Antiferromagnetism, spin-glass state, H–T phase diagram, and inverse magnetocaloric effect in Co ₂ RuO ₄

Abstract

Static and dynamic magnetic properties of normal spinel Co₂RuO₄= (Co²⁺)A[Co3+Ru3+]BO4are reported based on our investigations of the temperature (<i>T</i>), magnetic field (<i>H</i>) and frequency (<i>f</i>) dependence of the ac-magnetic susceptibilities and dc-magnetization (<i>M</i>) covering the temperature range<i>T</i>= 2 K-400 K and H up to 90 kOe. These investigations show that Co₂RuO₄exhibits an antiferromagnetic (AFM) transition at<i>T</i>_N∼ 15.2 K, along with a spin-glass state at slightly lower temperature (<i>T</i>_SG) near 14.2 K. It is argued that<i>T</i>_Nis mainly governed by the ordering of the spins of Co²⁺ions occupying the<i>A</i>-site, whereas the exchange interaction between the Co²⁺ions on the<i>A</i>-site and randomly distributed Ru³⁺on the<i>B</i>-site triggers the spin-glass phase, Co³⁺ions on the<i>B</i>-site being in the low-spin non-magnetic state. Analysis of measurements of<i>M</i>(<i>H</i>,<i>T</i>) for<i>T</i><<i>T</i>_Nare used to construct the<i>H</i>-<i>T</i>phase diagram showing that<i>T</i>_SGshifts to lower<i>T</i>varying as H^2/3.2expected for spin-glass state whereas<i>T</i>_Nis nearly<i>H</i>-independent. For<i>T</i>><i>T</i>_N, analysis of the paramagnetic susceptibility (<i>χ</i>) vs.<i>T</i>data are fit to the modified Curie-Weiss law,<i>χ</i>=<i>χ</i>₀+<i>C</i>/(<i>T</i>+<i>θ</i>), with<i>χ</i>₀= 0.0015 emu mol^-1Oe^-1yielding<i>θ</i>= 53 K and<i>C</i>= 2.16 emu-K mol^-1Oe^-1, the later yielding an effective magnetic moment<i>μ</i>_eff= 4.16<i>μ</i>_<i>B</i>comparable to the expected value of<i>μ</i>_eff= 4.24<i>μ</i>_<i>B</i>per Co₂RuO₄. Using<i>T</i>_N,<i>θ</i>and high temperature series for<i>χ</i>, dominant exchange constant<i>J</i>₁/<i>k</i>_<i>B</i>∼ 6 K between the Co²⁺on the<i>A</i>-sites is estimated. Analysis of the ac magnetic susceptibilities near<i>T</i>_SGyields the dynamical critical exponent<i>zν</i>= 5.2 and microscopic spin relaxation time<i>τ</i>₀∼ 1.16 × 10^-10sec characteristic of cluster spin-glasses and the observed time-dependence of<i>M</i>(<i>t</i>) is supportive of the spin-glass state. Large<i>M</i>-<i>H</i>loop asymmetry at low temperatures with giant exchange bias effect (<i>H</i>_EB∼ 1.8 kOe) and coercivity (<i>H</i>_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<i>T</i>_Nis interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) for<i>T</i>><i>T</i>_Nto inverse MCE for<i>T</i><<i>T</i>_Nobserved in Co₂RuO₄(and reported previously in other systems also) is related to the change in sign of (∂<i>M</i>/∂<i>T</i>) vs.<i>T</i>data.