| 1. |
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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. |
- 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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| 5. |
- 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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| 6. |
- Singha, A. D., et al.
(författare)
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Reentrant canonical spin-glass dynamics and tunable field-induced transitions in (GeMn)Co2O4 Kagomé lattice
- 2024
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 36:7
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Tidskriftsartikel (refereegranskat)abstract
- We report on the reentrant canonical semi spin-glass characteristics and controllable field-induced transitions in distorted Kagomé symmetry of (GeMn)Co2O4. This B-site spinel exhibits complicated, yet interesting magnetic behaviour in which the longitudinal ferrimagnetic (FiM) order sets in below the Néel temperature TFN ∼ 77 K due to uneven moments of divalent Co (↑ 5.33 μB) and tetravalent Mn (↓ 3.87 μB) which coexists with transverse spin-glass state below 72.85 K. Such complicated magnetic behaviour is suggested to result from the competing anisotropic superexchange interactions (JAB/kB ∼ 4.3 K, JAA/kB ∼ −6.2 K and JBB/kB ∼ −3.3 K) between the cations, which is extracted following the Néel's expression for the two-sublattice model of FiM. Dynamical susceptibility (χac (f, T)) and relaxation of thermoremanent magnetization, MTRM (t) data have been analysed by means of the empirical scaling-laws such as Vogel–Fulcher law and Power law of critical slowing down. Both of which reveal the reentrant spin-glass like character which evolves through a number of intermediate metastable states. The magnitude of Mydosh parameter (Ω ∼ 0.002), critical exponent zυ = (6.7 ± 0.07), spin relaxation time τ0 = (2.33 ± 0.1) × 10−18 s, activation energy Ea/kB = (69.8 ± 0.95) K and interparticle interaction strength (T0 = 71.6 K) provide the experimental evidences for canonical spin-glass state below the spin freezing temperature TF = 72.85 K. The field dependence of TF obtained from χac (T) follows the irreversibility in terms of de Almeida–Thouless mean-field instability in which the magnitude of crossover scaling exponent Φ turns out to be ∼2.9 for the (Ge0.8Mn0.2)Co2O4. Isothermal magnetization plots reveal two field-induced transitions across 9.52 kOe (HSF1) and 45.6 kOe (HSF2) associated with the FiM domains and spin-flip transition, respectively. Analysis of the inverse paramagnetic susceptibility after subtracting the temperature independent diamagnetic term (=−3 × 10−3 emu mol−1 Oe−1) results in the effective magnetic moment = 7.654 μB/f.u. This agrees well with the theoretically obtained = 7.58 μB/f.u. resulting the cation distribution in support of the Hund's ground state spin configuration and of Mn4+ and Co2+, respectively. The H–T phase diagram has been established by analysing all the parameters (TF(H), TFN(H), HSF1(T) and HSF2(T)) extracted from various magnetization measurements. This diagram enables clear differentiation among the different phases of the (GeMn)Co2O4 and also illustrates the demarcation between short-range and long-range ordered regions.
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| 7. |
- 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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| 8. |
- 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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| 9. |
- Pramanik, P., et al.
(författare)
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Neutron diffraction evidence for local spin canting, weak Jahn-Teller distortion, and magnetic compensation in Ti1-xMnxCo2O4 spinel
- 2020
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Ingår i: Journal of Physics. - : IOP PUBLISHING LTD. - 0953-8984 .- 1361-648X. ; 32:24
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Tidskriftsartikel (refereegranskat)abstract
- A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti1-xMnxCo2O4 in the temperature interval 1.6 K T 300 K. Our neutron diffraction study reveals cooperative distortions of the TO6 octahedra in the Ti1-xMnxCo2O4 system for all the Jahn-Teller active ions T = Mn3+ , Ti3+ and Co3+ , having the electronic configurations 3d(1), 3d(4) and 3d(6), respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x = 0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet-Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a < 1, in which the apical bond length d(c)(T-B-O) is longer than the equatorial bond length d(ab)(T-B-O) due to the splitting of the e(g) level of Mn3+ ions into and . For the composition x = 0.4, the distortion in the TBO6 octahedra is stronger as compared to x = 0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti0.6Mn0.4Co2O4 and Ti0.8Mn0.2Co2O4 sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti3+ , Mn3+ , and Co3+ occupy the octahedral, whereas, Co2+ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at T-COMP = 25.4 K in Ti0.8Mn0.2Co2O4 and a reentrant spin-glass behaviour in Ti0.6Mn0.4Co2O4 with a freezing temperature of 110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below T-COMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.
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| 10. |
- Yuan, J., et al.
(författare)
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Elevated plasma sclerostin is associated with high brain amyloid-β load in cognitively normal older adults
- 2023
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Ingår i: Npj Aging. - 2731-6068. ; 9:1
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Tidskriftsartikel (refereegranskat)abstract
- Osteoporosis and Alzheimer's disease (AD) mainly affect older individuals, and the possibility of an underlying link contributing to their shared epidemiological features has rarely been investigated. In the current study, we investigated the association between levels of plasma sclerostin (SOST), a protein primarily produced by bone, and brain amyloid-beta (A ss) load, a pathological hallmark of AD. The study enrolled participants meeting a set of screening inclusion and exclusion criteria and were stratified into A ss(n = 65) and A ss+ (n = 35) according to their brain A ss load assessed using A ss-PET (positron emission tomography) imaging. Plasma SOST levels, apolipoprotein E gene (APOE) genotype and several putative AD blood-biomarkers including A ss 40, A ss 42, A ss 42/A ss 40, neurofilament light (NFL), glial fibrillary acidic protein (GFAP), total tau (t-tau) and phosphorylated tau (p-tau181 and p-tau231) were detected and compared. It was found that plasma SOST levels were significantly higher in the A ss+ group (71.49 +/- 25.00 pmol/L) compared with the A ss- group (56.51 +/- 22.14 pmol/L) (P < 0.01). Moreover, Spearman's correlation analysis showed that plasma SOST concentrations were positively correlated with brain A ss load (. = 0.321, P = 0.001). Importantly, plasma SOST combined with A ss 42/A ss 40 ratio significantly increased the area under the curve (AUC) when compared with using A ss 42/A ss 40 ratio alone (AUC = 0.768 vs 0.669, P = 0.027). In conclusion, plasma SOST levels are elevated in cognitively unimpaired older adults at high risk of AD and SOST could complement existing plasma biomarkers to assist in the detection of preclinical AD.
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