Nondestructive Readout Scheme for Thin Film Memories

If a magnetic bit in a thin film memory array is driven in the hard direction, the changing flux produces eddy currents in the surrounding conducting surfaces. It has been pointed out by Pohm and others that the eddy field produced by these currents always tends to restore the bit magnetization to its original direction. If this eddy field were strong enough at the end of the word drive pulse, the effect could be made the basis of a scheme for nondestructive readout (NDRO). Some experiments were performed on structures where the eddy field was strong enough to restore the bit. The thin film plane used contained 50 X 50-mil X 500-A-thick Permalloy films with uniaxial anisotropy. The word line was completely wrapped around the plane. Word-line-to-wordline spacings of 1 1.25 and 7. 25 mils, and word-line widths of 50, 75, and 150 mils were used. The eddy field was measured at 50 and 100 nsec after the switching operation and, assuming an exponential eddy field decay, a time constant was calculated. Two theoretical approximations were considered. In both, the eddy currents were assumed to flow only in the word lines. In one, the thickness of the lines was assumed negligible. In the other approximation, the thickness of the striplines was considered, but was assumed to be of infinite width. The finite width, thin stripline approximation seems to be in better agreement with the measured values of the time constant which varies between 76 and 200 nsec. Accepted for the Air Force Franklin C. Hudson, Deputy Chief Air Force Lincoln Laboratory Office * This report is based on a thesis of the same title submitted to the Department of Electrical Engineering at the Massachusetts Institute of Technology on 19 August 1963, in partial fulfillment of the requirements for the degree of Master of Science.