A study of the martensitic phase transition in the shape memory alloy Ni2MnGa

A study of the martensitic phase transition in the shape memory alloy Ni2MnGa has been carried out. Ni2MnGa is one of the group of 'shape memory effect' alloys which are currently exciting considerable interest. The origin of this effect in the compound is in the phase change which takes place on cooling through T=200 K from the cubic L21 Heusler structure to a tetragonal phase. Recently the results of band structure calculations have been used to conclude that in Ni2MnGa the structural phase transition is driven by a band Jahn Teller distortion. A polarised neutron diffraction study has been undertaken to determine the magnetisation distribution in the cubic and tetragonal phases. The aim of the experiment was to determine whether the phase transformation is driven by a repopulation of the d electrons into band of different symmetry. In fact this was found to be the case and the results represent the first direct observation of the band Jahn Teller mechanism. A study of the way in which the transition takes place has been undertaken using unpolarised neutron diffraction from Ni2MnGa single crystal. The effect of electron concentration on the magnetic and structural properties of Ni2MnGa has been investigated by substitute the Ga by other elements. Polycrystalline specimens of Ni2MnGa, Ni2MnGa1-xSnx, Ni2MnGa1-xInx, Ni2Mn1-xVxGa, CO2Mn1-xVxGa, Cu2Mn1-xVxGa were prepared using an argon arc furnace. A single crystal of Ni2MnGa for use in the neutron experiments was grown using the Czochralski method. Heusler alloys are properly described as ternary intermetallic compounds. Ni2MnGa has the Heusler cubic (L21) structure with space group Fm3-barm, and is ferromagnetic with Curie temperature of 376 K, and a magnetic moment of 4.71 μB largely confined to the Mn sites. Different techniques; such as X-ray diffraction, SQUID magnetometer measurements, DSC (Differential Scanning Calorimetry), the contactless measurement of the electrical resistivity and neutron diffraction measurements have been used on these samples. (author)