A New High-Precision Optical Technique to Measure Magnetostriction of a Thin Magnetic Film Deposited on a Substrate

We have developed a sensitive tool for the measurement of the saturation magnetostriction λs of a thin soft-magnetic film deposited on a substrate. To our best knowledge, our measurement sensitivity is more than an order of magnitude better than prior art, and is adequate for measuring magnetostriction smaller than 10-7 in magnitude for a film as thin as 30 nm on a glass wafer of thickness 20 μm. To make a measurement, the sample wafer in the form of a rectangle is clamped near an edge, and a rotating magnetic field of constant amplitude (sufficient to saturate the film) is applied to the film in its plane; this rotating field causes the film to expand/contract in the direction of magnetization for positive/negative magnetostriction, 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 suitably incident onto the sample is used to sense this small periodic warpage; the reflected laser beam is incident onto a position sensor, whose output is normalized to the laser beam intensity (to reduce intensity fluctuation effects) and then fed to a phase-sensitive lock-in amplifier that is locked to twice the field rotation frequency. This sample vibration frequency is selected at a value where the vibration-noise spectrum is relatively quiet, and below the first mechanical resonance frequency of the sample-holder assembly to avoid complications in the signal analysis. To provide longterm reproducibility, a “deflection calibration” signal can be produced by translating the position sensor by small amounts (using a piezoelectric transducer) at a different frequency and lock-in detection of the sensor signal at the fundamental of this translation frequency. This tool has been developed, characterized, and applied to the measurement of the saturation magnetostriction of ultrathin soft-magnetic Permalloy-base films. © 1989 IEEE