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

Abstract

Ferrimagnetism in orthorhombic Ni₄, Nb₂ O_g below its Néel temperature. <i>TFN</i> ~ 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 > <i>TFN</i> and T < <i>TFN</i> 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 [x_ac,(f, 7, H)], and heat-capacity [C_p (T, H)] measurements are combined with theoretical analysis to provide new insights into the nature of ferrimagnetism in Ni₄, Nb₂ O_g. 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 O_g. Analysis of x_ac(T) shows the paramagnetic-to-ferrimagnetic transition occurs at 76.5 K <i>TFN</i>, which increases with applied field H as <i>TFN</i> ∝ H^0.35 due to the coupling of the ferromagnetic component with H. For T > <i>TFN</i>, 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 N_AA, = 26.31, N_BB = 8.59. and N_AB = 43.06, which, in turn, yield the antiferromagnetic exchange parameters: J_AA/kp = 4.27 K, J_BB/kp = 1.40 K, and J_AB/k_B = 6.98 K. For T < <i>TFN</i>, the M versus T data clearly show the magnetic compensation point at T_com ~ 33 K. The mathematical model presented here using the magnitudes of N_AA, N_BB and N_AB Correctly predicts the position of Toy as well the temperature variation of M both above and below T_COM. 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 — T_N)(-∝) yielding the critical exponent ∝ = 0.14(0.12) for <i>TFN</i> (T > <i>TFN</i>), 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 > <i>TFN</i> and T < T_com. but for T_com < T <i>TFN</i>, 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 <i>TFN</i> initially show a decreasing trend. becoming zero atT_COM, then followed by an increase and eventually becoming zero again at <i>TFN</i>.