AN APPROXIMATE FORMULA FOR THE RELAXATION-TIME OF A SINGLE-DOMAIN FERROMAGNETIC PARTICLE WITH UNIAXIAL ANISOTROPY AND COLLINEAR FIELD

Recently there have been renewed efforts to solve the Fokker-Planck eq uation resulting from Brown's model [Phys. Rev. 130, 1677 (1963)] of s ingle domain ferromagnetic particle relaxation for the case of uniaxia l anisotropy and zero external field. In particular, the usefulness of a simple analytic formula for the relaxation time based on the recipr ocal of the lowest non-zero eigenvalue resulting from this model has b een stressed. Here we suggest an improved analytic formula which we ex tend to the case of applied collinear field and compare it with the ex act numerical solution and previous analytic expressions. The results are presented in terms of the pre-factor dependence which is commonly taken as constant. The formula exhibits good agreement with the exact results throughout the full range of anisotropy and reduced external f ield values.