Temperature-dependent demagnetization behaviour in perpendicular exchange-coupled SmCo5/FeCo multilayers

Abstract

We study with finite temperature Monte Carlo simulation the temperature-dependent demagnetization behaviour of exchange-coupled multilayers of hard (SmCo5) and soft (FeCo) magnetic materials. The easy axis of the hard phase is perpendicular to the layers, as is motivated by recent experimental results. We perform simulations with and without the dipolar interaction and study the effect of finite temperature on the coercivity, the remanence and the energy density of the composites. We investigate these properties as functions of the soft layer thickness. We find that as the temperature is increased, the increase in the energy product with the addition of soft material becomes progressively smaller than expected because the increase in the remanent magnetization is less than anticipated for two reasons. Because of the different magnetization densities between the hard and the soft phases, magnetic charges are created at the hard–soft boundaries. To reduce the corresponding dipolar energy, at moderate temperatures the magnetization of the soft phase becomes tilted with respect to that of the hard phase; at relatively high temperatures vortex-like structures are created in the soft phase. Furthermore, the addition of soft material reduces the spin wave stiffness and increases the amount of thermal fluctuation. These two effects reduce the rate of increase of the magnetic moment and hence the maximum energy product. At low temperatures, the hysteresis curve shows a two-stage magnetization reversal process. Simulation results for the first instability field Hc1 are in reasonable agreement with theoretical nucleation field Hn estimates for a moderate amount of soft materials. The difference between Hc1 and Hn when the amount of soft material is small is explained. As the temperature is increased, the two-stage process disappears, most likely due to the formation of vortex-like structures in the soft phase.