High-Throughput Measurement of Magnetostriction using MEMS and Composition Spreads
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High-Throughput Measurement of Magnetostriction using MEMS and Composition Spreads Noble C. Woo, Department of Chemistry & Chemical Biology Bryan G. Ng, Department of Materials Science & Engineering R. B. van Dover, Department of Materials Science & Engineering Cornell University, Ithaca NY 14853, U.S.A. ABSTRACT While measurement of magnetostriction in bulk materials is readily accomplished using a strain gauge, measurement of this quantity for thin films presents a greater challenge, and typically involves measurement of the overall wafer curvature (for a film of uniform composition) as a function of field. In order to evaluate magnetostriction locally in composition-spread samples, we have developed a method using a dense array of pre-fabricated cantilever beams on a silicon substrate prepared using MEMS techniques. Differential strain in the thin film/cantilever system results in curvature which is detected using an optical (laser/position-sensitive-detector) system. A magnetic field is applied using two orthogonal Helmholtz coils, and the resulting deflection-field curves are used to determine the saturation magnetostriction λs as well as dλ/dH. Our composition-spread films are prepared using a three gun on-axis magnetron cosputtering system. The position-dependent composition is inferred using rate calibrations and verified with electron microprobe and Rutherford Backscattering Spectroscopy. Preliminary experiments have validated the technique and the system has been used to measure magnetostriction in the Ni-Fe system. Our approach can also be used to measure properties of giant magnetostrictive materials (e.g. TbFe/Fe multilayers) as a function of layer thicknesses, or thin film shape-memory alloys, including magnetic shape-memory alloys [1,2].
INTRODUCTION Magnetostriction is a basic property of ferromagnetic materials, whose physical dimensions change depending on the orientation of the magnetic moment with respect to the lattice. It is a small effect, typically on the order of tens of parts per million (ppm). In bulk materials it is straightforwardly measured using strain gauges; data have been collected for many common elements and alloys [3]. However, determining magnetostriction coefficients in thin films presents a greater challenge since they are much thinner than a typical substrate. A simple way to conduct this measurement is to make a precise determination of the curvature of the magnetic film-substrate bi-layer under an applied magnetic field. This typically involves measuring the curvature of the bending film when the magnetic film is magnetized to saturation. In the past, this approach has almost invariably been used to characterize films of uniform composition. We describe here a magnetostriction measurement system for thin films that has high
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spatial resolution (~1 mm) and so is suitable for high-throughput combinatorial materials science (continuous composition spread approach).
EXPERIMENTAL DETAILS & DISCUSSIONS There are a number of different ways to mea
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