Novel Magnetic Behaviour of the System Zn sub x Ca sub 1--x Fe sub 2 O sub 4

A new magnetic system Zn sub x Ca sub 1--x Fe sub 2 O sub 4 (x = 0.4 to 0.6) is reported having extremely unusual properties like small ordered moments approx = 0.13-0.23 mu sub B , large ordering temp. T sub c approx = 700-750K and surprising lack of hyperfine field (HF) at Fe for temp. greater than T* ( approx = 0.25T sub c ) in spite of bulk magnetization in the system. The HF appears below T* implying evolution of local moments far below T sub c . Sharp susceptibility maxima resembling those of spin-glasses are observed for x = 0.4 and 0.5, which may be attributed to the single domain--superparamagnetic (SD--SP) transition and the overall behaviour shows a cluster spin-glass type of ordering. 9 ref.--AA