A conductor crossing problem in magnetic bubble memories

Abstract

Bubble propagation across a conductor depends, among other considerations, on the magnetostatic energy of the bubble-Permalloy interaction and strain due to patterned metallization layers. Previously neglected, another consideration—bias field distortion in the neighborhood of the crossing—is discussed here. Poles induced by the bias field on a Permalloy element shaped to conform to a conductor produce field components parallel and transverse to the nominal bias. In traversing an isolated crossing at right angles to the conductor length and at a fixed altitude within the magnetic garnet, one calculates for the distortion bias maxima near the conductor edges and a minimum at the center. To obtain a tractable solution for the bias field as a function of coordinates a two-dimensional model was chosen for the conductor crossing: an infinite half space of infinitely permeable Permalloy with a circular depression. At points infinitely removed from the depression which houses the conductor the complex field H=Hx+jHy=jH0; that is Hx vanishes and Hy assumes the value H0 of the nominal bias field. Near the conductor centered at x=0, however, Hx is generally not zero, Hy(0,y) <H0, and Hy(x,y) exhibits a minimum at x=0 for fixed y. Using this model to calculate the bias in the vicinity of a 4-μm wide, 0.5-μm thick conductor, one finds for the bias field, averaged over a 3-μm thick garnet film, extremal values of 0.952H0 and 1.007H0. For current materials with H0?150 Oe the local bias reduction is 7.5 Oe. Because bubbles reside at the edges of Permalloy features, the field perturbations encountered in practice will be smaller than predicted by this model. Were there no forces on a bubble other than those associated with the coordinate dependent bias, a domain situated between maxima would move to the bias minimum and be trapped there. As designers strive for greater information density, this aspect of the conductor crossing problem may assume greater importance because of difficulty in linear scaling of conductor dimensions and the use of materials with higher bias requirements.