Magnetoelastic Coupling in NiMnGa Ferromagnetic Shape Memory Alloy
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NiMnGa alloys have attracted extensive attention because their ferromagnetic characteristic provides an additional degree of freedom to control both the shape memory effect and the multi-stage phase transformations in this Heusler system. Technically, along with the large magnetic-field-induced strains, NiMnGa alloys exhibit giant magnetocaloric effect due to their magnetic entropy changes associated with the coupled magnetostructural transitions. Fundamentally, a sequence of phase transformations, manifesting itself by a rich variety of physical anomalies on cooling to the martensitic transformation (MT) temperature TM, has been established. However, in comparison to the intensive studies of structural transformations, the magnetic properties of NiMnGa premartensite were hardly touched. The purpose of this research is to i) investigate the temperature dependence of the magnetic driving force of martensitic NiMnGa, which is a critical factor to determine the actuation temperature window of this material; and ii) understand the magnetoelastic coupling enhanced precursor effects, especially the unique magnetic behavior of NiMnGa premartensite. The singular point detection technique has been applied to determine the magnetic anisotropy constant K1 of a martensitic Ni49.0Mn23.5Ga27.5 (wt%) crystal. As expected, K1 increases with decreasing temperatures below TM of 276 K, following a magnetization power law K1(T)/K1(0)=(Ms(T)/Ms(0))3. However, the force required to initiate twin boundary motion increases exponentially with decreasing temperature. The combination of both temperature dependences leads to a very restricted temperature window for magnetic actuation using this alloy. The premartensitic transformation has been established by means of neutron powder diffraction and measurements of elastic constants of C44 and C'. The premartensitic phase has been verified by the stiffening of C44 prior to the MT. The slope change of C' at TC positively confirms that the precursor phenomena are enhanced by the magnetoelastic coupling. Magnetic Ni49.0Mn23.5Ga27.5 premartensite is characterized by the coexistence of a finite dc magnetic susceptibility and a vanishing magnetocrystalline anisotropy, distinguishing bcc NiMnGa from the typical magnetic soft materials. This property arises from the competition between the exchange forces of the host lattice and the strong local crystal fields stemming from the tweed.