LOW-TEMPERATURE MAGNETIC-RELAXATION OF ORGANIC COATED NIFE2O4 PARTICLES

We report measurements of relaxation rate of the remanent magnetizatio n versus temperature for NiFe2O4 particles coated with the surfactant oleic acid. It has been reported that the organic coating induces a st rong surface anisotropy resulting in surface spin canting and anomalou s hysteresis properties at low temperature as compared to the bulk mat erial. 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 b een predicted and observed only for materials with high magnetocrystal line anisotropy, so it is particularly interesting to observe it in Ni Fe2O4, which has a relatively low bulk anisotropy. It is suggested tha t the strong surface anisotropy induced by the organic coating may be responsible for tunneling in these particles.