Ti substituted La 0.67Ca 0.33MnO 3 ortho-perovskites: Dominant role of local structure on the electrical transport and magnetic properties

Abstract

We report the effects of local structure on the electrical transport and magnetic properties of La 0.67Ca 0.33Mn 1− x Ti x O 3 (0⩽ x⩽0.10) system. Linear increase in the lattice parameters, consequent expansion of unit cell and a monotonic decrease in the relative Mn 4+ concentration with x suggest that Ti 4+ predominantly replaces Mn 4+ in La 0.67Ca 0.33Mn 1− x Ti x O 3 ( 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 ( T c) decrease at a rate of ∼26 K/at% up to x=0.05. T c levels off for higher compositions. Modification of the major carrier concentration (decreased Mn 4+ 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 T c 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 La 0.67Ca 0.33MnO 3 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.