Thermal relaxation of the pinned layer magnetization in NiO spin valves

A variation of the switching field of the pinned layer (PL) magnetization i n 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 (0.4-120Hz) using quasistatic magnetoresistance (MR) transfer curve testin g and measurements of the relaxation of the resistance around the quasistat ic switching field of the pinned layer (10(-1)-10(5) s). The influence of t emperature was investigated between room temperature and 125 degrees C. The relaxation data were fit using a model based on an Arrhenius law thermal a ctivation with a single relaxation time. The variation in the switching fie ld of the pinned layer as a function of the applied field rate was compared to the Kurkijarvi model. The fit parameters in both models are the energy barrier height and the thermally activated volume of nucleation. The parame ters derived from both types of experiments agree remarkably well. The unde rstanding 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 laye r to very short (ns) pulses of field or high frequency (100 MHz) excitation s. (C) 1999 American Institute of Physics. [S0021-8979(99)25508-9].