Remnant magnetization above room temperature in the semiconductor Y 0.5Ca 0.5BaCo 4O 7

Abstract

The Y 0.5Ca 0.5BaCo 4O 7 compound exhibits four magnetic anomalies at the temperatures 387, 281, 52, and 14 K; all anomalies show characteristics typical for spin freezing into disordered states: frequency dependent transition temperature in the AC magnetic susceptibility together with relaxation of thermoremnant magnetization. Y 0.5Ca 0.5BaCo 4O 7 is a semiconductor with a small band-gap of 0.17 eV concluded from four-point conductivity measurements and the conductivity is proportional to T − 3 / 4 , suggesting an electron hopping mechanism involving Co 2+ and Co 3+ ions; at higher temperatures, double-exchange is the proposed reason for the strong magnetic interaction. At lower temperatures, super-exchange interactions start to compete with the double-exchange for domination and this competition together with geometrical frustrations in the structure are responsible for magnetic disorder down to 2 K. Covalence between Co and O is also discussed as additional reason for the strong magnetic interactions.