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 A 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 Hc would be necessary to lower the energy barrier so as to observe appreciable quantum tunneling of magnetization. The analysis shows that the crossover température T° from the thermal to the quantum régime increases with decreasing anisotropy field HA. This relation between T° and HA is inconsistent with the theoretical prediction