Unraveling the nature of ferrimagnetism and associated exchange interactions in distorted honeycomb Ni4Nb2O9
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
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.
Physical Review B