Unraveling the nature of ferrimagnetism and associated exchange interactions in distorted honeycomb Ni4Nb2O9

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Citation
Subhash Thota, Mohindar S. Seehra, Mouli Roy Chowdhury, Singh, H., Sayandeep Ghosh, Jena, S. K., Pramanik, P., Sarkar, T., Rawat, R. S., Rohit Medwal. & Weise, B. (2022). Unraveling the nature of ferrimagnetism and associated exchange interactions in distorted honeycomb Ni4Nb2O9. Physical Review B, 106(13), Article 134418. https://doi.org/10.1103/physrevb.106.134418
Author
Subhash Thota
•
Mohindar S. Seehra
•
Mouli Roy Chowdhury
•
Singh, H.
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Sayandeep Ghosh
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Jena, S. K.
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Pramanik, P.
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Sarkar, T.
•
Rawat, Rajdeep Singh 
•
Rohit Medwal
•
Weise, Bruno
Abstract
Ferrimagnetism 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.
Date Issued
2022
Journal
Physical Review B
DOI
10.1103/physrevb.106.134418