Chiral bobbers and skyrmions in epitaxial FeGe/Si(111) films

Rapid Communication

Chiral bobbers and skyrmions in epitaxial FeGe/Si(111) films

Adam S. Ahmed, James Rowland, Bryan D. Esser, Sarah R. Dunsiger, David W. McComb, Mohit Randeria, and Roland K. Kawakami

Phys. Rev. Materials 2, 041401(R) – Published 17 April 2018

Abstract

We report experimental and theoretical evidence for the formation of chiral bobbers—an interfacial topological spin texture—in FeGe films grown by molecular beam epitaxy. After establishing the presence of skyrmions in FeGe/Si(111) thin-film samples through Lorentz transmission electron microscopy and the topological Hall effect, we perform magnetization measurements that reveal an inverse relationship between the film thickness and the slope of the susceptibility $(d χ / d H)$ . We present evidence for the evolution as a function of film thickness $L$ from a skyrmion phase for $L < L_{D} / 2$ to a cone phase with chiral bobbers at the interface for $L > L_{D} / 2$ , where $L_{D} \sim 70 nm$ is the FeGe pitch length. We show using micromagnetic simulations that chiral bobbers, earlier predicted to be metastable, are in fact the stable ground state in the presence of an additional interfacial Rashba Dzyaloshinskii-Moriya interaction.

Received 26 June 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.2.041401

Physics Subject Headings (PhySH)

Dzyaloshinskii-Moriya interaction Magnetic susceptibility Phase diagrams Skyrmions Spin texture

DC susceptibility measurements Micromagnetic modeling Molecular beam epitaxy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Adam S. Ahmed¹, James Rowland¹, Bryan D. Esser^2,3, Sarah R. Dunsiger^4,5, David W. McComb^2,3, Mohit Randeria^1,*, and Roland K. Kawakami^1,†

¹Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA
²Center for Electron Microscopy and Analysis, The Ohio State University, Columbus, Ohio 43210, USA
³Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210, USA
⁴Center for Emergent Materials, The Ohio State University, Columbus, Ohio 43210, USA
⁵Department of Physics, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6

^*randeria.1@osu.edu
^†kawakami.15@osu.edu

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 2, Iss. 4 — April 2018

Reuse & Permissions

Access Options

Article Available via CHORUS

Download Accepted Manuscript

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Examples of two different topological spin textures. (a) Chiral bobber crystal confined to the surface where the bulk is the conical phase. Inset: A 3/4 turn of a chiral bobber shown above the cone phase. (b) Skyrmion crystal. Inset: A 3/4 turn of a single skyrmion tube shown to extend through the entire sample. The color wheel indicates the orientation of the in-plane spins that turn in a counterclockwise fashion. The center point (white) indicates a spin into the page, and the gray/transparent perimeter are spins pointing out of the page.
Reuse & Permissions
Figure 2
Series of LTEM images on a 55-nm-thick cross section of an epitaxial FeGe film at 260 K. Both the $[1 \bar{1} 0]$ crystal axis and the applied magnetic field point into the page. Different magnetic textures are shown for different field values. (a) 0 T: Helical phase. (b) 0.3 kOe: Coexistence of a skyrmion crystal and the helical phase. (c) 0.6 kOe: Only skyrmion crystal present. (d) 1.2 kOe: Field-polarized state. Scale bars are 250 nm.
Reuse & Permissions
Figure 3
Electrical and magnetic properties of a 35-nm epitaxial FeGe/Si(111) film. (a) Topological Hall resistivity vs applied magnetic field at 50 K is hysteretic in applied field: Up scan (black curve) and down scan (red curve). The black diamond represents the zero crossing at high fields. The data were antisymmetrized (see Supplemental Material [22]). (b) Average topological Hall resistivity as a function of field and temperature (10, 50, 100, 150, 200, and 260 K). The black line (comprising the extrapolated THE zero crossings) represents the boundary between region of topological density and the field-polarized regime. (c) Susceptibility obtained by numerical differentiation of $M$ vs $H$ curves. SQUID scans are performed after cooling in zero field and measuring with increasing field steps. Solid lines indicate inflection points and dashed lines indicate peaks. (d) Magnetic phase diagram determined by features in the susceptibility data.
Reuse & Permissions
Figure 4
(a)–(c) Susceptibility vs magnetic field measured at 10 K for 35-, 80-, and 500-nm thickness of FeGe, respectively. The field is applied along the surface normal (growth direction). (d) Susceptibility χ-χ (2.5 kOe) vs magnetic field taken at 10 K and plotted between 2.5 and 5 kOe for film thicknesses from 14 to 1000 nm. (e) Magnitude of $d χ / d H$ plotted vs inverse thickness $L_{D} / L$ , with $L_{D} = 70 nm$ . For thicknesses greater than 40 nm, $d χ / d H$ follows a 1/ $L$ trend indicative of a surface phenomenon. For thicknesses less than 40 nm, $d χ / d H$ deviates from the 1/ $L$ line (dashed red line) and is constant. Note: $M_{sat}$ is the saturation magnetization, and $H_{D}$ is the saturation field.
Reuse & Permissions
Figure 5
Applied field vs film thickness phase diagrams at $T = 0 K$ calculated from micromagnetics. (a) Phase diagram with bulk and interface DMI and $K_{eff} = 0$ (no anisotropy). With sufficient interface DMI, the chiral bobber/cone phase becomes the true ground state instead of the pure cone phase. For $L / L_{D} \sim 1 - 6$ , the chiral bobber/cone phase and the skyrmion phase have large regions of stability. (b) Phase diagram with bulk and interface DMI and $K_{eff} = 0.25$ (easy-plane anisotropy). The skyrmion phase is confined to low thicknesses, and the chiral bobber/cone phase dominates for $L / L_{D} \sim 1 - 6$ . In addition, the stacked spiral/cone phase replaces the helical phase at low fields.
Reuse & Permissions

Physical Review Materials