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

Maruthi R; Mohindar S Seehra; Sayandeep Ghosh; Rohit Medwal; Rajdeep S Rawat; Bruno Weise; Eun Sang Choi; Subhash Thota

doi:10.1088/1361-648X/ac4cec

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

J Phys Condens Matter. 2022 Feb 7;34(15). doi: 10.1088/1361-648X/ac4cec.

Authors

Maruthi R¹, Mohindar S Seehra², Sayandeep Ghosh¹, Rohit Medwal³, Rajdeep S Rawat³, Bruno Weise⁴, Eun Sang Choi⁵, Subhash Thota¹

Affiliations

¹ Department of Physics, Indian Institute of Technology Guwahati, 781039, Assam, India.
² Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, United States of America.
³ National Institute of Education, Nanyang Technological University, Singapore 637616, Singapore.
⁴ Leibniz-IFW Dresden, Institute for Complex Materials, D-01069 Dresden, Germany.
⁵ National High Magnetic Field Laboratory, Tallahassee, FL 32310-3706, United States of America.

PMID: 35045399
DOI: 10.1088/1361-648X/ac4cec

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.