Investigation of Eddy-Current-Induced Nondestructive Readout in Thin-Film Memories

Abstract

When a magnetic thin-film bit is switched, eddy currents arise, creating fields which tend to remagnetize the bit in its original direction. A structure in which these fields are strong enough to produce NDRO was given a preliminary investigation. The bits were nickel-iron 17–83 Permalloy, 50×50 mil by 500 Å, evaporated onto a glass substrate. The structure layers were: bit line, word line, planar sense line, bit, glass, and word line. If the word drive pulse was 100 nsec long or less, the structure with the most unfavorable geometry gave stable NDRO at drive currents up to 10 times as large as the normal DRO value. Scale factors are displayed and applied to show that NDRO is possible with structures much smaller than the one tested. The eddy fields could even remagnetize partially demagnetized bits. When the eddy field was measured as a function of time and geometry, the time constant and magnitude of the decay was a sensitive function of the word linewidth and separation, varying between 76 and 191 nsec. Experimental and theoretical evidence was found for a rapid transient decay mode which precedes the slower decay mode observed in the experiments.