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

Abstract

Static and dynamic magnetic properties of normal spinel Co2RuO4 = (Co2+) 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 TN ∼ 15.2 K, along with a spin-glass state at slightly lower temperature (TSG) near 14.2 K. It is argued that TN 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 < TN are used to construct the H–T phase diagram showing that TSG shifts to lower T varying as H2/3.2 expected for spin-glass state whereas TN is nearly H-independent. For T > TN, analysis of the paramagnetic susceptibility (χ) vs. T data are fit to the modified Curie–Weiss law, χ = χ0 + C/(T + θ), with χ0 = 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 μeff = 4.16 μB comparable to the expected value of μeff = 4.24 μB per Co2RuO4. Using TN, θ and high temperature series for χ, dominant exchange constant J1/kB ∼ 6 K between the Co2+ on the A-sites is estimated. Analysis of the ac magnetic susceptibilities near TSG yields the dynamical critical exponent zν = 5.2 and microscopic spin relaxation time τ0 ∼ 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 (HEB ∼ 1.8 kOe) and coercivity (HC ∼ 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed below TN 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 > TN to inverse MCE for T < TN observed in Co2RuO4 (and reported previously in other systems also) is related to the change in sign of (∂M/∂T) vs. T data.

High-pressure synthesis, spin-glass behaviour, and magnetocaloric effects in FexTi2S4 heideite sulphides

“…5, the peak of χ ac '(T) exhibits a frequency-dependence: the peak position shifts to higher temperatures when AC frequency increases, while the peak magnitude decreases. This is a typical behaviour reported for the SG systems 32,33 . For further clarification, the relative shift (k) in the freezing temperature can be used to describe the magnetic order of the system, which is denoted as 34 , where ΔT f is maximum in the freezing temperature.…”

Magnetic Ground State and Tunable Néel Temperature in the Spin ½ Linear Chain Antiferromagnet Co(OH)(2−x)Fx

“…[ 19 ] In previous studies, [ 29,30 ] S = 3 / 2 was assumed in the discussion of the results, although disagreement between the magnetic moment μ(Co 2+ ) obtained from the CW law above and NPD below was not resolved. [ 29 ] In recent articles on three systems containing Co 2+ ions, viz., GeCo 2 O 4 , [ 36 ] β‐Co(OH) 2 , [ 26 ] and Co 2 RuO 4 , [ 31 ] it is argued that the ground state has effective S = 1 / 2 for and not S = 3 / 2 based on Hunds’ rules. The low spin state is attributed to the distorted octahedral crystal field and SO interaction as, we argue, is also valid in Co(OH)F. As illustrated in Figure 4 a, the highly symmetric CoO 6 is always in a high spin state because the Co–O bond lengths are 0.2144 nm for CoO in all directions.…”

“…Cobaltous nitrate (Co(NO 3 ) 2 ·6H 2 O, China Medicament Co.) and urea (CO(NH 2 ) 2 , China Medicament Co.) were used as precursors of and and ammonium fluoride (NH 4 F, China Medicament Co.) was used to adjust the pH values of the solutions. [ 31 ] All the chemicals, i.e., Co(NO 3 ) 2 ·6H 2 O (1.2 mmol), NH 4 F (4 mmol), and CO(NH 2 ) 2 (6 mmol), were dissolved into 15 mL deionized water and stirred for 10 min. The solution was poured into a 25 mL autoclave, sealed, and maintained at 120 °C for 6 h. Pink was obtained.…”

“…was used to adjust the pH values of the solutions. [31] All the chemicals, i.e., Co(NO 3 ) 2 •6H 2 O (1.2 mmol), NH 4 F (4 mmol), and CO(NH 2 ) 2 (6 mmol), were dissolved into 15 mL deionized water and stirred for 10 min. The solution was poured into a 25 mL autoclave, sealed, and maintained at 120 C for 6 h. Pink CoðOHÞ 0.91 F 1.09 was obtained.…”

Anisotropic thermal expansion and electronic transitions in the Co3BO5 ludwigite

“…7, where the CW temperature is fixed θ = 11 K. 15 Both curves show a steep increase in the effective moment at low temperatures, changing the slope continuously and reaching near saturation at about 500 K. For higher temperatures, there is a constant slope whose value depends directly on the χ 0 value. 33…”