Observation of coherently coupled cation spin dynamics in an insulating ferrimagnetic oxide

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_0.65Zn_0.35Al_0.8Fe_1.2O₄) under continuous microwave excitation. Specifically, we employ time-resolved x-ray ferromagnetic resonance to separately probe Fe^2+/3+ and Ni²⁺ cations on different sublattice sites. Our results show that the precessing cation moments retain a rigid, collinear configuration to within $≈$2°. Moreover, the effective spin relaxation is identical to within <10% for all magnetic cations in the ferrite. Thus, we validate the oft-assumed “ferromagnetic-like” dynamics in the resonantly driven ferrimagnetic oxide: the magnetic moments from different cations precess as a coherent, collective magnetization, despite the high contents of nonmagnetic Zn²⁺ and Al³⁺ diluting the exchange interactions.