Browsing by Author "Chowdhury, Mouli Roy"
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- PublicationOpen AccessMagnetic field-temperature phase diagram, exchange constants andspecific heat exponents of the antiferromagnet MnNb₂O₆(2021)
;Maruthi, R. ;Ghosh, Sayandeep ;Seehra, Mohindar Singh ;Joshi, Deep C. ;Chowdhury, Mouli Roy ;Medwal, Rohit; ;Weise, BrunoThota, SubhashThis work presents the magnetic field-temperature (H–T) phase diagram, exchange constants, specific heat (CP) exponents and magnetic ground state of the antiferromagnetic MnNb2O6 polycrystals. Temperature dependence of the magnetic susceptibility χ (=M/H) yields the Néel temperature TN = 4.33 K determined from the peak in the computed ∂(χT)/∂T vs T plot in agreement with the transition in the CP vs T data at TN = 4.36 K. The experimental data of CP vs T near TN is fitted to CP = A|T − TN|−α yielding the critical exponent α = 0.12 (0.15) for T > TN (T < TN). The best fit of χ vs T data for T > 50 K to χ = χ0 + C/(T − θ) with χ0 = −1.85 × 10−4 emu mol−1 Oe−1 yields θ = −17 K, and C = 4.385 emu K mol−1 Oe−1, the latter giving magnetic moment μ = 5.920μB per Mn2+ ion. This confirms the effective spin S = 5/2 and g = 2.001 for Mn2+ and the dominant exchange interaction being antiferromagnetic in nature. Using the magnitudes of θ and TN and molecular field theory (MFT), the exchange constants J0/kB = −1.08 K for Mn2+ ions along the chain c-axis and J⊥/kB = −0.61 K as the interchain coupling perpendicular to c-axis are determined. These exchange constants are consistent with the expected χ vs T variation for the Heisenberg linear chain. The H–T phase diagram, mapped using the M–H isotherms and M–T data at different H combined with the reported data of Nielsen et al, yields a triple-point TTP (H, T) = (18 kOe, 4.06 K). The spin–flopped state above TTP and the forced ferromagnetism for H > 192 kOe are used to estimate the anisotropy energy HA ≈ 0.8 kOe.WOS© Citations 7 64 54 - PublicationMetadata onlyUnraveling the nature of ferrimagnetism and associated exchange interactions in distorted honeycomb Ni4Nb2O9(2022)
;Thota, Subhash ;Seehra, Mohindar S. ;Chowdhury, Mouli Roy ;Singh, H. ;Ghosh, Sayandeep ;Jena, Suchit Kumar ;Pramanik, P. ;Sarkar, T.; ;Medwal, RohitWeise, BrunoFerrimagnetism in orthorhombic Ni4, Nb2 Og below its Néel temperature. TFN ~ 76 K 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 > TFN and T < TFN 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 [xac,(f, 7, H)], and heat-capacity [Cp (T, H)] measurements are combined with theoretical analysis to provide new insights into the nature of ferrimagnetism in Ni4, Nb2 Og. 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 Ni4, Nb2 Og. Analysis of xac(T) shows the paramagnetic-to-ferrimagnetic transition occurs at 76.5 K TFN, which increases with applied field H as TFN ∝ H0.35 due to the coupling of the ferromagnetic component with H. For T > TFN, the Xdc versus T' data are fitted to the Néel's expression for ferrimagnets, yielding the g-factors for the two Ni2+ ions as ga = 2.47 and gp = 2.10. Also, the antiferromagnetic molecular field constants between the A and B sublattices were evaluated as NAA, = 26.31, NBB = 8.59. and NAB = 43.06, which, in turn, yield the antiferromagnetic exchange parameters: JAA/kp = 4.27 K, JBB/kp = 1.40 K, and JAB/kB = 6.98 K. For T < TFN, the M versus T data clearly show the magnetic compensation point at Tcom ~ 33 K. The mathematical model presented here using the magnitudes of NAA, NBB and NAB Correctly predicts the position of Toy as well the temperature variation of M both above and below TCOM. The data of Cp(T) versus T shows a A-type anomaly across Try. After subtracting the lattice contribution, the Cp(T) data are fitted to Cp = A(T — TN)(-∝) yielding the critical exponent ∝ = 0.14(0.12) for TFN (T > TFN), 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 > TFN and T < Tcom. but for Tcom < T TFN, the spin disorder increases with the increase in H. The temperature variation of the measured coercivity Hc(T) and remanence Mpr(T) from 1.9 K to TFN initially show a decreasing trend. becoming zero atTCOM, then followed by an increase and eventually becoming zero again at TFN.WOS© Citations 3 52