No abstract

Acoustic emission measurements during the martensitic transition of a Ni–Mn–Ga magnetic shape-memory alloy reveal the intermittent and jerky character of the transition. The distribution of the amplitude of the acoustic emission events shows power law behavior, which reflects the absence of characteristic scales in the process. In this paper we show that the distribution is affected by an applied magnetic field, which proves that the transition dynamics is strongly influenced by magnetostructural coupling taking place at multiple length scales. The martensitic start temperature and the power law exponent of the amplitude distribution are measured in dependence of the applied field.

This paper deals with the study of the acoustic emission generated during the intermediate ͑premartensitic͒ transition to a micromodulated phase that precedes the martensitic transition in a nearly stoichiometric ferromagnetic Heusler Ni 52 Mn 23 Ga 25 crystal under applied magnetic fields. In the range of applied fields, the field stabilizes the high-temperature cubic phase by shifting the transition to lower temperatures. At low fields the amplitude, energy, and duration distributions of the acoustic emission events show power-law behavior which reflect the absence of characteristic scales associated with the transition. The corresponding exponents increase with the applied field, which indicate a weakening of the first-order character of the transition. No acoustic emission has been detected above ϳ2.5 kOe which is close to the field at which magnetic saturation is reached.