Dynamic unidirectional anisotropy in cubic FeGe with antisymmetric spin-spin-coupling

Nicolas Josten,Michael Farle,Iliya Radulov,Thomas Feggeler,Ralf Meckenstock,Marina Spasova,Ke Chai,Oliver Gutfleisch,Benjamin Zingsem,Zi-An Li,Detlef Spoddig

doi:10.1038/s41598-020-59208-8

Abstract

Strong unidirectional anisotropy in bulk polycrystalline B20 FeGe has been measured by ferromagnetic resonance spectroscopy. Such anisotropy is not present in static magnetometry measurements. B20 FeGe exhibits inherent Dzyaloshinskii-Moriya interaction, resulting in a nonreciprocal spin-wave dispersion. Bulk and micron sized samples were produced and characterized. By X-band ferromagnetic resonance spectroscopy at 276 K ± 1 K, near the Curie temperature, a distribution of resonance modes was observed in accordance with the cubic anisotropy of FeGe. This distribution exhibits a unidirectional anisotropy, i.e. shift of the resonance field under field inversion, of KUD = 960 J/m3 ± 10 J/m3, previously unknown in bulk ferromagnets. Additionally, more than 25 small amplitude standing spin wave modes were observed inside a micron sized FeGe wedge, measured at 293 K ± 2 K. These modes also exhibit unidirectional anisotropy. This effect, only dynamically measurable and not detectable in static magnetometry measurements, may open new possibilities for directed spin transport in chiral magnetic systems.

Highlights

Strong unidirectional anisotropy in bulk polycrystalline B20 FeGe has been measured by ferromagnetic resonance spectroscopy
Non-centrosymmetric crystal structures, such as the B20 phase of FeGe1,2, can host chiral spin textures like magnetic skyrmions[3,4], which have been proposed as new structures for memory storage applications[5] at room temperature[6]
The space group P213 of the FeGe B20 phase has an inherent broken inversion symmetry, but does not impose chirality

Summary

Introduction

Strong unidirectional anisotropy in bulk polycrystalline B20 FeGe has been measured by ferromagnetic resonance spectroscopy. More than 25 small amplitude standing spin wave modes were observed inside a micron sized FeGe wedge, measured at 293 K ± 2 K. Spin waves propagating in opposite directions at the same frequency have different wavelengths leading to complex standing waves with a moving phase front This allows to detect modes, which would otherwise cancel and not be detectable in FMR. Cylinders were formed and a high pressure high temperature synthesis inside a Kawai-type[30] multianvil apparatus with Walker-type[31] module was applied This resulted in 95% polycrystalline B20 FeGe, confirmed by X-ray diffraction. The lift-off process used Gallium as cutting ions and resulted in a localized deposition of a maximum of 2.6% of Ga (as verified by EDX)

Methods

Findings

Conclusion