Antiferromagnetism, spin-glass state, H–T phase diagram, and inverse magnetocaloric effect in Co <sub>2</sub> RuO <sub>4</sub>
Sayandeep Ghosh, D. C. Joshi, P. Pramanik, Suchit Kumar Jena, Pittala Suresh, Tapati Sarkar, M. S. Seehra, Subhash Thota
Abstract
Abstract Static and dynamic magnetic properties of normal spinel Co 2 RuO 4 = (Co 2+ ) <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mi>A</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mrow> <mml:mo stretchy="false">[</mml:mo> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">o</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:msup> <mml:mrow> <mml:mtext>Ru</mml:mtext> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> </mml:mrow> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msub> </mml:math> 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 Co 2 RuO 4 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 Co 2+ ions occupying the A -site, whereas the exchange interaction between the Co 2+ ions on the A -site and randomly distributed Ru 3+ on the B -site triggers the spin-glass phase, Co 3+ 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 −1 Oe −1 yielding θ = 53 K and C = 2.16 emu-K mol −1 Oe −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 Co 2 RuO 4 . Using T N , θ and high temperature series for χ , dominant exchange constant J 1 / k B ∼ 6 K between the Co 2+ 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 effec