Browsing by Author "S. Banik"

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Magnetoresistance behavior of ferromagnetic shape memory alloy Ni1.75 Mn1.25 Ga
(2008) S. Banik; R. Rawat; P.K. Mukhopadhyay; B.L. Ahuja; Aparna Chakrabarti; P.L. Paulose; Sanjay Singh; Akhilesh Kumar Singh; D. Pandey; S.R. Barman
A negative-positive-negative switching behavior of magnetoresistance (MR) with temperature is observed in a ferromagnetic shape memory alloy Ni1.75 Mn1.25 Ga. In the austenitic phase between 300 and 120 K, MR is negative due to s-d scattering. Curiously, below 120 K MR is positive, while at still lower temperatures in the martensitic phase, MR is negative again. The positive MR cannot be explained by Lorentz contribution and is related to a magnetic transition. Evidence for this is obtained from ab initio density-functional theory, a decrease in magnetization and resistivity upturn at 120 K. Theory shows that a ferrimagnetic state with antiferromagnetic alignment between the local magnetic moments of the Mn atoms is the energetically favored ground state. In the martensitic phase, there are two competing factors that govern the MR behavior: a dominant negative trend up to the saturation field due to the decrease in electron scattering at twin and domain boundaries and a weaker positive trend due to the ferrimagnetic nature of the magnetic state. MR exhibits a hysteresis between heating and cooling that is related to the first-order nature of the martensitic phase transition. © 2008 The American Physical Society.
Phase diagram and electronic structure of Ni2+x Mn1-x Ga
(2006) S. Banik; Aparna Chakrabarti; U. Kumar; P.K. Mukhopadhyay; A.M. Awasthi; R. Ranjan; J. Schneider; B.L. Ahuja; S.R. Barman
We determine the phase diagram of Ni2+x Mn1-x Ga as a function of x (00.2 has been identified. The structure of this phase is found to be tetragonal from high-temperature x-ray-diffraction (XRD) studies. XRD, magnetization, and DSC measurements show that the paramagnetic tetragonal phase evolves from the lower temperature ferromagnetic tetragonal phase through a second-order phase transition without any change of structure. The unoccupied conduction band has been studied by inverse photoemission spectroscopy. It is dominated by Mn 3d -like states, and is in good agreement with FPLAPW based calculations. © 2006 The American Physical Society.
Structural studies of Ni2+x Mn1-x Ga by powder x-ray diffraction and total energy calculations
(2007) S. Banik; R. Ranjan; A. Chakrabarti; S. Bhardwaj; N.P. Lalla; A.M. Awasthi; V. Sathe; D.M. Phase; P.K. Mukhopadhyay; D. Pandey; S.R. Barman
The crystal structure of Ni2+x Mn1-x Ga has been studied as a function of composition (x) by powder x-ray diffraction. For Ni2.24 Mn0.75 Ga, where one-fourth of the Mn atoms are replaced by Ni, the experimentally determined lattice constants are in good agreement with theoretical equilibrium lattice constants calculated by minimization of total energy using full potential linearized augmented plane-wave method. For 0.15≤x≤0.35, a nonmodulated tetragonal martensitic phase is obtained at room temperature, whose lattice constant c increases and a decreases linearly with increasing x following Vegard's law. A 7M modulated monoclinic phase is obtained for x=0.2 due to annealing. The small width of the hysteresis of the martensitic transition shows its thermoelastic nature that is the characteristic of a shape memory alloy. Phase coexistence is observed for 0.1≤x≤0.15 at room temperature, confirming the first-order nature of the martensitic transition. © 2007 The American Physical Society.