Coffey, William T.Crothers, D. S. F.Kalmykov, Yuri P.Massawe, Estomih S.Waldron, J. T.2016-06-262016-06-261994Coffey, W.T., Crothers, D.S.F., Kalmykov, Y.P., Massawe, E.S. and Waldron, J.T., 1994. Exact analytic formula for the correlation time of a single-domain ferromagnetic particle. Physical Review E, 49(3), p.1869.http://hdl.handle.net/20.500.11810/2667Exact solutions for the longitudinal relaxation time T∥ and the complex susceptibility χ∥(ω) of a thermally agitated single-domain ferromagnetic particle are presented for the simple uniaxial potential of the crystalline anisotropy considered by Brown [Phys. Rev. 130, 1677 (1963)]. This is accomplished by expanding the spatial part of the distribution function of magnetic-moment orientations on the unit sphere in the Fokker-Planck equation in Legendre polynomials. This leads to the three-term recurrence relation for the Laplace transform of the decay functions. The recurrence relation may be solved exactly in terms of continued fractions. The zero-frequency limit of the solution yields an analytic formula for T∥ as a series of confluent hypergeometric (Kummer) functions which is easily tabulated for all potential-barrier heights. The asymptotic formula for T∥ of Brown is recovered in the limit of high barriers. On conversion of the exact solution for T∥ to integral form, it is shown using the method of steepest descents that an asymptotic correction to Brown’s high-barrier result is necessary. The inadequacy of the effective-eigenvalue method as applied to the calculation of T∥ is discussed.enJournal Article, Peer Reviewed10.1103/PhysRevE.49.1869