Determination of the tricritical point, H–T phase diagram and exchange interactions in the antiferromagnet MnTa₂O₆

Abstract

Using the analysis of the temperature and field dependence of the magnetization measurements, the H-T phase diagram, tricritical point and exchange constants of the antiferromagnetic (AFM) MnTa2O6 are determined in this work. Temperature dependence of magnetic susceptibility χ(T) yields the Néel temperature TN = 5.97 K determined from the peak in the ∂(χT)/∂T vs. T plot, in agreement with the TN = 6K determined from the peak in the Cp vs. T data. The experimental data of Cp vs. T near TN is fitted to Cp = A|T-TN|-α yielding the critical exponent α = 0.09 (0.13) for T > TN (T < TN). The χ-T data for T > 25 K fits well with the modified Curie-Weiss law: χ = χ0 + C/(T-θ) with χ0 = -2.12 × 10-4 emu.mol-1Oe-1 yielding θ = -24 K, and C = 4.44 emu.K.mol-1Oe-1, the later giving μ= 5.96 μB per Mn2+. This yields the effective spin S = 5/2 and g = 2.015 for Mn2+, in agreement with g = 2.0165 measured using ESR spectroscopy. Using the magnitudes of θ and TN and molecular field theory, the AFM exchange constants J0/kB = -1.5 ± 0.2 K and J⊥/kB = -0.85 ± 0.05 K for Mn2+ ions along the chain c-axis and perpendicular to the c-axis respectively are determined. The χ-T data when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. The H-T phase diagram is mapped using the M-H isotherms and M-T data at different H yielding the tricritical point TTP (H, T) = (17.0 kOe, 5.69 K) separating the paramagnetic, AFM, and spin-flop phases. At 1.5 K, the experimental magnitudes of the exchange field HE = 206.4 kOe and spin-flop field HSF = 23.5 kOe yield the anisotropy field HA = 1.34 kOe.