Quantum Tunneling of Magnetization in Single Domain CoFe2O4 Particles

We systematically investigated the macroscopic quantum tunneling (MQT) phenomena of magnetization in CoFe,O, particles with a diameter ranging from 30 to 92 A in zero extemal magnetic field to remove the substantial influence of energy dissipation. For all of our samples, we observed the MQT phenomena in zero external magnetic field, in spite of the anticipation that a large magnetic field H comparable with the coercive force He would be necessary to lower the energy barrier so as to observe appreciable quantum tunneling of magnetization. The analysis shows that the crossover temperature T' from the thermal to the quantum regime increases with decreasing anisotropy field H,. This relation between ?* and HA ii inconsistent with the theoretical prediction.