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

Static and dynamic magnetic properties of normal spinel Co₂RuO₄ = (Co²⁺)${}_A{\left[{Co}^{3+}{\text{Ru}}^{3+}\right]}_B{O}_4$ 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 H^2/3.2 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. (paper)