Thermal activation of magnetization reversal in spin-valve systems

Magnetization reversal in the pinned layer of exchange biased spin valves i s a complex process due to the exchange interaction between the ferromagnet ic layer and the antiferromagnetic layer. This interaction results in progr essive reversal of the antiferromagnetic layer as the magnetization of the ferromagnetic layer changes direction. This reversal of the antiferromagnet will effect the subsequent reversal of the ferromagnet. It is known that t his process is thermally activated but time dependence measurements are dif ficult to interpret, as the exchange field is nonconstant at many positions along the hysteresis curve. Measurements have been made of the time depend ence of the reversal of the antiferromagnetic layer by measuring the recoil loops, following different times spent with the ferromagnetic layer satura ted in the negative direction. In this manner, the exchange field can be as sumed to be constant during the reversal of the antiferromagnet. These meas urements show a shift of the loop of the pinned layer towards positive fiel ds. This shift in the loop is interpreted as being the result of reordering of the antiferromagnet. Increasing the temperature during the time spent a t saturation shows that the process is driven by thermal activation. Close examination of the degree of loop shift with time spent at saturation shows behavior consistent with thermal activation governed by a distribution of activation energies. At longer times and elevated temperatures, the behavio r of the antiferromagnet reversal suggests that this distribution is comple x and may be multimodal. The reversal process is, however, reversible even at high temperatures indicating that the elevated temperatures do not signi ficantly change the structure of the ferromagnetic-antiferromagnetic layers or the interface between them. Finally, measurements at 77 K show that the active portion of the energy barrier distribution will change significantl y at low temperatures. (C) 2001 American Institute of Physics.