Elasticity and magnetism of Ni sub(2) MnGa premartensitic tweed

Magnetic, magneto-elastic and elastic measurements were used to characterize the thermal evolution of the premartensite phase of Ni sub(2) MnGa. The premartensitic transition is shown to correspond to a sharp maximum of magnetostriction and a pronounced minimum of the (110) [Formulaomitted] shear stiffness (the [Formulaomitted] elastic coefficient); no additional softening of the [Formulaomitted] coefficient prior to the martensitic transformation was observed. In contrast, a significant softening of the (100) [Formulaomitted] stiffness (the [Formulaomitted] elastic coefficient) was observed in the vicinity of the martensitic transition, while this coefficient is fully unaffected by the premartensitic transition. A simple two-dimensional model of the tweed is presented to explain this mutual independence between the [Formulaomitted] and [Formulaomitted] shears and the effect of tweed formation on both of them. Premartensite transformation of cubic austenite is characterised by strong softening of the diagonal shear modulus, which is strongly affected by magnetic field. The martensitic transformation of premartensite, in contrast, is characterised by softening of the basal shear modulus while diagonal shear modulus steadily increases toward transformation. Seiner et al. show that a particular modulated structure of premartensite can explain this experimental observation.