Ti substituted La0.67Ca0.33MnO3 ortho-perovskites: Dominant role of local structure on the electrical transport and magnetic properties

We report the effects of local structure on the electrical transport and magnetic properties of La0.67Ca0.33Mn1−xTixO3 (0⩽x⩽0.10) system. Linear increase in the lattice parameters, consequent expansion of unit cell and a monotonic decrease in the relative Mn4+ concentration with x suggest that Ti4+ predominantly replaces Mn4+ in La0.67Ca0.33Mn1−xTixO3 (x⩽0.10). The ferromagnetic-metallic ground state modifies to a glassy insulator for x⩾0.05. No field induced metallic state could be discerned for x=0.10 even at a field of 8 T and at as low a temperature as 4.2 K. Ti substitution significantly enhances the colossal magnetoresistance (CMR) effect. Both the metal to insulator transition temperature and Curie temperature (Tc) decrease at a rate of ∼26 K/at% up to x=0.05. Tc levels off for higher compositions. Modification of the major carrier concentration (decreased Mn4+ concentration) seems to be insufficient to account for the observed reduction in the transition temperatures. This in turn emphasizes the significance of local structural effects: systematic elongation of the Mn–O bond lengths and decrease of Mn–O–Mn angles leading to strong suppression of itinerant ferromagnetism and metallicity of the compounds. The additional features in the ac susceptibility, viz., a broad shoulder just below Tc followed by a sharp decrease in the susceptibility signal at low temperatures and a non-closure of MR at zero field as high as ∼75% at 5 K indicate a frustrated magnetic ground state for x=0.07. Based on the inter-comparison of the structural, electrical transport and magnetic properties of the Mn site substituted La0.67Ca0.33MnO3 with iso-valent diamagnetic and paramagnetic ions, we argue that local structural effects have a decisive role to play, compared to the local spin coupling effects, in the ferromagnetic-metallic ground state of the CMR manganites.