Ferromagnetic NiMnGa and CoNiGa shape memory alloy films

Abstract:

This paper reports results on Heusler-type NiMnGa and CoNiGa ferromagnetic shape memory alloy films deposited by magnetron sputtering and pulsed laser deposition respectively, on silicon cantilever-type substrates. NiMnGa FMSMA films deposited by magnetron sputtering on silicon substrates show good ferromagnetic and shape memory properties if the substrate is heated above 350°C or if the films deposited at RT are annealed above 600°C. The growth of the film is columnar. TEM examination shows typical martensitic variants while the SAED pattern suggests an orthorhombic structure. CoNiGa FMSMA films deposited at 550°C by PLD are crystalline. Mechanical spectroscopy data proves the presence of a typical martensitic transformation that develops below room temperature: the phase transitions manifest themselves by typical changes of the modulus defect and displacement vs. temperature characteristics similar to those associated with the martensitic transformation in known shape memory alloy films. The change in the elastic modulus as a function of the magnetic field suggests that in both NiMnGa and CoNiGa films the spontaneous magnetization is clamped by the magnetic anisotropy field.