Magnetic field dependent cycloidal rotation in pristine and Ge doped CoCr$_2$O$_4$

TL;DR

This study investigates how magnetic fields influence the spin configuration and cycloidal rotation in pristine and Ge-doped CoCr₂O₄ thin films, revealing complex magnetic behaviors and differences induced by doping.

Contribution

It provides the first detailed comparison of magnetic field effects on spin structures in Ge-doped and pure CoCr₂O₄, highlighting the impact of doping on magnetic and multiferroic properties.

Findings

Ge doping slightly increases the ordering wave vector.

Unexpected reversal of spiral helicity observed under magnetic field cooling.

Multiple magnetic spirals indicated by helicity change with momentum transfer.

Abstract

We report a soft x-ray resonant magnetic scattering study of the spin configuration in multiferroic thin films of Co$_{0.975}$Ge$_{0.025}$Cr$_2$O$_4$ (Ge-CCO) and CoCr$_2$O$_4$ (CCO), under low- and high-magnetic fields, from 0.2 T up to 6.5 T. A characterization of Ge-CCO at a low magnetic field is performed and the results are compared to those of pure CCO. The ferrimagnetic phase transition temperature $T_C \approx 95$ K and the multiferroic transition temperature $T_S \approx 27$ K in Ge-CCO are comparable to those observed in CCO. In Ge-CCO, the ordering wave vector $\textit{(qq0)}$ observed below $T_S$ is slightly larger compared to that of CCO, and, unlike CCO, the diffraction intensity consists of two contributions that show a dissimilar x-ray polarization dependence. In Ge-CCO, the coercive field observed at low temperatures was larger than the one reported for CCO. In both compounds, an unexpected reversal of the spiral helicity and therefore the electric polarization was observed on simply magnetic field cooling. In addition, we find a change in the helicity as a function of momentum transfer in the magnetic diffraction peak of Ge-CCO, indicative of the presence of multiple magnetic spirals.