Competing anisotropies in the chiral cubic magnet Co$_8$Zn$_8$Mn$_4$ unveiled by resonant x-ray magnetic scattering

TL;DR

This study investigates the temperature-dependent magnetic anisotropies in the chiral cubic magnet Co$_8$Zn$_8$Mn$_4$, revealing that anisotropic exchange interactions and competing anisotropies influence the spiral pitch and skyrmion textures.

Contribution

The paper provides quantitative analysis of anisotropic exchange interactions and demonstrates their role alongside competing anisotropies in the magnetic behavior of Co$_8$Zn$_8$Mn$_4$.

Findings

Anisotropic exchange interaction increases fourfold from room temperature to 20 K.

Helical pitch varies by 5% within the (001) plane at 20 K.

Magnetocrystalline and exchange anisotropies compete, affecting helical vector orientation.

Abstract

The cubic $\beta$-Mn-type alloy Co$_8$Zn$_8$Mn$_4$ is a chiral helimagnet that exhibits a peculiar temperature-dependent behavior in the spiral pitch, which decreases from 130 nm at room temperature to 70 nm below 20 K. Notably, this shortening is also accompanied by a structural transition of the metastable skyrmion texture, transforming from a hexagonal lattice to a square lattice of elongated skyrmions. The underlying mechanism of these transformations remain unknown, with interactions potentially involved including temperature-dependent Dzyaloshinskii-Moriya interaction, magnetocrystalline anisotropy, and exchange anisotropy. Here, x-ray resonant magnetic small-angle scattering in vectorial magnetic fields was employed to investigate the temperature dependence of the anisotropic properties of the helical phase in Co$_8$Zn$_8$Mn$_4$. Our results reveal quantitatively that the magnitude of the anisotropic exchange interaction increases by a factor of 4 on cooling from room temperature to 20 K, leading to a 5% variation in the helical pitch within the (001) plane at 20 K. While anisotropic exchange interaction contributes to the shortening of the spiral pitch, its magnitude is insufficient to explain the variation in the spiral periodicity from room to low temperatures. Finally, we demonstrate that magnetocrystalline and exchange anisotropies compete, favoring different orientations of the helical vector in the ground state.