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

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 MnTa2O6 are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa2O6 sample verified its phase purity. Temperature dependence of the magnetic susceptibility chi (=M/H) yields the Neel temperature T (N) = 5.97 K determined from the peak in the computed partial differential (chi T)/ partial differential T vs T plot, in agreement with the T (N) = 6.00 K determined from the peak in the C (P) vs T data. 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 = 0.10(0.13) for T > T (N) (T < T (N)). The chi vs T data for T > 25 K fits well with the modified Curie-Weiss law: chi = chi (0) + C/(T - theta) with chi (0) = -2.12 x 10(-4) emu mol(-1) Oe(-1) yielding theta = -24 K, and C = 4.44 emu K mol(-1) Oe(-1), the later giving magnetic moment mu = 5.96 mu (B) per Mn2+ ion. This yields the effective spin S = 5/2 and g = 2.015 for Mn2+, in agreement with g = 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes of theta and T (N) and molecular field theory, the antiferromagnetic exchange constants J (0)/k (B) = -1.5 +/- 0.2 K and J (perpendicular to)/k (B) = -0.85 +/- 0.05 K for Mn2+ ions along the chain c-axis and perpendicular to the c-axis respectively are determined. The chi vs 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 T (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 field H (E) = 206.4 kOe and spin-flop field H (SF) = 23.5 kOe yield the anisotropy field H (A) = 1.34 kOe. These results for MnTa2O6 are compared with those reported recently in the isostructural MnNb2O6.

Automated summary

By analyzing the temperature and magnetic field dependence of magnetization in MnTa2O6, the magnetic field-temperature phase diagram, tricritical point, and exchange constants were determined. Experimental results were consistent with theoretical predictions and previous studies, confirming the phase purity of the material.