First-Principles Calculations of Phase Stability and Magnetic Properties of Ni-Mn-Ga-Ti Ferromagnetic Shape Memory Alloys

The main purpose of the present work is to explore the influence of the Ti addition on crystal structure, phase stability, magnetic properties and electronic structures of the Ni8Mn4-xGa4Tix (x is the number of Ti atoms in a unit cell, x = 0, 0.5, 1, 1.5 and 2) alloys by first-principles calculations, aiming at providing the theoretical data and directions for developing high performance ferromagnetic shape memory alloys (FSMAs) in this alloy system. The formation energy results indicate that the doped Ti preferentially occupies the Mn sites in Ni2MnGa alloy. With the increase of Ti content, the optimized lattice parameter of the ferromagnetic austenite increases regularly. For the martensitic phase, the lattice parameter a increases while c decreases, leading to a decreased c/a ratio. The paramagnetic and ferromagnetic austenitic phases both become stable because their formation energies (E-form) gradually decrease with the increasing amount of Ti. The experimentally reported decrease in the Curie temperature with increasing Ti content is derived from the decrease of the total energy difference between the paramagnetic and the ferromagnetic austenite. The total magnetic moment is mainly contributed by Mn, while the magnetic moments of Ga and Ti are nearly zero. The total magnetic moment decreases notably when Mn is gradually substituted by Ti because the atomic magnetic moment of Ti is much less than that of Mn, which is in fair consistent with the experimental observations. The intensity of up-spin total density of state (DOS) decreased dramatically with the increase of the Ti content; whereas the change of the down-spin part below E-F is not obvious. This feature gives rise to the decrease of the total magnetic moments in these alloys. The results of present work are particularly useful in guiding composition design and developing new type of magnetic shape memory alloy.