Magnetic field-temperature phase diagram, exchange constants and specific heat exponents of the antiferromagnet MnNbO.

This work presents the magnetic field-temperature (-) phase diagram, exchange constants, specific heat () exponents and magnetic ground state of the antiferromagnetic MnNbOpolycrystals. Temperature dependence of the magnetic susceptibility(=/) yields the Néel temperature= 4.33 K determined from the peak in the computed ∂()/∂vsplot in agreement with the transition in thevsdata at= 4.36 K. The experimental data ofvsnearis fitted to=|-|yielding the critical exponent= 0.12 (0.15) for>(<). The best fit ofvsdata for> 50 K to=+/(-) with= -1.85 × 10emu molOeyields= -17 K, and= 4.385 emu K molOe, the latter giving magnetic moment= 5.920per Mnion. This confirms the effective spin= 5/2 and= 2.001 for Mnand the dominant exchange interaction being antiferromagnetic in nature. Using the magnitudes ofandand molecular field theory (MFT), the exchange constants/= -1.08 K for Mnions along the chain-axis and/= -0.61 K as the interchain coupling perpendicular to-axis are determined. These exchange constants are consistent with the expectedvsvariation for the Heisenberg linear chain. The-phase diagram, mapped using the-isotherms and-data at differentcombined with the reported data of Nielsen, yields a triple-point(,) = (18 kOe, 4.06 K). The spin-flopped state aboveand the forced ferromagnetism for> 192 kOe are used to estimate the anisotropy energy≈ 0.8 kOe.