Thermal activation in microwave-assisted magnetization switching and its effect on the switching behavior of granular media

Microwave-assisted switching (MAS) has gathered fundamental interest and has been explored for magnetic recording since the switching field can be reduced by exciting magnetization with a microwave field in the GHz range. We theoretically study the effect of thermal activation in MAS and apply the developed analytical method to examine the result of MAS experiments on CoCrPt-SiO2 granular media. The effective energy barrier height for magnetization switching under a microwave field is calculated from the Landau-Lifshitz-Gilbert equation. The calculated energy barrier shows that the thermal activation process plays a crucial role in MAS and modifies the dependences of the switching field on the microwave frequency and amplitude. The switching field decreases with increasing the microwave frequency up to the critical frequency, at which point the switching field abruptly increases. Thermal activation significantly increases the critical frequency and makes the slope of the frequency dependence of the switching field gradual. The maximum assistance effect increases with increasing the microwave field amplitude and is almost independent of the thermal activation. The analytical method quantitatively explains the gradual microwave frequency dependence and high critical frequency observed in MAS experiments on granular media. These results indicate the importance of thermal activation to optimize MAS behavior for practical use.