Thermal relaxation of the pinned layer magnetization in NiO spin valves

Abstract

A variation of the switching field of the pinned layer (PL) magnetization in NiO based spin valves was observed as a function of the rate of variation of the applied field. This phenomenon was characterized at low frequencies (0.4–120 Hz) using quasistatic magnetoresistance (MR) transfer curve testing and measurements of the relaxation of the resistance around the quasistatic switching field of the pinned layer (10−1–105 s). The influence of temperature was investigated between room temperature and 125 °C. The relaxation data were fit using a model based on an Arrhenius law thermal activation with a single relaxation time. The variation in the switching field of the pinned layer as a function of the applied field rate was compared to the Kurkijärvi model. The fit parameters in both models are the energy barrier height and the thermally activated volume of nucleation. The parameters derived from both types of experiments agree remarkably well. The understanding of this low frequency magnetic behavior of spin valves allows one to gain insight into the long-term relaxation of the magnetization of the PL in MR readback heads, as well as into the magnetic response of this layer to very short (ns) pulses of field or high frequency (100 MHz) excitations.