Magnetic phase transition and magnetoresistive effect in $Nd0.6Ca0.4(Mn1−xMex)O3$ (Me=Cr, Al, Ti, Nb)

The properties of $Nd0.6Ca0.4(Mn1−xMex)O3$ $(Me = Cr3+,$ $Al3+,$ $Ti4+,$ $Nb5+)$ are investigated. It is shown that the indicated substitutions destroy the long-range order in the positions of the $Mn3+$ and $Mn4+$ ions, but the properties depend sharply on the type of substituent ion. The replacement of a small fraction of the $Mn3+$ ions by $Cr3+$ $(0.04⩽x⩽0.1)$ leads to antiferromagnet–ferromagnet and insulator–metal transitions. At chromium concentrations $x⩾0.1$ the solid solutions again become nonconducting and exhibit the properties of inhomogeneous ferrimagnets in which the magnetic moments of the chromium and manganese 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 antiparallel to the moments of the Mn ions. All of the samples containing chromium have a large magnetoresistive effect at temperatures below the Curie point. The replacement of manganese by $Al3+,$ $Ti4+,$ or $Nb5+$ ions leads to a transition from the antiferromagnetic state to a spin glass state with no change in conductivity. The data obtained indicate that the superexchange interactions between the chromium and manganese ions is of an antiferromagnetic character.