Micromagnetics of Laminated Permalloy Films

Abstract

A single-layer Permalloy strip with a transverse magnetic easy axis generally has 90° closure domains and associated Bloch walls. However, for certain applications, transversely magnetized single-domain states may be preferred. Laminating the Permalloy by alternating magnetic and nonmagnetic sublayers can eliminate closure domains by permitting flux closure through the nonmagnetic spacers along the edges of the strip. A new kind of 90° wall lying along the edge describes the region where flux closure takes place. The profile and energy of this edge curling wall (ECW) follow from micromagnetic principles. The ECW is observed in an optical Kerr microscope. Conditions are derived energetically favoring single transverse domains with ECW’s over either multiple closure domains or single longitudinal domains that are undesirable because of their low permeability. Computed figures illustrate how the maximum permissible spacer thickness depends on Permalloy sublayer thickness, uniaxial anisotropy, width of the magnetic strip, mismatch of Permalloy thickness, and perpendicular anisotropy. The case with strip width W = 100 pm and vanishing magnetostriction or stress should require only a few sublayers. However, if W is very small, or if stress-induced or some other form of perpendicular anisotropy is excessive, then attainment of single transversely magnetized domains requires many sublayers. Some of these conclusions are supported by microscopic observations. © 1988 IEEE