Determination of the tricritical point,-phase diagram and exchange interactions in the antiferromagnet MnTaO.

Using the analysis of the temperature and magnetic field dependence of the magnetization () measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (-) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTaOare determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTaOsample verified its phase purity. Temperature dependence of the magnetic susceptibility(=/) yields the Néel temperature= 5.97 K determined from the peak in the computed ∂()/∂vsplot, in agreement with the= 6.00 K determined from the peak in thevsdata. The experimental data ofvsnearis fitted to=|-|yielding the critical exponent= 0.10(0.13) for>(<). Thevsdata for> 25 K fits well with the modified Curie-Weiss law:=+/(-) with= -2.12 × 10emu mol Oeyielding= -24 K, and= 4.44 emu K mol Oe, the later giving magnetic moment= 5.96 per Mnion. This yields the effective spin= 5/2 and= 2.015 for Mn, in agreement with= 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes ofandand molecular field theory, the antiferromagnetic exchange constants/= -1.5 ± 0.2 K and/= -0.85 ± 0.05 K for Mnions along the chain-axis and perpendicular to the-axis respectively are determined. Thevsdata when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. The-phase diagram is mapped using the-isotherms and-data at differentyielding the tricritical point(,) = (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= 206.4 kOe and spin-flop field= 23.5 kOe yield the anisotropy field= 1.34 kOe. These results for MnTaOare compared with those reported recently in the isostructural MnNbO.