Characterizing the magnetic anisotropy constant of spinel cobalt ferrite nanoparticles

Superconducting quantum interference device (SQUID) magnetometry and Mossbauer spectroscopy have been combined to determine the magnetic anisotropy constant K of spinel CoFe2O4 nanoparticles. The anisotropy constant is 2.23 x 10(6) erg/cm(3) for CoFe2O4 nanoparticles with a mean diameter of 8.5 nm. A relaxation-time constant of tau(0) = 1.90 x 10(-14) s has also been obtained. The magnetic anisotropy distribution function of the nanoparticles has been extracted from the temperature dependence of remanence decay in SQUID measurements. The population percentage of the superparamagnetically relaxed nanoparticles at each temperature can be calculated from this anisotropy distribution function. Such a population calculation from SQUID measurements can also fit the results from Mossbauer spectroscopic measurements even though these two measuring methods are fundamentally distinct. Such a successful fit implies that the anisotropy distribution function from our SQUID measurement is truly an intrinsic feature of the nanoparticles. (C) 2000 American Institute of Physics. [S0003-6951(00)01224-9].