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| 2. |
- Nayak, S., et al.
(författare)
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Magnetic compensation, field-dependent magnetization reversal, and complex magnetic ordering in Co2TiO4
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:21
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Tidskriftsartikel (refereegranskat)abstract
- The complex nature of magnetic ordering in the spinel Co2TiO4 is investigated by analyzing the temperature and magnetic field dependence of its magnetization (M), specific heat (C-p), and ac magnetic susceptibilities chi' and chi ''. X-ray diffraction of the sample synthesized by the solid-state reaction route confirmed the spinel structure whereas x-ray photoelectron spectroscopy shows its electronic structure to be Co2TiO4 = [Co2+][Co3+ Ti3+]O-4. From analysis of the temperature dependence of the dc paramagnetic susceptibility, the magnetic moments mu(A) = 3.87 mu(B) and mu(B) = 5.19 mu B on the A and B sites are determined with mu(B) in turn yielding mu(Ti3+) = 1.73 mu(B) and mu(Co3+) = 4.89 mu(B). Analysis of the dc and ac susceptibilities combined with the weak anomalies observed in the C-p vs T data shows the existence of a quasi-long-range ferrimagnetic state below T-N similar to 47.8K and a compensation temperature T-comp similar to 32 K, the latter characterized by sign reversal of magnetization with its magnitude depending on the applied magnetic field and the cooling protocol. Analysis of the temperature dependence of M (field cooled) and M (zero field cooled) data and the hysteresis loop parameters is interpreted in terms of large spin clusters. These results in Co2TiO4, significantly different from those reported recently in isostructural Co2SnO4 = [Co2+][Co2+ Sn4+]O-4, warrant further investigations of its magnetic structure using neutron diffraction.
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| 3. |
- Thota, S., et al.
(författare)
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On the nature of magnetic state in the spinel Co2SnO4
- 2015
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:16
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Tidskriftsartikel (refereegranskat)abstract
- In the spinel Co2SnO4, coexistence of ferrimagnetic ordering below T-N similar or equal to 41K followed by a spin glass state below T-SG similar or equal to 39K was proposed recently based on the temperature dependence of magnetization M(T) data. Here new measurements of the temperature dependence of the specific heat C-P(T), ac-susceptibilities chi'(T) and chi ''(T) measured at frequencies between 0.51 and 1.2 kHz, and the hysteresis loop parameters (coercivity H-C(T) and remanence M-R(T)) in two differently prepared samples of Co2SnO4 are reported. The presence of the Co2+ and Sn4+ states is confirmed by x-ray photoelectron spectroscopy (XPS) yielding the structure: Co2SnO4 = [Co2+][Co2+Sn4+]O-4. The data of C-P versus T shows only an inflection near 39K characteristic of spin-glass ordering. The analysis of the frequency dependence of ac-magnetic susceptibility data near 39K using the Vogel-Fulcher law and the power-law of the critical slowing-down suggests the presence of spin clusters in the system which is close to a spin-glass state. With a decrease in temperature below 39K, the temperature dependence of the coercivity H-C and remanence M-R for the zero-field cooled samples show both H-C and M-R reaching their peak magnitudes near 25 K, then decreasing with decreasing T and becoming negligible below 15K. The plot of C-P/T versus T also yields a weak inflection near 15 K. This temperature dependence of HC and remanence MR is likely associated with the different magnitudes of the magnetic moments of Co2+ ions on the 'A' and 'B' sites and their different temperature dependence.
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| 4. |
- Nayak, S., et al.
(författare)
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Low-temperature anomalous magnetic behavior of Co2TiO4 and Co2SnO4
- 2016
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 120:16
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Tidskriftsartikel (refereegranskat)abstract
- We report the low-temperature anomalous magnetic behavior of ferrimagnetic spinels cobalt orthotitanate (Co2TiO4), which exhibits magnetic compensation behavior across 31.74 K, and cobalt orthostannate (Co2SnO4) exhibiting two sequential magnetic transitions, namely (i) ferrimagnetic to paramagnetic transition with Neel temperature T-N similar to 41 K and reentrant spin-glass behavior with glass transition temperature T-SG similar to 39 K. The Arrott plot (H/M versus M-2) criterion has been used to extricate the order of sequential magnetic transitions occurring below TN. Negative slopes of the Arrott plots below 32 K, metamagnetic-like character of the M-H isotherms, anomalies in the specific-heat (C-P T-1 versus T) below 15 K, and a zero-crossover of isothermal magnetic-entropychange (Delta S) signify the presence of pseudo first-order discontinuous magnetic phase transition in the low-temperature regime 5K <= T <= 32 K. The dc- and ac-susceptibilities of both Co2TiO4 and Co2SnO4 are interpreted in terms of frozen-spin-clusters, which are responsible for very large magnitudes of the coercivity H-C similar to 20 kOe and bipolar-exchange bias H-EB similar to -20 kOe observed below 10 K.
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| 5. |
- Pramanik, P., et al.
(författare)
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Cationic distribution, exchange interactions, and relaxation dynamics in Zn-diluted MnCo2O4 nanostructures
- 2019
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Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 125:12
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Tidskriftsartikel (refereegranskat)abstract
- We report an experimental investigation of the electronic structure and magnetic properties of bulk and nanosized MnCo2O4 diluted with Zn. The cationic distribution for tetrahedral A-site dilution is (Co1-yA2+ZnyA2+)(A)[Mn3+Co3+](B)O-4 +/-delta, whereas B-site dilution results in (Co2+)(A)[Mn1-xB3+ZnxB2+Co3+](B)O4-delta. The strength of exchange interaction J(ij) between the magnetic ions in a bulk spinel lattice decreases by similar to 15% for A-site dilution relative to the undiluted compound; however, B-site dilution results in an enhancement in J(ij) by 17%. The frequency and temperature dependence of dynamic-susceptibility [chi(ac)(f, T)] studies of nanostructured compounds reveals the existence of spin-glass like behavior below the freezing temperature T-F similar to 125.7 K (for x(B) = 0.2) and 154.3 K (y(A) = 0.1). Relaxation time tau follows the Power-Law variation with a dynamical critical exponent zv = 6.17 and microscopic spin relaxation time tau(o) = 4.4 x 10(-15) s for x(B) = 0.2 (for y(A) = 0.1, zv = 5.2 and tau(o) = 5.4 x 10(-13) s). The amplitude and peak position in chi(ac)(T) decreases with an increase in the DC bias field, which indicates that the spin-glass phase can survive in the presence of low fields forming a critical line with an exponent 2/3. This behavior is similar to the de Almeida-Thouless (AT-line) analysis in the T-H phase diagram which supports the existence of spin-glass like behavior below T-F in these Zn diluted spinels.
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| 6. |
- Joshi, D. C., et al.
(författare)
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Nature of magnetic ordering in nanocomposites of Zn1-pNipO and NiO
- 2018
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Ingår i: Physica. E, Low-Dimensional systems and nanostructures. - : ELSEVIER SCIENCE BV. - 1386-9477 .- 1873-1759. ; 103, s. 46-52
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Tidskriftsartikel (refereegranskat)abstract
- A detailed study of the nature of magnetic exchange interactions and anisotropy in Zn-1_pNipO/NiO (0 <= p <= 1) two-phase nanocomposites is reported. The exchange constants |J(1)| and |J(2)| have been determined using the molecular field approximation for the two sublattice model which yields |J(2)| similar to 11.26 meV and |J(1)| similar to 7.17meV for p = 1, consistent with type-II antiferromagnetic ordering (i.e. |J2|>>|J(1)|). A novel core-shell surface layer model has been employed to explain the nature of magnetic anisotropy in these nanocomposites which yields anisotropy constants K-surface= 2.42 erg/cm(2), K-bulk = 2.61 x 10(6) erg/cm(3) and K-Shell = 9.56 x 10(5) erg/cm(3) for shell thickness d(Sh) similar to 1.34 nm (p = 1).
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| 7. |
- Mazrui, N. M., et al.
(författare)
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Enhanced availability of mercury bound to dissolved organic matter for methylation in marine sediments
- 2016
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037. ; 194, s. 153-162
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Tidskriftsartikel (refereegranskat)abstract
- The forms of inorganic mercury (HgII) taken up and methylated by bacteria in sediments still remain largely unknown. From pure cultures studies, it has been suggested that dissolved organic matter (DOM) may facilitate the uptake either by acting as a shuttle molecule, transporting the HgII atom to divalent metal transporters, or by binding HgII and then being transported into the cell as a carbon source. Enhanced availability of Hg complexed to DOM has however not yet been demonstrated in natural systems. Here, we show that HgII complexed with DOM of marine origin was up to 2.7times more available for methylation in sediments than HgII added as a dissolved inorganic complex (HgII(aq)). We argue that the DOM used to complex HgII directly facilitated the bacterial uptake of HgII whereas the inorganic dissolved HgII complex adsorbed to the sediment matrix before forming bioavailable dissolved HgII complexes. We further demonstrate that differences in net methylation in sediments with high and low organic carbon content may be explained by differences in the availability of carbon to stimulate the activity of Hg methylating bacteria rather than, as previously proposed, be due to differences in HgII binding capacities between sediments. © 2016 Elsevier Ltd
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| 8. |
- Padam, R., et al.
(författare)
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Magnetic phase diagram of Co(Cr1-xAlx)2O-4 (x=0.0-1.0)
- 2017
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 122:7
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Tidskriftsartikel (refereegranskat)abstract
- We report the role of Al substitution in the magnetic properties of spinel CoCr2O4 by means of temperature dependent dc and ac magnetization and heat capacity measurements. Various compositions (0.0 <= x <= 1.0) of polycrystalline Co(Cr1-xAlx)(2)O-4 samples have been prepared by sol-gel processing and their crystal structure was investigated by X-ray diffraction which was found to crystallize in the normal cubic spinel structure. For x <= 0.1, the system exhibits multiple magnetic orderings (long range ferrimagnetic ordering T-C, spin-spiral ordering T-S, and lock-in transition T-L), similar to that of the parent compound, CoCr2O4. However, all the compositions between x = 0.1 and 0.5 exhibit long range ferrimagnetic ordering below T-C and also a short range order at low temperature. Spin-glass like ordering was noticed between x = 0.6 and 0.8 due to the diluted B-site occupancy, whereas the end compound CoAl2O4 (x = 1.0) shows antiferromagnetic behavior. On the basis of these results, we propose a magnetic phase diagram for the Co(Cr1-xAlx)(2)O-4 series as a function of the Al content (x) and measuring temperature (T).
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| 9. |
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