No abstract

The reversal time of the magnetization of antiferromagnetic nanoparticles is evaluated for a uniform magnetic field applied at an arbitrary angle to the easy axis by using the adaptation of the Kramers escape rate theory to fine ferromagnetic particles given by Coffey et al. ͓Adv. Chem. Phys. 117, 483 ͑2001͒; Phys. Rev. E 63, 021102 ͑2001͔͒. The resulting analytic formula yields an accurate approximation to the reversal time for all values of the damping and agrees favorably with the numerically exact solution of the corresponding Fokker-Planck equation for the evolution of the probability density function of magnetization orientations.

The longitudinal relaxation time and spectrum of the complex magnetic susceptibility of single domain ferromagnetic particles with triaxial ͑orthorhombic͒ anisotropy are calculated by averaging the GilbertLangevin equation for the magnetization of an individual particle and by reducing the problem to that of solving a system of linear differential-recurrence relations for the appropriate equilibrium correlation functions. The solution of this system is obtained in terms of matrix continued fractions. It is shown that in contrast to the linear magnetic response of particles with uniaxial anisotropy, there is an inherent geometric dependence of the complex susceptibility and the relaxation time on the damping parameter arising from coupling of longitudinal and transverse relaxation modes. Simple analytic equations, which allow one to understand the qualitative behavior of the system and to accurately predict the spectrum of the longitudinal complex susceptibility in wide ranges of the barrier height and dissipation parameters, are proposed.