Magnetic Field-Temperature Phase Diagram, Exchange Constants And Specific Heat Exponents Of The Antiferromagnet Mnnb2o6

This work presents the magnetic field-temperature (H-T) phase diagram, exchange constants, specific heat (C (P)) exponents and magnetic ground state of the antiferromagnetic MnNb2O6 polycrystals. Temperature dependence of the magnetic susceptibility chi (=M/H) yields the Neel temperature T (N) = 4.33 K determined from the peak in the computed partial differential (chi T)/ partial differential T vs T plot in agreement with the transition in the C (P) vs T data at T (N) = 4.36 K. The experimental data of C (P) vs T near T (N) is fitted to C (P) = A|T - T (N)|(-alpha ) yielding the critical exponent alpha = 0.12 (0.15) for T > T (N) (T < T (N)). The best fit of chi vs T data for T > 50 K to chi = chi (0) + C/(T - theta) with chi (0) = -1.85 x 10(-4) emu mol(-1) Oe(-1) yields theta = -17 K, and C = 4.385 emu K mol(-1) Oe(-1), the latter giving magnetic moment mu = 5.920 mu (B) 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 theta and T (N) and molecular field theory (MFT), the exchange constants J (0)/k (B) = -1.08 K for Mn2+ ions along the chain c-axis and J (perpendicular to)/k (B) = -0.61 K as the interchain coupling perpendicular to c-axis are determined. These exchange constants are consistent with the expected chi 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 T (TP) (H, T) = (18 kOe, 4.06 K). The spin-flopped state above T (TP) and the forced ferromagnetism for H > 192 kOe are used to estimate the anisotropy energy H (A) approximate to 0.8 kOe.