S.R. Barman

Structural Studies on Mn Excess and Ga Deficient Ni-Mn-Ga

We report the structural studies on Mn excess and Ga deficient Ni2Mn1+zGa1-z specimens with z= 0, 0.05, 0.1, 0.15, 0.2 and 0.25. The crystal structure at room temperature was determined by the x-ray diffraction (XRD). Rietveld analysis has been performed to obtain the lattice parameters. For z= 0, 0.05 and 0.1, a cubic austenitic phase is observed. For 0.15≤ z ≤0.25, a tetragonal martensitic phase is obtained, whose lattice constant c increases and a decreases linearly with increasing z following Vegard’s law. Phase coexistence is observed for z= 0.15, confirming the first-order nature of the martensitic transition.

Magneto-Transport and Magnetic Properties of Ni-Mn-Ga

We report a detailed investigation of the magneto-transport and magnetic properties of Mn excess Ni-Mn-Ga using the resistivity and magnetization measurements. Magnetoresistance (MR) has been measured in the ferromagnetic state for different compositions in the austenitic, premartensitic and martensitic phases. With Mn doping in Ni2-yMn1+yGa, a decrease in magnetization and MR has been found, since the doped Mn atoms in Ni position are in the antiferromagnetic configuration with the Mn atoms in Mn position. MR for the parent stoichiometric composition Ni2MnGa varies almost linearly with field in the austenitic and pre-martensitic phases, and shows a cusp-like shape in the martensitic phase. This has been explained by the changes in twin and domain structures in the martensitic phase. Hysteresis in the heating and cooling cycles is a characteristic of the first order nature of the martensitic phase transition.

Signature of Austenitic to Martensitic Phase Transition in Ni2MnGa in Mn and Ni K-Edge XANES Spectra

The XANES studies at Mn, Ni and Ga K-edge of Ni2MnGa compound have been carried out at room and low temperatures. The Mn K-edge and Ni K-edge spectra shows modulation in the post edge features when the sample is cooled below martensitic transition temperature. It is strongly reflected in the XANES of Mn K-edge where the peak after the edge gets totally suppressed when the sample is in martensitic phase. This peak shows a hysteretic behaviour when thermal cycling was done across the martensitic transition temperature. This clearly shows that the peak height is a measure of austenitic phase present at a particular temperature. This demonstrates the strong correlations of electronic states and crystal structures in these compounds.

Structural, Thermal and Magnetic Properties of Ga Excess Ni-Mn-Ga

The martensitic transition and the ferro- to paramagnetic transition have been studied in a series of Ga excess Ni-Mn-Ga specimens [Ni2-xMnGa1+x (0.4≤ x≤ 0.9)] by differential scanning calorimetry and magnetization measurements. The martensitic transition exhibits a hysteresis whose width is similar to Ni2MnGa, indicating that the transition is thermoelastic. The latent heat of transformation is comparable with other Ni-Mn-Ga alloys. A substantial increase in the martensitic transition temperature is observed due to Ga doping. Interestingly, the x-ray diffraction pattern of all the compositions studied show a modulated martensitic structure in the martensitic phase.

Electronic and structural properties of ferromagnetic shape memory alloys studied by density functional theory

The electronic and structural properties of different members of the Ni-Mn-Ga family calculated by ab initio density functional theory are discussed. From total energy calculations, we show that the martensitic phase is the stable low temperature phase. Moreover, occurrence of ferromagnetic and paramagnetic martensitic phases for Ni2MnGa and Ni2.25Mn0.75Ga, respectively, are explained. Modifications in the density of states near the Fermi level EF are observed across the martensitic transition for Ni2MnGa, as well as in Mn2NiGa. While Ni2MnGa is ferromagnetic, we find Mn2NiGa to be ferrimagnetic. The calculated lattice constants and the magnetic moments are in good agreement with experiment.

Electronic structure of buried Co-Cu interface studied with photoemission spectroscopy

Depth profiling type of measurement has been performed on the Co(100 A)/Cu(50 A) bilayer thin film. Valence band photoemission spectra were recorded at 50 eV photon energy as a function of sputtering time. The motivation of the present work is to understand the electronic structure of the buried Co/Cu interface and the nature of intermixing in the Co and Cu layers. X-ray reflectivity and transmission electron microscopy corroborate with the photoemission results and shows a very broad intermixed Co-Cu interface. The valence band of intermixed Co/Cu interface shows the Co and Cu 3d states which are considerably shifted towards higher and lower binding energy, respectively, as compared to the bulk elemental Co and Cu 3d states. The experimental observations are explained with the help of calculations based on projected augmented wave pseudopotential method using density functional theory. The origin and the shift of feature in the valence band of the Co-Cu interface are mainly due to the formation of two di...

Mapping of Magnetic Domains by MFM in Ni 2 MnGa

Influence of structural transition in the evolution of the magnetic domains in the ferromagnetic shape memory alloy system Ni2+xMn1-xGa is reported here using Magnetic Force Microscopy (MFM) studies. Studies reported are with two samples with their martensite transition temperature TM less than and greater than the Curie temperature Tc. Present results show an evolution of MFM across the Tc with a clear twin domains and sub domain structures inside the twins. The higher spatial resolution of MFM (~50nm) as compared to optical microscope (400nm) is useful in probing the domain walls. Force derivative of the MFM signal that may be used as an order parameter seems to scale the onset of magnetic order in the system. One can clearly see the vanishing of the MFM patterns for T>Tc. Results are discussed in the light of models available for tip-sample interactions that track the local magnetization.

Textural Ordering in NiTi, Ni-Fe-Ti, and Ni-Mn-Ga Shape Memory Alloys - Kinetics of Intra- and Inter-Domain Processes

A detailed kinetics study of the first-order structural transition in virgin NiTi, Ni47Fe3Ti50, and Ni2+xMn1-xGa (x= 0 and 0.26) manifests the varying role of renucleation-driven austenitic growth with the doping-induced disorder and the magnetization state. The austenite transitions were investigated using differential scanning calorimeter (DSC) at heating rates spanning over a decade. They revealed the existence of two Arrhenius processes, with their relative presence, nucleationbarrier energies, and validity-timescales suggesting that both intra- and inter-domain texturalorderings undergo de-structuring. In the stoichiometric Ni2MnGa, a single low-energy barrier ( ) fast kinetics observed may be attributed wholly to the short-distance textural order-disorder (a near absence of bigger, inter-domain interactions). On the other hand, two distinct Arrhenicities are found in equal strength in NITINOL (NiTi) and Ni47Fe3Ti50, and in unequal proportion in Ni2.26Mn0.74Ga, over the full range of temperature scanning rates covered (q= 2.5 to 50°C/min). The relatively fast nucleation-driven growth dominates higher T-scanning rates, with lower barrier activation (qhi) (albeit > , due to a change in the twins’ character). Another kinetics with higher barrier energy (qlo) manifests at slow heatings. The crossover in Ni47Fe3Ti50 is interpreted as increase in the (disorder-induced) A-domain-size dispersion, which also causes a broadening of the transition. Parameters characterizing the kinetics of various specimens are examined; comparisons of the relative energy/time scales of inter- and intra-domain processes made, and their transition/crossover temperature discussed.

FIRST PRINCIPLES DENSITY FUNCTIONAL INVESTIGATION OF SUPPORTED TUNGSTEN CLUSTER (Wn; n = 1 TO 6) ON ANCHORED GRAPHITE (0001) SURFACE

Size-selected Wn clusters can be deposited firmly on a graphite (0001) surface using a novel technique, where the positive ions (of the same metal atom species) embedded on the graphite surface by ion implantation, act as anchors. The size selected metal clusters can then soft land on this anchored surface m [Hayakawa et al., 2009]. We have carried out a systematic theoretical study of the adsorption of Wn (n = 1-6) clusters on anchored graphite (0001) surface, using state-of-art spin-polarized density functional approach. In our first-principles calculations, the graphite (0001) surface has been suitably modeled as a slab separated by large vacuum layers. Wn clusters bond on clean graphite (0001) surface with a rather weak Van-der-Waals interaction. However, on the anchored graphite (0001) surface, the Wn clusters get absorbed at the defect site with a much larger adsorption energy. We report here the results of our first-principles investigation of this supported Wn cluster system, along with their reactivity trend as a function of the cluster size (n).

Domain Structures across the Martensitic Transformation in Ni2+xMn1-xGa

Evolution of domain structures across the martensitic transition (Tm) in the ferromagnetic shape memory alloy system Ni-Mn-Ga is studied using an optical microscope with a temperature variation. Compositions chosen have Tm < Tc, Tm = Tc and Tm > Tc, (Tc=Curie temperature) so that one can compare the nature of martensitic domains. There are no appreciable domain structures when Tm < Tc as compared to the one with Tm > Tc. However, giant morphological changes in the form of appearance of well-developed domains that are propagating with different directions are seen for the composition in which Tm=Tc. The results are discussed in the light of Magnetic Force Microscopy observations as well as giant entropy changes known to occur on samples with co-occurrence of Tm and Tc.