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| 2. |
- Thota, S., et al.
(författare)
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Unraveling the nature of ferrimagnetism and associated exchange interactions in distorted honeycomb Ni4Nb2O9
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:13
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Tidskriftsartikel (refereegranskat)abstract
- Ferrimagnetism in orthorhombic Ni4Nb2O9 below its Neel temperature, T-FN similar to 76K is reported to result from two inequivalent Ni2+ ions having different magnetic moments. However, a clear understanding of the temperature variation of its magnetization [M(T)] for T > T-FN and T < T-FN in terms of a single set of exchange parameters is still lacking. In this work, experimental results obtained from a detailed analysis of the temperature and magnetic field dependence of magnetization [M(T, H)], ac-magnetic susceptibility [chi(ac)( f, T, H)], and heat-capacity [C-P(T, H)] measurements are combined with theoretical analysis to provide new insights into the nature of ferrimagnetism in Ni4Nb2O9. X-ray diffraction/Rietveld analysis of the prepared sample yielded the structural parameters of the orthorhombic crystal in agreement with previous studies, whereas x-ray photoelectron spectroscopy confirmed the Ni2+ and Nb5+ electronic states in Ni4Nb2O9. Analysis of chi(ac)(T) shows the paramagnetic-to-ferrimagnetic transition occurs at 76.5 K (T-FN), which increases with applied field H as T-FN proportional to H-0.35 due to the coupling of the ferromagnetic component with H. For T > T-FN, the chi(dc) versus T data are fitted to the Neel's expression for ferrimagnets, yielding the g-factors for the two Ni2+ ions as g(A) = 2.47 and g(B) = 2.10. Also, the antiferromagnetic molecular field constants between the A and B sublattices were evaluated as N-AA = 26.31, N-BB = 8.59, and N-AB = 43.06, which, in turn, yield the antiferromagnetic exchange parameters: J(AA)/k(B) = 4.27 K, J(BB)/k(B) = 1.40 K, and J(AB)/k(B) = 6.98 K. For T < T-FN, the M versus T data clearly show the magnetic compensation point at T-COM similar to 33 K. The mathematical model presented here using the magnitudes of NAA, NBB, and NAB correctly predicts the position of T-COM as well the temperature variation of M both above and below T-COM. The data of C-P(T) versus T shows a lambda-type anomaly across T-FN. After subtracting the lattice contribution, the C-P(T) data are fitted to C-P = A(T - T-N)((-alpha)) yielding the critical exponent alpha = 0.14(0.12) for T < T-FN (T > T-FN), which is a characteristic of second-order phase transition. Magnetic entropy changes determined from the M-H isotherms shows that the applied field H enhances the magnetic ordering for T > T-FN and T < T-COM, but for T-COM < T < T-FN, the spin disorder increases with the increase in H. The temperature variation of the measured coercivity H-C(T) and remanence M-R(T) from 1.9 K to T-FN initially show a decreasing trend, becoming zero at T-COM, then followed by an increase and eventually becoming zero again at T-FN.
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| 3. |
- 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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| 4. |
- Jena, S. K., et al.
(författare)
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Spin-liquid state with precursor ferromagnetic clusters interacting antiferromagnetically in frustrated glassy tetragonal spinel Zn0.8Cu0.2FeMnO4
- 2023
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 35:37
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Tidskriftsartikel (refereegranskat)abstract
- Spinels (AB(2)O(4)) with magnetic ions occupying only the octahedral B sites have inherent magnetic frustration which inhibits magnetic long-range order (LRO) but may lead to exotic states. Here we report on the magnetic properties of the tetragonal spinel Zn0.8Cu0.2FeMnO4, the tetragonality resulting from the Jahn-Teller active Mn3+ ions. X-ray diffraction and x-ray photoelectron spectroscopy of the sample yielded the composition (Zn-0.8(2+) Cu-0.2(2+)) A [Fe-0.4(2+) Fe-0.6(3+) Mn3+](B)O4-delta. Analysis of the temperature dependence of magnetization (M), ac magnetic susceptibilities (chi' and chi''), dc susceptibility (chi), heat capacity C-p, and neutron diffraction (ND) measurements show complex temperature-dependent short-range order (SRO) but without LRO. The data of chi vs. T fits the Curie-Weiss law: chi = C/(T - theta) from T = 250 K to 400 K with theta similar or equal to 185 K signifying dominant ferromagnetic (FM) coupling with the FM exchange constant J/k(B) = 17 K, and C = 3.29 emu K mol(-1) Oe(-1) yielding an effective magnetic moment mu(eff) = 5.13 mu B resulting from the high-spin states of Cu2+ (A site) and Fe2+ (B site), while the B site trivalent ions Mn3+ and Fe3+ are in their low-spin states. The extrapolated saturation magnetization obtained from the M vs. H data at T = 2 K is explained using the spin arrangement (Cu2+down arrow)(A)[Fe2+down arrow, Fe3+down arrow, Mn3+down arrow](B) leading to FM clusters interact antiferromagnetically at low temperatures. Temperature dependence of d(chi T)/ dT shows the onset of ferrimagnetism below similar to 100 K and peaks near 47 K and 24 K. The relaxation time t obtained from temperature and frequency dependence of chi'' when fit to the power law and Vogel-Fulcher laws confirm the cluster spin-glass (SG) state. The magnetic field dependence of the SG temperature T-SG (H) follows the equation: T-SG (H) = T-SG (0) [ 1- AH(2/phi)] with T-SG (0) = 46.6 K, A = 8.6 x 10(-3) Oe(-0.593) and phi = 3.37. The temperature dependence of hysteresis loops yields coercivity H-C similar to 3.8 kOe at 2 K without exchange-bias, but HC decreases with increase in T becoming zero above 24 K, the T-SG(H) for H = 800 Oe. Variations of C-p vs. T from 2 K to 200 K in H = 0 and H = 90 kOe do not show any peak characteristic of LRO. However, after correcting for the lattice contribution, a broad weak peak typically of SRO becomes evident centered around 40 K. For T < 9 K, Cp varies as T-2; a typical signature of spin-liquids (SLs). Comparison of the ND measurements at 1.7 K and 79.4 K shows absence of LRO. Time dependence of thermo-remanent magnetization MTRM(t) studies below 9 K reveal weakening of the inter-cluster interaction with increase in temperature. A summary of these results is that in Zn0.8Cu0.2FeMnO4, ferromagnetic clusters interact antiferromagnetically without LRO but producing a cluster SG state at T-SG(0) = 46.6 K, followed by SL behavior below 9 K.
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| 5. |
- 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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