Theoretical relaxation times of large superparamagnetic particles with cubic ansiotropy

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Abstract
The superparamagnetic relaxation time is calculated for spherical particles with cubic magnetocrystalline anisotropy in zero applied field. Computation is extended to larger particle size, namely larger energy barriers, than could be evaluated in the previous work. Still, no asymptotic formula could be found to describe the behavior in this size region. It is found that the increase of the relaxation time with increasing particle size is much faster than can be obtained from a simple exponential, from the asymptotic formula for uniaxial anisotropy, or from a formula obtained by an analogous procedure to the uniaxial case. Unless these asymptotic formulas take over only for much larger barriers than studied here, it seems that the complications imposed by the cubic anisotropy potential cannot be ignored.

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