Magnetic ordering and magnetoresistive effect in La1−x Srx(Mn1−y Mey)O3 perovskites (Me = Nb, Mg)

Abstract

Perovskites of composition La1−xSrx(Mn1−x/2Nbx/2)O3 and La0.49Sr0.51(Mn1−yNby)O3 have been synthesized and investigated. The substitution of nonmagnetic niobium ions for manganese was shown to lead to a transition from the metallic into the insulating state due to a decrease in the number of dissimilar (different-valence) manganese atoms in the lattice. In spite of the high resistivity, the niobium-containing perovskites exhibit a large magnetoresistive effect and ferromagnetic ordering. Small additions of Nb5+ to La0.49Sr0.51MnO3 stimulate the transition from the antiferromagnetic into the ferromagnetic state, whereas the substitution of Mg2+ for Mn stabilizes the antiferromagnetic state. It is supposed that the ferromagnetism in the insulating perovskites at hand is due to the positive superexchange of the Mn3+-O-Mn3+ type, and the magnetoresistive effect owes to the intergranular transfer of spin-polarized charge carriers and the suppression of magnetic nonuniformities by an applied magnetic field near TC.