L. S. Sharath Chandra

Elevating the temperature regime of the large magnetocaloric effect in a Ni–Mn–In alloy towards room temperature

We have prepared a Ni50(Mn,2%Cr)34In16 alloy by substituting Mn by Cr in a Ni50Mn34In16 alloy. A large isothermal magnetic entropy change is observed across the austenite?martensite phase transition in this new alloy for a moderate field change of 50?kOe. In comparison with the parent Ni50Mn34In16 alloy where the peak in isothermal entropy change amounts to nearly 19?J?kg?1?K?1, in this Ni50(Mn,2%Cr)34In16 alloy the peak value of the isothermal entropy change is much larger, 24.4?J?kg?1?K?1. Moreover, this large magnetocaloric effect in the Ni50(Mn,2%Cr)34In16 alloy occurs at ambient temperature near 294?K as compared with the temperature regime of the magnetocaloric effect being near 240?K in the parent Ni50Mn34In16 alloy. Thus the partial substitution of Mn by Cr in the Ni50Mn34In16 alloy is shown to be an effective method for enhancing the potential of the alloy system for near-room temperature applications.

Electrical and magnetic transport properties of Fe3O4 thin films on a GaAs(100) substrate

Thin films of magnetite (Fe3O4) are grown on single crystal GaAs (100) substrate by pulsed laser deposition. X ray diffraction (XRD) result shows the (111) preferred orientation of the Fe3O4 film and x-ray photoelectron spectroscopy confirm the presence of single phase Fe3O4 in the film. The electrical transport property of the film shows the characteristic Verwey transition at 122 K and below 110 K, the transport follows variable range hopping type conduction mechanism. The film shows room temperature magnetization hysteresis loop suggesting the ferrimagnetic behavior of the film with saturation magnetization value close to 470 emu/cc.

Enhanced critical parameters of nanocarbon doped MgB2 superconductor

The high field magnetization and magneto transport measurements are carried out to determine the critical superconducting parameters of MgB2-xCx system. The synthesized samples are pure phase and the lattice parameters evaluation is carried out using the Rietveld refinement. The R-T(H) measurements are done up to a field of 140 kOe. The upper critical field values, Hc2 are obtained from this data based upon the criterion of 90% of normal resistivity i.e. Hc2=H at which Rho=90%Rho; where RhoN is the normal resistivity i.e., resistivity at about 40 K in our case. The Werthamer-Helfand-Hohenberg (WHH) prediction of Hc(0) underestimates the critical field value even below than the field up to which measurement is carried out. After this the model, the Ginzburg Landau theory (GL equation) is applied to the R-T(H) data which not only calculates the Hc2(0) value but also determines the dependence of Hc2 on temperature in the low temperature high field region. The estimated Hc(0)=157.2 kOe for pure MgB2 is profoundly enhanced to 297.5 kOe for the x=0.15 sample in MgB2-xCx series. Magnetization measurements are done up to 120 kOe at different temperatures and the other parameters like irreversibility field, Hirr and critical current density Jc(H) are also calculated. The nano carbon doping results in substantial enhancement of critical parameters like Hc2, Hirr and Jc(H) in comparison to the pure MgB2 sample.

Simple and precise thermoelectric power measurement setup for different environments.

We report here on a simple but precise thermoelectric power measurement setup that can be adapted for different environments. This setup has been extensively used for cryogen-free environment to measure a variety of samples. It is made simple to load and hold the sample between two copper blocks by a spring-shaft arrangement. The usable range of measurements is a few microV/K to a few hundreds of microV/K. The salient features of the setup in achieving good precision both in natural warm-up/cooldown and controlled measurements are (i) the continuous DeltaT control across the sample by a chromel-AuFe(0.07%) thermocouple and (ii) the measurements of emf generated across the sample and the thermocouple using a nanovoltmeter and a scanner system. The versatile nature of the setup is further demonstrated by employing it in a magnetic field environment up to 140 kOe. The precision achieved using this system is highlighted for a few systems of current interest.

Do the grain boundaries of β-In2S3 thin films have a role in sub-band-gap photosensitivity to 632.8nm?

Highly photoconducting β-In2S3 thin films with conducting grain boundaries were obtained, using “chemical spray pyrolysis” technique. By varying the atomic ratio of the precursor solution used for spray pyrolysis, the photoconductivity of these films could be tailored. Conducting grain boundaries were found only for samples with a specific stoichiometry and these films exhibited photoresponse to intrinsic and extrinsic excitation wavelengths in the range of 325–532nm. Postdeposition vacuum annealing of these films enhanced the grain boundary conductivity, caused the films to exhibit persistent photoconductivity for both intrinsic and extrinsic excitations and extended the extrinsic photoresponse to wavelengths beyond 632.8nm. Photoresponse to excitation wavelength of 632.8nm was observed in films with and without conducting grain boundaries which proved that the extrinsic photoresponse to this wavelength was an effect associated with the defect chemistry of the β-In2S3.

Resistive broadening in sulfur doped FeTe

An analysis of resistive broadening in the presence of magnetic fields up to 14 T for sulfur doped FeTe superconductors is presented. FeTe shows an abrupt change in resistivity at 70 K due to a structural transition. Vanishing of the structural transition and the appearance of superconductivity at ~ 10 K and 9.7 K are seen in FeTe0.9S0.1 and FeTe0.8S0.2 respectively. The upper critical field and coherence lengths are estimated using the Werthamer–Helfand–Hohenberg and Ginzburg–Landau theories for different criteria for the transition temperatures. The estimated activation energy for thermally activated flux flow (TAFF) is an order of magnitude smaller than for the rare earth (R) based RFeAsO1 − xFx system, which indicates weaker pinning than for the RFeAsO1 − xFx system. The flux flow activation energy shows power law behavior with the two different exponents for fields above and below H = 6 T for FeTe0.9S0.1 and H = 8 T for FeTe0.8S0.2. The fluctuation conductivity is analyzed using Aslamazov–Larkin theory and lowest Landau level (LLL) theory, respectively, for zero and nonzero magnetic fields. Incidentally, the field above which 2D LLL scaling is observed in these systems coincides with the crossover field observed in TAFF resistivity.

Magnetic irreversibility and pinning force density in the Ti-V alloys

We present a study of critical current density estimated through dc magnetization measurements in the superconducting alloys Ti60V40 and Ti70V30. The magnetization is irreversible below the irreversibility field (BIrr), which is different from the upper critical field for the alloys. Additionally, the alloys are found to exhibit a peak effect in magnetization below the upper critical field. The critical current densities of the alloys estimated from the magnetization results decrease strongly with increasing magnetic field. The pinning force density follows a universal scaling relation with respect to the magnetic field divided by the BIrr. The field dependence of the pinning force density is analyzed in terms of the size of the grains of the main β phase, the possible presence of dislocation arrays within the grains of the main phase, the presence of additional metallurgical phases, and the configuration of the grain boundaries in the system. The temperature dependence of critical current density is also...

Impurity band conduction in FeSi1−xAlx

Precise resistivity and thermopower measurements across the metal insulator transition in Al doped correlated semiconductor FeSi are reported. Doping of carriers results in the emergence of electronic states at the Fermi level. For sufficient concentration of dopant, these states form into an impurity band. The properties of such systems are governed by this impurity band at low temperatures and the semiconducting bands at higher temperatures. Here we show the applicability of such a two-band structure to account for the temperature dependence of resistivity and thermopower in metals near the metal insulator transition.

High field paramagnetic effect in the superconducting state of Ti0.8V0.2 alloy

We report on the superconducting state of Ti0.8V0.2 alloy, highlighting an anomalous magnetic response in the flux-line lattice or the mixed state. The value of magnetization while cooling down the alloy in the presence of high magnetic field is lower than that obtained while warming it up from the lowest temperature in the presence of the same magnetic field. This is just the opposite to the thermo-magnetic hysteresis observed due to homogeneous flux-line pinning. This anomalous effect appears below a characteristic temperature that shifts towards a lower temperature with the increase in applied magnetic field. The value of magnetization measured at a constant temperature after cooling down the sample in the presence of magnetic field increases appreciably with time in the temperature–field domain where this anomalous effect is observed. These results indicate that the present Ti0.8V0.2 alloy exhibits a high field paramagnetic effect resulting from inhomogeneous flux pinning. Optical metallography and x-ray diffraction measurements show the formation of stress induced martensitic phase in the alloy, which could result in the inhomogeneous flux pinning. The temperature dependence of magnetization after annealing the sample after mechanical processing showed the usual properties of a type-II superconductor. This supports the argument that the inhomogeneous distribution of the stress induced martensitic phase is the reason for the existence of the high field paramagnetic effect in Ti0.8V0.2 alloy.

Study of the dynamical features of the austenite-martensite phase transition in the Ni50(Mn, 1%Fe)34In16 alloy using scanning Hall probe imaging

We have performed scanning Hall probe imaging experiments to study the martensite to austenite phase transition in the Ni50(Mn, 1%Fe)34In16 alloy as a function of temperature and magnetic field. We observe that the martensite and austenite phase regions are separated by a distinct interface. The relative growth of phase across the phase transition is associated with the movement of this interface. The movement of the interface becomes arrested at low temperature, which leads to the formation of a “magnetic glass” state in the alloy. The dynamics of the martensite to austenite phase transition in the Ni50(Mn, 1%Fe)34In16 alloy is found to be qualitatively different when the transition is field induced than what it is when the same transition is induced by temperature. While both nucleation and growth of the martensite phase is observed during the austenite to martensite phase transition in the alloy during cooling down, the martensite to austenite phase transition during warming up appears to be growth ori...