ON THE CALCULATION OF THE NEEL RELAXATION-TIME IN UNIAXIAL SINGLE-DOMAIN FERROMAGNETIC PARTICLES

Nanometre-sized particles of ferrite, commonly used in magnetic fluids , are single-domain. The direction of magnetic moment of these small, uniaxial, ferromagnetic particles is known to fluctuate due to thermal agitation, and can relax through the Neel-type relaxation mechanism. The relaxation time of such fluctuations is usually determined by mean s of Brown's equations for high and low barrier heights. More recently , modified equations catering for a continuous range of barrier height s have been proposed. Comparison of these equations shows that, even i n the most extreme case only a factor of approximately 1.7 distinguish es the corresponding eigenvalues (which represent the inverse of the r elaxation time). It is concluded that the major source of error in pre dicting the relaxation time arises, not primarily due to the particula r equations used, but because of the large uncertainty in obtaining pr ecise experimental data needed to determine the components, f0 and sig ma, of these equations. For example, for a small change in anisotropy constant K by a factor of 2.5 (typical values for the system considere d here are (2-5) x 10(4) J m-3), the calculated values of Neel relaxat ion times using Brown's equation differ by a factor of about 37, corre sponding to times of 1.6 x 10(-7) to 4.3 x 10(-9). An experimental val ue of 5 x 10(-9) s determined from the frequency of the maximum of the loss-peak of the imaginary part of the complex susceptibility is at t he outer limit of these calculated values.