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

Abstract

Using the analysis of the temperature and magnetic field dependence of the magnetization (M) measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (H-T) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTa₂O₆are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa₂O₆sample verified its phase purity. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureT_N= 5.97 K determined from the peak in the computed ∂(χT)/∂TvsTplot, in agreement with theT_N= 6.00 K determined from the peak in theC_PvsTdata. The experimental data ofC_PvsTnearT_Nis fitted toC_P=A|T-T_N|^-αyielding the critical exponentα= 0.10(0.13) forT>T_N(T<T_N). TheχvsTdata forT> 25 K fits well with the modified Curie-Weiss law:χ=χ₀+C/(T-θ) withχ₀= -2.12 × 10^-4emu mol^-1 Oe^-1yieldingθ= -24 K, andC= 4.44 emu K mol^-1 Oe^-1, the later giving magnetic momentμ= 5.96 μ_Bper Mn²⁺ion. This yields the effective spinS= 5/2 andg= 2.015 for Mn²⁺, in agreement withg= 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes ofθandT_Nand molecular field theory, the antiferromagnetic exchange constantsJ₀/k_B= -1.5 ± 0.2 K andJ_⊥/k_B= -0.85 ± 0.05 K for Mn²⁺ions along the chainc-axis and perpendicular to thec-axis respectively are determined. TheχvsTdata when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. TheH-Tphase diagram is mapped using theM-Hisotherms andM-Tdata at differentHyielding the tricritical pointT_TP(H,T) = (17.0 kOe, 5.69 K) separating the paramagnetic, antiferromagnetic, and spin-flop phases. At 1.5 K, the experimental magnitudes of the exchange fieldH_E= 206.4 kOe and spin-flop fieldH_SF= 23.5 kOe yield the anisotropy fieldH_A= 1.34 kOe. These results for MnTa₂O₆are compared with those reported recently in the isostructural MnNb₂O₆.

Keywords: H–T phase diagram; antiferromagnet; exchange interactions; tricritical point.