Magnetic phase transition and magnetoresistive effect in Nd0.6Ca0.4(Mn1-xMex)O-3 (Me=Cr, Al, Ti, Nb)

The properties of Nd0.6Ca0.4(Mn1-xMex)O-3 (Me = Cr3+, Al3+, Ti4+, Nb5+) are investigated. It is shown that the indicated substitutions destroy the lon g-range order in the positions of the Mn3+ and Mn4+ ions, but the propertie s depend sharply on the type of substituent ion. The replacement of a small fraction of the Mn3+ ions by Cr3+ (0.04 less than or equal to x less than or equal to 0.1) leads to antiferromagnet-ferromagnet and insulator-metal t ransitions. At chromium concentrations x greater than or equal to 0.1 the s olid solutions again become nonconducting and exhibit the properties of inh omogeneous ferrimagnets in which the magnetic moments of the chromium and m anganese ions are directed opposite to each other. For T < 30 K there is a phase transition in which the magnetic moments of the Nd3+ ions order antip arallel to the moments of the Mn ions. All of the samples containing chromi um have a large magnetoresistive effect at temperatures below the Curie poi nt. The replacement of manganese by Al3+, Ti4+, or Nb5+ ions leads to a tra nsition from the antiferromagnetic state to a spin glass state with no chan ge in conductivity. The data obtained indicate that the superexchange inter actions between the chromium and manganese ions is of an antiferromagnetic character. (C) 2000 American Institute of Physics. [S1063-777X(00)00301-7].