Studies on Ni-Mn-Ga ferromagnetic shape memory alloys

Ferromagnetic shape memory alloys (FSMA’s) have emerged as an important class of smart material in recent years because they exhibit interesting physics as well as properties of technological importance such as large magnetic field induced reversible strain (MFIS)  magnetoresistance  and magnetocaloric effect.  Ni2MnGa is an important FSMA, which shows a large MFIS (10%) and fast actuation in the martensite phase.  The large magnetic field induced strain is associated with the structural modulation in the martensite phase. Ni2MnGa shows martensitic and magnetic transitions at 210 K and 373 K, respectively. The martensitic transition in Ni2MnGa is preceded by the appearance of a premartensite phase around T= 260 K, which also shows modulation but of shorter wavelength.  A controversy about the nature of modulation has been solved by using high resolution x-ray diffraction data. The analysis of  high resolution x-ray diffraction data confirms that both the premartensite and martensite phases of Ni2MnGa are incommensurate modulated.[2] The stoichiometric Mn2NiGa has high magnetic transition temperature (588 K) and martensite transition close to room temperature (270 K). In Mn2NiGa antiferromagnetic coupling between two types of Mn atoms gives rise to interesting physical properties. The spin-valve-like asymmetric magnetoresistance at room temperature has been observed  for Mn2NiGa.[3] The asymmetry in magnetoresistance persists down up to 5K. The antisite disorder between Mn and Ga gives rise to the ferromagnetic nano-cluster in the Mn2NiGa ferrimagnetic matrix which causes asymmetry in MR with change in the direction of field.  Although  Ni2MnGa has  high potential for application as magnetic actuators,  brittleness and low transition temperatures of this material has necessitated the search for new alloys with similar MFIS, but with higher transition temperatures and ductility.  The initial theoretical and experimental results for new Pt doped Ni-Mn-Ga (Ni1.8Pt0.2MnGa) alloy show that it can be a good alternative to Ni¬2MnGa. Ni1.8Pt0.2MnGa has higher martensite transition temperature (285 K) compared to the Ni2MnGa (210 K). Neutron diffraction measurement shows that it has 7M modulated martensite structure in the martensite phase which indicates the possibility of magnetic field induced strain.[4] The magnetization and magnetocaloric studies on Ni1.8Pt0.2MnGa show the existence of large magnetocrystalline anisotropy in the martensite phase, which is required for magnetic field induced strain effect. 

References:
[1]  A. Sozinov, A. A. Likhachev, N. Lanska and K. Ullakko, Appl. Phys. Lett. 80, 1746 (2002).
[2]  Sanjay Singh et al. (To be submitted) .
[3]  Sanjay Singh, R. Rawat, S. Esakki Muthu, S. W. D’Souza, E. Suard, A. Senyshyn, S. Banik, P. Rajput, S. Bhardwaj, A. M. Awasthi, R. Ranjan, S. Arumugam, D. L. Schlagel, T. A. Lograsso, Aparna Chakrabarti and S. R. Barman,  Phys. Rev. Lett. (In press).
[4]  Sanjay Singh,  K. R. A. Ziebeck, E. Suard,  P. Rajput,  S. Bhardwaj,  A. M. Awasthi, and S. R.  Barman, Appl. Phys. Lett. (In press)