Measurement of Film Relaxation Time

Abstract

Magnetic film memories and other devices depend upon the relaxation from a state of hard direction magnetization to an easy direction state in the presence of a small easy direction ``tipping'' field HL. The relaxation times of five Permalloy films of thicknesses 0.1, 0.25, 0.4, 0.5, and 1.0 μ have been measured with an apparatus of switching field turn-off time <0.5 nsec and a detection risetime of 0.85 nsec. The relaxation time is defined as the time for a 10–90% easy direction flux change. It is found that the relaxation time varies roughly inversely with easy direction field and is to some extent independent of the initial hard direction field (8 Oe for the following remarks). The tangent slope Sw to the 1/τs vs HL curve at a field HL=1 Oe fits the relation Sw=1+10d2 Oe nsec, where d is the film thickness in μ, within experimental error (∼20%). The Sw predicted from the Landau-Lifshitz equation for the rotational model with λ=108 cps is 1 Oe nsec. The rotational model also predicts that τs approaches 2 nsec for HL→0. The limiting τs for the real films varied from about 7 nsec for the 0.4-μ film to about 40 nsec for the 0.25-μ film with no thickness correlation. In addition, the 0.1-, 0.25-, and 0.5-μ films exhibited a distinct slow-switching component for |HL| <Hdisp (Hdisp is the field HL necessary for a 90% relaxation in one direction). Observation of the final state of the 0.1- and 0.25-μ films by the Bitter technique revealed a fine-scale domain structure for |HL| <Hdisp and a single domain state for |HL| ≳Hdisp. The two thicker films were electrodeposited on a BeCu substrate, while the three thinner films were evaporated on a glass substrate. Since all five fit the same Sw vs d relation, it would appear that substrate-induced damping is not large.