions and antiferromagnetic exchange between adjacent interlayers involving Si and Ge atoms in connection to the Tb3+ atoms., Work at the University of Zaragoza is supported by the Spanish Ministerio de Ciencia, Innovación y Universidades MAT2017-82970-C2-2-R and Spanish DGA (Grant No. E26). Work at the University of Cantabria is supported by the Spanish Ministerio de Ciencia, Innovación y Universidades MAT2017-83631-C3-R. J.H.B. would like to thank CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement.

Resumen del trabajo presentado al 10th International Symposium on Metallic Multilayers (MML), celebrado en Madrid (España) del 17 al 21 de junio de 2019.

Resumen del póster presentado al 10th International Symposium on Metallic Multilayers (MML), celebrado en Madrid (España) del 17 al 21 de junio de 2019., Inverse Spin-Hall effect (ISHE) has been widely exploited during last decades to convert spin currents into electrically detectable charge currents. The origin of this effect lies in the spin-orbit coupling, and usually transition metals with high atomic number such as Pt, Au or W have been used as spin-current detectors. However, conductive oxides constitute a promising alternative; not only because of their chemical stability, ease of fabrication and lower cost, but also because of their higher electrical resistivities, which can be an asset when the quantity of interest is the output voltage after the spin-to-charge conversion. This is because the measured voltage is proportional to the incoming spin current as follows Δ𝑉ISHE ∝ 𝜃SH 𝜌𝐶𝐼𝑆 which means that not only the spin-to-charge conversion efficiency —the so-called spin Hall angle, 𝜃𝑆H— affects the performance as detector, but also the electrical resistivity. Fujiwara et al. in Ref.[1] proposed IrO2 as a promising material for spin-current detection, finding 𝜃𝑆H ∼ 0.04 for sputtering-grown polycrystalline IrO2. Surprisingly, later on Qiu et al. reported that the performance of IrO2 in detecting thermally excited spin currents in spin-Seebeck experiments (SSE) was extremely low compared to Pt. This apparent contradiction indicates that spin-to-charge conversion in IrO2 is not a closed subject. Trying to better understand how ISHE works in IrO2, we have performed further SSE experiment using as spin detector IrO2 grown by means of a different technique (Pulsed Laser Deposition, PLD), studying the effect of IrO2 thickness, temperature and underlying magnetic material and substrate. The results allow us to establish the dominant mechanism in ISHE and point out significant differences in behaviour of PLD-grown IrO2 with respect to previously reported IrO2 prepared by sputtering.

Resumen del póster presentado al 10th International Symposium on Metallic Multilayers (MML), celebrado en Madrid (España) del 17 al 21 de junio de 2019., 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.

Resumen del trabajo presentado al 14th International Workshop on Magnetism & Superconductivity at the Nanoscale, celebrado en Coma-Ruga, El Vendrell (España) del 1 al 6 de julio de 2018., Spin Seebeck effect (SSE) is defined as the excitation of a spin current in a magnetic ordered material by its subjection to a thermal gradient. This spin current has been proved to be carried by magnetic excitations -magnons- rather than mobile carriers, and it is usually detected by its conversion into an electrical current by means of the Inverse Spin Hall effect in an attached metallic and non-magnetic material. Thus, SSE is a complex phenomenon whose output is the result of the interplay between several parameters characterizing magnetic, electrical, magnonic, and thermal properties of the involved materials and interfaces, making its quantitative analysis far from straightforward. Here we report a series of experiments and calculations intended to enable this quantitative analysis of longitudinal SSE (LSSE) in epitaxial thin films of ferrimagnetic insulating maghemite (γ-Fe2O3). The use of an insulating ferrimagnet overcomes theoretical and experimental complications, and brings some advantages for practical applications We study the temperature dependence of the stationary LSSE, and correlate it with the thermal conductivity of both the magnetic thin-film and the substrate, measured by the 3ω method. The influence of the magnetic layer thickness is also determined and considered in the analysis. The results are fitted to theory, granting us the access to the value of different parameters influencing the spin transport through the samples and the output ISHE voltage. The knowledge of these quantities is of great relevance because it brings to light the contribution of the underlaying mechanisms to the studied effect and points out the direction in which materials and devices should be engineered in order to improve the LSSE signal.

In this report, a successful thermodynamical model was employed to understand the structural transition in Er5Si4, able to explain the decoupling of the magnetic and structural transition. This was achieved by DFT calculations, which were used to determine the energy differences at 0 K, using a LSDA + U approximation. It was found that the M structure is the stable phase at low temperatures, as verified experimentally with a value of ΔF0 = −0.262 eV. Finally, a variation of Seebeck coefficient (∼6 μV) was determined at the structural transition, which allows to conclude that the electronic entropy variation is negligible in the transition., This work was partially supported by the projects FEDER/POCTI n0155/94, PTDC/CTM/NAN/5414/2014 from Fundação para a Ciência e Tecnologia (FCT), Portugal. The authors thank the Portuguese Foundation for Science and Technology (FCT) for grants SFRH/BD/88440/2012 (JHB) and SFRH/BD/97591/2013 (MSB), SFRH/BD/97591/2013 (MBB), SFRH/BPD/82059/2011 (JNG), IF/01089/2015 (JSA). This work was developed within the scope of the project CICECO‐Aveiro Institute of Materials, POCI‐01‐0145‐FEDER‐007679 (FCT Ref. UID /CTM /50011/2013), financed by national funds through the FCT/MEC and when appropriate co‐financed by FEDER under the PT2020 Partnership Agreement. C.M. acknowledges the support of the Fundación ARAID. This work was supported by the Spanish Ministry of Science (through Project Nos. MAT2014‐ 51982‐C2‐R and C1‐R, including FEDER funding) and the Aragon Regional government (Project No. E26).

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., 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., Work at the University of Zaragoza is supported by the Spanish Ministry of Science (MAT2011-27553-C02, MAT2011-27573-C04) and Spanish DGA (Grant No. E26). Work at the Ames Laboratory is supported by the Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the Office of Science of the US Department of Energy. Ames Laboratory is operated by Iowa State University of Science and Technology for the US Department of Energy under Contract No. DE-AC02-07CH11358.

Resumen del trabajo presentado a la "XI Reunión Nacional de Electrocerámica" celebrada en Zaragoza (España) del 19 al 21 de junio de 2013.-- et al., 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.

Resumen del trabajo presentado al "European Congress and Exhibition on Advanced Materials Processes" celebrado en Sevilla (España) del 8 al 13 de septiembre de 2013.-- et al., 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 ferroelectricity). Electric measurements were carried out to further investigate the ferroelectric nature of SMO films, thus assessing the possibility of non-d0 cations off-centring.

Resumen del póster presentado a la 19th International Conference on Magnetism celebrada en Korea del 8 al 13 de julio de 2012., 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.

et al., 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., Work at the University of Zaragoza was supported by the Spanish Ministry of Science (MAT2008-06567-C02) and Spanish DGA (Grant No. E26). N. Marcano acknowledges Spanish CSIC (JAE-doc program) for financial support. Work at the Ames Laboratory was supported by the Office of Basic Energy Sciences,Materials Sciences and EngineeringDivision of theOffice of Science of theUSDepartment of Energy. Ames Laboratory is operated by Iowa StateUniversity of Science and Technology for the US Department of Energy under Contract No. DE-AC02-07CH11358.

Resumen del póster presentado al American Physical Society March Meeting celebrado en Boston (US) del 27 de febrero al 2 de marzo de 2012.-- et al., 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., Support by INA, Spain; “El Patrimonio Autón. Fondo Nal. de Financiamiento CT&I. FJC,” Contract RC 275-2011; 1106-48-925531 Colciencias project.

8 páginas, 7 figuras, 1 tabla.-- PACS number(s): 75.30.Sg, 71.70.Ch, 75.30.Kz, 75.10.Dg, We present a detailed study of the temperature dependence of the electrical resistivity [ρ(T)] in the range 13–300 K for the Ho5(SixGe1-x)4 system. Three distinct ρ(T) behaviors are observed, associated with different magnetic and crystallographic structures along the series. In the samples with an antiferromagnetic phase (AFM) one observes a shoulder near the Néel temperature (TN) attributed to the formation of a gap on the Fermi surface. This gap is analyzed using a phenomenological two-band model for an AFM with distinct atomic and magnetic periodicities, and its effect seems to extend well above TN. We also found the presence of short-range magnetic clusters in the paramagnetic (PM) phase. On the ferromagnetic (FM) materials, the distinct ρ(T) scattering contributions (phonon, magnetic, and residual terms) are extracted from the measurements, with ρ(T) mainly dominated by electron spin scattering. An additional contribution is also observed, arising from the strong crystal field effect in these materials. The effect is mainly observed in the PM phase, leading to a curvature on ρ(T) in this phase. Using a two-level crystal field model the corresponding gap was estimated for the different Si/Ge ratio samples, revealing that the crystal field splitting increases linearly with Si content., Work partially supported by the projects POCI/CTM/61284/2004, PTDC/CTM/NAN/115125/2009, and FEDER/POCTI n0155/94 from Fundaçâo para a Ciência ex Tecnologia (FCT), Portugal. A.M.P. thanks FCT for Grant No. SFRH/BPD/63150/2009. C.M. acknowledges the support of the Fundación ARAID. The financial support of the Spanish MEC (MAT2008-06567-C02) and DGA (Grant No. E26) is acknowledged.

In this article, we review our recent research on Fe3O4 epitaxial thin films and Fe3O4/MgO/Fe epitaxial heterostructures. More specifically, we report on the magnetotransport properties of Fe3O4 epitaxial films in a wide range of film thicknesses and temperatures, focusing on the anomalous, planar and ordinary Hall effects. We also summarize our insight on the origin of the enhanced magnetic moment found in ultra-thin magnetite films (thickness t < 5 nm). Finally, our work on the growth, and structural and magnetic characterization of heteroepitaxial Fe3O4/MgO/Fe trilayers is presented.

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., This work was supported by the ERATO “Spin Quantum Rectification Project” (Grant No. JPMJER1402) and PRESTO “Phase Interfaces for Highly Efficient Energy Utilization” (Grant No. JPMJPR12C1) from JST, Japan; Grant-in-Aid for Scientific Research (A) (Grant No. JP15H02012), Grant-in-Aid for Scientific Research on Innovative Area, “Nano Spin Conversion Science” (Grant No. JP26103005) and Grant-in-Aid for Research Activity Start-up (No. JP18H05841) from JSPS KAKENHI, Japan; H2020-MSCA-RISE-2016 SPICOLOST (Grant No. 734187); and the Spanish Ministry of Economy and Competitiveness (Grant No. MAT2017-82970-C2, including FEDER) and the Aragon regional government (E26), Spain. The microscopy works were conducted in the LMA at INA, Universidad de Zaragoza. A.A. acknowledges MINECO [through Project Nos. FIS2016-78591-C3-1-R (SKYTRON), PCIN-2015-111 (SOgraph), and SEV-2016-0686] and Comunidad de Madrid through Grant No. S2018/NMT-4321 (NANOMAGCOST-CM).

[EN] Since its birth marked by the discovery of the Giant Magnetoresistance (GMR) in 1988, the field of spintronics has evolved rapidly, maturing and giving rise to new subfields. The ultimate goal of spintronics is the development of a new paradigm with potential to overcome the limits imposed by the Moore’s law in conventional electronics. This paradigm is based on the use of the spin degree of freedom, together with the electrical charge. A functional spintronic technology requires the capability to control three fundamental operations: generation, transport and detection of spin angular momentum. Therefore, spin dynamics are focus of an intense research aiming at the enhancement of the performance of these three operations. In particular, spin currents are a fundamental object in the field of spintronics. Furthermore, pure spin currents, which are not accompanied by a charge flow, allow to propagate spin without Joule power dissipation losses. In this regard, pure spin currents carried by collective magnetic excitations (magnons) bring the possibility of insulator-based spintronics using electrically insulating materials with long-range magnetic order (MOIs). This thesis is devoted to the study of thermal spin transport, i.e., the coupling between spin and heat currents, which constitutes the area of research of the spintronics subfield of spin caloritronics or thermal spintronics. This field is envisaged to significantly contribute to the development of a new generation of highly efficient thermoelectric devices; to this end, a deeper understanding on the fundamental mechanisms governing thermal spin transport is still required. Through the work developed in this thesis, the spin Seebeck effect (SSE) in maghemite (γ-Fe2O3) -based low-dimensional nanostructures has been exhaustively investigated. The SSE is one of the most prominent transport phenomenon integrating the field of spin caloritronics, as it enables the direct generation of a magnon spin current upon the application of a thermal gradient in magnetic materials. SSE is observed in FM/NM bilayers, where FM is a magnetically ordered material and NM is a paramagnetic or diamagnetic metal (most often, Pt). In general, strategies aimed at improving the SSE efficiency can be targeted at three levels: (1) the heat-to-spin current conversion in the FM layer; (2) the interface level, comprising both interfacial heat-to-spin current generation and spin transfer from FM to NM; and (3) spin current detection at NM. This last step is usually performed by means of spin-to-charge conversion via the inverse spin Hall effect (ISHE) enabled by spin-orbit coupling (SOC). The first part of this thesis is focused on the development as thin films of materials of interest within this topic. In particular, the preparation of high quality maghemite and iridium(IV) oxide (IrO2) thin films is investigated. Additionally, the fabrication of Y3Fe5O12 (YIG) by an affordable chemical method is also addressed. Maghemite and YIG constitute examples of MOIs holding an already successful history of applications in other fields. Meanwhile, IrO2 is a NM metal with strong SOC and high electrical resistivity making it an appealing candidate for spin current detection via the ISHE. Then, the SSE is extensively investigated in γ-Fe2O3/Pt bilayers of nanometric thickness. A thorough depiction of the SSE is reached, accounting for both contributions to the thermal spin current: the one originated at the FM/NM interface, and the one originated at the thickness of γ-Fe2O3 due to the thermally induced magnon accumulation. To this end, the influence of different transport parameters is discussed. Furthermore, a second method for measuring the SSE in steady heating conditions is implemented. This alternative method is based on the current-induced heating approach, in which the NM material holds a triple role: heater, thermometer, and spin-to-charge converter. The experimental equivalence between both methods is analyzed. Additionally, this current-induced method enables the simultaneous detection of the recently discovered spin Hall magnetoresistance (SMR), which is hence characterized in the γ-Fe2O3/Pt bilayers. Secondly, the ultrafast dynamics of the interfacial SSE is investigated by means of the Terahertz Emission Spectroscopy all-optical technique (TES). In this case, three different FM/NM structures using FM materials with different degree of electrical conductivity are used: insulating maghemite, half-metallic magnetite and metallic iron. The comparison of the photoinduced thermal spin currents in each sample enables the characterization of the time scale at which these spin currents rise and decay. Based on their different dynamics, the SSE and the spin-dependent version of the thermoelectric Seebeck effect (spin-dependent Seebeck effect, SDSD) are differentiated. The SDSE-associated thermal spin current is carried by electrons, in contrast to the magnonic nature of spin currents excited by the SSE, and thus SDSE is operative only in conductive materials. Finally, the third level of the SSE —the spin current detection in the NM layer—is addressed using IrO2 as NM in γ-Fe2O3/IrO2 bilayers of nanometric thickness. To date, a few experimental works studying spin-to-charge conversion in polycrystalline or amorphous IrO2 have been reported. In contrast, IrO2 thin films investigated in this thesis are strongly textured in a preferential direction, and the role of the different SOC mechanisms contributing to ISHE in this kind of samples is analyzed. Besides, the striking differences found between ISHE spin-to-charge conversion in textured samples with respect to previous studies in polycrystalline or amorphous IrO2 are discussed. The results open the door to the interesting possibility of tuning the desired functionality of high-resistance spin-Hall-based devices., [ES] Desde su nacimiento marcado por el descubrimiento de la magnetorresistencia gigante (GMR) en 1988, el campo de la espintrónica ha evolucionado rápidamente, madurando y dando lugar a nuevas subdisciplinas. El objetivo final de la espintrónica es el desarrollo de un nuevo paradigma con potencial para superar las limitaciones impuestas en la electrónica convencional por la ley de Moore. Este paradigma se basa en el uso del grado de libertad de espín, junto con la carga eléctrica. Una tecnología espintrónica requiere, para ser funcional, de la capacidad para controlar tres operaciones fundamentales: generación, transporte y detección del momento angular de espín. De esta manera, la dinámica de espín es el foco de una intensa investigación con el objeto de aumentar la eficiencia en estas operaciones. En concreto, las corrientes de espín son un objeto fundamental en el campo de la espintrónica. Mas aún, las corrientes puras de espín, que no van acompañadas por una corriente de carga, permiten la propagación de espín sin pérdidas por disipación Joule. En este sentido, las corrientes puras de espín portadas por excitaciones magnéticas colectivas (magnones) brindan la posibilidad de una espintrónica basada en aislantes usando materiales eléctricamente aislantes con orden magnético de largo alcance (MOIs). Esta tesis se ha dedicado al estudio del transporte térmico de espín, es decir, a la interacción entre corrientes de espín y de calor, que constituye el área de investigación de la subdisciplina (dentro de la espintrónica) de la caloritrónica de espín o termoespintrónica. Se prevé que este campo contribuya significativamente al desarrollo de una nueva generación de dispositivos termoeléctricos altamente eficientes. Para ello, todavía es necesario profundizar más en la comprensión de los mecanismos fundamentales que gobiernan el transporte térmico de espín. A través del trabajo desarrollado en esta tesis, se ha estudiado de manera exhaustiva el efecto Seebeck de espín (SSE) en nanoestructuras de baja dimensionalidad basadas en maghemita (gamma-Fe2O3). El SSE es uno de los fenómenos de transporte más destacados en el campo de la caloritrónica de espín, puesto que permite la generación directa de una corriente de espín magnónica al aplicar un gradiente térmico en materiales magnéticos. El SSE se observa en bicapas FM/NM, donde FM es un material con orden magnético de largo alcance y NM es un metal paramagnético o diamagnético (habitualmente, Pt). En general, las estrategias que pretendan mejorar la eficiencia del SSE pueden dirigirse a tres niveles: (1) conversión de corriente de calor en corriente de espín en la capa FM; (2) al nivel interfacial, que comprende tanto la conversión de corriente de calor en corriente de espín a este nivel como la transferencia de espín desde la capa FM hacia la capa NM; y (3) la detección de la corriente de espín en la capa NM. Este último paso se realiza habitualmente mediante la conversión de corriente de espín en corriente de carga a través del efecto Hall de espín inverso facilitado por el acoplamiento espín-órbita (SOC). La primera parte de esta tesis se centra en el desarrollo de materiales de interés dentro de este tópico en forma de películas delgadas. En particular, se investiga la preparación de películas delgadas de alta calidad de maghemita de y óxido de iridio (IV) (IrO2). Además, se aborda también la fabricación de Y3Fe5O12 (YIG) mediante un método químico rápido y económico. La maghemita y el YIG constituyen ejemplos de MOIs que ya han sido aplicados exitosamente en otros campos. Por su parte, el IrO2 es un material NM con elevado SOC y elevada resistividad eléctrica, propiedades que lo convierten en un candidato prometedor para la detección de corriente de espín mediante ISHE. A continuación, se investiga extensamente el SSE en bicapas gamma-Fe2O3/Pt de espesor nanométrico. Como resultado, se consigue elaborar una descripción precisa del SSE, que tiene en cuenta las dos contribuciones a la corriente espín térmica: la originada en la intercara FM/NM, y la originada en el espesor de gamma -Fe2O3 debido a la acumulación de magnones inducida térmicamente. Para ello, se analiza la influencia de diferentes parámetros de transporte. Además, se implementa un segundo método de medida del SSE en condiciones de calentamiento estable. Este método alternativo se basa en el enfoque del calentamiento inducido por corriente, en el que el material NM cumple una función triple: calentamiento, termometría y conversión de corriente de espín en corriente de carga. Por último, se analiza la equivalencia experimental entre los dos métodos. Además, el método de inducción por corriente permite la detección simultánea de la recientemente descubierta magnetorresistencia Hall (SMR), que es asimismo caracterizada en las bicapas gamma-Fe2O3/Pt. En segundo lugar, se investiga la dinámica ultrarrápida del SSE de intercara por medio de la técnica óptica de Espectroscopía de Emisión de Terahercios (TES). En este caso, se emplean tres estructuras FM/NM usando para la capa FM materiales con diferente grado de conductividad eléctrica: maghemita (aislante), magnetita (mediometal) y hierro (metal). La comparación entre las corrientes de espín térmicas fotoinducidas en cada muestra permite la caracterización de la escala temporal en las que estas corrientes se originan y decaen. Basándose en sus diferentes dinámicas, el SSE y la versión dependiente de espín del efecto Seebeck termoeléctrico (efecto Seebeck dependiente de espín, SDSE) se pueden distinguir y separar, dentro de un mismo experimento. La corriente térmica de espín asociada al SDSE es portada por electrones, a diferencia de las corrientes de espín excitadas mediante SSE, que son portadas por magnones. Por lo tanto, el SDSE solo se da en materiales conductores. Por último, la tesis aborda el tercer nivel del SSE —la detección de la corriente de espín en la capa NM— usando IrO2 para la capa NM en bicapas gamma-Fe2O3/IrO2. Hasta la fecha, sólo se han reportado unos pocos trabajos experimentales que estudian la conversión de espín a carga en IrO2 policristalino o amorfo. Las muestras de IrO2 estudiadas en esta tesis, por el contrario, presentan una estructura cristalina fuertemente texturada en una dirección preferencial; en la presente tesis se investiga por tanto el papel de los diferentes mecanismos originados por el SOC y que contribuyen al ISHE en este tipo de muestras. Además, se analizan las llamativas diferencias encontradas en los procesos de conversión de corriente de espín en corriente de carga encontradas en las muestras texturadas con respecto a resultados previos en IrO2 policristalino o amorfo publicados en la literatura. Estas diferencias abren la puerta a la interesante posibilidad de controlar la funcionalidad de dispositivos basados en el efecto Hall de espín en materiales altamente resistivos., Ayudas para la formación de profesorado universitario (programa FPU) del Ministerio de Educación, Cultura y Deporte. Referencia: FPU014/02546 (Predoctoral research grant of the Spanish Government). Ayudas de movilidad para estancias breves en centros en el extranjero para beneficiarios del programa FPU. Referencia: EST17/00382 (Grant for research stays abroad of the Spanish Government). Ayuda del Programa Ibercaja-CAI de Estancias de investigación. Referencia: CB 11/18 (Grant for research stays of the Ibercaja-CAI Foundation).

Tesis Doctoral presentada por Lorena marín Mercado en el Departamento de Física de la Materia Condensada de la Universidad de Zaragoza; Instituto de Nanociencia de Aragón (INA), Instituto de Ciencia de Materiales de Aragón (ICMA), Consejo Superior de Investigaciones Científicas (CSIC)., 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.

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 magnetoelectronics. Most of the systems studied have been produced by means of a Pulsed Laser Deposition (PLD) and sputtering system. PLD is a suitable deposition technique for the growth of high-quality oxide thin films, and as a PLD-sputtering system by Neocera Inc. company is available at the Instituto de Nanociencia de Aragón (INA) in Zaragoza (Spain) and fully operative since June 2005, this system will represent the fundamental instrument of the Thesis. This book is composed of five chapters, being the first one dedicated to the description of the experimental techniques used in this research, giving an special importance to the PLD technique.