Large magnetoresistance in (AA′) 2FeReO 6 double perovskites

Abstract

We review the main structural, magnetic and magnetotransport properties of the intriguing (AA′) 2FeReO 6 magnetic double perovskites. As the average cation size decreases, the crystallographic structure at room temperature evolves from cubic [(AA′) 2=Ba 2, Ba 1.5Sr 0.5, BaSr, Ba 0.5Sr 1.5] to tetragonal [(AA′) 2=Sr 2] and monoclinic [(AA′) 2=Ca 0.5Sr 1.5, CaSr, Ca 1.5Sr 0.5, Ca 2]. The Curie temperature increases anomalously from ≈303 K for Ba 2 to ≈522 K for Ca 2 in sharp contrast with the observed behaviour in the isostructural compounds (AA′) 2FeMoO 6. Other anomalous features in the (AA′) 2FeReO 6 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 T S 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 T S a negative magnetoresistance effect associated to the melting of the low-temperature phase and a positive magnetoresistance effect only present in (AA′) 2=Ca 2 below T ∼ 50 K .