| 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. |
- Dokala, Ravi Kiran, et al.
(author)
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Elastic strain control of electronic structure, and magnetic properties of [Pr1-xCaxMnO3/SrTiO3](15) superlattices
- 2020
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In: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 127:17
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Journal article (peer-reviewed)abstract
- We report the growth, electronic structure, and in-plane magnetic properties of pulsed laser deposition grown 2D superlattice structures [Pr 0.7Ca 0.3MnO 3/SrTiO 3] 15 and [Pr 0.5Ca 0.5MnO 3/SrTiO 3] 15 on (001) oriented SrTiO 3 and LaAlO 3 single crystal substrates. The x-ray reflectivity measurements reveal well-defined interfaces between the manganite and titanate layers along with the existence of Kiessig fringes, providing the evidence for the smooth periodic superlattice structure. The reciprocal space mapping provides signature of tetragonal distortion in all the superlattices. The electronic structure determined from the x-ray photoelectron spectroscopy reveals divalent Sr and Ca, tetravalent Ti, and mixed valent Mn with a pronounce shift of binding energy peaks toward the higher energy side in the superlattices grown on (001) oriented LaAlO 3 as compared to those grown on SrTiO 3. These superlattices exhibit highly anisotropic ferromagnetic character. We used the law of approach to saturation to determine the anisotropy field (H K) and cubic anisotropy constant ( K 1) for all the investigated superlattices. This analysis yields the highest H K similar to 9kOe and K 1 similar to 8 x 10 5erg/cc for the [Pr 0.7Ca 0.3MnO 3/SrTiO 3] 15 superlattice system. Furthermore, significant enhancement of the overall magnetic moment and a decrease in T C ( < 100K) was observed in the case of LaAlO 3 grown superlattice, which indicates a substantial role of residual elastic strain on the magnetic ordering. Our results indicate that the strain induced elongation of MnO 6 octahedra leads to finite possibility of non-orthogonal overlapping of orbitals in the presence of large crystal field splitting of e g levels, which, in turn, causes suppression of the ferromagnetic double exchange interaction.
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| 3. |
- 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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| 4. |
- Jena, Suchit Kumar, et al.
(author)
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Slow spin dynamics of cluster spin-glass spinel Zn(Fe1-xRu (x))(2)O-4 : role of Jahn-Teller active spin-1/2 Cu2+ ions at B-sites
- 2022
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In: Journal of Physics. - : IOP Publishing Ltd. - 0953-8984 .- 1361-648X. ; 34:40
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Journal article (peer-reviewed)abstract
- We report the slow spin dynamics of cluster spin-glass (SG) spinel Zn(Fe1-xRux)(2)O-4 by means of detailed dc-magnetization and ac-susceptibility studies combined with the heat capacity analysis. Two specific compositions (x= 0.5, 0.75) have been investigated in detail along with the substitution of Jahn-Teller (JT) active spin-1/2 Cu2+ ions at B-sites. Measurements based on the frequency and temperature dependence of ac-susceptibility (chi(ac)(f, T)) and the subsequent analysis using the empirical scaling laws such as: (a) Vogel-Fulcher law and (b) Power law reveal the presence of cluster SG state below the characteristic freezing temperature T-SG (17.77 K (x= 0.5) and 14 K (x= 0.75)). Relaxation dynamics of both the compositions follow the non-mean field de Almeida-Thouless (AT)-line approach (T-SG(H) = T-SG(0)(1 - AH(2/phi))), with an ideal value of phi = 3. Nevertheless, the analysis of temperature dependent high field dc-susceptibility, chi(hf) (2 kOe <= H-DC <= 20 kOe, T) provides evidence for Gabay-Toulouse type mixed-phase (coexistence of SG and ferrimagnetic (FiM)) behaviour. Further, in the case of Cu0.2Zn0.8FeRuO4 system, slowly fluctuating magnetic clusters persist even above the short-range FiM ordering temperature (T-FiM) and their volume fraction vanishes completely across similar to 6T(FiM). This particular feature of the dynamics has been very well supported by the time decay of the thermoremanent magnetization and heat-capacity studies. We employed the high temperature series expansion technique to determine the symmetric exchange coupling (J(S)) between the spins which yields J(S) = -3.02 x 10(-5) eV for Cu0.2Zn0.8FeRuO4 representing the dominant intra-sublattice ferromagnetic interactions due to the dilute incorporation of the JT active Cu2+ ions. However, the antiferromagnetic coupling is predominant in ZnFeRuO4 and Cu0.2Zn0.8Fe0.5Ru1.5O4 systems. Finally, we deduced the magnetic phase diagram in the H-DC - T plane using the characteristic parameters obtained from the field variations of both ac- and dc-magnetization measurements.
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| 5. |
- Sasidharan, Sajitha, et al.
(author)
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Anisotropic Ferromagnetic Organic Nanoflowers
- 2022
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 126:19, s. 8511-8518
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Journal article (peer-reviewed)abstract
- We report a weak anisotropic ferromagnetic behavior in a purely organic molecule at room temperature, a property rarely reported in organic nanomaterials. The reported 1,2-bis(tritylthio)ethane, forming plate- and organic-flower-like morphologies at the nanolevel, is the first organic crystal with an inherent magnetic property at 300 and 2 K. However, at low temperatures, the magnetization value [Mmax(T) ∼116 emu/mol at 2 K] increases drastically at 3 orders higher compared to 300 K. Interestingly, the system exhibits strong anisotropy with an anisotropic constant, K1 ∼3.25 × 103 erg/cc, and anisotropy field, HK ∼3.25 kOe. Below 10 K, this system displays unusual temperature dependence of the coercive field [HC(T)] and remanence magnetization [MR(T)] with a hysteresis-peak anomaly (T∗ ∼10-15 K) due to the enhanced spin-orbit coupling. The maximum HC and MR at T∗ were HC = 220 Oe and MR ∼12 emu/mol, respectively. Beyond T*, HC(T) and MR(T) drop continuously and become negligible as the measurement temperature approaches 300 K. Our results demonstrate that the triphenyl molecules can be further exploited for the design and synthesis of organic magnets for possible applications in spintronics and memory storage devices.
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