Large magnetically induced strains in Ni_(50)Mn_(28.7)Ga_(21.3) driven with collinear field and stress

摘要

Despite the huge magnetic-field-induced strain (MFIS) of up to 9.5% exhibited by certain Ni-Mn-Ga alloys, their usefulness in applications is severely hindered by the electromagnet device needed for driving the alloys with a magnetic field and orthogonal loading stress. In this paper we present macroscopic measurements obtained from a single crystal of Ni_(50)Mn_(28.7)Ga_(21.3) which demonstrate a large reversible MFIS of -4100 ppm when the alloy is driven with quasistatic magnetic fields and fixed compressive stresses applied collinearly along the [001] axis. This collinear configuration marks a fundamental difference with prior research in the field and points to the existence in this alloy of stable bias or residual stresses—likely associated with pinning sites in the alloy—which provide the energy necessary to restore the original variant state when the field is reversed. We present macroscopic magnetomechanical measurements which show a decrease of the MFIS with increasing stress loading and a stiffening of the alloy with increasing dc fields. The latter behavior is phenomenologically similar to the ΔE effect in magnetostrictive materials. The large reversible MFIS and tunable stiffness properties exhibited by this alloy could enable practical Ni-Mn-Ga actuators for high-deflection, low-force applications which due to being driven by a solenoid transducer are more compact, energy efficient, and faster than their electromagnet counterpart. A thermodynamic model is presented which qualitatively characterizes the decay in MFIS with increasing compressive external load and provides a starting point for the characterization, design, and control of the proposed Ni-Mn-Ga devices.