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.

Automated summary

This study presents the magnetic field-temperature phase diagram, exchange constants, and magnetic ground state of antiferromagnetic MnNb2O6 polycrystals. Experimental data agrees with theoretical analysis, providing values for the Neel temperature, critical exponent, magnetic moment, and exchange constants. The study also maps out the H-T phase diagram and estimates the anisotropy energy of the material.