The effect of stress and sample shape on the magnitude and frequency of magnetomechanical acoustic emission

The processes that can generate acoustic waves in a sample subjected to a changing magnetic field are discussed. An applied magnetic field with a fast rise time is found to be most efficient for generating magnetomechanical acoustic emission (MAE). The amplitude of the events correlates with the magnetostriction of the material. The shape of a sample affects both the level of magnetization, which can be achieved in a given applied field, and the acoustic modes, which the sample can support. It is demonstrated that MAE excites the fundamental longitudinal resonance of rodlike samples and the way the sample is held affects both the magnitude and frequency of these resonances. When the sample is subjected to tensile stress, the amplitude of the MAE decreases rapidly. However, it is shown that this decrease can be accounted for by an increase in the degree of clamping of the sample ends and that the normalized MAE actually increases initially with tension.