Ferrimagnetism of the magnetoelectric compound Cu$_2$OSeO$_3$ probed by $^{77}$Se NMR

TL;DR

This study uses $^{77}$Se NMR to investigate the magnetic phase transition in Cu$_2$OSeO$_3$, revealing a ferrimagnetic state with no symmetry breaking, supported by theoretical modeling of exchange interactions.

Contribution

It provides the first detailed NMR characterization of the ferrimagnetic transition in Cu$_2$OSeO$_3$ and develops a mean field theory based on microscopic exchange interactions.

Findings

Identification of a ferrimagnetic state with 3/4 and 1/4 Cu$^{2+}$ spin alignment.

No observable symmetry reduction during the magnetic transition.

Quantitative estimates of exchange interactions ($J_ ext{afm}$ and $J_ ext{fm}$).

Abstract

We present a thorough $^{77}$Se NMR study of a single crystal of the magnetoelectric compound Cu$_2$OSeO$_3$. The temperature dependence of the local electronic moments extracted from the NMR data is fully consistent with a magnetic phase transition from the high-T paramagnetic phase to a low-T ferrimagnetic state with 3/4 of the Cu$^{2+}$ ions aligned parallel and 1/4 aligned antiparallel to the applied field of 14.09 T. The transition to this 3up-1down magnetic state is not accompanied by any splitting of the NMR lines or any abrupt modification in their broadening, hence there is no observable reduction of the crystalline symmetry from its high-T cubic \textit{P}2$_1$3 space group. These results are in agreement with high resolution x-ray diffraction and magnetization data on powder samples reported previously by Bos {\it et al.} [Phys. Rev. B, {\bf 78}, 094416 (2008)]. We also develop a mean field theory description of the problem based on a microscopic spin Hamiltonian with one antiferromagnetic ($J_\text{afm}\simeq 68$ K) and one ferromagnetic ($J_\text{fm}\simeq -50$ K) nearest-neighbor exchange interaction.