LOW-TEMPERATURE MAGNETIC-RELAXATION OF ORGANIC COATED NIFE2O4 PARTICLES

We report measurements of relaxation rate of the remanent magnetization versus temperature for NiFe2O4 particles coated with the surfactant oleic acid. It has been reported that the organic coating induces a strong surface anisotropy resulting in surface spin canting and anomalous hysteresis properties at low temperature as compared to the bulk material. We have measured, at temperatures down to 400 mK, relaxation of remanent magnetization which extrapolates to a nonzero value at zero temperature. Macroscopic quantum tunneling at these temperatures has been predicted and observed only for materials with high magnetocrystalline anisotropy, so it is particularly interesting to observe it in NiFe2O4, which has a relatively low bulk anisotropy. It is suggested that the strong surface anisotropy induced by the organic coating may be responsible for tunneling in these particles.