Abstract
We report the charge state modification effects at the Mn site on the ground state properties of colossal magnetoresistive manganites. Ta5+ substitution results in an appreciable increase in the lattice parameters and unit cell volume due to increased Mn3+ concentration. The ferromagnetic-metallic ground state modifies to a cluster glass insulator for $x \geqslant 0.05$ . The reduction in the transition temperatures with increasing x is ∼39 K/at.%. Besides the modification of majority carrier concentration due to increased Mn3+ concentration and enhanced local structural effects, the local electrostatic potential of the substituent seems to contribute to the unusually strong reduction of the transition temperatures of the compounds. Thermo magnetic irreversibility just below Curie temperature (Tc), non-saturation of magnetization, two distinct magnetic transitions in ac susceptibility in an appropriate static field: close to Tc and other at low temperature (the spin freezing temperature (Tg)) and non-stationary dynamics with a characteristic maximum in the magnetic viscosity close to Tg confirm a cluster glass state for $x \geqslant 0.05$ . These results find additional support from a linear low temperature magnetic specific heat of x = 0.10 with a characteristic broad maximum close to Tg.
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