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Magnetization reversal in the pinned layer of exchange biased spin valves is a complex process due to the exchange interaction between the ferromagnetic layer and the antiferromagnetic layer. This interaction results in progressive 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 this process is thermally activated but time dependence measurements are difficult to interpret, as the exchange field is nonconstant at many positions along the hysteresis curve. Measurements have been made of the time dependence of the reversal of the antiferromagnetic layer by measuring the recoil loops, following different times spent with the ferromagnetic layer saturated in the negative direction. In this manner, the exchange field can be assumed to be constant during the reversal of the antiferromagnet. These measurements show a shift of the loop of the pinned layer towards positive fields. This shift in the loop is interpreted as being the result of reordering of the antiferromagnet. Increasing the temperature during the time spent at 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 behavior of the antiferromagnet reversal suggests that this distribution is complex and may be multimodal. The reversal process is, however, reversible even at high temperatures indicating that the elevated temperatures do not significantly 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 significantly at low temperatures.

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An x-ray diffraction investigation of the structure of β′-NiAl alloys was conducted to define the state of order, identify the lattice sites occupied by compositional vacancies, and measure both static and vibrational components of atomic displacements, all as functions of temperature and composition. Long-range order was found to be virtually complete up to 1000 °C and remained substantial up to the melting points of the alloys. Compositional vacancies present in Al-rich β′ alloys were found to occupy Ni lattice sites only, at least to 1000 °C. Static atomic displacements were minimal at stoichiometry, increased with initial Ni or Al additions, and then decreased with further Ni or Al additions, with the maximum displacement about midway between stoichiometry and the β′ phase boundaries. Vibrational atomic displacements were also minimal at stoichiometry, but in contrast to static displacements, increased continually as either Ni or Al was added. Atomic displacements were correlated with known mechanical properties of β′-NiAl alloys.

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