Study of Magnetic and Magnetotransport Properties of Epitaxial MnPtGa and Mn2Rh(1-x)Ir(x)Sn Heusler Thin Films

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 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 14