Thesis advisor: Stephen Wilson / The magnetic interaction in materials generally can be categorized into two extremes: localized and itinerant. This work will focus on the electronic and magnetic properties of two prototypical magnetic compounds, which fall into the opposite extremes, i:e:, the spin-orbit coupled Mott insulator Sr₂IrO₄ (Sr214) described by the localized Heisenberg model and the itinerant helical (nearly-ferromagnetic) metal MnSi pictured with band or Stoner magnetism. The single layered cuprate analogue Sr₂IrO₄ has attracted considerable attentions in recent years, due to its unusual electronic and magnetic properties and the potential to access superconducting states. The exotic jeff = 1/2 ground state for the Ir⁴⁺ (5d⁵) ions results from the delicate balance of competing/cooperating energy scales, such as the stronger spin-orbit coupling (SOC) in 5d materials as compared to 3d transition metal oxides (TMOs), crystal electric field (CEF) splitting and electron-electron correlations. Superconducting states are theoretically predicted to be achievable if sufficient carriers are introduced into this spin-orbit assisted compound, which later triggers tremendous experimental works toward the realization of superconductivity. Here in this work a combined study of various probes, such as transport, magnetization, X-ray and neutron scattering measurements, focusing on the electronic and magnetic properties, is presented in the perturbed spin-orbit coupled Mott (SOM) state. Specifically in electron doped (Sr₁₋ₓLaₓ)₂IrO₄, a detailed mapping of magnetism with respect to electron doping is presented, demonstrating the gradual transition from long range magnetic order in parent state, to intermediate short range order, and eventually into the incommensurate (IC) spin density wave (SDW) state with increasing electron doping. Our picture supports the conjecture that the quenched Mott phases in electron-doped Sr₂IrO₄ and hole doped La₂CuO₄ share common competing electronic phases. On the other hand, the prototypical itinerant metal MnSi is examined by inelastic neutron scattering (INS). Our experimental data directly demonstrate the collapse of linear spin wave theory for localized Heisenberg magnets in the large energy limit, although the low energy dispersion is still described by the ferromagnetic spin wave theory. Most importantly, our observations display the chimney-like dispersion spectrum up to the energy scale of at least 240 meV, which is more than one order of magnitude larger than the Heisenberg interaction energy scale. For the first time, solid characterizations of Stoner excitations in itinerant helimagnet (nearly ferromagnetic) have been demonstrated up to an exceedingly large energy scale. Our intriguing results will greatly promote further understanding and exploration of Stoner excitations in itinerant magnets. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The aim of the current thesis was to investigate the preparation of MnSi film on Si substrates. The preparation process includes room temperature sputtering Mn films with different thicknesses and flash-lamp annealing with different energy density (annealing temperature). Systematic investigations on their structural, electrical, magnetic, and magneto-transport properties were performed. The key findings are summarized below: Thin films with the B20-MnSi phase on Si(100) substrates were fabricated for the first time. They exhibit magnetic skyrmion behaviour. In comparison with Si(111) substrates, Si(100) substrates are more preferred from the practical application point of view. The nucleation of B20-MnSi on Si(100) is believed to be triggered by the fast solid-state phase reaction between Mn and Si via ms-range flash-lamp annealing. Compared with the corresponding bulk material, our films show an increased Curie temperature of around 43 K. The magnetic and transport measurements reveal that skyrmions in B20-MnSi on Si(100) made by sub-seconds solid-state reaction are stable within much broader field and temperature windows than bulk MnSi. The parasitic MnSi1.7 phase can be further minimized or eliminated by optimizing the annealing conditions, the quality of the deposited Mn film, and its interface with the Si substrate. Our work demonstrates a promising route for the fabrication of B20-type transition metal silicides for integrated and/or hybrid spintronic applications on Si(100) wafers, which are more preferable for industry applications. The growth of MnSi films on Si(111) substrates has been widely realized by solid phase epitaxy or molecular beam epitaxy since the lattice mismatch and symmetry fit better. One problem is the parasitic MnSi1.7 phase. By controlling the reaction parameters using strongly non-equilibrium flash lamp annealing, we have achieved full control over the phase formation of Mn-silicides in thin films from single-phase B20-MnSi or MnSi1.7 to mixed phases. The obtained films are highly textured and reveal sharp interfaces to the Si substrate. The obtained B20-MnSi films exhibits a high Curie temperature of 41 K. The skyrmion phase can be stabilized over broad temperature and magnetic field ranges. We propose flash-lamp-annealing-induced transient reaction as a general approach for phase separation in transition-metal silicides and germanides and for growth of B20-type films with enhanced topological stability. By comparing the magnetic properties of MnSi films grown on both Si(111) and Si(100) substrates by ourselves and by others in literature, we found one common feature. It is the increased Curie temperature of around 41-43 K for all MnSi films. It is much higher than 29.5 K for bulk MnSi. We try to understand the puzzling Curie temperature widely reported in MnSi films. We have prepared MnSi films with a large variation regarding their thickness, crystallinity, strain and phase separation. Particularly, polycrystalline MnSi films on Si(100) and textured MnSi films on Si(111), both with different mixture ratio with MnSi1.7 have been grown and systematically characterized. Surprisingly, all obtained MnSi films exhibit a high Curie temperature at around 43 K. The skyrmion phase has also been detected in these films. However, we find no correlation between the increased Curie temperate and the film thickness, strain, lattice volume or the mixture with MnSi1.7. Our work has not provided a conclusive picture for this question, but is rather calling a revisit, especially to the effect by the interface, stoichiometry and point defects. Further studies are essential to understand the B20 transition-metal silicide/germanides films and therefore to utilize them for spintronic applications.:Contents Abstract iii Kurzfassung v 1. Introduction 1 1.1 B20 compounds and magnetic skyrmions 1 1.2 B20 MnSi with magnetic Skyrmions 8 1.2.1 Crystallization process 10 1.2.2 Phase diagram of Mn-Si binary compounds 13 1.2.3 Bulk B20-MnSi 14 1.2.4 B20-MnSi thin film 18 1.2.5 B20-MnSi nanowire 25 1.3 Fast annealing method 27 1.4 Objectives and the structure of the thesis 30 2. Experiment 32 2.1 Sample preparation 32 2.1.1 DC magnetron sputtering 32 2.1.2 Sub-second annealing 35 2.2 Structure characterization: X-ray diffraction 40 2.3 Property characterization 41 2.3.1 Magnetic properties 41 2.3.2 Magneto-transport properties 44 3. B20-MnSi films grown on Si(100) substrates with magnetic skyrmion signature 46 3.1 Introduction 46 3.2 Experiment 47 3.3 Results and Discussions 48 3.4 Conclusions 56 4. Phase selection in Mn-Si alloys by fast solid-state reaction with enhanced skyrmion stability 57 4.1 Introduction 57 4.2 Experiment 59 4.3 Results 61 4.3.1 MnSi and MnSi1.7 phase reaction 61 4.3.2 Magnetic Skyrmion 68 4.3.3 Discussion 76 4.4 Conclusion 78 5. On the Curie temperature of MnSi films 80 5.1 Introduction 80 5.2 Experiment 82 5.3 Results 83 5.4 Conclusion 89 6. Summary and outlook 90 6.1 Summary 90 6.2 Outlook 91 6.2.1 Film thickness effect on formation of (111)-textured B20-MnSi 91 6.2.2 MnSi1.7% influence on Skyrmion stability 96 6.2.3 Preparation of other transition-metal monosilicides and germanides 98 Acknowledgement 99 References 101 Publication list 117 Curriculum Vitae 119 Erklärung 120

MnSi(111) films were grown on Si(111) substrates by solid phase epitaxy (SPE) and molecular beam epitaxy (MBE) to determine their magnetic structures. A lattice mismatch of -3.1% causes an in-plane tensile strain in the film, which is partially relaxed by misfit dislocations. A correlation between the thickness dependence of the Curie temperature (TC) and strain is hypothesized to be due to the presence of interstitial defects. The in-plane tensile strain leads to an increase in the unit cell volume that results in an increased TC as large as TC = 45 K compared to TC = 29.5 K for bulk MnSi crystals. The epitaxially induced tensile stress in the MnSi thin films creates an easy-plane uniaxial anisotropy. The magnetoelastic coefficient was obtained from superconducting quantum interference device (SQUID) magnetometry measurements combined with transmission electron microscopy (TEM) and x-ray diffraction (XRD) data. The experimental value agrees with the coefficient determined from density functional calculations, which supports the conclusion that the uniaxial anisotropy originates from the magnetoelastic coupling. Interfacial roughness obscured the magnetic structure of the SPE films, which motivated the search for a better method of film growth. MBE grown films displayed much lower interfacial roughness that enabled a determination of the magnetic structure using SQUID and polarized neutron reflectometry (PNR). Out-of-plane magnetic field measurements on MBE grown MnSi(111) thin films on Si(111) substrates show the formation of a helical conical phase with a wavelength of 2?/Q = 13.9 ± 0.1 nm. The presence of both left-handed and right-handed magnetic chiralities is found to be due to the existence of inversion domains that result from the non-centrosymmetric crystal structure of MnSi. The magnetic frustration created at the domain boundaries explains an observed glassy behaviour in the magnetic response of the films. PNR and SQUID measurements of MnSi thin films performed in an in-plane magnetic field show a complex magnetic behaviour. Experimental results combined with theoretical results obtained from a Dzyaloshinskii model with an added easy-plane uniaxial anisotropy reveals the existence of numerous magnetic modulated states that do not exist in bulk MnSi. It is demonstrated in this thesis that modulated chiral magnetic states can be investigated with epitaxially grown MnSi(111) thin films on insulating Si substrates, which offers opportunities to investigate spin-dependent transport in chiral magnetic heterostructures based on this system.