Resonance-like magnetic excitations in spinel ferrimagnets FeCr ₂O₄ and NiCr₂O₄ observed by neutron scattering

We performed powder neutron scattering experiments on spinel ferrimagnets ACr₂O₄ with magnetic and Jahn-Teller A²⁺ ions (A = Fe, Ni, Cu). In both FeCr₂O₄ and NiCr ₂O₄, 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 ACr ₂O₄ with nonmagnetic and non-Jahn-Teller A²⁺ ions (A = Mg, Zn). In a high-temperature magnetic order phase the resonance-like inelastic scattering transformed into quasielastic scattering. In CuCr ₂O₄ 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.