Forward Modeling of Thermally Activated Single-Domain Magnetic Particles Applied to First-Order Reversal Curves

Theoretical first-order reversal curves (FORCs) were generated by numerically solving a thermally activated Stoner-Wohlfarth model for assemblages of randomly oriented magnetic particles. The thermally activated Stoner-Wohlfarth model extends previous models based on Preisach theory. The new numerical simulations show that the shapes of reversal curves and the FORC distributions are significantly modified by the effect of the thermal energy only if superparamagnetic particles are predominant. However, most assemblages containing moderate amounts of superparamagnetic particles are hardly distinguishable from stable single-domain assemblages. The most relevant thermal effect is a reduction of coercivity that translates in a shift of the FORC distribution toward the origin. Not all of the distinctive characteristics previously predicted for superparamagnetic grain assemblages were confirmed by our calculations, and most of the observed modifications due to thermal effects can be considered minor. A direct comparison with hysteresis parameters shows that these simpler experiments can be equally effective in characterizing viscous and superparamagnetic particles.