Superconducting quantum interference device (SQUID) magnetometry and Mössbauer spectroscopy have been combined to determine the magnetic anisotropy constant K of spinel CoFe2O4 nanoparticles. The anisotropy constant is 2.23×106erg/cm3 for CoFe2O4 nanoparticles with a mean diameter of 8.5 nm. A relaxation-time constant of τ0=1.90×10−14s 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 Mössbauer 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.

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