Hysteresis and limiting cycles in a high frequency ac field

Numerical solutions of Brown’s Fokker-Planck equation are used to analyze the response of a superparamagnetic particle to an ac applied field. The nature of the response is governed by the rate of intrawell relaxation processes: At low driving frequencies they maintain the system close to a quasistationary state, and the response is Markovian. By contrast, probability redistribution effects taking place at high frequencies lead to a non-Markovian response mode which displays memory phenomena. The high frequency limiting cycles are analyzed in detail, and the transition to conventional low frequency hysteresis loops is discussed.