Thesis advisor: Stephen Wilson / The work performed within this thesis is divided into two parts, each focusing primarily on the study of magnetic phase behavior using neutron scattering techniques. In first part, I present transport, magnetization, and neutron scattering studies of materials within the iridium oxide-based Ruddelsden-Popper series [Srn+1IrnO3n+1] compounds Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). This includes a comprehensive study of the doped bilayer system Sr3(Ir1-xRux )2O7. In second part, I present my studies of the effect of uniaxial pressure on magnetic and structural phase behavior of the iron-based high temperature superconductor Ba(Fe1-xCox)2As2. Iridium-based 5d transition metal oxides host rather unusual electronic/magnetic ground states due to strong interplay between electronic correlation, lattice structure and spin-orbit effects. Out of the many oxides containing iridium, the Ruddelsden-Popper series [Srn+1IrnO3n+1] oxides are some of the most interesting systems to study both from the point of view of physics as well as from potential applications. My work is focused on two members of this series Sr3Ir2O7 (n=2) and Sr2IrO4 (n=1). In particular, our combined transport, magnetization and neutron scattering studies of Sr3Ir2O7 (n=2) showed that this system exhibits a complex coupling between charge transport and magnetism. The spin magnetic moments form a G-type antiferromagnetic structure with moments oriented along the c-axis, with an ordered moment of 0.35±0.06 µB/Ir. I also performed experiments doping holes in this bilayer Sr3(Ir1-xRux)2O7 system in order to study the role of electronic correlation in these materials. Our results show that the ruthenium-doped holes remain localized within the Jeff=1/2 Mott insulating background of Sr3Ir2O7, suggestive of `Mott blocking' and the presence of strong electronic correlation in these materials. Antiferromagnetic order however survives deep into the metallic regime with the same ordering q-vector, suggesting an intricate interplay between residual AF correlations in the Jeff=1/2 state and metallic nanoscale hole regions. Our results lead us to propose an electronic/magnetic phase diagram for Sr3(Ir1-xRux)2O7 system showing how the system moves from Jeff=1/2 antiferromagnetic Mott insulator (Sr3Ir2O7) to paramagnetic Fermi liquid metal (Sr3Ru2O7). On the other hand, our neutron scattering measurements on Sr2IrO4 (n=1), a prototypical Jeff=1/2 Mott insulator, showed that the spins arranged antiferromagnetically in ab-plane with an ordered moment comparable to that of Sr3Ir2O7. The second part of my work is comprised of a neutron scattering-based study of the Ba(Fe1-xCox)2As2 system, a bilayer family of iron-based high temperature superconductors. Undoped, this system exhibits either simultaneous or nearly simultaneous magnetic and structural phase transitions from a high temperature paramagnetic tetragonal phase to low temperature orthorhombic antiferromagnetic phase. With the gradual suppression of these two temperatures, the superconducting phase appears with the highest TC obtained just beyond their complete suppression. It has been proposed that these coupled magnetostructural transitions are secondary manifestations which arise as a consequence of electronic nematic ordering that occurs at a temperature higher than either of them. My work is mainly focused on probing the spin behaviors coupling to this electronic nematic phase. I devised a small device to apply uniaxial pressure along an in-plane high symmetry axis and studied the magnetic and structural behavior in series of Ba(Fe1-xCox)2As2 compounds via neutron scattering in presence of uniaxial pressure. There is an upward thermal shift in the onset of structural and magnetic transition temperature caused by this uniaxial pressure which is surprisingly insensitive to cobalt concentration in the absolute scale. Furthermore, on the first order side of the phase diagram (below the tricritical point), the structural and magnetic transitions are decoupled with magnetic transition following structural distortion. This study suggests the importance of both spin-lattice and orbital-lattice interactions in these families of compounds. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18445号 / 理博第4005号 / 新制||理||1577(附属図書館) / 31323 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 前野 悦輝, 教授 石田 憲二, 教授 田中 耕一郎 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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Magnetic materials are of great research interest because of their potential applications. Most Mn-based compounds exhibit magnetic ordering, being antiferromagnetic or ferromagnetic depending on their crystal structure. Many of these compounds have complex non-collinear magnetic structures that can give rise to exotic and robust phenomena. The scope of this thesis encompasses two independent projects on exploring single-crystalline Mn-based compounds with magnetic properties: (i) the study of the thickness-dependent magnetic textures in ferromagnetic Mn1.4PtSn by means of Focused Ion Beam (FIB) for sample shaping and Magnetic Force Microscopy (MFM) for imaging, and (ii) the experimental demonstration of an anomalous Hall effect in non-collinear antiferromagnetic Mn3Pt, revealed with the aid of uniaxial pressure tuned in-situ. The first chapter motivates the study of magnetic materials and introduces the theoretical framework on which they are understood. In particular, refers to the energy contributions of magnetic origin and gives an overview of the Hall effect and how it is used to probe magnetic properties, from ferromagnetism to non-collinear antiferromagnetism and non-coplanar spin textures (such as the so-called skyrmions). The second chapter is dedicated to the ferromagnetic compound Mn1.4PtSn. It starts by introducing concepts important in the context of magnetic domains. A variety of magnetic textures are discussed, in particular antiskyrmions which differ from regular skyrmions by their internal structure. A material-specific introduction is given, starting by its discovery as the first antiskyrmion-hosting compound (when in thin-plate shape) and including recent literature showing by means of neutron scattering how magnetic domains in bulk single crystals are best described as anisotropic fractals. This study complements our first observations in real-space MFM images of the magnetic texture in this material. The detailed study of the dependence of the magnetic domains as a function of sample thickness is presented and analyzed. The third and final chapter focuses on antiferromagnetic Mn3Pt. To motivate the experiment, the theoretical study that predicts the presence of an intrinsic zero-field anomalous contribution to the Hall effect for this material is introduced. Next, the experimental investigation of single crystals of Mn3Pt is presented, where a Hall effect dominated by the ordinary contribution in the temperature range from 10 to 300 K is found. Thereafter, the response of the Hall effect to uniaxial pressure tuned in-situ is explored. When the sample is compressed, a hysteresis is observed to open up. The magnitude of this anomalous Hall conductivity (when compressing the sample by ∼0.2 GPa) is estimated to be at least ∼ 10 Ω-1cm-1 at room temperature and ∼ 40 Ω-1cm-1 at 100 K, and it is demonstrated that the measured value originates in the antiferromagnetic structure, rather than in a stress-induced ferromagnetism.:1 Introduction 1 1.1 Overview of elemental properties . . . . . . . . . . . . . . . . 1 1.1.1 Notes on Mn . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.2 Notes on Pt . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.3 Notes on Sn . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Magnetic Interactions . . . . . . . . . . . . . . . . . . . . . . 5 1.2.1 Zeeman interaction . . . . . . . . . . . . . . . . . . . . 5 1.2.2 Magnetostatic energy . . . . . . . . . . . . . . . . . . . 5 1.2.3 Magnetic anisotropy . . . . . . . . . . . . . . . . . . . 6 1.2.4 Magnetoelastic coupling . . . . . . . . . . . . . . . . . 7 1.2.5 Exchange interaction . . . . . . . . . . . . . . . . . . . 8 1.2.6 Antisymmetric exchange . . . . . . . . . . . . . . . . . 10 1.3 Antiferro-, ferri- and helimagnets . . . . . . . . . . . . . . . . 11 1.4 Hall effect in magnetism . . . . . . . . . . . . . . . . . . . . . 14 1.4.1 Geometrical phase in quantum mechanics . . . . . . . 14 In the context of the anomalous Hall effect . . . . . . 16 1.4.2 Complementary anomalous Hall theories . . . . . . . . 18 Skew scattering . . . . . . . . . . . . . . . . . . . . . . 18 Inelastic scattering . . . . . . . . . . . . . . . . . . . . 18 Side jump . . . . . . . . . . . . . . . . . . . . . . . . . 18 Spin chirality mechanism . . . . . . . . . . . . . . . . 19 I The uniaxial ferromagnet Mn1.4PtSn 21 2 Mn1.4PtSn 23 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.2 Background physics . . . . . . . . . . . . . . . . . . . . . . . . 27 2.2.1 Topology in magnetism . . . . . . . . . . . . . . . . . 27 2.2.2 Domain theory . . . . . . . . . . . . . . . . . . . . . . 29 Domain refinement . . . . . . . . . . . . . . . . . . . . 31 2.2.3 Literature overview . . . . . . . . . . . . . . . . . . . . 32 SANS studies on bulk Mn1.4PtSn . . . . . . . . . . . . 34 2.3 Experimental methods . . . . . . . . . . . . . . . . . . . . . . 37 2.3.1 Sample preparation . . . . . . . . . . . . . . . . . . . . 37 2.3.2 Lamellae fabrication . . . . . . . . . . . . . . . . . . . 37 2.3.3 Magnetic Force Microscopy . . . . . . . . . . . . . . . 38 History . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Operating principle . . . . . . . . . . . . . . . . . . . . 39 Specifications for our experiments . . . . . . . . . . . . 40 2.4 Results and discussions . . . . . . . . . . . . . . . . . . . . . . 40 2.4.1 Bulk samples characterization . . . . . . . . . . . . . . 40 Mn1.4Pt0.9Pd0.1Sn polycrystal . . . . . . . . . . . . . . 40 Mn1.4PtSn single crystal . . . . . . . . . . . . . . . . . 43 Mn1.4PtSn single crystal in applied field . . . . . . . . 45 Mn1.4PtSn single crystal below TSR . . . . . . . . . . . 46 2.4.2 Lamellae characterization . . . . . . . . . . . . . . . . 48 Thickness dependence . . . . . . . . . . . . . . . . . . 48 Temperature dependence . . . . . . . . . . . . . . . . 54 Magnetic field dependence . . . . . . . . . . . . . . . . 56 2.5 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . 63 II The non-collinear antiferromagnet Mn3Pt under strain 65 3 Mn3Pt 67 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3.2 Background physics . . . . . . . . . . . . . . . . . . . . . . . . 69 3.2.1 Thin film study of Mn3Pt . . . . . . . . . . . . . . . . 71 3.2.2 Our contribution . . . . . . . . . . . . . . . . . . . . . 73 3.3 Experimental methods . . . . . . . . . . . . . . . . . . . . . . 74 3.4 Results and discussions . . . . . . . . . . . . . . . . . . . . . . 75 3.4.1 Characterization of unstrained crystals . . . . . . . . . 75 3.4.2 Elastic response of Mn3Pt single crystals . . . . . . . . 79 Electrical transport response to strain . . . . . . . . . 81 3.4.3 Onset of AHE in single crystals under uniaxial pressure 84 Sample III4 . . . . . . . . . . . . . . . . . . . . . . . . 84 Sample IV1 . . . . . . . . . . . . . . . . . . . . . . . . 89 Sample IV2 . . . . . . . . . . . . . . . . . . . . . . . . 91 3.4.4 Temperature dependence of the AHE . . . . . . . . . . 94 3.4.5 Elastic limit of Mn3Pt . . . . . . . . . . . . . . . . . . 98 3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 A On Mn3Pt resistivity 101 B On Mn3Pt sample mounting 103