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

We performed powder neutron scattering experiments oil spinel ferrimagnets ACr(2)O(4) with magnetic and Jahn-Teller A(2+) ions (A = Fe, Ni, Cu). In both FeCr2O4 and NiCr2O4, although geometric frustration had been expected to vanish so far, resonance-like magnetic excitation modes were discovered around Q similar or equal to 1.4 angstrom(-1) and E similar or equal to 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(2)O(4) with nonmagnetic and non-Jahn-TellerA(2+) ions (A = Mg, Zn). In a high-temperature magnetic order phase the resonance-like inelastic scattering transformed into quasielastic scattering. In CuCr2O4 no such diffusive scattering was observed. We interpret the resonance-like magnetic excitations as a dynamical spin-frustration effect, and discuss it necessary condition for the appearance of them.