MAGNETOMECHANICAL MEASUREMENTS OF MAGNETICALLY SOFT AMORPHOUS MATERIALS

Amorphous materials in the form of ribbons, wires and thin films require special care to be taken in measurements of magnetostriction, the field dependence of elastic moduli and the magnetomechanical coupling factor. Problems arise in these measurements because of the sample geometry and, in some cases, because of the low anisotropy commonly found in amorphous ferromagnetic materials. This paper describes the methods that have been adapted or designed to measure the main magnetomechanical properties of magnetically soft, amorphous alloys of Fe, Co and Ni, in the three forms generally available: ribbons typically 20-50 mum thick and 1-25 mm wide; wires typically 30-130 mum in diameter; and films 0.01-10 mum thick, deposited on substrates 25-250 mum thick. The methods are described in sufficient detail, supported by numerous references, to enable readers to make a critical choice between them, taking into account such things as the temperature range, sensitivity and geometry. Magnetostriction measurements are classified as direct or indirect, depending on whether the strain is measured directly or the magnetostriction is deduced from a measurement of some other property dependent upon strain. Direct methods enable the magnetostrictive strain to be measured as a function of applied field-the so-called engineering magnetostriction; indirect methods are suitable for measuring only the saturation magnetostriction. The direct methods covered are the strain gauge, three-terminal and cantilever capacitance dilatometry, optical methods and the tunnelling tip method. Indirect methods include the small-angle magnetization rotation (SAMR) method, susceptibility measurements, the Wiedemann effect and strain-modulated ferromagnetic resonance (SMFMR). The methods described for measuring the field dependence of Young's modulus (the DELTAE effect) and magnetomechanical coupling factor k are the resonance-antiresonance method, including the use of the impedance or admittance circle, the vibrating reed method and ultrasonic wave velocity methods. The use of a torsional pendulum for measuring the shear modulus and internal friction is also described.