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.