Ferromagnetic Resonance and Magnetization Studies on Ferrimagnetic Double Perovskites A $_{2}$ FeReO $_{6}$ (A = Ca, Sr, Ba)

We present the results of an investigation of the magnetic properties A $_{2}$ FeReO $_{6}$ (A = Ca, Sr, Ba). We have studied the magnetization, high-temperature susceptibility and the ferromagnetic resonance response. The Ba and Sr systems are conducting while the Ca material is an insulator. The ordering temperature of the Ca materials is highest (\sim 540 K) and decreases to 315 K for the Ba system. The high-temperature susceptibilit suggests a large moment, and the Curie-Weiss temperature is very close to the ordering temperature, as expected for a simple ferromagnet, although the ferromagnetic moment is somewhat less than expected. The spin dynamics are rather peculiar: the ferromagnetic resonance results suggest a magnetically inhomogeneous material. We discuss these results in terms of cation disorder.