In this paper, we report the signature of spin gapless semiconductor (SGS) in CoFeMnSi that belongs to the Heusler family. SGS is a new class of magnetic semiconductors which have a band gap for one spin subband and zero band gap for the other, and thus are useful for tunable spin transport based applications. We show various experimental evidences for SGS behavior in CoFeMnSi by carefully carrying out the transport and spin-polarization measurements. SGS behavior is also confirmed by first-principles band-structure calculations. The most stable configuration obtained by the theoretical calculation is verified by experiment. The alloy is found to crystallize in the cubic Heusler structure (LiMgPdSn type) with some amount of disorder and has a saturation magnetization of 3.7 μ B /f.u. and Curie temperature of ∼620 K. The saturation magnetization is found to follow the Slater-Pauling behavior, one of the prerequisites for SGS. Nearly-temperature-independent carrier concentration and electrical conductivity are observed from 5 to 300 K. An anomalous Hall coefficient of 162 S/cm is obtained at 5 K. Point contact Andreev reflection data have yielded the current spin-polarization value of 0.64, which is found to be robust against the structural disorder. All these properties strongly suggest SGS nature of the alloy, which is quite promising for the spintronic applications such as spin injection as it can bridge the gap between the contrasting behaviors of half-metallic ferromagnets and semiconductors.

We report the structure, magnetic property and spin polarization of CoFeMnGe equiatomic quaternary Heusler alloy. The alloy was found to exist in the L2 1 structure with considerable amount of DO 3 disorder. Thermal analysis result indicated the Curie temperature is about 711K without any other phase transformation up to melting temperature. The magnetization value was close to that predicted by the Slater-Pauling curve. Current spin polarization of P = 0.70 ± 0.1 was deduced using point contact Andreev reflection (PCAR) measurements. Halfmetallic trend in the resistivity has also been observed in the temperature range of 5 K to 300 K. Considering the high spin polarization and Curie temperature, this material appears to be promising for spintronic applications.Key words: Heusler alloy, Mössbauer spectroscopy, Slater-Pauling rule, Spin Polarization, Half-metal. PACS: 75.50.Bp, 75.30.Cr, 75.47.Np, 76.80.+y, 75.76 The polycrystalline bulk sample of an equiatomic CoFeMnGe (CFMG) alloy was prepared by arc melting of stoichiometric quantities of constituent elements in an inert atmosphere. The ingot was melted several times to increase the chemical homogeneity and the final weight loss was less than 1%. To increase the homogeneity, the as-cast alloy was annealed under vacuum for 14 days at 1073 K and then quenched in cold water. The crystal structure was investigated by x-ray diffraction with Cu K α radiation at room temperature. 57 FeMössbauer spectra at room temperature were recorded using a constant acceleration spectrometer with 25 mCi 57 Co(Rh) radioactive source. The spectrometer was calibrated by using natural iron foil of 25 µm before measuring the sample. The obtained spectra were analyzed using PCMOS-II least-squares fitting program. Curie temperature and the structural transition temperature (if any) were probed by using the differential thermal analysis (DTA)with a heating rate of 20 K min -1 . Magnetization measurements at 3 K and 300 K were performed by using a vibrating sample magnetometer (VSM) attached to a physical property measurement system (PPMS, Quantum design). Current spin polarization measurements were done by using the point contact Andreev reflection (PCAR) technique. 19 Sharp Nb tips prepared by electrochemical polishing were used to make point contacts with the sample.Spin polarization of the conduction electrons was obtained by fitting the normalized conductance G(V)/G n curves to the modified Blonder-Tinkham-Klapwijk (BTK) model. 20 A multiple parameter least squares fitting was carried out to deduce spin polarization (P) using dimensionless interfacial scattering parameter (Z), superconducting band gap (∆) and P as variables. Fig. 1 shows the Rietveld refinement of powder XRD pattern recorded at room temperature. The lattice parameter of the alloy was found to be 5.75 Å, which is in agreement with the earlier reports on CFMG. 14 As one can see, superlattice reflections, (111) and (200), are very weak for this alloy. When Z element is from the same period of the periodic system...

Bismuth-modified manganese dioxide (BMD) cathodes are shown to exhibit good cycling characteristics with a theoretical two-electron capacity in rechargeable alkaline cells. With an aim to understand the discharge-charge mechanisms, the BMD cathodes are characterized by X-ray diffraction, scanning electron microscopy, and wet-chemical analysis at various levels of discharge and charge during the first two cycles and after various numbers of cycles. It is found that a well-ordered, crystalline birnessite MnO2 is formed at the end of first charge, irrespective of the initial form of the manganese oxide. The discharge-charge mechanism involves a reversible conversion of birnessite MnO2 to MnOOH to normalMnfalse(OH)2 in the subsequent cycles. Wet-chemical analyses demonstrates for the first time that the discharge/charge process in rechargeable alkaline cells involves a reversible dissolution/incorporation of K+ ions from/into the cathode lattice into/from the electrolyte. The incorporation of the K+ ions into the lattice appears to stabilize a well-ordered birnessite structure during charge. © 2002 The Electrochemical Society. All rights reserved.

Magnetic anomalies corresponding to the Verwey transition and reorientation of anisotropic vacancies are observed at 151 K and 306 K, respectively, in NiCoFe2O4 nanoparticles (NPs) synthesized by a modified-solvothermal method followed by annealing. Cationic disorder and spherical shape induced non-stoichiometry suppress the Verwey transition in the as-synthesized NPs. On the other hand, reorientation of anisotropic vacancies is quite robust. XRD and electron microscopy investigations confirm a single phase spinel structure and the surface morphology of the as-synthesized NPs changes from spherical to octahedral upon annealing. Rietveld analysis reveals that the Ni(2+) ions migrate from tetrahedral (A) to octahedral (B) sites upon annealing. The Mössbauer results show canted spins in both the NPs and the strength of superexchange is stronger in Co-O-Fe than Ni-O-Fe. Magnetic force images show that the as-synthesised NPs are single-domain whereas the annealed NPs are multi-domain octahedral particles. The FMR study reveals that both the NPs have a broad FMR line-width; and resonance properties are consistent with the random anisotropy model. The broad inhomogeneous FMR line-width, observation of the Verwey transition, tuning of the magnetic domain structure as well as the magnetic properties suggest that the NiCoFe2O4 ferrite NPs may be promising for future generation spintronics, magneto-electronics, and ultra-high-density recording media as well as for radar absorbing applications.

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