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 (CP) exponents and magnetic ground state of the antiferromagnetic MnNb2O6polycrystals. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureTN= 4.33 K determined from the peak in the computed ∂(χT)/∂TvsTplot in agreement with the transition in theCPvsTdata atTN= 4.36 K. The experimental data ofCPvsTnearTNis fitted toCP=A|T-TN|-αyielding the critical exponentα= 0.12 (0.15) forT>TN(T<TN). The best fit ofχvsTdata forT> 50 K toχ=χ0+C/(T-θ) withχ0= -1.85 × 10-4emu mol-1Oe-1yieldsθ= -17 K, andC= 4.385 emu K mol-1Oe-1, the latter giving magnetic momentμ= 5.920μBper Mn2+ion. This confirms the effective spinS= 5/2 andg= 2.001 for Mn2+and the dominant exchange interaction being antiferromagnetic in nature. Using the magnitudes ofθandTNand molecular field theory (MFT), the exchange constantsJ0/kB= -1.08 K for Mn2+ions along the chainc-axis andJ⊥/kB= -0.61 K as the interchain coupling perpendicular toc-axis are determined. These exchange constants are consistent with the expectedχvsTvariation for the Heisenberg linear chain. TheH-Tphase diagram, mapped using theM-Hisotherms andM-Tdata at differentHcombined with the reported data of Nielsenet al, yields a triple-pointTTP(H,T) = (18 kOe, 4.06 K). The spin-flopped state aboveTTPand the forced ferromagnetism forH> 192 kOe are used to estimate the anisotropy energyHA≈ 0.8 kOe.