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

Static and dynamic magnetic properties of normal spinel Co₂RuO₄= (Co²⁺)A[Co3+Ru3+]BO4are 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 rangeT= 2 K-400 K and H up to 90 kOe. These investigations show that Co₂RuO₄exhibits an antiferromagnetic (AFM) transition atT_N∼ 15.2 K, along with a spin-glass state at slightly lower temperature (T_SG) near 14.2 K. It is argued thatT_Nis mainly governed by the ordering of the spins of Co²⁺ions occupying theA-site, whereas the exchange interaction between the Co²⁺ions on theA-site and randomly distributed Ru³⁺on theB-site triggers the spin-glass phase, Co³⁺ions on theB-site being in the low-spin non-magnetic state. Analysis of measurements ofM(H,T) forT<T_Nare used to construct theH-Tphase diagram showing thatT_SGshifts to lowerTvarying as H^2/3.2expected for spin-glass state whereasT_Nis nearlyH-independent. ForT>T_N, analysis of the paramagnetic susceptibility (χ) vs.Tdata are fit to the modified Curie-Weiss law,χ=χ₀+C/(T+θ), withχ₀= 0.0015 emu mol^-1Oe^-1yieldingθ= 53 K andC= 2.16 emu-K mol^-1Oe^-1, the later yielding an effective magnetic momentμ_eff= 4.16μ_Bcomparable to the expected value ofμ_eff= 4.24μ_Bper Co₂RuO₄. UsingT_N,θand high temperature series forχ, dominant exchange constantJ₁/k_B∼ 6 K between the Co²⁺on theA-sites is estimated. Analysis of the ac magnetic susceptibilities nearT_SGyields the dynamical critical exponentzν= 5.2 and microscopic spin relaxation timeτ₀∼ 1.16 × 10^-10sec characteristic of cluster spin-glasses and the observed time-dependence ofM(t) is supportive of the spin-glass state. LargeM-Hloop 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 belowT_Nis interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) forT>T_Nto inverse MCE forT<T_Nobserved in Co₂RuO₄(and reported previously in other systems also) is related to the change in sign of (∂M/∂T) vs.Tdata.