Ultrafast Optical and Magneto-Optical Kerr Effect Spectroscopy on Condensed Matter

In this masters project, an ultrafast optical spectroscopy system has been constructed and tested. The system was built as an extension to an existing spectroscopy system, centered around a mode-locked Ti:Sapphire femtosecond laser, optically amplified by a Q-switched, frequency-doubled Nd:YLF laser. The previous system was based on principles of nonlin- ear optics, and was used to measure third-order nonlinear effects. The goal of the project was to create a system that could perform measurements of magnetic and third-order non- linear properties in condensed matter. The work lead to improvements in detection and signal analysis, and inclusion of magneto-optical measurements based on the longitudinal magneto-optical Kerr effect. The functionality of the system was experimentally verified through measurements on permalloy (Ni0.8Fe0.2) grown on fused silica (SiO2), optically coated with zirconium dioxide (ZrO2). Optical demagnetization was also tested, and the demagnetization and recovery transients were resolved in time, using ultrafast optical mea- surement techniques. This proved to be feasible with the current system. Further work is however required to gain a detailed analysis of the time-resolved measurements. The ultrafast nonlinear optical capabilities were also tested through measurements on deionized water (H2O) and acetone ((CH3)2CO). Several samples with films of iron-doped zinc sulfide (Fe:ZnS) on sapphire (Al2O3) substrate were characterized, on request from another group of researchers. The third-order nonlinear susceptibility was calculated based on these measurements, and was found to conform well with values obtained by others. The Raman spectrum of water was also calculated based on nonlinear measurements, to test the possibility of retrieving spectral information from the time-resolved measurements. The bandwidth of the laser was 14 THz, and it is believed that spectral features as high as 12 THz could be resolved in the analysis of the Raman spectrum

Thickness dependent uniaxial magnetic anisotropy due to step-edges in (1 1 1)-oriented La0.7Sr0.3MnO3 thin films

The magnetic anisotropy of films of La0.7Sr0.3MnO3 grown on vicinal (1 1 1)-oriented SrTiO3 substrates are investigated. For temperatures above the tetragonal – cubic structural phase transition temperature of the substrate, a step edge induced uniaxial magnetic anisotropy is found at remanence, with a thickness-driven change in easy axis direction, from perpendicular to the step edges to parallel to the step edges with increasing thickness. The anisotropy constant for the investigated (1 1 1)-oriented samples is of the same magnitude as for previously reported (0 0 1)-oriented samples. The data is discussed in the framework of in-plane rotations of the oxygen octahedra resulting in a uniaxial anisotropy. Furthermore, the magnetic anisotropy is sensitive to the structural phase transition at 105 K of the substrate, and the anisotropy constant increases drastically as the temperature is lowered below 105 K

Effect of (111)-oriented strain on the structure and magnetic properties of La2/3Sr1/3MnO3 thin films

Using strain, i.e. subtle changes in lattice constant in a thin film induced by the underlying substrate, opens up intriguing new ways to control material properties. We present a study of the effects of strain on structural and ferromagnetic properties of (1 1 1)pc-oriented La0.7Sr0.3MnO3 epitaxial thin films grown on NdGaO3, SrTiO3, and DyScO3 substrates. (The subscript pc denotes the pseudo-cubic symmetry.) The results show that La0.7Sr0.3MnO3 assumes a monoclinic unit cell on NdGaO3 and DyScO3 and a rhombohedral unit cell on SrTiO3. For La0.7Sr0.3MnO3 on NdGaO3 and DyScO3 a uniaxial magnetic anisotropy is found, while La0.7Sr0.3MnO3 on SrTiO3 is magnetically isotropic. The Néel model is used to explain the anisotropy of the thin films on NdGaO3 and SrTiO3, however, for La0.7Sr0.3MnO3 on DyScO3 the effect of octahedral rotations needs to be included through the single ion model. Through examination of the Curie temperature of the strained films we suggest that (1 1 1)-strain has a different effect on the Jahn–Teller splitting of e g and t 2g electron levels than what is seen in (0 0 1)pc-oriented La0.7Sr0.3MnO3 thin films

Dislocation-Driven Relaxation Processes at the Conical to Helical Phase Transition in FeGe

The formation of topological spin textures at the nanoscale has a significant impact on the long-range order and dynamical response of magnetic materials. We study the relaxation mechanisms at the conical-to-helical phase transition in the chiral magnet FeGe. By combining macroscopic ac susceptibility measurement, surface-sensitive magnetic force microscopy, and micromagnetic simulations, we demonstrate how the motion of magnetic topological defects, here edge dislocations, impacts the local formation of a stable helimagnetic spin structure. Although the simulations show that the edge dislocations can move with a velocity up to 100 m/s through the helimagnetic background, their dynamics are observed to disturb the magnetic order on the time scale of minutes due to randomly distributed pinning sites. The results corroborate the substantial impact of dislocation motions on the nanoscale spin structure in chiral magnets, revealing previously hidden effects on the formation of helimagnetic domains and domain walls

Magnetic and geometric control of spin textures in the itinerant kagome magnet Fe3 Sn2

Magnetic materials with competing magnetocrystalline anisotropy and dipolar energies can develop a wide range of domain patterns, including classical stripe domains, domain branching, and topologically trivial and nontrivial (skyrmionic) bubbles. We image the magnetic domain pattern of Fe3Sn2 by magnetic force microscopy and study its evolution due to geometrical confinement, magnetic fields, and their combination. In Fe3Sn2 lamellae thinner than 3 μm, we observe stripe domains whose size scales with the square root of the lamella thickness, exhibiting classical Kittel scaling. Magnetic fields turn these stripes into a highly disordered bubble lattice. Complementary micromagnetic simulations quantitatively capture the magnetic field and thickness dependence of the magnetic patterns, reveal strong reconstructions of the patterns between the surface and the core of the lamellae, and identify the observed bubbles as skyrmionic bubbles. Our results imply that geometrical confinement together with competing magnetic interactions can provide a path to fine-tune and stabilize different types of topologically trivial and nontrivial spin structures in centrosymmetric magnets

Detection of Topological Spin Textures via Nonlinear Magnetic Responses

Topologically nontrivial spin textures, such as skyrmions and dislocations, display emergent electrodynamics and can be moved by spin currents over macroscopic distances. These unique properties and their nanoscale size make them excellent candidates for the development of next-generation race-track memory and unconventional computing. A major challenge for these applications and the investigation of nanoscale magnetic structures in general is the realization of suitable detection schemes. We study magnetic disclinations, dislocations, and domain walls in FeGe and reveal pronounced responses that distinguish them from the helimagnetic background. A combination of magnetic force microscopy (MFM) and micromagnetic simulations links the response to the local magnetic susceptibility, that is, characteristic changes in the spin texture driven by the MFM tip. On the basis of the findings, which we explain using nonlinear response theory, we propose a read-out scheme using superconducting microcoils, presenting an innovative approach for detecting topological spin textures and domain walls in device-relevant geometries