  Two perovskite-derived materials, CaCu3Mn4O12 and CaCu3Ti4O12, have drawn
much recent interest due to their magnetoresistive, dielectric, and
magnetoelectronic characteristics. Here we present initial theoretical insights
into each of these points, based on first principles, density functional based
calculations. Our results predict CCMO to have a spin-asymmetric energy gap,
which leads to distinct temperature- and magnetic field-dependent changes in
properties, and helps to account for its observed negative magnetoresistivity.
We have studied CCTO primarily to gain insight into the exchange coupling in
both these compounds, where the conventional superexchange coupling vanishes by
symmetry for both nearest and next nearest Cu-Cu neighbors, a consequence of
the structure. In CCTO, it is necessary to go to 5th Cu-Cu neighbors to obtain
a (superexchange) coupling that can provide the coupling necessary to give
three dimensional order. Non-superexchange mechanisms may be necessary to
describe the magnetic coupling in this structural class.
