Magnetic field-temperature phase diagram, exchange constants andspecific heat exponents of the antiferromagnet MnNb₂O₆

Abstract

This 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.