  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.
