Os compostos intermetálicos do tipo R IND. 6 Fe IND. 14-X Al IND. X (R = Nd, La) são antiferromagnéticos e apresentam uma transição do tipo metamagnética em campos da ordem de 2-3 Tesla. Com objetivo de caracterizar suas propriedades magnéticas, amostras policristalinas forma investigadas via medidas de magnetização, magneto-resistência, magnetostricção e difração de nêutrons. As amostras foram preparadas por meio de forno a arco e submetidas a tratamentos térmicos de 750 GRAUSC por 4 semanas. A estrutura cristalina foi identificada como sendo tetragonal do tipo La IND. 6 Co IND. 11 GA IND. 3 (grupo especial 14/mcm). Em contraste com os compostos baseados em La, as amostras de Nd apresentaram uma grande histerese magnética e um efeito dependente da história magnética na região de T<30 K, e um ordenamento dos momentos magnéticos de Nd a 70 K. As T IND. N foram determinadas por meio de medidas de suscetibilidade magnética e se encontram na região de 150-310 K e 100-230 K para os compostos de Nd e La, respectivamente. A transição metamagnética foi observada em todas as amostras analisadas, e foi observada uma variação de 3-5% da resistência elétrica no momento da transição, sugerindo uma estrutura magnética em forma de camadas magnéticas. Infelizmente através das medidas de difração de nêutrons, não foi possível determinar a estrutura magnética das amostras. Entretanto, no composto Nd IND. 6 Fe IND. 10 Al IND. 4, essas medidas nos indicaram uma mudança da estrutura espacial por meio da alteração do grupo espacial do 14/mcm para P42/mcm na região de temperatura de 70 K, onde foi observado um pico bastante saliente em nossas medidas de suscetibilidade. Por meio de medidas de espectroscopia Mössbauer não foi observado nenhum comportamento anômalo nos parâmetros hiperfinos do Fe, indicando ser esta um fenômeno isolado dos momentos do Nd. ) Um outro resultado obtido por difração de nêutrons foi a presença de uma reflexão magnética (001) que cresce na região abaixo de 30 K, indicando a existência de uma componente dos spins paralela ao plano basal. Nenhuma anomalia magnética foi observada nas amostras de La. Esse crescimento do pico (001) sugere uma tendência dos momentos do Nd a se alinharem na direção do plano basal, podendo ser esta a explicação para o efeito de irreversibilidade observado nesta região de temperatura / Rare earth-based intermetallic compounds R IND.6Fe IND.14-xAl IND.x with R=(Nd, La) are antiferromagnetic and undergo a metamagnetic transition at 2~3 T applied fields. In order to further characterize their magnetic behavior, polycrystalline samples were investigated with magnetometry, magneto resistance, magnetostriction and neutron diffraction. Samples have been prepared by arc melting under argon and annealed at 750°C for four weeks. The crystal structure is tetragonal of La IND.6Co IND.11Ga IND.3-type (space group 14/mcm). In contrast to La-based compounds, Nd IND.6Fe IND.14-xAl IND.x exhibits a large transition hysteresis and magnetic history effects at T<30 K, and antiferromagnetic ordering of Nd moments at 70 K. The T IND.N was determined by magnetic susceptibility measurements in the 150-310 K and 100-230 K range for the Nd and La compounds, respectively. The metamagnetic transition was observed for all samples and a ~3-5% resistivity drop was found to occur concomitantly, suggesting a magnetic multilayer-like structure. Unfortunately the magnetic structure of this compound could not be determined in this work by neutron diffraction. However, neutron diffraction measurements in the Nd IND.6Fe IND.10Al IND.4 compound revealed a change of space group from 14/mcm to P4 IND.2/mcm at 70 K, where the magnetic susceptibility exhibits a sharp peak. Mossbauer spectroscopy measured in the 4.2-100 K range has shown no evidence of Fe spin rotation, suggesting an isolated Nd phenomenon. Another result from neutron diffraction measurements was the presence of a magnetic (001) reflection that grows below 30 K, indicating the development of a spin component parallel to the basal plane. No such effect was observed with La-based samples. This increase is likely to be due to a canting of Nd moments towards the basal plane. The coincidence of this effect with the onset temperature for irreversible behavior in magnetic measurements is consistent with this interpretation.

Submitted by Erika Demachki (erikademachki@gmail.com) on 2015-05-22T21:34:47Z No. of bitstreams: 2 Dissertação - Leandro Tolentino Coutrim - 2015.pdf: 2861900 bytes, checksum: 25e74c53b1811b169e93359d4ec98858 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Erika Demachki (erikademachki@gmail.com) on 2015-05-22T21:36:09Z (GMT) No. of bitstreams: 2 Dissertação - Leandro Tolentino Coutrim - 2015.pdf: 2861900 bytes, checksum: 25e74c53b1811b169e93359d4ec98858 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2015-05-22T21:36:09Z (GMT). No. of bitstreams: 2 Dissertação - Leandro Tolentino Coutrim - 2015.pdf: 2861900 bytes, checksum: 25e74c53b1811b169e93359d4ec98858 (MD5) license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Previous issue date: 2015-02-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work we describe the synthesis of the novel series La2−xCaxCoIrO6 and its structural, electronic and magnetic characterization. The main objective of this project is to investigate the magnetic evolution of the system when modified due to the chemical substitution of Ca at La site, which can induce changes in the magnetic interactions. The policristaline samples were synthesized by Solid State Reaction process, and studied combining global characterization techniques as powder x-ray diffraction, temperature dependence of magnetic susceptibility and magnetization as a function of applied magnetic field. All compounds crystallize in monoclinic structure, with space group P21/n. Magnetic susceptibility results indicate that the superexchange antiferromagnetic interaction between Co2+ and Ir4+ in La2CoIrO6 lead to ferrimagnetic behavior due to the different magnetic moment of the transition metal ions. For the Ca2+ doped at La3+ site compounds, a magnetic evolution is observed with doping, induced by Co and Ir valence changes. As expected, this system presents a strong correlation between its structural, electronic and magnetic properties, in a way that combining the different results of each compound we can explain its behavior and understand the big picture of this system. / Neste trabalho descrevemos a síntese e a caracterização estrutural, eletrônica e magnética dos compostos policristalinos da série inédita La2−xCaxCoIrO6. O principal objetivo deste projeto foi investigar a evolução da magnetização do sistema quando os materiais são modificados a partir da substituição química do Ca no sítio do La, o que pode induzir mudanças na natureza das interações magnéticas. As amostras policristalinas foram sintetizadas pela técnica de Reação de Estado Sólido convencional, e estudadas combinando-se técnicas de caracterização globais como difração de raios X de pó, magnetização em função da temperatura e em função do campo magnético aplicado, e também calor específico em função da temperatura. Todos os compostos da série cristalizam-se em estrutura monoclínica, com grupo espacial P21/n. As medidas de susceptibilidade magnética indicaram que a interação de supertroca antiferromagnética entre os metais de transição Co2+ e Ir4+ leva o composto La2CoIrO6 a apresentar comportamento ferrimagnético devido aos diferentes momentos magnéticos apresentados pelos dois íons, com TC em torno de 100K. Para os compostos dopados com Ca2+ no sítio do La3+, observou-se mudanças nas propriedades magnéticas, induzidas pelas alterações nas valências dos íons de Co e Ir. Como esperado, este sistema apresenta uma forte correlação entre as suas propriedades estruturais, eletrônicas e magnéticas, de modo que a partir da interpretação conjunta dos dados acerca da estrutura e da magnetização, podemos entender o comportamento de cada composto da série.

The evolution of electronics has become the staple thrust of modern scientific innovation: a need for advancing materials engineered for our equally rapidly advancing needs and computing requirements has fueled recent wealth of new materials. Here, I use the ideals of exotic materials design to answer this need, specifically for 2D materials. Two-dimensional (2D) van der Waals materials with in-plane anisotropy are of great interest for directional transport of charge and energy. I perform solid state synthesis to produce several such materials: an intrinsic antiferromagnet, superatomic semiconductors, and a polytype system with a component that displays the possibilities of Weyl nodes.The former, chromium sulfur bromide (CrSBr), is first synthesized, then fully studied structurally, compositionally, electronically, and magnetically. Second harmonic generation (SHG), more advanced than older techniques such as magneto-optical Kerr spectroscopy or Raman spectroscopy, allows us to fully understand the magnetic symmetry in this system as an interlayer antiferromagnetic and intralayer ferromagnetic in-plane anisotropic material. I also introduce published work in which we integrate CrSBr into different devices to show the utility of this fundamental research into a more practical application setting. It is used to stimulate more magnetic response from graphene — promising ultra-thin magnetic memory or sensory devices in future projects. Applying strain and external magnetic fields provides another tuning knob through which to access different functional modalities. In the latter third of this dissertation, we report a layered van der Waals semiconductor with in-plane anisotropy built upon the superatomic units of Mo₆S₃Br₆ (MSB), a robust construction with a direct gap of 1.64 eV. Next, MSB and Re₆Se₈Cl₂, another analogous superatomic vdW material, are potential candidates for optoelectronic applications; we qualify this by studying their Auger dynamics as a measure of quantum efficiency. Finally, layered van der Waals (vdW) materials belonging to the MM’Te₄ structure class have recently received intense attention due to their ability to host exotic electronic transport phenomena, such as in-plane transport anisotropy, Weyl nodes, and superconductivity. In summary, we have discovered two ternary exfoliatable vdW TMD polytypes with the composition TaFeTe₄, one of which (ꞵ) shows the prerequisite symmetry elements to be a type-II Weyl semimetal. This dissertation is a treatise to solid state synthesis, exploration into the more exotic spectrum of 2D materials, and robust and eclectic methods used to paint a full picture of different magnetic and electronic systems within.

We have studied two different systems of compounds, YPd2Al3 and CenTmIn3n+2m. Polycrystalline YPd2Al3, a new member of the REPd2Al3 system, was prepared by arc melting. The X-ray diffraction confirmed that YPd2Al3 crystallizes in the hexagonal PrNi2Al3-type structure as the other REPd2Al3 compounds. Magnetization, AC susceptibility, specific heat and resistivity measurements revealed superconductivity below Ts ≈ 2.2 K. The second part of the work was focused on studies of Ce2PdIn8 and Pd-doped CenRhIn3n+2. Single crystals of Ce2Rh1-xPdxIn8 with x = 0, 0.10, 0.15, 0.30, 0.45, 0.85, 1 and CeRh1-xPdxIn5 with x = 0, 0.1, 0.25 were prepared by solution growth method. The quality of crystals was confirmed by microprobe analysis and X-ray diffraction. The effect of Pd doping on magnetism of CenRh1-xPdxIn3n+2 was studied by specific heat, magnetization and resistivity measurements. The Pd doping gradually suppresses the Néel temperature in both systems, however the effect is stronger in Ce2Rh1-xPdxIn8. Temperature dependence of resistivity of CeRh0.75Pd0.25In5 was studied in pressure up to 2.2 GPa. Similar to CeRhIn5, the antiferromagnetism is gradually suppressed by the applied pressure, while superconductivity is induced and coexists with antiferromagnetism.

Tato práce je zaměřená na studium magnetických vlastností tří r·zných sloučenin na bázi uranu obsahujících 5f elektrony (U4Ru7Ge6, UAu2Si2 a UIrGe). V rámci této práce byly, za pomoci r·zných metod, připraveny vysoce kvalitní krystaly těchto sloučenin. Vlastnosti těchto systém· byly studovány r·znými objemovými meto- dami (magnetizace, měrné teplo, teplotní roztažnost, elektrický transport) a také neu- tronovým a rentgenovým rozptylem při širokém spektru vnějších podmínek (nízká teplota, vysoké magnetické pole, vysoký tlak). Kombinace těchto metod odhalila komplexní chování těchto systém· a pomohla sestrojit jejich magnetické fázové diagramy. První studovanou sloučeninou je feromagnet U4Ru7Ge6vykazující velmi nízkou mag- netokrystalovou anizotropii, která je neočekávaná pro sloučeninu na bázi uranu. To se promítá do izotropní závislosti téměř všech měřených fyzikálních vlastností. Bylo zjištěno, že osa snadné magnetizace se mění v uspořádaném stavu a tento jev je spojen s anomáliemi v teplotní roztažnosti ukazující na možnou rhomboedrickou distorzi. Ta vede k vytvoření dvou odlišných pozic uranu s odlišným magnetickým momentem. Tato před- pově¤ byla potvrzena teoretickými výpočty a pomocí polarizované neutronové difrakce. Rozdíl magnetických moment· na dvou odlišných uranových pozicích je zp·soben...

Title: Magnetism in the non-centrosymmetric uranium compound: UIrSi3 Author: RNDr. Jaroslav Valenta Department: Department of Condensed Matter Physics Supervisor of the doctoral thesis: RNDr. Jiří Prchal, Ph.D., DCMP Abstract: The study presented in this thesis is focused on UIrSi3 which is one of two known uranium compounds that crystallize in a non-centrosymmetric structure. This first study of UIrSi3 in single-crystalline form confirms antiferromagnetic order below TN = 41.7 K and reveals strong uniaxial anisotropy. The antiferromagnetic order is suppressed by application of a magnetic field along the c axis (0HC = 7.3 T at 2 K). In contrast, application of a magnetic field up to 14 T along the a axis gives paramagnetic response. The high TN, contrasting with the low 0HC, together with the opposite signs of the paramagnetic Curie temperatures of the a-axis and c-axis paramagnetic susceptibility indicate a competition between antiferromagnetic and ferromagnetic interactions. The first-order metamagnetic transition at HC shows asymmetric hysteresis which reflects a complex antiferromagnetic ground state. With increasing temperature, the hysteresis becomes gradually smaller and vanishes at 28 K where the first-order transition changes to second-order transition which remains up to TN. The point where the...

This work is focused on the parent compounds of the Fe-chalcogenide superconductors. For this purpose poly- and single-crystalline forms of tetragonal β-FexSe, Fe1+yTe, Fe1+yTe1-xSex and Fe(1+y)-xMxTe (M = Ni, Co) have been prepared. Second focal points of this study are the low-temperature structural phase transitions and physical property changes in tetragonal Fe1+yTe which are induced by composition, external pressure, and cationic substitution.

Masters of Science / Rare-earth (RE) compounds have been an attractive subject, based on the unique electronic structures of the rare-earth elements. In particular, the RETX (RE = rare-earth, T = 3d/4d/5d, transition metals, and X = p – block elements) series is a large family of intermetallic compounds which crystallizes in different crystal structure depending on the constituents. Most of these compounds crystalize in the hexagonal, orthorhombic, and tetragonal crystal structure. On the other hand, the family of compounds RET2X2 adopted the tetragonal crystal structure of the ThCr2Si2 or the CaBe2Be2 with different space groups. Owing to the different crystal structure, these compounds show versatile magnetic and electrical properties such as Kondo effect, complex magnetic behaviour, valence fluctuation, unconventional and conventional superconductivity, heavy fermion behaviour, Fermi and non – Fermi liquid behaviour, metamagnetism, spin – glass, memory effect, crystal electric field (CEF), magnetoresistance and magnetocaloric effect. The history of magnetism reveals that it is closely related to practical applications and magnetic materials from the most vital components in many applications. These are memory devices, permanent magnets, transformer cores, magneto-mechanical devices and magneto-electronic devices. Recent additions to this list include magnetic refrigeration through the studies of magnetocaloric effect as well as spintronics. Magnetic refrigeration (MR) is an emerging technology and shows real potential to enter conventional markets and the principles of MR obeys the magnetocaloric effect (MCE), which is based on the effect caused by a magnetic field on the materials that accept the property of varying the magnetic entropy, as well as its temperature when varying the magnetic field. In this thesis, we report giant magnetocaloric effect and magnetic properties of NdPd2Al2 and RECuGa (RE = Nd, Dy, and Ho) compounds. These investigations were done through measurements of X – ray diffraction (XRD), magnetic susceptibility, ((T)), magnetization, (M(H)), isothermal magnetization, (M(H, T)), heat capacity, (Cp(T)) and electrical resistivity, ((T)). MCE has been studied from the isothermal magnetization and heat capacity measurements.The first chapter of the thesis describes the theoretical background from which the experimental results have been analyzed and interpreted. This is followed by the chapter which presents experimental details and methodology carried out in this thesis. Chapter three presents the results and discussion of the transport, magnetic and magnetocaloric properties of NdPd2Al2 compounds. XRD studies confirm the tetragonal CaBe2Ge2 – type structure with space group P4/nmm (No. 129). The results of (T), (T) and Cp(T) indicate a putative antiferromagnetic (AFM) phase transition at low temperature at, TN = 3 K. On the other hand, (T) data at high temperatures follow the Curie – Weiss relationship giving an effective magnetic moment close to that expected for the trivalent Nd3+ ion. The magnetization results indicate metamagnetic – like transition at a low field that bears a first-order character which corroborates with the Below – Arrott plots. Giant MCE was obtained for the NdPd2Al2 compound similar to those reported for potential magnetic refrigerant materials. Chapter four discusses the magnetic and thermodynamic properties of the series of compounds RECuGa where RE = Nd, Dy, and Ho. XRD studies indicate the orthorhombic CeCu2 – type crystal structure with space group Imma (No. 74) for all three compounds. Magnetic measurements indicate a putative AFM phase transition below 𝑇𝑁 = 7.1, 8.5, and 3.7 K for Nd, Dy, and Ho compounds, respectively. The high-temperature (T) data for all three compounds follow the Curie – Weiss relationship giving an effective magnetic moment close to that expected for the trivalent rare-earth ion. Again, large MCE were obtained for all three compounds similar to those reported for materials that can be used as magnetic refrigerant materials.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18058号 / 理博第3936号 / 新制||理||1567(附属図書館) / 30916 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 池田 隆介, 教授 川上 則雄, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Ferromagnetic (FM) thin-film heterostructures provide opportunities to investigate GHz magnetic dynamics and emerging magnetoelectric devices based on dynamic phenomena. An intriguing direction for these studies is the control of the flow of pure (chargeless) spin current flow in different magnetic systems. In this thesis, we focus on the excitation of pure spin currents using ferromagnetic resonance (FMR), also known as spin pumping, and their transport characteristics in magnetic heterostructures based on Ni₈₁Fe₁₉ (Permalloy, or Py) under a variety of circumstances. In Chapter 2, we present measurements of the anisotropy of the spin pumping effect in the Pt/Py/Pt system via variable-frequency, swept-field FMR. We find a very small anisotropy of enhanced Gilbert damping with sign opposite to a recent theory's prediction from the Rashba effect at the FM/Pt interface. In Chapter 3, we present an experimental and theoretical study of spin dynamics in the antiferromagnetically coupled Py/Ru/Py system. We show that, contrary to the behavior of the uniform mode in a saturated single-layer FM, the symmetric mode in unsaturated synthetic antiferromagnet (SAF) has approximately constant FMR linewidth as a function of frequency. This behavior can be explained mostly semiclassically by our model. In Chapter 4, we present an investigation of interfacial Gilbert damping due to the spin pumping effect in Py/W heterostructures with enriched α phase or β phase W. The spin mixing conductances (SMC) for W at interfaces with Py are found to be significantly lower than those for similar heavy metals such as Pd and Pt, but comparable to those for Ta, and independent of enrichment in the β phase. The experimental results also indicate that W, no matter of which phase, is a good spin sink in Py/W heterostructures. In Chapter 5, we describe explorations of the spin pumping effect in antiferromagnetic insulator (AFI)-based heterostructures using variable-temperature, variable-frequency FMR. We find a spin-pumping-induced damping enhancement for Py/Cu/CoO, Py/Cu/CoO/Cu/Pt and Py/Cu/NiO/Cu/Pt. Broad peaks have been observed in FMR linewidth difference as a function of temperature for Py/Cu/NiO, normalized to the linewidth of the reference Py sample. Our results indicate that some effects previously attributed to spin current flow are better described by a defect-related mechanism. Chapter 6 summarizes the various findings of spin-pumping effects in ferromagnetic thin film heterostructures and possible future work.