Antiferromagnetism, spin-glass state, H-T phase diagram, and inverse magnetocaloric effect in Co2RuO4.

Static and dynamic magnetic properties of normal spinel Co2RuO4 = (Co2+)[Formula: see text] are reported based on our investigations of the temperature (T), magnetic field (H) and frequency (f) dependence of the ac-magnetic susceptibilities and dc-magnetization (M) covering the temperature range T = 2 K-400 K and H up to 90 kOe. These investigations show that Co2RuO4 exhibits an antiferromagnetic (AFM) transition at T N ∼ 15.2 K, along with a spin-glass state at slightly lower temperature (T SG) near 14.2 K. It is argued that T N is mainly governed by the ordering of the spins of Co2+ ions occupying the A-site, whereas the exchange interaction between the Co2+ ions on the A-site and randomly distributed Ru3+ on the B-site triggers the spin-glass phase, Co3+ ions on the B-site being in the low-spin non-magnetic state. Analysis of measurements of M (H, T) for T < T N are used to construct the H-T phase diagram showing that T SG shifts to lower T varying as H2/3.2 expected for spin-glass state whereas T N is nearly H-independent. For T > T N, analysis of the paramagnetic susceptibility (χ) vs. T data are fit to the modified Curie-Weiss law, χ = χ 0 + C/(T + θ), with χ 0 = 0.0015 emu mol-1Oe-1 yielding θ = 53 K and C = 2.16 emu-K mol-1Oe-1, the later yielding an effective magnetic moment µ eff = 4.16 µ B comparable to the expected value of µ eff = 4.24 µ B per Co2RuO4. Using T N, θ and high temperature series for χ, dominant exchange constant J 1/k B ∼ 6 K between the Co2+ on the A-sites is estimated. Analysis of the ac magnetic susceptibilities near T SG yields the dynamical critical exponent zν = 5.2 and microscopic spin relaxation time τ 0 ∼ 1.16 × 10-10 sec characteristic of cluster spin-glasses and the observed time-dependence of M(t) is supportive of the spin-glass state. Large M-H loop asymmetry at low temperatures with giant exchange bias effect (H EB ∼ 1.8 kOe) and coercivity (H C ∼ 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed below T N is interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) for T > T N to inverse MCE for T < T N observed in Co2RuO4 (and reported previously in other systems also) is related to the change in sign of (∂M/∂T) vs. T data.

Strong interfacial polarization in ZnO decorated reduced-graphene oxide synthesized by molecular level mixing.

Globally, there is a great demand for energy storage materials and devices. In this context, charge storage capacitors are of great prominence. Metal oxide-graphene composites are excellent candidates for charge storage materials. This is because the dielectric properties of these composites can be controlled by the nature, dimensions and spatial distribution of the conductive components in these composites. ZnO decorated reduced-graphene oxide (r-GO) is synthesized and studied in this context. ZnO-r-GO composites are synthesized using molecular-level mixing. The composites are named as ZnO-0.1G, ZnO-0.2G and ZnO-0.3G in the order of increasing r-GO content. At 1 kHz, the dielectric permittivity (ε') values of ZnO-0.1G, ZnO-0.2G and ZnO-0.3G are nearly 11 (ε' = 114), 15 (ε' = 153) and 40 (ε' = 400) times greater than that of ZnO (ε' = 10). The strong interfacial polarization (Maxwell-Wagner polarization) in these composites is attributed to the presence of functional groups (which are polar in nature) on the r-GO sheets and also to the presence of lattice and/or topological defects in the r-GO. Temperature dependent electric modulus (M'') studies further confirm the observed interfacial polarization.