Observation of Coherently Coupled Cation Spin Dynamics in an Insulating Ferrimagnetic Oxide

TL;DR

This study demonstrates that in ferrimagnetic NiZnAl-ferrite, different magnetic cations precess coherently with minimal phase difference, validating the assumption of collective magnetization dynamics in such insulators under microwave excitation.

Contribution

It provides direct experimental evidence of coherent, collective precession of different cation spins in ferrimagnetic oxides using time-resolved x-ray ferromagnetic resonance.

Findings

Cation moments precess with a collinear configuration within 2 degrees.

Effective spin relaxation is nearly identical for all magnetic cations.

Supports the 'ferromagnetic-like' dynamics assumption in ferrimagnetic oxides.

Abstract

Many technologically useful magnetic oxides are ferrimagnetic insulators, which consist of chemically distinct cations. Here, we examine the spin dynamics of different magnetic cations in ferrimagnetic NiZnAl-ferrite (Ni$_{0.65}$Zn$_{0.35}$Al$_{0.8}$Fe$_{1.2}$O$_4$) under continuous microwave excitation. Specifically, we employ time-resolved x-ray ferromagnetic resonance to separately probe Fe$^{2+/3+}$ and Ni$^{2+}$ cations on different sublattice sites. Our results show that the precessing cation moments retain a rigid, collinear configuration to within $\approx$2$^\circ$. Moreover, the effective spin relaxation is identical to within $<$10% for all magnetic cations in the ferrite. We thus validate the oft-assumed ``ferromagnetic-like'' dynamics in resonantly driven ferrimagnetic oxides, where the magnetic moments from different cations precess as a coherent, collective magnetization.