Amorphous Magnetoelastic Materials
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MAGNETOELASTIC
MATERIALS
H. T. SAVAGE* AND MARILYN WUN-FOGLE** *BCS Inc., 1003g Sunnyvale Lane, Madison, WI 53713 **NSWC, Carderock, Code 684, Silver Spring, MD 20903-5000 ABSTRACT The outstanding feature of amorphous magnetoelastic alloys is the controllability of the magnetic anisotropy energy, Curie point, magnetostriction and magnetic moment. This control of material characteristics, achieved by magnetic and stress annealing plus changes in composition, is impossible in crystalline materials. The control allows the design of tactile and magnetic field sensors with special features and very high sensitivity. The materials discussed are prepared by rapid solidification through melt spinning in ribbon and wire geometries and magnetron sputtering onto substrates. As an example of the advantages of sputtered material, an accelerometer on silicon micro-cantilevers is shown. I-t has no coils. The basic magnetoelastic theory that governs tactile sensors is shown. Low-noise magnetic field sensors with novel twist anisotropies and Barkhausen instabilities in wires are discussed.
INTRODUCTION The outstanding characteristic of amorphous materials for device exploitation is the control of the magnetic anisotropy energy K. Using magnetic annealing, direction of the easy axis and uniformity can be controlled. 1 By altering alloy composition magnitudes can be controlled.' We will show later that a good figure of merit for magnetoelastic devices is (3Xs) 2E0 /2K, where E 0 is Young's modulus and X, is the magnetostriction constant. 2 In Fe based materials the magnetostriction cannot be made much larger than that of Ni (3 x 10-5). Control of Young's modulus is limited. The salient feature is that the magnitude of K, via magnetic annealing, can be made a 1000 times smaller than transition metal anisotropies at room temperature. Magnetic annealing, annealing above the Curie point and below the crystallization temperature in the presence of a magnetic field, greatly relieves the random internal stress. The direction of the easy axis (direction in which the magnetic anisotropy energy is a minimum) is in the direction of the annealing field. This degree of control is not possible in crystalline materials. The small value of K implies that a small applied magnetic field H will rotate the magnetic moment, thus making changes in length possible with very small field changes. Conversely microscopic changes in length rotate the magnetic moment appreciably, making high figure of merit strain gages possible. Control of the easy axis direction implies that the magnetic field can be applied in a convenient direction. 201 Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
Another feature is the possibility of achieving near-zero magnetostriction by altering composition. This is normally done by adding Co to iron-based alloys. In this way magnetically softer materials are prepared . The addition of Co increases cost greatly and reduces the room temperature magnetic moment. The anisotropy is not as uniform as in Fe based m
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