Remnant magnetization above room temperature in semiconducting   Y_0.5Ca_0.5BaCo_4O_7
  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. 
Martin Valldor