n- to p-type Sr2Fe1+xRe1-xO6 double perovskites: magnetoresistance and magnetoSeebeck

Abstract

The T-dependent Seebeck (S) of several samples belonging to the Sr2Fe1+xRe1-xO6 series (x≤0.33) has been investigated. All these polycrystalline samples synthesized by solid state reaction in closed vessels crystallize in the I4/m double perovskite structure. Though ferrimagnetic with TC>400 K, the decrease of their magnetization as x increases from x=0.00 is explained by the cation disorder induced by the deviation from the ideal 1 : 1 Re/Fe ratio for the 2a and 2b crystallographic sites. Interestingly, it is found that at room temperature, the S sign changes from S>0 to S<0 as x increases to about x=0.25. Despite that the magnitude of their magnetoresistance, near −20 % in 9T and at T=5 K, is not very sensitive to x, a positive magneto Seebeck effect is observed only for the samples exhibiting S(300 K)>0. This asymmetry in the magnetothermopower properties of the n- and p-type samples is interpreted in the framework of the spin polarized transport of these magnetoresistive double-perovskites.