Microwave-assisted switching of microscopic rings: Correlation between nonlinear spin dynamics and critical microwave fields

We have studied the spin dynamics of microscopic permalloy rings at GHz frequencies. Increasing the irradiation power, we observe first nonlinear spin dynamics and second microwave-assisted switching (MAS). We explore the MAS phase diagram as a function of microwave power and frequency f and, in particular, extract the critical microwave field h(c)(f). Its frequency dependence reflects characteristic eigenfrequencies from both the linear and nonlinear spin-wave spectrum. By comparing h(c)(f) with the different susceptibilities, we gain insight into the microscopic processes which might be the basis of a predictive theory of MAS.