Electronic transfer in Sr₂FeMoO₆ perovskites

The magnetic properties of Sr₂FeMoO₆ oxides are investigated over a broad temperature range extending from 10 K to 800 K. It is found that the magnetization fully saturates at low temperatures at a value of 3.75 µ_B. In the paramagnetic regime, above the Curie temperature (T_C≈420 K), the magnetic susceptibility can be well described by a Curie-Weiss law. It is found that the experimental effective moment µ*_eff is gradually reduced under a field, an effect that can be attributed to some non-intrinsic behaviour. By means of detailed data analysis, it is shown that the paramagnetic effective moment µ_eff is close to that expected for a 3d⁶:4d⁰ atomic configuration. This result is interpreted as due to some electronic charge transfer from the itinerant 4d⁰(Mo) towards the 3d⁵(Fe) orbitals in the paramagnetic phase. We have also observed that the paramagnetic Curie temperature θ_P is positive and that θ_P≈T_C, thus suggesting that the nearest-neighbour magnetic interactions are ferromagnetic in nature. These observations strongly support a double-exchange picture for the ferromagnetic coupling in these oxides.