A. K. Rastogi

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.

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.

Peculiar magnetism of transition metal cluster compounds

We discuss a class of solids which have in their structure fairly separated tetrahederal clusters of transition metals: vanadium and molybednum. With decreasing temperature these compounds undergo a structural transition from cubic to rhombohederal phase at a temperature Ts, followed by a transition to a ferromagnetic phase at a lower temperarure Tc. Magnetic measurements indicate the presence of moments of spin half per cluster. Between Tc and Ts susceptibility shows large departures from Curie-Weiss behavior and specific heat has an unusually large non-phononic contribution. We present a model in which each cluster acts as a unit associated with a magnetic moment of spin half and a structural degree of freedom related to distortions of the tetrahedron. Our model accounts for all the observed features including effects due to external pressure and magnetic field.

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.

Correlation between magnetoresistivity and magnetic entropy change in the 3D-Heisenberg ferromagnet CuCr2Te4

We report a correlation between magnetoresistivity [Δρmag = ρmag(H,T) − ρmag(0,T)] and magnetic entropy change [ΔSM = SM(H,T) − SM(0,T)] in the metallic spinel CuCr2Te4 which orders ferromagnetically below TC = 320 K. We find that Δρmag = KΔSM for several temperatures around TC with a temperature independent K, which suggests that magnetoresistivity arising from the suppression of spin disorder scattering scales with ΔSM. However, unlike expected theoretically, a clear correlation is not obtained between Δρmag and square of the reduced magnetization (M/Msat)2, where Msat is saturation magnetization at 0 K. A maximum magnetoresistance of −3.56% for μ0ΔH = 5 T and magnetic entropy change of −1.57 J/kg K occur at TC, which are typical values for itinerant ferromagnets. Critical exponent analysis using modified Arrott plots yield β=0.371±0.005 and γ=1.312±0.02 that are close to the 3D-Heisenberg (β=0.365,γ=1.336, and δ=4.66) model.

High field Mössbauer measurements on CeFe2 and Ce(Fe0.965 Al0.035)2

Magnetization and high field Mössbauer experiments were performed to study the ground state and the metamagnetic transition in Ce(Fe, Al)2. A model is proposed for the arrangement of the Fe-moments which includes the reported finite Ce-moment.

A fluctuating state and the critical behavior of the depinning transition in driven vortex matter in superconductors

A status report is presented on explorations in 2H-NbS2 which have revealed a new protocol that generates vortex states corresponding to two limiting ends of critical current density (Jlc(H) and Jhc (H)) for a given vortex density deep in the mixed state. An ordered vortex array set in motion with transport current I (Ilc < I < Ihc) explores its other option of non-moving jammed state of higher end Ihc while steadily enhancing the driving force. Attempts to further unjam the re-pinned state unmask (i) a moving fluctuating state and (ii) the diverging (life) times along with an underlying critical behaviour at the threshold limit of de-pinning the disordered-jammed state, from both below as well as above Ihc.

Electronic Transport Properties of Layered Antiferromagnet CuCrS2

Antiferromagnetic layered compound CuCrS2 has been prepared in single phase. Structural details with x‐ray diffraction and electronic transport properties from 15 to 300 K have been studied. X‐ray diffraction with reitveld refinement gave significant concentration of Cr occupying inter‐layer octahedral site. Antiferromagnetism is marginally affected by chromium disorder. The electrical resistivity in polycrystalline pellet shows non‐metallic non‐insulating behavior while the single crystal flake is metallic. We interpret the temperature dependence of electrical resistivity and thermoelectric power due to the localization of carriers and the formation of magnetic polarons in the paramagnetic phase of CuCrS2 due to the Cr disorder.