Suppression of itinerant ferromagnetism and cluster-glass behaviour of colossal magnetoresistive Ta-substituted La0.67Ca0.33MnO3

We discuss the modified magnetic state of single-phase La0.67Ca0.33Mn1-xTaxO3 (x≤0.10) colossal magnetoresistive compounds with orthorhombic Pnma symmetry. Pentavalent Ta substitution results in an extraordinarily strong reduction of the magnetic phase transition temperatures (∼39 K/at.%) in the ferromagnetic-metallic regime (x≤0.03) and suppresses the metal-insulator transition for x≥0.05. Many experimental features show evidence for the cluster-glass behaviour of the compounds with x≥0.05. These features are large thermomagnetic irreversibility just below the Curie temperature (Tc), monotonic increase of the field cooled magnetization down to 4.2 K, non-saturation of magnetization, and the observation of two distinct magnetic transitions in ac susceptibility in an appropriate dc field-one close to Tc and the other at a lower temperature (the spin-freezing temperature (Tg)). These results find additional support from non-equilibrium spin dynamics with a maximum in magnetic viscosity at Tg and a linear low-temperature magnetic specific heat with a characteristic broad maximum close to Tg