Step Stress Aging of Plated Wire Memories

In spite of their advanced state of development, there is still uncertainty concerning the long term stability of magnetic film memories. As early as 1958 E. N. Mitchell showed that the anisotropy of 1539 1540 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1968 permalloy thin films could be modified by a magnetic anneal at moderate temperatures. 1 Chang, Gianola, and Sagal subsequently pointed out that such a phenomenon could be detrimental to the lifetime of a magnetic film memory element, and t h a t in the case of plated wire intolerable changes in magnetic anisotropy and coercivity occur in freshly plated films. 2 These changes result in an increased minimum digit current for reliable writing along with a serious decrease in the digit disturb level. The result is a monotonic reduction in operating margins that, in time, can lead to a complete loss of range. It was, however, also shown t h a t the magnitude of the rate of change of magnetic properties could be substantially reduced by following electrodeposition with a stabilization anneal in an easy direction field. 2,3 From a practical standpoint, a stabilization anneal has been found essential and is now in general use in plated wire fabrication. It is worth noticing that a post-deposition anneal is automatically provided in vacuum deposited magnetic films, since the latter are deposited on a hot substrate which is allowed to cool in the vacuum system. From a simple physical viewpoint the effectiveness of the stabilization anneal should increase with annealing temperature and annealing time.