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Magnetization oscillations in a spherical ferromagnetic particle with unidirectional surface and uniaxial bulk anisotropies are considered. Due to the surface spin pinning, the uniform Larmor precession is replaced by a spatially modulated mode. The corresponding dispersion equation is solved in the weak pinning limit. The result shows that in the particle FMR spectrum, the surface effect is equivalent to the presence of an additional intrinsic size-dependent field coaligned with the constant magnetizing one. The experimental evidence obtained on maghemite ferrofluids supports this conclusion.

Signal-to-noise properties of a periodically driven noisy magnetic bistable system—superparamagnetic particles with uniaxial anisotropy—are investigated in the framework of the Fokker-Planck equation solved both analytically and numerically. The system is subject to a constant ~bias! field that imparts even harmonics into its low-frequency magnetic spectrum. A comparative study of the linear and quadratic susceptibilities and stochastic resonances ~SR! is carried out. We show that the quadratic SR is much sharper than the linear one and unlike the latter is essentially frequency dependent. @S1063-651X~97!10411-1#

Monte Carlo modeling of superparamagnetic kinetics is addressed with regard to quantification of the simulation steps in units of real time. For the classical N\'eel case (field-free relaxation of the magnetic moment in a single-domain uniaxial particle), a modification of the Nowak-Chantrell-Kennedy formula is proposed enabling the calculations with the variation amplitude of the magnetic moment independent of the particle anisotropy. This relation is obtained from comparison of the Monte Carlo simulated relaxation against the solution of the kinetic equation for the particle magnetic moment. The same idea is applied to a strongly nonequilibrium process: steady re-magnetization of a superparamagnetic particle under an ac field. It is found that in the latter case a linear relationship can be established between the number of simulation steps and the real-time scale (the field period) that holds fairly well in wide frequency and anisotropy parameter intervals.

On the basis of Brown's kinetic equation a consistent study of the regimes of cyclic magnetization reversal of single-domain particles with a uniaxial anisotropy is performed. The applied field is harmonic and linearly polarized, its amplitude equals the maximal coercive force of a Stoner-Wohlfarth particle. The dynamic magnetic hysteresis loops are obtained for a particle (oriented particle ensemble), whose easy-magnetization axis is tilted to the field direction under an arbitrary angle. It is shown that the Stoner-Wohlfarth regime (often termed as quasistatic) is able to describe the behavior of a nanoparticle only in a quite limited material and external parameter range. The developed approach has at least two major merits: it enables one to consider dynamic magnetic hysteresis in the temperature-frequency domains inaccessible with the aid of approximate methods, and provides a tool to test the accuracy of the latter.

Dynamic birefringence in a ferrofluid subjected to crossed bias (constant) and probing (pulse or ac) fields is considered, assuming that the nanoparticles have finite magnetic anisotropy. This is done on the basis of the general Fokker-Planck equation that takes into account both internal magnetic and external mechanical degrees of freedom of the particle. We describe the orientation dynamics in terms of the integral relaxation time of the macroscopic orientation order parameter. To account for an arbitrary relation between the bias (external) and anisotropy (internal) fields, an interpolation expression for the integral relaxation time is proposed and justified. A developed description is used to interpret the measurements of birefringence relaxation in magnetic fluids with nanoparticles of high (cobalt ferrite) and low (maghemite) anisotropy. The proposed theory appears to be in full qualitative agreement with all the experimental data available.