Heusler compounds are cubic intermetallics with a wide range of interesting properties, which are closely related to the structure of the material. In addition, several exotic physical phenomena have been predicted for different compositions in the family, but have not been experimentally realized. By and large, the lack of success in realization of various properties are due to the issues with intrinsic material attributes, which have been difficult to resolve as the relationship between them is not well understood. The aim of this work has been to unravel the entanglement between the intrinsic material attributes of cobalt-based quaternary Heusler compounds such as the structure, defects (disorder), chemical inhomogeneities etc., and the resulting physical properties.

Heusler compounds are cubic intermetallics with a wide range of interesting properties, which are closely related to the structure of the material. In addition, several exotic physical phenomena have been predicted for different compositions in the family, but have not been experimentally realized. By and large, the lack of success in realization of various properties are due to the issues with intrinsic material attributes, which have been difficult to resolve as the relationship between them is not well understood. The aim of this work has been to unravel the entanglement between the intrinsic material attributes of cobalt-based quaternary Heusler compounds such as the structure, defects (disorder), chemical inhomogeneities etc., and the resulting physical properties.

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