2 SPIN-FREEZING BEHAVIOR IN CONCENTRATED AMORPHOUS FERRITE OXIDES

Amorphous CoFe2O4 with metal ions coordinated in a manner similar to the spinal crystal was prepared by the ion-beam sputter deposition method. The study of magnetic properties was carried out by dc- and ac-susceptibility measurements and Mossbauer spectroscopy. The strong spin-freezing behavior was observed in 80-Hz, 1-Oe ac susceptibility at two spin-freezing temperatures: 284 K (T(f1)), which is considered to be caused by a random anisotropy of Co ions coordinated in distorted octahedral sites, and 86 K (T(f2)), which might be caused by the frustration of antiferromagnetic interactions due to the amorphous structure. The dc susceptibility showed irreversibility below T(f1) which decreased with increasing applied field. However, a distinct change was not observed at T(f2). The Mossbauer spectrum revealed that some of the paramagnetic Fe ions were frozen below T(f1), with relaxation phenomena, and that the residual paramagnetic ions were frozen at T(f2). The time decay of the thermoremanent magnetization observed below T(f1) showed different behavior compared to that above and below T(f2).