| 1. |
- Chowdhury, Mouli Roy, et al.
(author)
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Antiferromagnetic short-range order and cluster spin-glass state in diluted spinel ZnTiCoO4
- 2022
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In: Journal of Physics. - : IOP Publishing Ltd. - 0953-8984 .- 1361-648X. ; 34:27
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Journal article (peer-reviewed)abstract
- The nature of magnetism in the doubly-diluted spinel ZnTiCoO4 = (Zn2+)( A ) [Ti4+Co2+]( B )O-4 is reported here employing the temperature and magnetic field (H) dependence of dc susceptibility (chi), ac susceptibilities (chi ' and chi ''), and heat capacity (C (p)) measurements. Whereas antiferromagnetic (AFM) Neel temperature T (N) = 13.9 K is determined from the peak in the partial differential (chi T)/ partial differential T vs T plot, the fit of the relaxation time tau (determined from the peak in the chi '' vs T data at different frequencies) to the Power law: tau = tau (0) [(T - T (SG))/T (SG)](-z nu ) yields the spin glass freezing temperature T (SG) = 12.9 K, z nu similar to 11.75, and tau (0) similar to 10(-12) s. Since the magnitudes of tau (0) and z nu depend on the magnitude of T (SG), a procedure is developed to find the optimum value of T (SG) = 12.9 K. A similar procedure is used to determine the optimum T (0) = 10.9 K in the Vogel-Fulcher law: tau = tau (0) exp[E (a)/k (B)(T - T (0))] yielding E (a)/k (B) = 95 K, and tau (0) = 1.6 x 10(-13) s. It is argued that the comparatively large magnitude of the Mydosh parameter omega = 0.026 and k (B) T (0)/E (a) = 0.115 (MUCH LESS-THAN1) suggests cluster spin-glass state in ZnTiCoO4 below T-SG. In the C (p) vs T data from 1.9 K to 50 K, only a broad peak near 20 K is observed. This and absence of lambda-type anomaly near T (N) or T (SG) combined with the reduced value of change in magnetic entropy from 50 K to 1.9 K suggests only short-range AFM ordering in the system, consistent with spin-glass state. The field dependence of T (SG) shows slight departure (phi similar to 4.0) from the non-mean-field Almeida-Thouless line T (SG)(H) = T (SG)(0) (1 - AH (2/phi )). Strong temperature dependence of magnetic viscosity S and coercivity H (C) without exchange bias, both tending to zero on approach to T (SG) from below, further support the spin-glass state which results from magnetic dilution driven by diamagnetic Zn2+ and Ti4+ ions leading to magnetic frustration. Magnetic phase diagram in the H-T plane is established using the high-field magnetization data M(H, T) for T < T (N) which reveals rapid decrease of T (SG) with increase in H whereas decrease in T (N) with increase in H is weaker, typical of AFM systems. For T > T (N), the data of chi vs T are fit to the modified Curie-Weiss law, chi = chi (0) + C/(T + theta), with chi (0) = 3.2 x 10(-4) emu mol(-1) Oe(-1) yielding theta = 4 K and C = 2.70 emu K mol(-1) Oe(-1). This magnitude of C yields effective magnetic moment = 4.65 mu (B) for Co2+, characteristic of Co2+ ions with some contribution from spin-orbit coupling. Molecular field theory with effective spin S = 3/2 of Co2+ is used to determine the nearest-neighbor exchange constant J (1)/k (B) = 2. 39 K AFM and next-nearest-neighbor exchange constant J (2)/k (B) = -0.66 K (ferromagnetic).
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| 2. |
- Ghosh, Sayandeep, et al.
(author)
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Antiferromagnetism, spin-glass state, H-T phase diagram, and inverse magnetocaloric effect in Co2RuO4
- 2020
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In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 32:48
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Journal article (peer-reviewed)abstract
- Static and dynamic magnetic properties of normal spinel Co2RuO4= (Co2+)A[Co3+Ru3+](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 rangeT= 2 K-400 K and H up to 90 kOe. These investigations show that Co2RuO4 exhibits an antiferromagnetic (AFM) transition at T-N similar to 15.2 K, along with a spin-glass state at slightly lower temperature (T-SG) near 14.2 K. It is argued thatT(N)is mainly governed by the ordering of the spins of Co2+ ions occupying theA-site, whereas the exchange interaction between the Co2+ ions on theA-site and randomly distributed Ru(3+)on theB-site triggers the spin-glass phase, Co3+ ions on theB-site being in the low-spin non-magnetic state. Analysis of measurements ofM(H,T) for TT-N, analysis of the paramagnetic susceptibility (chi) vs.Tdata are fit to the modified Curie-Weiss law,chi=chi(0)+C/(T+theta), with chi(0)= 0.0015 emu mol(-1)Oe(-1)yielding theta= 53 K andC= 2.16 emu-K mol(-1)Oe(-1), the later yielding an effective magnetic moment mu(eff)= 4.16 mu(B)comparable to the expected value of mu(eff)= 4.24 mu(B)per Co2RuO4. Using T-N,theta and high temperature series for chi, dominant exchange constant J(1)/k(B)similar to 6 K between the Co(2+)on theA-sites is estimated. Analysis of the ac magnetic susceptibilities near T-SG yields the dynamical critical exponent z nu= 5.2 and microscopic spin relaxation time tau(0)similar to 1.16 x 10(-10)sec characteristic of cluster spin-glasses and the observed time-dependence ofM(t) is supportive of the spin-glass state. LargeM-Hloop asymmetry at low temperatures with giant exchange bias effect (H-EB similar to 1.8 kOe) and coercivity (H-C similar to 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed belowT(N)is interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) forT>T(N)to inverse MCE forT
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