T. S. Tripathi

Thermoelectric Properties of Layer-Antiferromagnet CuCrS2

The electrical, thermal conductivity and Seebeck coefficient of the quenched, annealed and slowly cooled phases of the layer compound CuCrS2 have been reported between 15K to 300K. We also confirm the antiferromagnetic transition at 40K in them by our magnetic measurements between 2K and 300K. The crystal flakes show a minimum around 100K in their in-plane resistance behavior. For the polycrystalline pellets the resistivity depends on their flaky texture and it attains at most 10 to 20 times of the room temperature value at the lowest temperature of measurement. The temperature dependence is complex and no definite activation energy of electronic conduction can be discerned. We find that the Seebeck coefficient is between 200-450 microV/K and is unusually large for the observed resistivity values of between 5-100 mOhm-cm at room temperature. The figure of merit ZT for the thermoelectric application is 2.3 for our quenched phases, which is much larger than 1 for useful materials. The thermal conductivity K is mostly due to lattice conduction and is reduced by the disorder in Cu- occupancy in our quenched phase. A dramatic reduction of electrical and thermal conductivity is found as the antiferromagnetic transition is approached from the paramagnetic region, and K subsequently rises in the ordered phase. We discuss the transport properties as being similar to a doped Kondo-insulator.

Magnetocaloric effect and magnetothermopower in the room temperature ferromagnet Pr0.6Sr0.4MnO3

We have investigated magnetization(M), magnetocaloric effect(MCE) and magnetothermopower(MTEP) in polycrystalline Pr0.6Sr0.4MnO3, which shows a second-order paramagnetic to ferromagnetic transition near room temperature (TC = 305 K). However, field-cooled M(T) within the long range ferromagnetic state shows an abrupt decrease at TS = 86 K for H < 3 T. The low temperature transition is first-order in nature as suggested by the hysteresis in M(T) and exothermic/endothermic peaks in differential thermal analysis for cooling and warming cycles. The anomaly at TS is attributed to a structural transition from orthorhombic to monoclinic phase. The magnetic entropy change is negative at TC but changes to positive at TS. Thermopower (Q) is negative from 350 K to 20 K, shows a rapid decrease at TC and a small cusp around TS in zero field. The MTEP reaches a maximum value of 25% for deltaH = 3 T around TC which is much higher than 15% dc magnetoresistance for the same field change. A linear relation between MTEP and magnetoresistance, and between delta Sm and Delta Q are found near TC. Further, ac magnetotransport in low dc magnetic fields (H less than or equal to 1 kOe), critical analysis of the paramagnetic to ferromagnetic transition and scaling behavior of the magnetic entropy change versus a reduced temperature under different magnetic fields are also reported.

An experimental setup for the simultaneous measurement of thermoelectric power of two samples from 77 K to 500 K

We report on an experimental setup for the simultaneous measurement of the thermoelectric power (TEP) of two samples in the temperature range from 77 K to 500 K using optimum electronic instruments. The setup consists of two rectangular copper bars in a bridge arrangement for sample mounting, two surface mount (SM) chip resistors for creating alternate temperature gradient, and a type E thermocouple in differential geometry for gradient temperature (ΔT) measurement across the samples. In addition, a diode arrangement has been made for the alternate heating of SM resistors using only one DC current source. The measurement accuracy of ΔT increases with the differential thermocouple arrangement. For the calibration of the setup, measurements of TEP on a high purity (99.99%) platinum wire and type K thermocouple wires Chromel and Alumel have been performed from 77 K to 500 K with respect to copper lead wires. Additionally, this setup can be utilized to calibrate an unknown sample against a sample of known absolute TEP.

Atomic layer deposition of transparent semiconducting oxide CuCrO2 thin films

Atomic layer deposition (ALD) is a vital gas-phase technique for atomic-level thickness-controlled deposition of high-quality thin films on various substrate morphologies owing to its self-limiting gas–surface reaction mechanism. Here we report the ALD fabrication of thin films of the semiconducting CuCrO2 oxide that is a highly prospective candidate for transparent electronics applications. In our process, copper 2,2,6,6-tetramethyl-3,5-heptanedionate (Cu(thd)2) and chromium acetyl acetonate (Cr(acac)3) are used as the metal precursors and ozone as the oxygen source. Smooth and homogeneous thin films with an accurately controlled metal composition can be deposited in the temperature range of 240–270 °C; a post-deposition anneal at 700–950 °C in an Ar atmosphere then results in well crystalline films with a delafossite structure. Electrical transport measurements confirm the p-type semiconducting behavior of the films. The direct bandgap is determined from UV-vis spectrophotometric measurements to be 3.09 eV. The observed transmittance is greater than 75% in the visible range.

Magnetic entropy change and critical exponents in double perovskite Y2NiMnO6

Abstract We report the magnetic entropy change ( Δ S M ) and the critical exponents in the double perovskite manganite Y 2 NiMnO 6 with a ferromagnetic to paramagnetic transition T C ~ 85 K . For a magnetic field change Δ H = 80 kOe , a maximum magnetic entropy change Δ S M = − 6.57 J / kg K is recorded around TC. The critical exponents β = 0.363 ± 0.05 and γ=1.331±0.09 obtained from power law fitting to spontaneous magnetization MS(T) and the inverse initial susceptibility χ 0 − 1 ( T ) satisfy well to values derived for a 3D-Heisenberg ferromagnet. The critical exponent δ = 4.761 ± 0.129 is determined from the isothermal magnetization at TC. The scaling exponents corresponding to second order phase transition are consistent with the exponents from Kouvel–Fisher analysis and satisfy Widom׳s scaling relation δ = 1 + ( γ / β ) . Additionally, they also satisfy the single scaling equation M ( H , ϵ ) = ϵ β f ± ( H / ϵ β + γ ) according to which the magnetization-field-temperature data around TC should collapse into two curves for temperatures below and above TC.

Enhancement of thermoelectric power of PbTe:Ag nanocomposite thin films

The present study focuses on the enhancement of thermoelectric power of PbTe:Ag nanocomposite thin films, synthesized by the thermal evaporation technique. Thermoelectric measurements were carried out from room temperature to 400 K. It is observed that Ag addition improves the thermoelectric power and crystalline nature of the PbTe thin films. Synchrotron based X-ray diffraction was performed to confirm the phases of the Pb–Ag–Te alloy. This was further reconfirmed by X-ray photoelectron spectroscopy (XPS) and showed the precipitation of Pb on the surface of the PbTe:Ag films. The enhancement of thermoelectric power is thus attributed to the formation of Ag2−xTe alloy and the precipitation of Pb nanostructures on the surface. The origin of such enhancement is understood based on the phenomenon of energy dependent filtering of charge carriers.

Effect of chromium disorder on the thermoelectric properties of layered-antiferromagnet CuCrS2

Abstract Layered-antiferromagnetic compound CuCrS2 has been prepared by different methods. The analysis of X-ray diffraction patterns of different samples gave significant amount of vacancy-disorder of Cr-atoms within the layers. Extended period of sintering above 900 °C increases the transfer of Cr-atoms to the interstitial sites between the layers. This disorder has marginal effect on the Antiferromagnetic properties. The electrical conductivity is increased and the thermoelectric power remains positive and quite high between 150–400 μV/K in the paramagnetic state around room temperature with increase in disorder in different samples. We interpret the temperature dependence of electrical resistivity and thermoelectric power due to the localization of carriers by interstitial defects and the formation of magnetic polarons in the paramagnetic phase of CuCrS2.

Anomalous thickness-dependent optical energy gap of ALD-grown ultra-thin CuO films

Usually an inverse square relation between the optical energy gap and the size of crystallites is observed for semiconducting materials due to the strong quantum localization effect. Coulomb attraction that may lead to a proportional dependence is often ignored or considered less important to the optical energy gap when the crystallite size or the thickness of a thin film changes. Here we report a proportional dependence between the optical energy gap and the thickness of ALD-grown CuO thin films due to a strong Coulomb attraction. The ultrathin films deposited in the thickness range of 9-81 nm show a p-type semiconducting behavior when analyzed by Seebeck coefficient and electrical resistivity measurements. The indirect optical energy gap nature of the films is verified from UV-vis spectrophotometric measurements. A progressive increase in the indirect optical energy gap from 1.06 to 1.24 eV is observed with the increase in the thickness of the films. The data are analyzed in the presence of Coulomb attractions using the Brus model. The optical energy gap when plotted against the cubic root of the thickness of the films shows a linear dependence.

The study of magnon-drag effects in the thermopower of CuCr2X4 (X = S, Se and Te)

The electrical resistivity ρ and thermopower S of metallic ferromagnets CuCr2X4 (X=S,Se or Te) are measured at low temperatures and in an external magnetic field of 1.5 Tesla. These properties are dominated by strong scattering effects in their ferromagnetic phase, resulting in (a) ρ(T) ~A T2dependence with very large value of coefficient A ≈ 1 − 3×10−8 Ω-cm/K2 in resistivity and (b) a peak in the thermopower around TC/3 . Thermopower is further increased by crystalline order and by the alignment of magnetic domains in an external magnetic field as has been found for Cu1+x Cr2Te4. We have interpreted these effects as due to strongly coherent momentum conserving electron-magnon scattering giving a large T2 – contribution in resistivity and a large drag effect in thermopower.

Phase evolution and electrical properties of Co–Sb alloys fabricated from Co/Sb bilayers by thermal annealing and ion beam mixing

An investigation was carried out to understand the phase evolution and study the structural, morphological, optical and electrical properties of Co-Sb alloys fabricated by two different approaches: (a) thermal annealing and (b) ion-beam mixing followed by post annealing. The as-deposited and 100 MeV Ag ion beam irradiated Co/Sb bilayer thin films were subjected to thermal annealing from 200 to 400 °C for 1 hour. The Rutherford backscattering spectrometry (RBS) results showed partial mixing for the thermally annealed films and complete mixing for the irradiated and post annealed films at 400 °C. The XRD and RAMAN measurements indicated the formation of Co-Sb alloy, with ∼70% concentration of CoSb3 phase in the irradiated post annealed sample at 400 °C. The band gaps of the annealed and post irradiated annealed Co-Sb alloys were determined using UV-visible spectroscopy. Electrical and thermoelectric power measurements were performed in the temperature range of 300-420 K. It was observed that the alloys formed by ion-beam induced mixing exhibited higher electrical conductivity and thermoelectric power than the as-deposited and thermally annealed Co/Sb bilayer thin films.