Stress relief mechanism of aging in plated-wire memories

Results are presented which identify a dominant aging mechanism of plated-wire memory elements in the absence of magnetic fields. This mechanism, i.e., stress relief in the Permalloy film, can occur even in elements stabilized against field-induced aging. The evidence for stress relief consists of aging and strain sensitivity measurements on samples of different Permalloy composition. In aging experiments under vacuum at 230°C, Ni-rich films show an increase in anisotropy field Hk, while Fe-rich films show a decrease. A stress relief model is described which relates such aging changes to the initial film stress the cylindrical geometry, and the tension strain sensitivity. Using this model, agreement is found between film stress deduced from the Hkaging changes, and film stress from direct measurements, reported in the literature. Films which are judged to have zeromagnetostrictive composition (ZMC) on the basis of torsion sensitivity are found to show nonzero tension sensitivity. This is satisfactorily accounted for by circumferential composition inhomogeneity, which is shown analytically to explain the observed decrease in anisotropy field with tension. It is concluded that reduction of such inhomogeneity by appropriate deposition techniques is favorable to both smaller strain sensitivity and increased aging stability.