Ghosh, S and Joshi, DC and Pramanik, P and Jena, SK and Pittala, S and Sarkar, T and Seehra, MS and Thota, S
(2020)
*Antiferromagnetism, spin-glass state, H-T phase diagram, and inverse magnetocaloric effect in Co2RuO4.*
In: Journal of Physics Condensed Matter, 32
(48).

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## Abstract

Static and dynamic magnetic properties of normal spinel Co2RuO4 = (Co2+)A Co3+ Ru3+BO4 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 â�¼ 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 (Ï�) vs. T data are fit to the modified Curie-Weiss law, Ï� = Ï� 0 + C/(T + Î¸), with Ï� 0 = 0.0015 emu mol-1Oe-1 yielding Î¸ = 53 K and C = 2.16 emu-K mol-1Oe-1, the later yielding an effective magnetic moment Î¼ eff = 4.16 Î¼ B comparable to the expected value of Î¼ eff = 4.24 Î¼ B per Co2RuO4. Using T N, Î¸ and high temperature series for Ï�, dominant exchange constant J 1/k B â�¼ 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 zÎ½ = 5.2 and microscopic spin relaxation time Ï� 0 â�¼ 1.16 Ã� 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 â�¼ 1.8 kOe) and coercivity (H C â�¼ 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 (â��M/â��T) vs. T data. Â© 2020 IOP Publishing Ltd.

Item Type: | Journal Article |
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Publication: | Journal of Physics Condensed Matter |

Publisher: | IOP Publishing Ltd |

Additional Information: | Copyright to this article belongs to IOP Publishing Ltd |

Keywords: | Antiferromagnetism; Carbon dioxide; Glass; Glass transition; Ions; Magnetic moments; Magnetic susceptibility; Phase diagrams; Ruthenium compounds; Spin glass, AC Magnetic susceptibility; Analysis of measurements; Antiferromagnetic transition; Dynamical critical exponent; Effective magnetic moments; Exchange bias effects; Paramagnetic susceptibility; Randomly distributed, Magnetocaloric effects |

Department/Centre: | Division of Physical & Mathematical Sciences > Physics |

Date Deposited: | 26 Nov 2020 10:49 |

Last Modified: | 26 Nov 2020 10:49 |

URI: | http://eprints.iisc.ac.in/id/eprint/66963 |

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