Large magnetoresistance in (AA′)2FeReO6 double perovskites

We review the main structural, magnetic and magnetotransport properties of the intriguing (AA′)2FeReO6 magnetic double perovskites. As the average cation size decreases, the crystallographic structure at room temperature evolves from cubic [(AA′)2=Ba2, Ba1.5Sr0.5, BaSr, Ba0.5Sr1.5] to tetragonal [(AA′)2=Sr2] and monoclinic [(AA′)2=Ca0.5Sr1.5, CaSr, Ca1.5Sr0.5, Ca2]. The Curie temperature increases anomalously from ≈303 K for Ba2 to ≈522 K for Ca2 in sharp contrast with the observed behaviour in the isostructural compounds (AA′)2FeMoO6. Other anomalous features in the (AA′)2FeReO6 series are: the large magnetic anisotropy, the large magnetoelastic coupling and the semiconducting behaviour of the monoclinic compounds. The monoclinic compounds undergo a structural/magnetic transition at TS below 125 K. Three different magnetoresistance mechanisms have been identified: the intergrain negative magnetoresistance effect, which is present across the whole series of compounds, and in the case of the monoclinic compounds below TS a negative magnetoresistance effect associated to the melting of the low-temperature phase and a positive magnetoresistance effect only present in (AA′)2=Ca2 below T∼50K.