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Magnetic transitions, exchange constants, spin-flop transition, and spin-phonon coupling in Ising-like antiferromagnet ${Mn}_{4} {Ta}_{2} O_{9}$

Harshita Singh ¹, Mohindar S. Seehra ², Arjyama Bordoloi ³, Bruno Weise ⁴, Tapati Sarkar ⁵, Vasant Sathe⁶, Wilfrid Prellier ⁷, Sobhit Singh ^3,8, and Subhash Thota ^1,*

¹Department of Physics, Indian Institute of Technology Guwahati 781039, Assam, India
²Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA
³Department of Mechanical Engineering, University of Rochester, Rochester, New York 14627, USA
⁴Leibniz-IFW Dresden, Institute for Complex Materials, D-01069 Dresden, Germany
⁵Department of Materials Science and Engineering, Uppsala University, SE-751 03 Uppsala, Sweden
⁶UGC DAE Consortium for Scientific Research, Indore 452 001, India
⁷Laboratoire CRISMAT, ENSICAEN, CNRS, Université de Caen, Normandie Université, Caen, France
⁸Materials Science Program, University of Rochester, Rochester, New York 14627, USA

^*Contact author: subhasht@iitg.ac.in

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Phys. Rev. Materials 10, 024405 – Published 12 February, 2026

DOI: https://doi.org/10.1103/xx7d-xvly

Abstract

${Mn}_{4} {Ta}_{2} O_{9}$ (MTO) is a magnetoelectric material with Ising-like antiferromagnetism (AFM) in which an applied magnetic field ( $H$ ) induces electric polarization and an applied electric field produces magnetization. Here, we report results from our detailed investigations of the temperature ( $T$ ) and $H$ dependence of magnetization ( $M$ ) and heat capacity of MTO. These results show AFM ordering in MTO below $T_{N} \sim$ 102 K followed by additional transitions near $T_{S 1} \sim$ 50 K and $T_{S 2} \sim$ 20 K. The $M$ vs $H$ variations at 3 K show the presence of weak ferromagnetism (FM) below $T_{S 1}$ superposed on AFM, with coercivity becoming zero for $T \geq T_{S 1}$ . This weak FM for $T \leq T_{S 1}$ is likely due to two inequivalent ${Mn}^{2 +}$ ions present in MTO. The magnetic susceptibility $χ$ vs $T$ data for $T > T_{N}$ is used to determine two dominant AFM exchange constants $J_{2} / k_{B}$ = –11.3 K and $J_{3} / k_{B}$ = –22.9 K, which are in good agreement with those determined from the first-principles density-functional theory ( $DFT + U$ ) calculations. It is argued that the $H$ -induced transition observed at 3 K at the critical field $H_{c} \sim$ 20 kOe is a spin flop transition whose surprising shift to higher $H$ values with increasing $T$ is explained in terms of $T$ dependence of anisotropic $χ$ , magnetic anisotropy, and exchange fields. The temperature dependence of magnetic specific heat and entropy shows the presence of short-range spin correlations ordering up to 140 K, a $λ$ -type anomaly at $T_{N}$ , and a broad anomalous peak near 40 K covering the region of $T_{S 1}$ and $T_{S 2}$ . To further explore the nature of these transitions at $T_{N}, T_{S 1}$ , and $T_{S 2}$ , the $T$ dependence of various Raman modes of MTO was measured and compared with the lattice dynamics calculations from $DFT + U$ . For $T > T_{N}, T$ dependencies of the frequency and linewidth of the Raman modes are described by the three-phonon anharmonic approximation with considerable departures in these quantities observed at $T_{N}, T_{S 1}$ , and $T_{S 2}$ , which are then related to the strong spin-phonon coupling evident in MTO.

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Magnetic transitions, exchange constants, spin-flop transition, and spin-phonon coupling in Ising-like antiferromagnet ${Mn}_{4} {Ta}_{2} O_{9}$

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Magnetic transitions, exchange constants, spin-flop transition, and spin-phonon coupling in Ising-like antiferromagnet Mn4Ta2O9

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Magnetic transitions, exchange constants, spin-flop transition, and spin-phonon coupling in Ising-like antiferromagnet ${Mn}_{4} {Ta}_{2} O_{9}$