Neel relaxation uniaxial anisotropy

In this section we consider the dependence of the relaxation time on the internal anisotropy of a single domain ferromagnetic particle. Thus it is applicable only to Neel relaxation. We consider the simplest form of anisotropy, namely uniaxial anisotropy described by the potential... 

Here we note that this result differs from that of Raikher and Shliomis [17] (Eq. (42) of their paper) where the relaxation time used in the denominator is simply the Neel relaxation time Tjv(o-). The relaxation time used here is Tj (ct), the (perpendicular field-dependent) relaxation time in the presence of uniaxial anisotropy which from Eq. (7.58) to 0(cr) from the binomial expansion is (see Eq. 6.78)... 

In sufficiently small ferro- or ferrimagnetic particles with an effective uniaxial anisotropy per unit volume Ka the relaxation of the magnetization will vary exponentially with the temperature following a Neel-Brown process characterized by a relaxation time given by... 

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