Browsing by Author "Ghosh, Sayandeep"
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- PublicationMetadata onlyDetermination of the tricritical point, H-T phase diagram and exchange interactions in the antiferromagnet MnTa2O6(2022)
;Maruthi, R. ;Seehra, Mohindar S. ;Ghosh, Sayandeep ;Medwal, Rohit; ;Weise, Bruno ;Choi, Eun SangThota, SubhashUsing the analysis of the temperature and magnetic field dependence of the magnetization (M) measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (H–T) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTa2O6 are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa2O6 sample verified its phase purity. Temperature dependence of the magnetic susceptibility χ (=M/H) yields the Néel temperature TN = 5.97 K determined from the peak in the computed ∂(χT)/∂T vs T plot, in agreement with the TN = 6.00 K determined from the peak in the CP vs T data. The experimental data of CP vs T near TN is fitted to CP = A|T − TN|−α yielding the critical exponent α = 0.10(0.13) for T > TN (T < TN). The χ vs T data for T > 25 K fits well with the modified Curie–Weiss law: χ = χ0 + C/(T − θ) with χ0 = −2.12 × 10−4 emu mol−1 Oe−1 yielding θ = −24 K, and C = 4.44 emu K mol−1 Oe−1, the later giving magnetic moment μ = 5.96 μB per Mn2+ ion. This yields the effective spin S = 5/2 and g = 2.015 for Mn2+, in agreement with g = 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes of θ and TN and molecular field theory, the antiferromagnetic exchange constants J0/kB = −1.5 ± 0.2 K and J⊥/kB = −0.85 ± 0.05 K for Mn2+ ions along the chain c-axis and perpendicular to the c-axis respectively are determined. The χ vs T data when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. The H–T phase diagram is mapped using the M–H isotherms and M–T data at different H yielding the tricritical point TTP (H, T) = (17.0 kOe, 5.69 K) separating the paramagnetic, antiferromagnetic, and spin-flop phases. At 1.5 K, the experimental magnitudes of the exchange field HE = 206.4 kOe and spin-flop field HSF = 23.5 kOe yield the anisotropy field HA = 1.34 kOe. These results for MnTa2O6 are compared with those reported recently in the isostructural MnNb2O6.WOS© Citations 6 255 - 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 - PublicationOpen AccessMagnetic ground state and exchange interactions in the Ising chain ferromagnet CoNb2O6(2021)
;Thota, Subhash ;Ghosh, Sayandeep ;Maruthi, R. ;Joshi, Deep C. ;Medwal, Rohit; Seehra, Mohindar SinghReported here are measurements and analysis of the magnetization (M) versus temperature (1.9–400 K) in magnetic fields H up to 90 kOe for a polycrystalline sample of Ising chain ferromagnet Co Nb2 O6 with TC = 2.9K. For T > TC, the fit of magnetic susceptibility χ = M /H (H = 300 Oe) to χ = χ0 + C / (T−Θ) with χ0 = 0.0009 emu mol–1 Oe–1 determined from high -T data yields Θ = 11K and magnetic moment μ = 4.994μB per Co2+ ion calculated from experimental C = 3.12 emu K mol–1 Oe–1. Values of g obtained from μ2 / μB2 = g2S (S + 1) for spin S = 1/2 and 3/2 are used to determine μZ = g S μB and compared with μZ obtained from saturation magnetization and neutron diffraction for T ≪ TC. This analysis of the data for both above and below TC shows that the ground state of Co2+ in Co Nb2 O6 has the effective spin S = 1/2 and not S = 3/2 expected from Hund's rules, the S = 1/2 ground state resulting from the combined effects of a noncubic crystalline field and spin-orbit coupling. The fit of the data for T > TC to χ = χ0 + (C/T) exp (J0 / 2kBT) valid for an Ising chain with S = 1/2 yields the intrachain ferromagnetic exchange constant J0 / kB = 6.2K, whereas the g value with S = 1/2 and the experimental critical fields for spin flips yields interchain antiferromagnetic exchange constants J1 / kB = −0.42K and J2 / kB = − 0.67K.WOS© Citations 16Scopus© Citations 17 64 53 - PublicationMetadata onlySubstrate orientation dependent characteristics of half-metallic and metallic superlattices [La0. 7Sr0. 3MnO3/LaNiO3] 10(2022)
;Das, Shaona ;Tanguturi, Ranganadha Gopalarao ;Ghosh, Sayandeep ;Dokala, Ravi Kiran ;Medwal, Rohit ;Gupta, Surbhi ;Yan, Zhuo ;Qi, Yan; Thota, SubhashWe report a detailed study on the orientation dependent growth characteristics, electronic structure, transport, magnetic, and vibrational excitations in atomically flat interfaces of [La0.7Sr0.3MnO3/LaNiO3]10 superlattices (SLs) coherently grown on (001/011/111)-SrTiO3 substrates by the pulsed laser deposition technique. X-ray reflectometry confirms the periodic superlattice stacks from the Kiessig interference fringes and well-defined even interfaces between the nickelate and manganite layers. A complex local atomic environment across the interfaces was noticed, yet trivalent La, divalent Sr, and mixed valent Ni2+/3+ and Mn3+/4+ electronic states prevail at the core level with enhanced relative intensity ratio of the Mn ions in the superlattices grown on (111) oriented SrTiO3 substrates as compared to those grown on (001) and (011) oriented SrTiO3. The temperature (5≤T≤300K) dependence of electrical resistivity 𝜌(𝑇) analysis reveals 3D variable range hopping model [𝜌(T)=𝜌0exp(T0/T)(1/4)] with large magnitude of hopping energies (≥40 meV) for the SL-111 system associated with the high energy gap developed by the accumulation of disorderness in the individual constituents of polar layers. Moreover, all SL systems exhibit reduced ferromagnetic ordering temperatures (67≤T𝐶≤110K) with a low-temperature anomaly (11.4≤T∗≤22K) and a substantial enhancement in the effective exchange interaction (J𝑒𝑓𝑓∼3.52meV) having altered ground state-spin configuration S∼1/2 different from S=3/2 of La0.75Sr0.25MnO3. Nevertheless, the SL-011 system exhibits large anisotropy field H𝐾∼18kOe and cubic anisotropy constant K1∼9.3×103J/m3 in comparison to the other two orientations. The second order two-phonon interaction driven by the local polaronic distortion causes significant changes in the vibrational excitations of the investigated system. Nonetheless, most of the Raman modes follow the substrate-induced, highly oriented epitaxial growth pattern except for two modes 𝜈4 (326cm−1) and 𝜈8 (728cm−1), which slightly differ in the case of SL-111 superlattices.WOS© Citations 2Scopus© Citations 2 111 - 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