The impact of antisite disorder on the double perovskites

Several double perovskite materials exhibit high ferromagnetic Tc,
half-metallicity and significant low-field magnetoresistance. The
potential usefulness of these materials is, however, frustrated by the
inevitable antisite disorder. The result is a strong dependence of
physical properties on preparative conditions. We have studied the
impact of spatially correlated antisite disorder, as observed
experimentally, on the double perovskites. In the ferromagnetic phase,
antisite disorder suppresses the saturation magnetisation, leaves the
Tc mostly unaffected, but leads to a metal-insulator transition in the
electronic ground state.  The insulating samples shows huge low-field
negative magnetoresistance at low temperature.

On the other hand antiferromagnetic phases (A and G type), that are
predicted in the clean limit are actually suppressed less strongly
than the ferromagnetic phase by the antisite disorder. The
antiferromagnetic phases are metallic, and, remarkably, more
conducting than the ferromagnetic phase for similar antisite
disorder. Beyond the modest field needed for the suppression of long
range antiferromagnetic (A type) order, the system shows almost
tenfold increase in resistivity near Tc. The large positive
magnetoresistance, though suppressed gradually, survives the presence
of significant antisite disorder. The principle that we uncover behind
this 'colossal positive magnetoresistance' should be applicable to
other local moment metals that show field driven suppression of
non-ferromagnetic order.