Temperature- and field-dependent evolution of micromagnetic structure in ferromagnetic shape-memory alloys

Abstract

Ferromagnetic shape-memory actuator materials offer an ingenious route to actuation by an applied magnetic field rather than the slow process of shape change by temperature in conventional shape-memory alloys. These actuator materials combine high strains with fast reaction times. This work shows the detailed temperature- and field-dependent evolution of the micromagnetic structure. Results provide a new perspective on understanding and modeling these important classes of actuator materials---the micromagnetic structure of these alloys truly resembles a magnetic ``mosaic'' composed of tiles of the transformed martensite phase (twins). Also, direct micromagnetic evidence of field-induced martensitic phase transformation is shown. This field-induced martensite transformation was found to be completely reversible in analogy with the well-known pseudoelastic behavior in conventional shape-memory alloys. Remarkably, the field-induced transformation occurs at fields $(800\phantom{\rule{0.3em}{0ex}}\mathrm{Oe})$ that are much lower than theoretically predicted values of several thousand oersteds.