Precise measurements of a magnetostriction coefficient of a thin soft-magnetic film deposited on a substrate

Abstract

A novel magnetostriction measurement tool is developed for a thin soft-magnetic film on a nonmagnetostrictive substrate. The sensitivity is at least an order of magnitude better than prior art, and is adequate to measure a magnetostriction coefficient as small as 10-7-10 -8 for a film as thin as 30 nm on a glass wafer of thickness 200 μm. To make a measurement, the sample wafer in the form of a rectangle is clamped near an edge, and a saturating rotating magnetic field of constant amplitude is applied in its plane; this rotating field causes the film to expand/contract in the direction of magnetization for a positive/negative magnetostriction coefficient and results in a periodic "warpage" of the sample at a frequency that is twice the rotation frequency of the field. A continuous helium-neon laser beam incident on the sample is used to sense this small periodic warpage. The reflected laser beam is detected by a position sensor; its output is normalized to the laser beam intensity to account for intensity fluctuation effects, and then fed to a phase-sensitive lock-in amplifier that is referenced to twice the field rotation frequency. This tool has been developed, characterized, optimized for noise reduction by suitable selection of frequencies and suspension techniques, and applied to measure the saturated magnetostriction coefficients of ultrathin soft-magnetic Permalloy-based films.