Double perovskites with ferromagnetism above room temperature

We review the structural, magnetic and transport properties of double perovskites (A2BB'O6) with ferromagnetism above room temperature. Ferromagnetism in these compounds is explained by an indirect B-O-B'-O-B exchange interaction mediated by itinerant electrons. We first focus on the BB' = FeMo-based double perovskites, with Sr2FeMoO6 (TC = 420 K) being the most studied compound. These compounds show metallic behaviour and low magnetic coercivity. Afterwards, we will focus on B' = Re compounds, where the significant orbital moment of Re plays a crucial role in the magnetic properties, for example in the large magnetic coercivity and magnetostructural coupling. More specifically, we first discuss the A2FeReO6 series, with maximum TC = 520 K for Ca2FeReO6, which shows a tendency to semiconducting behaviour. Finally, we describe the Sr2(Fe1-xCrx)ReO6 series, with maximum TC = 625 K for Sr2CrReO6, which is the highest TC in an oxide compound without Fe. This compound is metallic. We discuss the impact of these materials for spin electronics in the light of their high spin polarization at the Fermi level and metallicity. In particular, we focus on the large intergrain magnetoresistance effect observed in polycrystalline samples and the possible implementation of these materials as electrodes in magnetic tunnel junctions. (topical review)