The impact of antisite disorder on the double perovskites

by Dr Viveka Nand Singh (HRI, Allahabad)

Friday 23 Dec 2011, 14:30 → 15:30 Asia/Kolkata

A304 (Colaba Campus)

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.