Magnetic structure variations during giant magnetoresistance training in spin valves with picoscale antiferromagnetic layers

Microscopic models of exchange bias focus on the formation of domains in the antiferromagnet or the ferromagnet, or on a small induced moment in the antiferromagnet. Previous giant magnetoresistance (GMR) measurements, however, reveal exchange bias and training effects in CoFe-based spin valves with antiferromagnetic IrMn layers as thin as 0.4 nm. Polarized neutron reflectometry studies of a related spin valve with a 1.6 nm IrMn layer were carried out for several points along the GMR hysteresis curve to probe separately the free and pinned CoFe layers. These measurements confirm that the two ferromagnetic CoFe layers are aligned in parallel in saturating fields. During the first field cyle, regions of high resistance correspond to an antiparallel alignment of the CoFe layers as expected. Significant changes in this antiparallel structure are observed during the second field sweep, and a magnetic spiral forms and persists in the pinned CoFe layer. High-field saturation seems to reduce the effectiveness of the pinning and thus gives rise to training. These results have implications for the origin of exchange bias in spin valves with thin antiferromagnetic layers.