RESONATING TORQUE MICROBALANCE DEVELOPED FOR IN SITU MEASUREMENTS OF FERROMAGNETIC FILMS WITH SUB-MONOLAYER SENSTIVITY.

Work at NIST to develop ultra-sensitive magnetometers based on micromechanical sensors has led to a new instrument for in situ measurements of ferromagnetic films. In particular, measurements of thin-film properties critical to the development of read head sensors and magnetic recording media are being investigated.

The production and development of many contemporary magnetic devices require that consistent growth conditions be maintained during thin-film deposition processing steps. Typically, film properties are determined ex situ with induction-field (B-H) loopers that measure the [M.sub.s][t.sub.f] product for the film, where [M.sub.s] and [t.sub.f] are the saturation magnetization and the thickness of the film, respectively. The goal of this project is to develop an instrument that depends on inexpensive, batch-fabricated, micromechanical substrates for quantitative measurements with sub-monolayer magnetic moment sensitivity.

NIST scientists are the developers of an instrument that measures the magnetic torque on a film as it is being deposited onto a single-crystal silicon micro-cantilever. An optical fiber interferometer is used to measure the deflection of the cantilever. Optic-fiber detectors work well in the high noise environment typical of deposition systems. The magnetic torque is applied near the mechanical resonance of the cantilever to take advantage of the quality factor enhancement of the mechanical torque signal. Dynamic feedback is used to balance the magnetic torque by applying a mechanical force at the base of the cantilever that is just equal and opposite to the magnetic torque. The dynamic feedback approach minimizes the mass loading and the effects of temperature-dependent elastic modulus that change the resonant frequency of the cantilever during deposition. The cantilevers were custom designed for this application and fabricated in the new NIST micro-electromechanical sensors fabrication facility in Boulder.

The technique provides a way to make quantitative measurements of the saturation magnetization of thin-film samples with very small total magnetic moments. The Brownian motion of the cantilever sensor fundamentally limits its ultimate sensitivity; at room temperature this corresponds to a 0.02 nm thick ferromagnetic film with the current cantilever geometry.