Magnetic Film with Helical Anisotropy

Abstract

A helical anisotropy film is characterized by the induced anisotropy of the form K sin²[π(z)−2πz/d], where K is the anisotropy constant, θ is the magnetization orientation, z is the coordinate along the direction of film thickness, and d is the film thickness. It has been predicted by solving Euler's equation that for film thickness greater than the critical value 2π(A/K)^½ (A=exchange constant), stable states may occur in which the magnetization orientation θ follows approximately the helical variation of the easy axis. The magnetic characteristics of a helical film can be clearly demonstrated by the M‐H loops of the top or bottom layers, as obtainable with a Kerr magneto‐optic apparatus. The transitions from a saturated state to a helical state and from one helical state to another result in multiple‐threshold M‐H loops. Nonmagnetostrictive Permalloy films have been electroplated onto a slowly rotating substrate in the presence of a dc orienting field. Samples of 3 μ or more thickness show the helical anisotropy characteristics while a sample of less than 0.5‐μ thickness behaves as a uniaxial film. Furthermore, control samples of each thickness have also been electroplated on a stationary substrate. They only show the usual uniaxial anisotropy property.