Quantum tunneling of magnetization in single domain CoFe₂O₄ particles

We systematically investigated the macroscopic quantum tunneling (MQT) phenomena of magnetization in CoFe₂O₄ particles with a diameter ranging from 30 to 92 Å in zero external 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 H_c 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_A. This relation between T* and H_A is inconsistent with the theoretical prediction.