Transitory ultrasonic absortion in multiferroic Ni-Mn-Ga alloy

Abstract

[eng] In this work we create in 10M Ni-Mn-Ga martensitic samples a special martensitic variant structure consisting of only 3 twins separated by two a/c twin boundaries: Type I and Type II, with relatively low and very high mobility, respectively. The domain engineered structure thus created allows us to investigate the dynamics of a single highly mobile a/c twin boundary. We show that temperature variations induce in our samples an intense transitory absorption at ultrasonic frequencies ca. 105 Hz, peaking around 215 K. We also prove the existence of mobile obstacles to the motion of Type II twin boundary under quasistatic thermal stresses. Observation of conventional (structural) transitory damping at a frequency 105 Hz is not allowed, despite rather low strain amplitude used, due to the inverse frequency dependence of this damping component. We argue that the transitory absorption registered at rather high ultrasonic frequencies is associated with rearrangement of magnetic domain structure, associated with motion of Type II a/c twin boundary. The uncovered transitory internal friction term of magnetic origin is a new category that falls within the classes of magnetomechanical damping and transitory internal friction.