Tunable gigahertz dynamics of low-temperature skyrmion lattice in a chiral magnet

TL;DR

This study investigates the GHz spin dynamics of low-temperature skyrmions in Cu$_2$OSeO$_3$, revealing tunable excitations and the crucial role of magnetocrystalline anisotropy in their stabilization and mode hybridization.

Contribution

It demonstrates how field cycling can generate and tune low-temperature skyrmions and elucidates the influence of magnetocrystalline anisotropy on their dynamic modes.

Findings

Low-temperature skyrmion excitations exhibit CW, CCW, and breathing modes below 40 K.

Mode intensities are tunable via the number of field cycles.

Magnetocrystalline anisotropy is key to mode hybridization.

Abstract

Recently, it has been shown that the chiral magnetic insulator Cu$_2$OSeO$_3$ hosts skyrmions in two separated pockets in temperature and magnetic field phase space. It has also been shown that the predominant stabilization mechanism for the low-temperature skyrmion (LTS) phase is via the crystalline anisotropy, opposed to temperature fluctuations that stabilize the well-established high-temperature skyrmion (HTS) phase. Here, we report on a detailed study of LTS generation by field cycling, probed by GHz spin dynamics in Cu$_2$OSeO$_3$. LTSs are populated via a field cycling protocol with the static magnetic field applied parallel to the $\langle{100}\rangle$ crystalline direction of plate and cuboid-shaped bulk crystals. By analyzing temperature-dependent broadband spectroscopy data, clear evidence of low-temperature skyrmion excitations with clockwise (CW), counterclockwise (CCW), and breathing mode (BR) character at temperatures below $T$ = 40 K are shown. We find that the mode intensities can be tuned with the number of field-cycles below the saturation field. By tracking the resonance frequencies, we are able to map out the field-cycle-generated LTS phase diagram, from which we conclude that the LTS phase is distinctly separated from the high-temperature counterpart. We also study the mode hybridization between the dark CW and the BR modes as a function of temperature. By using two Cu$_2$OSeO$_3$ crystals with different shapes and therefore different demagnetization factors, together with numerical calculations, we unambiguously show that the magnetocrystalline anisotropy plays a central role for the mode hybridization.