Magnetic ordering and magnetoresistive effect in La1-xSrx(Mn1-yMey)O-3 perovskites (Me = Nb, Mg)

Perovskites of composition La1-xSrx(Mn1-x/2Nbx/2)O-3 and La0.49Sr0.51(Mn1-y Nby)O-3 have been synthesized and investigated. The substitution of nonmagn etic 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 dissi milar (different-valence) manganese atoms in the lattice. In spite of the h igh resistivity, the niobium-containing perovskites exhibit a large magneto resistive 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 ins ulating perovskites at hand is due to the positive superexchange of the Mn3 +-O-Mn3+ type, and the magnetoresistive effect owes to the intergranular tr ansfer of spin-polarized charge carriers and the suppression of magnetic no nuniformities by an applied magnetic field near T-C. (C) 2000 MAIK "Nauka/I nterperiodica".