Resonance-Like Magnetic Excitations in Spinel Ferrimagnets FeCr₂O₄ and NiCr₂O₄ Observed by Neutron Scattering

Abstract

We performed powder neutron scattering experiments on spinel ferrimagnets A Cr 2 O 4 with magnetic and Jahn–Teller A 2+ ions ( A = Fe, Ni, Cu). In both FeCr 2 O 4 and NiCr 2 O 4 , although geometric frustration had been expected to vanish so far, resonance-like magnetic excitation modes were discovered around Q ≃1.4 Å -1 and E ≃5 and 8 to 9 meV in a low-temperature magnetic order phase, similar to the local spin resonance modes early reported in highly frustrated spinel antiferromagnets A Cr 2 O 4 with nonmagnetic and non-Jahn–Teller A 2+ ions ( A = Mg, Zn). In a high-temperature magnetic order phase the resonance-like inelastic scattering transformed into quasielastic scattering. In CuCr 2 O 4 no such diffusive scattering was observed. We interpret the resonance-like magnetic excitations as a dynamical spin-frustration effect, and discuss a necessary condition for the appearance of them.