S. D. Kevan

Tailoring magnetic energies to form dipole skyrmions and skyrmion lattices

Author(s): Montoya, SA; Couture, S; Chess, JJ; Lee, JCT; Kent, N; Henze, D; Sinha, SK; Im, MY; Kevan, SD; Fischer, P; McMorran, BJ; Lomakin, V; Roy, S; Fullerton, EE | Abstract:

Synthesizing skyrmion bound pairs in Fe-Gd thin films

We show that properly engineered amorphous Fe-Gd alloy thin films with perpendicular magnetic anisotropy exhibit bound pairs of like-polarity, opposite helicity skyrmions at room temperature. Magnetic mirror symmetry planes present in the stripe phase, instead of chiral exchange, determine the internal skyrmion structure and the net achirality of the skyrmion phase. Our study shows that stripe domain engineering in amorphous alloy thin films may enable the creation of skyrmion phases with technologically desirable properties.

Soft x-ray ptychography studies of nanoscale magnetic and structural correlations in thin SmCo5 films

High spatial resolution magnetic x-ray spectromicroscopy at x-ray photon energies near the cobalt L3 resonance was applied to probe an amorphous 50 nm thin SmCo5 film prepared by off-axis pulsed laser deposition onto an x-ray transparent 200 nm thin Si3N4 membrane. Alternating gradient magnetometry shows a strong in-plane anisotropy and an only weak perpendicular magnetic anisotropy, which is confirmed by magnetic transmission soft x-ray microscopy images showing over a field of view of 10 μm a primarily stripe-like domain pattern but with local labyrinth-like domains. Soft x-ray ptychography in amplitude and phase contrast was used to identify and characterize local magnetic and structural features over a field of view of 1 μm with a spatial resolution of about 10 nm. There, the magnetic labyrinth domain patterns are accompanied by nanoscale structural inclusions that are primarily located in close proximity to the magnetic domain walls. Our analysis suggests that these inclusions are nanocrystalline Sm2...

Electron states and the spin density wave phase diagram in Cr(1 1 0) films

Chromium films offer an excellent system to study the impact of dimensional confinement on physical properties associated with the spin-density-wave (SDW) ground state observed in bulk materials. These properties are also of some technological importance since chromium is a common component of thin film magnetic structures. We prepared chromium (1?1?0) films of high crystalline quality on a W(1?1?0) substrate with a wedge-shaped thickness profile so that the impact of confinement can be systematically studied. We have characterized these films using a combination of low-energy electron diffraction and microscopy as well as high-resolution angle-resolved photoemission spectroscopy. We have probed the Fermi surface and the nesting vectors therein that are relevant to the SDW ground state. We find these to predict accurately the observed bulk SDW periodicity. We have also characterized the SDW periodicity in the film directly by measuring the splitting between backfolded bands, and we find that this periodicity deviates markedly from the bulk periodicity for thinner films at higher temperatures. We have systematically mapped the SDW incommensurability and phase diagram as a function of both film thickness and temperature. We find commensurate and incommensurate phases that are separated by nearly continuous transitions. Our results suggest a simple model to explain the delicate interplay between commensurate and incommensurate phases that involves a balance between SDW stabilization energy and surface and interface energetics.

Surface states and spin density wave periodicity in Cr(110) films

Using angle-resolved photoemission, we have mapped the dispersion relations and Fermi contours for surface-localized electron states onto clean and hydrogen-covered Cr(110) surfaces. In particular, we have probed the relationship between hydrogen adsorption and the evolution of the spin density wave (SDW) periodicity in chromium thin films observed previously. We find qualitatively similar surface band dispersion relations to those on W(110) and Mo(110), although with a narrower bandwidth, broader spectral features, and a smaller impact from the spin–orbit interaction. We compare our results to existing first-principles calculations and find a significant disagreement for a surface band that produces a prominent surface Fermi contour. Upon hydrogen adsorption, the Fermi contour for a particular surface band becomes well nested at a wave vector that stabilizes a commensurate SDW. We suggest that a competition between commensurate two-dimensional (2D) and incommensurate 3D Fermi surface nesting plays an important role in the SDW energetics in thin Cr(110) films.

Nanosecond X-Ray Photon Correlation Spectroscopy on Magnetic Skyrmions.

We report an x-ray photon correlation spectroscopy method that exploits the recent development of the two-pulse mode at the Linac Coherent Light Source. By using coherent resonant x-ray magnetic scattering, we studied spontaneous fluctuations on nanosecond time scales in thin films of multilayered Fe/Gd that exhibit ordered stripe and Skyrmion lattice phases. The correlation time of the fluctuations was found to differ between the Skyrmion phase and near the stripe-Skyrmion boundary. This technique will enable a significant new area of research on the study of equilibrium fluctuations in condensed matter.

Microscopic return point memory in Co/Pd multilayer films

We report soft x-ray speckle metrology measurements of microscopic return point and complementary point memory in Co/Pd magnetic films having perpendicular anisotropy. We observe that the domains assemble into a common labyrinth phase with a period that varies by nearly a factor of two between initial reversal and fields near saturation. Unlike previous studies of similar systems, the ability of the film to reproduce its domain structure after magnetic cycling through saturation varies from loop to loop, from position to position on the sample, and with the part of the speckle pattern used in the metrology measurements. We report the distribution of memory as a function of field and discuss these results in terms of the reversal process.

Resonant properties of dipole skyrmions in amorphous Fe/Gd multilayers

Author(s): Montoya, SA; Couture, S; Chess, JJ; Lee, JCT; Kent, N; Im, MY; Kevan, SD; Fischer, P; McMorran, BJ; Roy, S; Lomakin, V; Fullerton, EE | Abstract:

Altered magnetism and new electronic length scales in magneto-electric La2/3Sr1/3MnO3–BiFeO3 heterointerface

We map out the charge-spin density profile of magneto-electric La2/3Sr1/3MnO3 (LSMO)–BiFeO3 (BFO) heterostructure using soft x-ray resonant magnetic reflectivity. We show that the spatial extent of interface orbitals can extend over a few lattice periods even for an atomically sharp interface. While LSMO magnetization is depleted at the interface, BFO does develop a weak magnetic moment mostly near the interface, probably due to a proximity-induced charge-transfer process. Our study reveals that simultaneous control of electronic and magnetic interfaces is essential in realizing the potential of oxide devices.

Observation of Skyrmions at Room-temperature in Amorphous Fe/Gd Films

Magnetic bubbles, investigated in the 1970s as potential data storage media 1 , have seen a recent surge in scientific inquiry due to the their topological spin textures [1]. A specific type of bubble with whirling magnetic spins (see Fig. 1) called a skyrmion is the major driving force behind these investigations. Skyrmions are characterized by a non-zero chirality or topological charge, given by Ns = ∫ m ∙ (∂xm × ∂ym)dxdy, where m is the normalized in-plane magnetization m = M(x, y) |M(x, y)| ⁄ . These spin textures were first observed in MnSi using neutron scattering [2-4]. Much of the excitement surrounding skyrmions is due to their high potential for application in spintronics [5,6]. Yu et al. observed skyrmion motion in FeGe using LTEM at current densities as low as 10 A・m, ~10 orders of magnitude lower than that required to move domain walls in ferromagnetic devices making them promising for racetrack memory applications [5-7]. While previous magnetic skyrmion research has largely been focused on non-centrosymmetric crystals where the Dzyaloshinskii-Moriya interaction (DMI) stabilizes the skyrmion phase, here we present a LTEM study of an amorphous ferromagnetic material with perpendicular magnetic anisotropy, consisting of an Fe/Gd multilayer film.