El objetivo de esta tesis es estudiar en forma teórica las propiedades estructurales, energéticas, termodinámicas y magnéticas de la aleación FeRh utilizando cálculos de primeros principios, debido a su notable transición de fase de primer orden desde el estado antiferromagnético (AFM) al ferromagnético (FM) a temperatura cercana a la del ambiente. Para ello se analizó el caso del FeRh bulk extendiendo el estudio a las superficies aisladas en las direcciones cristalográficas [001], [110] y [111] y luego a los films soportados en epitaxis sobre el óxido MgO(001). Los resultados estructurales y magnéticos indican que la transición metamagnética es acompañada por un aumento del 2.4% en el volumen de la celda, de un 9.6% en el momento magnético de espín del Fe y lo más importante, de la aparición del momento magnético del Rh. A pesar de lo originalmente publicado en la literatura, la fase AFM muestra una distorsión de celda cúbica a tetragonal y a ortorrómbica, resolviendo las inestabilidades observadas en las curvas de las estructuras de bandas de fonones de la estructura cúbica correspondiente. La estabilidad de la configuración AFM ortorrómbica se corroboró con simulaciones de dinámica molecular. A partir de los espectros fonónicos se obtuvieron las propiedades térmicas del sistema. Los estudios se complementan con el análisis de las propiedades termoeléctricas de la aleación. Para los films ultradelgados la estabilidad relativa de los crecidos en las direcciones [001] y [110] es similar. Por el contrario, los films generados en la dirección [111] presentan valores menos favorables en términos energéticos. En particular, para los films FeRh(001), el análisis de energía de anisotropía magnetocristalina indica que la magnetización perpendicular al plano resulta ser la más favorable, salvo para la terminación de films en Rh, configuración AFM, donde se observa una proyección de la magnetización en el plano. En el caso de los films soportados, resultan más favorables las terminaciones en Fe con menores energías de adhesión. En función de todo lo analizado se puede concluir que los films terminados en Rh presentan en la mayoría de los casos comportamientos inestables y hasta impredecibles dependiendo de la cantidad de capas de Rh que conforman el film. / The objective of this thesis is to perform a theoretical study of the structural, energetic, thermodynamic and magnetic properties of the FeRh alloy using first principle calculations, due to its remarkable first-order phase transition from the antiferromagnetic state (AFM) to the ferromagnetic (FM) at a temperature close to that of the environment. It is for this reason that the case of FeRh bulk was analyzed, extending the study to the isolated surfaces in the crystallographic directions [001], [110] and [111] and then to the films supported in epitaxis on MgO(001) oxide. The structural and magnetic results indicate that the metamagnetic transition is accompanied by a 2.4% increase in cell volume, a 9.6% in the magnetic spin moment of Fe atom and, most importantly, the appearance of the Rh magnetic moment. Despite what was originally published in the literature, the AFM phase shows a distortion of cubic cell to tetragonal and orthorhombic, resolving the instabilities observed in the phonon band structures of the corresponding cubic structure. The stability of the orthorhombic AFM configuration was corroborated with molecular dynamics simulations. The thermal properties of the system were obtained from the phonon spectra. The studies are complemented by the analysis of the thermoelectric properties of the alloy. For ultra-thin films, the relative stability of the films grown in [001] and [110] directions are similar. On the contrary, films generated in the [111] direction present less favorable energy values. In particular, for FeRh(001) films, the magnetocrystalline anisotropy energy analysis indicates that the magnetization perpendicular to the plane x is found to be the most favorable, except for films -Rh terminated, AFM configuration, where a projection of the magnetization is observed in the plane. In the case of supported films, those -Fe terminated with lower adhesion energies are more favorable. Based on everything analyzed, it can be concluded that –Rh terminated films present in most cases unstable and even unpredictable behaviors depending on the number of Rh layers that make up the film.

Bilayer magnetic nanostructures are currently of interest in a variety of applications due to the ability to combine complementary properties of each layer. One key area is data storage where extending hard disk drive (HDD) storage density may be achieved by storing each bit of data in an individual magnetic nanostructure. Whilst several magnetometry techniques are capable of measuring the properties of arrays of magnetic nanostructures, very few are sensitive enough to measure an individual magnetic nanostructure. An electrical technique termed anomalous Hall effect (AHE) magnetometry is used in this work due to its high sensitivity. In this technique the structure is fabricated on a Hall cross, and a current is applied and the transverse voltage measured whilst sweeping an external magnetic field. Bilayer magnetic nanostructures formed from separated Co/Pd and Co/Ni multilayers were measured for the first time with this technique, which showed that an asymmetry is seen in their hysteresis loops when the two layers are magnetically decoupled. It is demonstrated that this is due to a combination of a very small giant magnetoresistance (GMR) effect present in the magnetic nanostructure, and the Hall cross offset voltage which results from small imperfections in the shape of the cross. A finite element simulation is used to provide quantitative evidence for this model. These results indicate that asymmetry will be seen in Hall measurements of other materials which exhibit magnetoresistance. Bilayers of FePt and FeRh were also investigated as these materials are of interest for a future HDD system where the structure is heated, and the FeRh undergoes a ferromagnetic transition causing it to assist the switching of the FePt. These thin films are difficult to deposit whilst achieving chemical ordering in both layers. FeRh/FePt films are deposited, before greater ordering is demonstrated with an FePt/FeRh film. It is demonstrated that AHE magnetometry can provide a similar level of measurement information of bilayer nanostructures of these materials as bulk magnetometry techniques can provide of the thin film.

A liga de FeRh apresenta uma transição de fase magneto-estrutural de primeira ordem próxima da temperatura ambiente, fazendo deste material um forte candidato para aplicações de gravação magnética termicamente assistida ou em refrigeradores magnéticos. Em baixas temperaturas o FeRh apresenta uma fase antiferromagnética enquanto em altas temperaturas apresenta uma fase ferromagnética. Cientes disto, neste trabalho foram analisados as transições de fase de filmes de FeRh, porém quando na presença de nanofios de Ni. Para isso foram fabricados filmes de FeRh, próximos da composição equiatômica, sobre uma matriz nanoporosa de Al 2 O 3 com nanofios de Ni. A matriz nanoposora foi produzida por um processo de anodização em dois passos, sendo preenchido os poros com Ni via eletrodeposição. O filme de FeRh foi crescido por deposição via magnetron sputtering, em duas temperaturas distintas, 525 e 600 o C. Um tratamento térmico in situ a 600 o C, por uma hora, foi feito no filme depositado na mesma temperatura. Curvas de magnetização em função de campo magnético aplicado, em temperaturas fixas, mostraram comportamentos característicos de filmes ou nanofios quando medidos nas direções preferenciais de cada componente, respectivamente, sugerindo um forte acoplamento via magnetoestrição entre os elementos. Foi analisada a dependência da transição de fase com relação a condições de magnetização dos nanofios de Ni, sendo observado clara dependência do comportamento da transição de fase com relação à estas condições. A transição de fase do FeRh é dada por nucleações e, posteriormente, por um deslocamento das paredes de domínio adjacentes a nucleação. Este comportamento é bem descrito pela somatória de duas distribuições gaussianas, sendo ajustadas na primeira derivada da magnetização em função da temperatura. Estes ajustes mostraram um claro comportamento da distribuição de nucleação com relação a magnetização remanente dos nanofios. Tal comportamento não tinha sido relatado até este momento. O filme depositado em 525 o C se mostrou mais suscetível a magnetização remanente dos nanofios do que o filme depositado em 600 o C. Posteriormente, medidas de transição de fase sob campo magnético intenso mostraram uma forte relação entre as temperaturas críticas de transição e a intensidade de campo aplicado, sendo deslocada a curva de transição para temperaturas mais baixas devido o incremento do campo. O filme depositado em 525 o C apresentou uma transição mais larga e uma menor magnetização em altas temperaturas do que o filme depositado em 600 o C. / The FeRh alloy presents a first order magnetic-structural phase transition close the room temperature, making this material a strong candidate to heat-assisted magnetic recording or magnetic refrigerators. At low temperaturas the FeRh the FeRh shows a antiferromagnetic phase while at high temperatures shows a ferromagnetic phase. Aware of this, in this work were analyzed the FeRh films phase transition, but under the Ni nanowires presence. For this were maked FeRh films, close the equiatomic composition, over a nanoporous matrix of Al 2 O 3 with Ni nanowires. The nanoporou matrix were produced by a two-step anodization process and the nanoporou fille with Ni via eletrodeposition. The FeRh film were grown by deposition via magnetron sputtering at two different temperatures, 525 e 600 o C. Annealing in situ at 600 o C for one hour was done in the film deposited at same temperature. Magnetization curves as function of applied magnetic field, at fixed temperatures, showed characteristic behaviors of the films or nanowires where measured in the preferential direction of each componente, respectively, suggest a strong magnetostriction coupling between elements. The phase transition dependence was analyzed with respect to magnetization conditions of Ni nanowires, been observed a clear dependence of the phase transition behavior with these conditions. The FeRh phase transition is given by nucleations and followed by a displacement of domain walls adjacents to the nucleation. This behavior is well defined for the sum of two gaussians distributions, been fitted to the first derivative of magnetization as function of temperature. These fittings showed a clear behavior of the nucleation distribution with repect to a nanowires remanent magnetization. Such behavior has never been reported before. The film deposited at 525 o C showed more susceptible to nanowires remanentc magnetization than film deposited at 600 o C. Subsequently meadures of phase transition under high magnetic fields showed a strong relation between the critical temperatures of depostion and the field intensity, being displaced to lower temperatures the transition curve with the field increment. The film deposited at 525 o C presented a broader transition with lower magnetization at high temperatures than film deposited at 600 o C.

The equiatomic FeRh alloy undergoes a first-order phase transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state at about 370 K with a small thermal hysteresis of about 10 K around the phase transition. The transition is accompanied by a unit cell volume expansion about 1% in the c lattice parameter. During the transition the new phase nucleates in the matrix of the original phase by reaching the critical temperature followed by a growth in size upon increasing temperature further. Therefore, to understand the transition process with more details, it is desirable to investigate the nucleation and growth of both phases within the first order phase transition. In the present thesis the main focus is on the growth of FeRh thin films by means of Molecular Beam Epitaxy (MBE) technique and characterization of the magnetic and structural properties. To develop an understanding of the phase transformation in FeRh thin films the ways in which one can tune it were investigated. The following aspects concerning the FeRh system have been examined here: 1) influence of annealing temperature on the magnetic and structural response, 2) effect of film thickness on the first-order phase transition temperature as well as the saturation magnetization, 3) influence of chemical composition on the magnetic properties and 4) magnetic field-induced phase transition. To get insight to details of the transition process the magnetization dynamic has been addressed by performing Ferromagnetic resonance (FMR) experiment across the phase transition. FMR measurements determined the existence of two areas with different magnetic properties inside the film. A huge temperature difference for the beginning of the phase transition in comparison with the static magnetization measurement was observed for the equiatomic FeRh thin film prepared by MBE. Tuning of the AFM to FM phase transition in the FeRh thin film by means of low-energy/low fluence Ne+ ion irradiation was studied. Ion irradiation technique offers a quantitative control of the degree of chemical disorder by adjusting the ion fluence applied, while the penetration depth of the disordered phase can be adjusted by the ion-energy. The main results of ion irradiation are the shifting of the phase transition temperature to lower temperature and irradiation with 3×1014 ion/cm2 leads to the disappearance the AFM phase completely.

The spintronic materials graphene and FeRh are of great scientific and technological interest due to their unique properties. Graphene's remarkable electronic transport and low spin interaction suggest it could be a near-perfect spin-transport material, while the equiatomic alloy FeRh undergoes a first-order antiferromagnetic (AF) to ferromagnetic (FM) phase transition when heated through a critical temperature ~370 K. Combining these materials could lead to a single multifunctional spin injection, transport and detection device in which a range of stimuli - heat, magnetic field, strain etc. - could be used to manipulate the device state. However, realisation of such a multifunctional device is extremely challenging. This thesis describes the progress made in developing a novel method of spin injection into graphene, and details a study of the metamagnetic phase transition in FeRh nanowires suitable for use as spin injection and detection electrodes. The measured values of spin lifetime and spin diffusion length in graphene are an order of magnitude lower than those predicted theoretically. In this project, a novel 1D contact geometry was investigated to determine whether the dwelling of spins underneath tunnel barrier contacts was the cause of the discrepancy. Although these devices exhibited very high charge carrier mobility - indicating successful device fabrication, defect-free graphene flakes and low levels of contamination - no spin signals were observed. Through a thorough investigation of this unexpected result it was determined that the quality of the graphene/- ferromagnetic interface was limiting the polarisation of injected spin current. The use of FeRh as a novel spin injection and detection material was investigated through magnetic force microscopy imaging of the AF and FM phases during heating and cooling sweeps. The results from FeRh full-films showed a strong dependence on surface morphology, as certain surface types were observed to favour the FM phase. These behaviours were confirmed in patterned nanowire devices, which indicated that the dependence on surface topology dominated over spatial confinement effects. In order to perform these studies a magneto-transport measurement system capable of performing measurements over a wide temperature range 2 K - 500 K in a rotatable magnetic field of up to 750 mT was developed. The noise base of the completed system was measured at just 10% above the theoretical minimum level.

This work is dedicated to the study of two antiferromagnetic materials that are suitable for use in spintronic devices. In series of FeRh samples we studied the transition temperature between the antiferromagnetic and ferromagnetic phases. We developed a method based on material optical response for a quick determination of this temperature, which enabled us to study with a spatial resolution of 1 μm a magnetic inhomogeneity of prepared samples.We also developed a method for a determination of the Néel temperature and the magnetization easy axis position in thin films prepared from compensated antiferromagnetic metal. We successfully applied this method on an uniaxial sample of CuMnAs and we discussed its applicability for a research of samples with a biaxial magnetic anisotropy.

Komplexní magnetické materiály v nanoměřítku mají své nezastupitelné místo v moderních zařízeních, jako jsou digitální paměti nebo senzory. Moderní technologické procesy vyžadují porozumění a možnost kontroly moderních magnetických materiálů až na atomární úrovni. Jednou z možných cest je magnetická analýza za použití transmisní elektronové mikroskopie (TEM), která je unikátní díky možnosti zobrazování až v subatomárním měřítku. Tato práce popisuje možnosti zobrazování metamagnetických materiálů metodou TEM. Tyto materiály se vyznačují možností stabilizace více magnetických uspořádání najednou za daných vnějších podmínek. Modelovým systémem pro popis zobrazovacích možností metody TEM byly zvoleny tenké vrstvy metamagnetické slitiny FeRh. Tento materiál prochází při zahřívání fázovou přeměnou z antiferomagnetické do feromagnetické fáze. Podrobně jsou rozebrány procesy výroby vzorků, což je zásadní pro úspěšnou TEM analýzu. Pro magnetické zobrazování vzorků v TEMu je využita technika diferenciálního fázového kontrastu (DPC), umožňující přímé mapování rozložení magnetické indukce ve vzorku. Důsledně je diskutován vznik signálu v DPC, což je nezbytné pro porozumění a analýzu výsledných dat. FeRh vrstvy jsou podrobeny analýze struktury, chemického složení a především magnetických vlastností obou magnetických fází. Závěrem je představen proces přímého ohřevu metamagnetických vrstev v TEMu.

The equiatomic FeRh alloy undergoes a first-order phase transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state at about 370 K with a small thermal hysteresis of about 10 K around the phase transition. The transition is accompanied by a unit cell volume expansion about 1% in the c lattice parameter. During the transition the new phase nucleates in the matrix of the original phase by reaching the critical temperature followed by a growth in size upon increasing temperature further. Therefore, to understand the transition process with more details, it is desirable to investigate the nucleation and growth of both phases within the first order phase transition. In the present thesis the main focus is on the growth of FeRh thin films by means of Molecular Beam Epitaxy (MBE) technique and characterization of the magnetic and structural properties. To develop an understanding of the phase transformation in FeRh thin films the ways in which one can tune it were investigated. The following aspects concerning the FeRh system have been examined here: 1) influence of annealing temperature on the magnetic and structural response, 2) effect of film thickness on the first-order phase transition temperature as well as the saturation magnetization, 3) influence of chemical composition on the magnetic properties and 4) magnetic field-induced phase transition. To get insight to details of the transition process the magnetization dynamic has been addressed by performing Ferromagnetic resonance (FMR) experiment across the phase transition. FMR measurements determined the existence of two areas with different magnetic properties inside the film. A huge temperature difference for the beginning of the phase transition in comparison with the static magnetization measurement was observed for the equiatomic FeRh thin film prepared by MBE. Tuning of the AFM to FM phase transition in the FeRh thin film by means of low-energy/low fluence Ne+ ion irradiation was studied. Ion irradiation technique offers a quantitative control of the degree of chemical disorder by adjusting the ion fluence applied, while the penetration depth of the disordered phase can be adjusted by the ion-energy. The main results of ion irradiation are the shifting of the phase transition temperature to lower temperature and irradiation with 3×1014 ion/cm2 leads to the disappearance the AFM phase completely.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20939号 / 理博第4391号 / 新制||理||1631(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 小野 輝男, 教授 吉村 一良, 教授 島川 祐一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

In these diploma thesis magnetically ordered materials are studied with the prospect of their application in spintronics. Specifically, we investigated metallic alloy FeRh, which undergoes a magnetic phase transition from antife- romagnetic phase to feromagnetic one around 100◦ C. This phenomenon can be readily used in memory devices. Laser spectroscopy is used as a nondestructive method without need of any electrical contacts. Magnetic properties of FeRh are studied by magnetooptical effects including quadratic magnetic linear dichroism. The measured polarization rotations are of the order of miliradians, therefore, the detection is realized by an optical bridge. At first, we concentrated on discrimina- ting of various magnetooptical effects from each other. The second part is focused on the phase transition induced by different means. Firstly, by heating the whole sample, secondly by illuminating the sample locally by continuous laser.