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Spintronics or spin-electronics attempt to utilize the electronic spin degree of freedom to make advanced materials and devices for the future. Heusler materials are considered very promising for spintronics applications as many highly spin-polarized materials potentially exist in this family. To accelerate materials discovery and development, The Materials Genome Initiative (https://www.mgi.gov/) was undertaken in 2011 to promote theory-driven search of new materials. In this thesis work, we outline our effort to develop several new materials that are predicted to be 100% spin-polarized (half-metallic) and thermodynamically stable by theory. In particular, two Mn-based Heusler families were investigated: Mn2CoZ (Z= Ga, Sb, Ge) and Mn2FeZ (Z=Si,Ge), where the latter is potentially a new Heusler family. These materials were synthesized using the arc-melting technique and their crystal structure was investigated using the X-ray diffraction (XRD) method before and after appropriate annealing of the samples. Preliminary magnetometry measurements are also reported. We first developed a heat-treatment procedure that could be applied to all the Mn-based compounds mentioned above. Mn2CoGa was successfully stabilized in the cubic inverse-Heusler phase with a=5.869 Å and magnetic moment of 2.007 /fu. This is in good agreement with prior literature reports [1]. However, cubic phases of Mn2CoSb and Mn2CoGe could not be stabilized within the annealing temperature range that is accessible in our lab. We successfully synthesized a cubic Mn2FeSi phase using an annealing procedure similar to Mn2CoGa. The measured cubic lattice parameter of Mn2FeSi was 5.682 Å. This is the first experimental report of this material to the best of our knowledge. Detailed analysis of relative intensities of different X-ray peaks revealed that the structure is most likely in an inverse Heusler phase, in agreement with theory. However, a substantial atomic-level disorder was also uncovered from XRD analysis that requires further investigation to understand its effect on its magnetism and half-metallicity. Mn2FeGe showed the existence of non-cubic phases that substantially weakened at high annealing temperatures.

Cílem této práce bylo studium Heuslrových sloučenin pomocí optických a magneto-optických (MO) metod. V případě první sloučeniny, Co2FeGa0.5Ge0.5, jsme studovali výskyt strukturálního disorderu za pomoci spektroskopické elipsometrie, MO spektroskopie a MO Kerrovy magnetome- trie. Experimentální výsledky jsme poté porovnali s teoretickými modely. Doplnili jsme také výsledky našich spolupracovník·, a to včetně X-ray difrakce, atomic-force mikroskopie a ab initio výpočt·, abych potvrdili naši interpretaci naměřených výsledk·. Bylo zjištěno, že strukturální disorder je vskutku pozorovatelný pomocí metod, kterých jsme využili. Jeho výskyt se projevil změnou elektronové struktury vzork·. V případě druhé sloučeniny, Fe2MnGa, jsme vyšetřovali vliv obsahu Fe složky na optické a MO vlastnosti tohoto materiálu. Zjistili jsme, že atomy Fe ovlivňují koncentraci volných elektron·, čímž dodávají sloučenině kovový charakter (v porovnání s Ni2MnGa). Zároveň zvyšují optickou a MO odezvu v blízké infra-červené oblasti spektra. Dále bylo zjištěno, že vzorek s nejvyšším obsahem Fe má nulovou magnetizaci. 1

A new tetragonal Heusler compound Mn1.4PtSn is synthesized. Crystal growth techniques that require growth directly from melt, such as Bridgman method, always result in microtwinned crystals. To get microtwin free crystals, another technique, flux method is employed, where growth can be done far below the melting point and martensitic transition temperature. The flux method results in successful large microtwin free crystals of Mn1.4PtSn. The single-crystal diffraction is done on a small piece of single crystals of Mn1.4PtSn. From structural analysis, it is found out that the crystal structure of Mn1.4PtSn is the first tetragonal superstructure in the family of Heusler compounds. The superstructure reflections are clearly observed in the powder X-ray diffraction patterns. Direction-dependent magnetic properties are measured. The compound is found to undergo two magnetic transitions. First, at 392 K, which corresponds to Curie temperature and second, at 170 K, which corresponds to the spin-reorientation transition temperature. The saturation magnetic moment at 2 K is very large of 4.7 µB/f.u. The refinement of powder neutron diffraction reveals that in the temperature range of 170 to 392 K, the magnetic structure is collinear ferromagnet whereas below 170 K, it is a non-coplanar spin structure. The magnetic moment, obtained from refinement, is close to the saturation moment obtained from magnetization. The electric transport properties are studied along the different crystallographic directions of the compound. The longitudinal resistivity measurement indicates that the compound is metallic and reveals the magnetic transitions at the same temperature as seen in the magnetization. An overall negative magnetoresistance of 3 percent is found. The Hall resistivity measurements reveal the presence of a large topological Hall resistivity (THE) of 0.9 µΩ cm and -0.1 µΩ cm for the magnetic field applied along [100] and [001], respectively. Two types of contributions in the THE for the field along [100] are seen. One that follows the quadratic form of longitudinal resistivity and second, that is independent of longitudinal resistivity. Anomalous Hall conductivity is found to be 250 and 165 Ω-1cm-1 for the field along [100] and [001], respectively. This value is close to the value obtained from theoretical calculations. The topological Hall conductivity is found to be approximately the same as its anomalous analog. A new series of polycrystalline samples with iridium substitution at the place of platinum in Mn1.4PtSn are prepared. The structural characterization show the crystal structure of these compounds is the same as Mn1.4PtSn, therefore, they also possess the tetragonal superstructure form. Magnetic properties, along with powder neutron diffraction data, reveal that the magnetic structure changes from out-of-plane ferromagnet to in-plane ferrimagnet with Ir-substitution. All the compounds are found to have metallic character. A large anomalous Hall conductivity of 405 Ω-1cm-1 is found for compound Mn1.4Pt0.7Ir0.3Sn. Three new series of compounds are prepared as an attempt to fill the vacancies present in the crystal structure of Mn1.4PtSn with transition-metal elements cobalt, nickel, and copper. The tetragonal superstructure survives up to 0.2 cobalt addition, 0.4 nickel addition and 0.6 copper addition. Further addition of elements leads to transformation to the inverse cubic Heusler structure. The magnetic properties show that the compounds with tetragonal structure have spin-reorientation transition, which is absent in the compounds with cubic structure. A new compound Mn1.7Pt0.8In is discovered. The single crystals are prepared by flux-method. Upon structural analysis from single-crystal refinement, it is found that the crystal structure is 3 × 3 × 3 superstructure of a Heusler structure and is so far the largest discovered in the Heusler family of compounds. Two magnetic transitions are revealed in the magnetization measurements. First, at 330 K, which corresponds to Curie temperature and second, at 220 K, which corresponds to spin-reorientation transition. The magnetic moment is 0.4 µB/Mn at 2 K and 0.07 µB/Mn at 300 K. Such a low moment might be due to possible compensated ferrimagnetic structure. Therefore, the compound is a potential candidate for spintronics devices.

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.

Crystalline systems formed with transition metal elements tend to exhibit strong magneto-structural coupling that gives rise to unusual but exciting physical phenomena in these materials. In this dissertation, we present our findings from the studies of structural and physical properties of single phase compounds Co2MnSi, Ni16Mn6Si7 and Mn(Ni0.6Si0.4)2. In addition, the stability of a Ni2MnSi composition in a multiphase system is discussed by both theoretical and experimental approaches. All the works have been conducted with a focus on explaining the fundamental behaviors of these systems that have not been adequately addressed by other studies in the literature. We present an experimental and theoretical investigation of the half-metallic Heusler compound, Co2MnSi to address disorder occupancies and magnetic interactions in the material. Contrary to previous studies, our neutron diffraction refinement of the polycrystalline sample reveals almost identical amount of Mn and Co antisite disorders of ~6.5% and ~7.6%, respectively which is also supported explicitly by our first-principles calculations on the system with defects. A reduction of the net moment of Co2MnSi due to an antiferromagnetic interaction introduced by disordered Mn is observed by our theoretical study. The neutron refinements at 298 K, 100 K, and 4 K further supports such reduction of moments. The work also reports the growth of single crystal by the Czochralski method and determination of a Curie temperature of ~1014 K measured by both the electrical resistivity and dilatometry measurement. Studies of a Ni2MnSi Heusler system reveal two new systems i.e., the Ni16Mn6Si7 G-phase and the Mn(Ni0.6Si0.4)2 based Laves phase with complex crystal structures. These systems exhibit strong magneto-structural coupling that could lead to interesting physical behaviors. The lack of thorough understanding of the properties of these materials inspired us to undertake the present studies. We address the geometrically frustrated two-dimensional magnetic structure and spin canted weak ferromagnetic behavior of Ni16Mn6Si7. Our magnetization and specific heat measurements on a Czochralski grown single crystal sample depicts the paramagnetic to antiferromagnetic transition at 197 K, and a second phase change at 50 K. Furthermore, a gradual drop of zero field cooled magnetic susceptibility is observed below 6 K that is associated with the spin freezing effect. The neutron diffraction on the polycrystalline powder samples at the temperatures of interest reveals that the antiferromagnetism is governed by the magnetic ordering of the Mn ions in the octahedral network. Below the Néel temperature of 197 K, the 2/3 of Mn atom moments form a two-dimensional magnetic arrangement, while the 1/3 moments remain geometrically frustrated. The phase transition at 50 K is found to be associated with the reorientation of the 2D moments to a canted antiferromagnetic state and development of ordering of the frustrated paramagnetic ions. Magnetization measurements as a function of temperature and magnetic field in principal directions, permit to determine the anisotropic magnetic behavior of Ni16Mn6Si7 in terms of the magnetic structure obtained by the neutron diffraction measurements. We also report an irreversible smeared spin-flop type transition for the system at a higher magnetic field. The diffuse scattering due to the short-range ordering is a commonly occurring phenomenon in Laves phase materials. The occurrence of such distinct atomic arrangement can considerably influence the physical behavior of the material. Nevertheless, no structural reconstruction of such atomic distribution in Laves phase has ever been reported in the literature. In this work, we present the structural ordering, and the associated physical behavior of an antiferromagnetic Ni-Mn-Si Laves phase with a composition Mn(Ni0.6Si0.4)2. The possibility of unique short-range ordering in the material is first concluded based on our single crystal diffraction analysis. With the high-resolution transmission electron microscopy and electron energy loss spectroscopy analysis, our work resolves the distinct atomic ordering of the Laves phase system. The investigations reveal the origin of the short-range ordering to arise from a unique arrangement between Ni and Si. The study also presents the atomic resolution mapping of the Si atoms which has never been reported by any previous studies. With further electrical conductivity measurement, we find one of the consequences of the unique ordering reflected in a semiconducting like temperature dependence of the compound. The neutron diffraction at 298 K suggests Mn(Ni0.6Si0.4)2 to be a strong antiferromagnetic system, which is further supported by the successive magnetic susceptibility measurement. The Néel temperature is determined to be 550 K. We also address the stability of the hypothetical ferromagnetic Heusler compound Ni2MnSi which has been proposed to be a stable system by numerous theoretical studies. Our first-principles work corroborates those studies with a negative formation enthalpy of -1.46 eV/formula unit. However, after numerous attempts to synthesize the composition, we conclude that a single phase Heusler Ni2MnSi compound cannot form under ambient conditions. Our results show that the system crystallizes as a mixture of the two Ni-Mn-Si compounds, i.e., the Ni16Mn6Si7 type G-phase and Mn(Ni0.6Si0.4)2 based Laves phase. Our work provides a possible explanation for the unstable Ni2MnSi Heusler compound with the calculation of formation enthalpy of the hypothetical Heusler system in terms of the computed energies of the neighboring phases Ni16Mn6Si7 and Mn(Ni0.6Si0.4)2. / Thesis / Doctor of Philosophy (PhD)

Abstract After the discovery of topological insulators (TIs), it has come to be widely recognized that topological states of matter can actually be widespread. In this sense, TIs have established a new paradigm about topological materials. Recent years have seen a surge of interest in topological semimetals, which embody two different ways of generalizing the effectively massless electrons to bulk materials. Dirac and, particularly, Weyl semimetals should support several transport and optical phenomena that are still being sought in experiments. A number of promising experimental results indicate superconductivity in members of half-Hesuler semimetals which realize the mixing singlet and triplet pairing symmetry. We now turn to results we got through the work on topological semimetals. This work presents quantum high field transports on Dirac and Weyl topological semimetals including Sr1-yMn1-zSb2 (y, z < 0.1), YbMnBi2 and TaP. In case of Sr1-yMn1-zSb2 (y, z < 0.1), massless relativistic fermion was reported with m* = 0.04-0.05m0. This material presented a ferromagnetic order for in 304 K < T < 565 K, but a canted antiferromagnetic order with a net ferromagnetic component for T < 304 K. These are considered striking features of Dirac fermions For YbMnBi2, we reported the unusual interlayer quantum transport behavior in magnetoresistivity, resulting from the zeroth LL mode observed in this time reversal symmetry breaking type II Weyl semimetal. Also, for Weyl semimetal TaP the measurements probed multiple Fermi pockets, from which nontrivial π Berry phase and Zeeman splitting were extracted. Our ultra-low penetration depth measurements on half-Heuslers YPdBi and TbPdBi revealed a power- law behavior with n= 2.76 ± 0.04 for YPdBi samples and n=2.6 ± 0.3 for TbPdBi sample. We may conclude the exponent n > 2 implies nodless superconducting gap in our samples. Also, we found that despite the increase in magnetic correlations from YPdBi to TbPdBi, superconductivity remains robust in both systems which indicates that AF fluctuations do not play a major role in superconducting mechanism.

Manganese-based Heusler compounds display intriguing fundamental physical properties, determined by the delicate balance of magnetic interactions that give rise to real and reciprocal-space topology, sparking the interest in their potential application in the spin-based technology of the future. In this thesis, a thorough study of thin films of two Mn-based Heusler compounds, the hexagonal MnPtGa and inverse tetragonal Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) system, was performed. The observation of Néel-type skyrmions in single-crystalline MnPtGa motivated our interest in the growth and characterization of thin films of this compound. The films were deposited by magnetron sputtering on (0001)-Al2O3 single crystalline substrates, achieving the epitaxial growth of the Ni2In-type hexagonal crystal structure (P6_3/mmc space group, no. 194). Two thermally-induced magnetic transitions were identified in MnPtGa thin films: below the ordering temperature (T_C=273 K) the system becomes ferromagnetic, followed by a spin-reorientation transition at T_sr=160 K, adopting a spin-canted magnetic structure. Resorting to single-crystal neutron diffraction (SCND), we were able to resolve the magnetic ground state of our MnPtGa thin films. The Mn magnetic moments were found to tilt 20 degrees away from the c-axis, forming a commensurate magnetic structure with a ferromagnetic component along the crystallographic c-axis and a staggered antiferromagnetic one in the basal plane. This further demonstrated the applicability of a bulk technique, such as SCND, to the study of magnetic structures in thin films. Additionally, the perpendicular magnetic anisotropy (PMA) in the system was determined by magnetometry technique. Electrical magnetotransport measurements were performed in a thickness series of MnPtGa thin films. A non-monotonous anomalous Hall conductivity (AHC) was observed, whose intrinsic Berry-curvature origin was elucidated by means of first-principle calculations. We further observed by magnetic force microscopy technique the nucleation of irregular magnetic bubbles under the application of a magnetic field. We tentatively link their appearance to the onset of an additional electron scattering mechanism contributing to the transverse resistivity. In the second part of this thesis, the inverse tetragonal Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) system was investigated. The films were grown on MgO(100) single crystalline substrates, promoting the epitaxial growth of the tetragonal structure (I-4m2 space group, no. 119). We primarily focused on the impact of the systematic substitution of iridium on the structural, magnetic and electrical (magneto)transport properties of the system. A compression of the basal lattice parameters and elongation of the c-axis, accompanied by larger crystallographic disorder, was observed as the Ir content (x) increased, altering the Mn-Mn exchange interactions and therefore the magnetic properties of the compound. Mn2RhSn have two thermally-induced magnetic transitions: first, to a collinear ferrimagnetic state below the Curie temperature (T_C=280 K), followed by a spin-reorientation transition at T_sr=80 K to a noncollinear state, determined by two inequivalent Mn sublattices. A reduction of both T_C and T_sr was observed, as well as a tendency towards a hard-axis ferromagnet and therefore larger PMA as the Ir content of the films was increased. Additionally, a reduction of the saturation magnetization suggest a change of the magnitude of the spin canting upon Ir-substitution. The electrical magnetotransport properties of the Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) thin films were acquired and analyzed in a wide temperature and magnetic field range. A strongly temperature and composition dependent non-monotonous AHC was found, suggesting two regimes in the electronic transport: (i) a nearly x-independent regime dominated by intrinsic Berry-curvature and (ii) a strongly x-dependent regime suggesting a more relevant role from extrinsic mechanisms contributing to the AHC. On the other hand, the Mn2Rh(0.95)Ir(0.05)Sn bulk system is known to host magnetic skyrmion and antiskyrmion phases. We indirectly assessed the impact of the systematic Ir-substitution on the (anti)skyrmionic phases through the analysis of the sign of the topological Hall effect in our thin films. A tendency towards the suppression of the low-T skyrmion phase stabilized by magnetic dipole-dipole interaction, and subsistence of the high-T antiskyrmion phase in Mn2Rh(1-x)Ir(x)Sn thin films was found as x > 0.2, which can be interpreted as a change of magnitude of the anisotropic DMI in this tetragonal D_2d system upon Ir-substitution. We have thus demonstrated that the magnetic and topological properties of the Mn2Rh(1-x)Ir(x)Sn system can be tailored upon chemical substitution, showing a strongly intertwined relation between composition, crystal and electronic structure, with the emergence of exotic magnetic phases, ultimately reflected in their electrical transport signatures.:Abstract iii Abbreviations iv Symbols vi Preface xii 1 Fundamentals 1 1.1 Noncollinear magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Magnetic interactions in solids . . . . . . . . . . . . . . . . . . . 2 1.1.1.1 Exchange interaction . . . . . . . . . . . . . . . . . . . 2 1.1.1.2 Dzyaloshinsky-Moriya interaction . . . . . . . . . . . . 3 1.1.1.3 Magnetic anisotropy . . . . . . . . . . . . . . . . . . . 4 1.1.1.4 Magnetic dipolar interaction . . . . . . . . . . . . . . . 5 1.1.2 Spin-reorientation transition . . . . . . . . . . . . . . . . . . . . 5 1.1.3 Magnetic skyrmions and antiskyrmions . . . . . . . . . . . . . . 6 1.1.3.1 Antiskyrmions in Heusler compounds . . . . . . . . . . 8 1.2 Magnetic Heusler compounds . . . . . . . . . . . . . . . . . . . . . . . 9 1.2.1 Cubic crystal structure . . . . . . . . . . . . . . . . . . . . . . . 10 1.2.2 Distorted crystal structures . . . . . . . . . . . . . . . . . . . . 10 1.2.2.1 Tetragonal Heusler compounds . . . . . . . . . . . . . 11 1.2.2.2 Hexagonal Heusler compounds . . . . . . . . . . . . . 11 1.3 Charge and spin transport in ferromagnets . . . . . . . . . . . . . . . . 13 1.3.1 The two-current model . . . . . . . . . . . . . . . . . . . . . . . 13 1.3.2 The Hall effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.3.2.1 Anomalous Hall effect . . . . . . . . . . . . . . . . . . 15 1.3.2.2 Topological Hall effect . . . . . . . . . . . . . . . . . . 17 1.4 Neutron scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.4.1 Thermal Neutrons . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.4.1.1 Scattering cross sections . . . . . . . . . . . . . . . . . 19 1.4.1.2 The four-circle diffractometer . . . . . . . . . . . . . . 23 xv 1.4.2 Magnetic neutron scattering . . . . . . . . . . . . . . . . . . . . 24 2 Experimental Techniques 29 2.1 Magnetron sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.1.1 Thin films growth modes . . . . . . . . . . . . . . . . . . . . . . 32 2.1.2 Thin films microstructure . . . . . . . . . . . . . . . . . . . . . 33 2.2 X-ray characterization of thin films . . . . . . . . . . . . . . . . . . . . 34 2.2.1 Geometry of the X-ray diffractometer . . . . . . . . . . . . . . . 35 2.2.2 Radial θ-2θ scan . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.3 ϕ -scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.4 Rocking curves (ω-scans) . . . . . . . . . . . . . . . . . . . . . . 36 2.2.5 X-ray reflectivity (XRR) . . . . . . . . . . . . . . . . . . . . . . 37 2.3 Composition analysis: energy dispersive X-ray spectroscopy (EDS) . . . 38 2.4 Surface characterization: atomic and magnetic force microscopy . . . . 38 2.5 D10 thermal neutron diffractometer . . . . . . . . . . . . . . . . . . . . 39 2.6 SQUID-VSM magnetometry . . . . . . . . . . . . . . . . . . . . . . . . 40 2.7 Electrical (magneto-)transport measurements . . . . . . . . . . . . . . 41 3 Noncollinear magnetism in MnPtGa epitaxial thin films 43 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.2 MnPtGa thin films: growth and characterization . . . . . . . . . . . . . 45 3.2.1 Growth conditions . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.2.2 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.3 Magnetic properties of MnPtGa thin films . . . . . . . . . . . . . . . . 49 3.3.1 Thermal evolution of the magnetic structure . . . . . . . . . . . 49 3.3.2 Field dependent magnetization . . . . . . . . . . . . . . . . . . 50 3.3.3 Single-crystal neutron diffraction in MnPtGa thin films . . . . . 52 3.3.3.1 Ferromagnetic phase . . . . . . . . . . . . . . . . . . . 54 3.3.3.2 Noncollinear phase . . . . . . . . . . . . . . . . . . . . 55 3.4 Electronic band structure of h-MnPtGa . . . . . . . . . . . . . . . . . . 57 3.5 Electrical magnetotransport properties of MnPtGa thin films . . . . . . 59 3.5.1 Zero field longitudinal resistivity . . . . . . . . . . . . . . . . . . 60 3.5.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.5.3 Magnetic transitions under a magnetic field . . . . . . . . . . . 64 3.6 Intrinsic origin of the anomalous Hall effect . . . . . . . . . . . . . . . . 65 3.6.1 Scaling of the anomalous Hall conductivity vs. σxx . . . . . . . 68 3.7 Spin textures in MnPtGa thin films . . . . . . . . . . . . . . . . . . . . 73 3.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4 Tuning the magnetic and topological properties of Mn2Rh1−xIrxSn epitaxial thin films 83 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.2 Growth and characterization of Mn2Rh1−xIrxSn thin films . . . . . . . 86 4.2.1 Growth conditions and Ir substitution . . . . . . . . . . . . . . 86 4.2.2 Crystal structure of Mn2Rh1−xIrxSn . . . . . . . . . . . . . . . . 87 4.3 Tuning the magnetic properties of the Mn2Rh1−xIrxSn system . . . . . 91 xvi 4.3.1 Thermal magnetic transitions . . . . . . . . . . . . . . . . . . . 91 4.3.2 Increasing the magnetic anisotropy under Ir-substitution . . . . 92 4.4 Electrical (magneto-)transport properties of Mn2Rh1−xIrxSn thin films 94 4.4.1 Zero-field longitudinal resistivity and spin reorientation transition 94 4.4.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.4.3 Hall effects: from ordinary to anomalous & topological . . . . . 96 4.4.3.1 Ordinary Hall effect . . . . . . . . . . . . . . . . . . . 97 4.4.3.2 Anomalous Hall effect . . . . . . . . . . . . . . . . . . 98 4.4.3.3 Competing mechanisms in the AHC of the Mn2Rh1−xIrxSn system . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.4.3.4 Scaling of the AHC with the magnetization . . . . . . 101 4.4.3.5 Topological Hall effect . . . . . . . . . . . . . . . . . . 102 4.5 Tuning the (Anti-)Skyrmion phases . . . . . . . . . . . . . . . . . . . . 106 4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 5 Conclusions & Outlook 111 List of Figures 117 List of Tables 120 List of Publications 124 Aknowledgements 124 Bibliography 127 Eigenständigkeitserklärung 147