An analysis of ferromagnetic shape memory alloys produced by melt spinning, sputtering and pulsed laser deposition.

摘要

Ferromagnetic shape memory alloys (FSMA) have recently attracted much interest and research because of their large magnetic-field induced strain. Strains of up to 10% have been reported in single crystals of Ni2MnGa at room temperature. The mechanism of twin boundary motion is understood to be responsible for the strain in FSMA’s and theoretically allows for a full crystal lattice distortion strain, which is 10% in the case of orthorhombic martensites and 5-6% in the case of tetragonal martensites. These strains however are only achievable in single crystals and much smaller strains have been observed in polycrystalline samples. Here, competition of the randomly orientated grains restrict the cooperative twin boundary motion that is evident in single crystals.Melt spun ribbons are often highly textured and may offer a method of producing ribbons in a form useful as a start material for applications, i.e. basis of a useful ‘bulk’ compact form. Magnetic and structural properties of melt spun Ni-Mn-Ga have previously been reported. Applied magnetic field has been shown to affect the transformation strain on cooling. Ni-Mn-Ga alloys form a Heusler (L21) lattice which, on cooling, undergoes a diffusionless transformation beginning at the martensite start temperature Ms into either a tetragonal ororthorhombic martensite, both of which are highly twinned. The reverse transformation begins at the austenite start temperature As and both transformations occur over a range of approximately 10K. The type of martensite formed depends strongly on the composition of the sample and its thermal history. The influence of atomic order in Ni-Mn-Ga alloys has also been studied. It has been found that the degree of atomic order plays a large part in determining the martensitic transition temperature. This temperature is suppressed significantly in the disordered B2 structure where there is little correlation of the Mn and Ga atoms at the body center. Ni2MnGa undergoes a B2 to L21 (Heusler) transition at 1071K on cooling. In this work the structural and magnetic properties of melt spun Ni-Mn-Ga ribbons are presented and the effect of an applied magnetic field on the transformation strain is shown for the case of annealed ribbons under tensile stress.It is the intention of this work to explore the properties of melt-spun ribbons of the ferromagnetic shape memory alloys Ni-Mn-Ga and Ni-Fe-Ga with the addition of Tb. A range of compositions of both alloys have been created and analysed for both structural and magnetic properties. Melt spun ribbons are usually either amorphous or nanocrystalline, so the effect of heat treatments to both relieve stress and re-crystallise the ribbons is also explored. This provides some insight into how the crystal structure and magnetic properties evolve with annealing temperature. Thin films of these ferromagnetic shape memory alloys were also produced by both sputtering and pulsed laser deposition in order to characterise their behaviour and gain some insight into the growth conditions necessary to successfully produce thin films of ferromagnetic shape memory alloys. This was done because thin films grown at low temperatures can have similar properties to the melt spun ribbons, in that they are nanocrystalline or amorphous. It was hoped that producing and analysing these thin films would yield some more information about the behaviour of polycrystalline ferromagnetic shape memory alloys.