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.     

State space analysis of stochastic response cumulants

The technique of linear state space analysis is extended to allow computation of response cumulants of any order. In particular, equations are derived which govern the evolution of the cumulants for the response of a linear system excited by a delta correlated process, which generally causes nonnormal response. The technique also applies if the excitation can be viewed as the output of a linear filter with a delta correlated input. When applied to find second cumulants (i.e., covariances), the method gives the usual Lyapunov matrix equation. The method is illustrated by formulation of the state space equations for the third-order cumulants of the response of an SDF oscillator.     

Quasi-static switching of plated wires in external magnetic fields

Switching thresholds of plated wires were determined with slowly applied external magnetic fields, using both torquemeter and ac techniques. The samples included wires having helical easy-direction angles of 0°, 24°, 30°, and 60°. A study was made of the dependence of switching threshold HCon the angle θ between applied field and wire axis. Good agreement was found with the relationH_{C}(theta)=H_{C}(0)/cos thetafor all helical angles and for θ as high as 85°. It is concluded that with the cylindrical film geometry only the axial component of external field is effective in quasi-static switching over wide ranges of field direction and helical angle.