Large negative magnetoresistance in a ferromagnetic shape memory alloy : Ni_{2+x}Mn_{1-x}Ga

5% negative magnetoresistance (MR) at room temperature has been observed in bulk Ni_{2+x}Mn_{1-x}Ga. This indicates the possibility of using Ni_{2+x}Mn_{1-x}Ga as magnetic sensors. We have measured MR in the ferromagnetic state for different compositions (x=0-0.2) in the austenitic, pre-martensitic and martensitic phases. MR is found to increase with x. While MR for x=0 varies almost linearly in the austenitic and pre-martensitic phases, in the martensitic phase it shows a cusp-like shape. This has been explained by the changes in twin and domain structures in the martensitic phase. In the austenitic phase, which does not have twin structure, MR agrees with theory based on s-d scattering model.Comment: 3 pages, 3 figures, Appl. Phys. Lett 86, 202508 (2005

Electronic excitations on metal surfaces and nano-structures

Collective electron excitations on a free-electron-like metal surface like Al have been studied by X-ray photoemission spectroscopy. In this review, we discuss photoemission studies on quantization of electron states due to confinement in nano-structures like epitaxial Na films on Al(111). The electronic structure of Ar nanobubbles embedded in the sub-surface region of Al has been discussed. For Al, an asymmetric line-shape is observed for both monopole surface and bulk plasmon in good agreement with theory. The relative contributions of the intrinsic, extrinsic and the interference processes to the surface plasmon intensity are determined from theoretical plasmon line-shape calcu lations and angle-dependent photoemission. The characteristics of the multipole plasmon mode are also discussed. Using angle resolved photoemission, Na thin films on Al(111) have been studied for different thicknesses. We find features in the valence band spectra that behave like quantum well resonances in a narrow photon energy range where the overlayer collective excitations are observed. These resonances are observed because of quantum confinement due to a potential step at the Na/Al interface and the dynamical enhancement of the electric field in the overlayer. In case of argon nano-bubbles in Al, we find that the Ar 2p binding energy and the Doniach-Sunjic asymmetry of the core level line shape vary systematically as functions of Ar+ implantation energy and number of ions bombarded (fluence). These observations are explained by relating the strength of Al conduction electron screening of the core -hole created in the photoemission final state to the size of the Ar nano-bubbles

Optimal operating conditions and characteristics of acetone/CaF_2 detector for inverse photoemission spectroscopy

Performance and characteristics of a band-pass photon detector using acetone gas and CaF_2 window (acetone/CaF_2) have been studied and compared with an ethanol/MgF_2 detector. The optimal operating conditions are found to be 4 mbar acetone pressure and 745+/-20 V anode voltage. The count rate obtained by us is about a factor of 3 higher than what has been reported earlier for the acetone detector. Unlike other gas filled detectors, this detector works in the proportional region with very small dead time (4 micro sec). A detector band-pass of 0.48+/-0.01 eV FWHM is obtained.Comment: Review of Scientific Instruments 76, 066102 (2005

Spectral functions in doped transition metal oxides

We present experimental photoemission and inverse photoemission spectra of SrTiO$_{3- \delta}$ representing electron doped $d^0$ systems. Photoemission spectra in presence of electron doping exhibit prominent features arising from electron correlation effects, while the inverse photoemssion spectra are dominated by spectral features explainable within single-particle approaches. We show that such a spectral evolution in chemically doped correlated systems is not compatible with expectations based on Hubbard or any other similar model. We present a new theoretical approach taking into account the inhomogeneity of the `real' system which gives qualitatively different results compared to standard `homogeneous' models and is in quantitative agreement with experiments.Comment: 10 pages; 1 tex file+4 postscript files (to appear in Europhysics Letters

Influence of Ni doping on the electronic structure of Ni_2MnGa

The modifications in the electronic structure of Ni_{2+x}Mn_{1-x}Ga by Ni doping have been studied using full potential linearized augmented plane wave method and ultra-violet photoemission spectroscopy. Ni 3d related electron states appear due to formation of Ni clusters. We show the possibility of changing the minority-spin DOS with Ni doping, while the majority-spin DOS remains almost unchanged. The total magnetic moment decreases with excess Ni. The total energy calculations corroborate the experimentally reported changes in the Curie temperature and the martensitic transition temperature with x.Comment: 4 pages, 4 figures, accepted in Phys. Rev.

Premartensite to martensite transition and its implications on the origin of modulation in Ni2MnGa ferromagnetic shape memory alloy

We present here results of temperature dependent high resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in Ni2MnGa. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase, and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval (~40K), discontinuous change in the unit cell volume as also in the modulation wave vector across the transition temperature and considerable thermal hysteresis in the characteristic transition temperatures. The temperature variation of the modulation wave vector q shows smooth analytic behaviour with no evidence for any devilish plateau corresponding to an intermediate or ground state commensurate lock-in phases. The existence of the incommensurate 7M like modulated structure down to 5K suggests that the incommensurate 7M like modulation is the ground state of Ni2MnGa and not the Bain distorted tetragonal L10 phase or any other lock-in phase with a commensurate modulation. These findings can be explained within the framework of the soft phonon model

Modulated structure in the martensite phase of Ni1.8Pt0.2MnGa: a neutron diffraction study

7M orthorhombic modulated structure in the martensite phase of Ni1.8Pt0.2MnGa is reported by powder neutron diffraction study, which indicates that it is likely to exhibit magnetic field induced strain. The change in the unit cell volume is less than 0.5% between the austenite and martensite phases, as expected for a volume conserving martensite transformation. The magnetic structure analysis shows that the magnetic moment in the martensite phase is higher compared to Ni2MnGa, which is in good agreement with magnetization measurement

Fabrication of an inverse photoemission spectrometer to study unoccupied electronic states

We discuss the performance of an inverse photoemission spectrometer that has been recently fabricated in our laboratory. The photon detector is of band-pass type with acetone gas filling and CaF2 window (acetone/ CaF2). We determine the optimal operating conditions of the detector to be 4 mbar acetone pressure and 745 ± 20 V anode voltage. At these operating conditions, the count rate is improved by a factor of three to what has been reported earlier by Funnemann and Merz. We show that unlike other gas-filled detectors, acetone/CaF2 detector works in the proportional region. Its dead time is negligible and addition of multiplier gas like argon worsens its performance. The performance of this detector has been compared with an ethanol/MgF2 detector. High count rate, reasonable resolution, negligible dead time, no requirement of multiplier or quench gas, satisfactory stability, and ease of handling make the acetone/CaF2 detector an attractive candidate for use in inverse photoemission spectroscopy (IPES). A low-energy electron gun of Stoffel-Johnson type has been fabricated and the beam current profile has been determined as a function of electron kinetic energy. The IPES spectra of polycrystalline Ag and Ta are shown and compared with data in the literature. By fitting the IPES Fermi edge of Ag, we find the overall resolution of the spectrometer to be 0.55 eV and the band-pass of acetone/CaF2 detector to be 0.48 eV FWHM

Collective and single-particle excitations in the photoyield spectrum of Al

Using angle- and energy-resolved photoyield spectroscopy, we investigate the properties of the multipole plasmon excitation. At higher energies, a systematic dependence of the photoyield on the photon angle of incidence is observed and explained on the basis of classical Fresnel theory, indicating the possibility of obtaining information about optical constants from such measurements. A feature above the multipole plasmon is assigned to the excitation of a bulk plasmon by the photon field