Charge state modification in Mn site substituted CMR manganites: strong deleteriousinfluence on the ferromagnetic–metallic state

Abstract

The effect of charge state modification at the Mn site on the physicalproperties of CMR manganites is reported. With a view to avoidingadditional complexity of local spin coupling effects, Mn site substitution ofLa0.67Ca0.33MnO3 is carried out with appropriate diamagneticions—Zn2+, Zr4+, Ta5+ and W6+—of different valence states. The substitution results in size changes of the unit cell andenhanced local structural distortions, which increase in the order Zn, Zr, Ta and W.The ground state is ferromagnetic–metallic below a certain critical concentrationxc of the substituents, beyond which the magnetic ground stateshows a glassy behaviour. The phase transition temperatures(TMI andTc) decreasewith substitution, but to different extents. The observed suppression rates of the Curie temperature,Tc, of∼39 K/at.% and∼45 K/at.% respectivelyfor Ta5+ and W6+ substituted compounds are the highest reported in the Mn site substituted CMR manganites.Besides the modification of majority carrier concentration due to the increased (decreased)Mn3+ concentration and enhanced local structural effects, the local electrostatic potential of thesubstituents seems to contribute to the unusually strong reduction in the itinerantferromagnetism and the observed glassy states.