Exchange-Geometry Instability in Hg₂MnTeO₆ Double Perovskites: CM→ICM Magnetic Transition

En Pei Liu^1,2*, Sagar Mal Kumawat³, Chin Wei Wang⁴, Chi Cheng Lee⁵, Tzu Hao Liu³, Angel M. Arévalo-López⁶, Wei Tin Chen^2,7,8, Chien Lung Huang^3,9,8

¹Department of Physics, National Taiwan University, Taipei, Taiwan
²Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
³Department of Physics, National Cheng Kung University, Tainan, Taiwan
⁴National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁵Department of Physics, Tamkang University, New Taipei, Taiwan
⁶UMR-8181-UCCS-Unité de Catalyse et Chimie du Solide-Université Lille, CNRS, Centrale Lille, ENSCL, Université Artois, Lille, France
⁷Center of Atomic Initiative for New Materials, National Taiwan University, Taipei, Taiwan
⁸Taiwan Consortium of Emergent Crystalline Materials, National Science and Technology Council, Taipei, Taiwan
⁹Center for Quantum Frontiers of Research & Technology (QFort), National Cheng Kung University, Tainan, Taiwan

* Presenter:En Pei Liu, email:a24502p@gmail.com

Hg₂MnTeO₆ (HMTO) provides a clean platform to investigate how tiny structural change can redirect collective magnetism. In HMTO, Mn⁺ moments form a frustrated fcc network and are separated by nonmagnetic Te⁶⁺, crystalized in a rhombohedral space group R-3. Upon cooling, the “unlock” of a commensurate (CM) antiferromagnet to an incommensurate (ICM) state. The observation from magnetic susceptibility and neutron powder diffraction evidence no crystallographic symmetry changes and in the absence of Jahn–Teller activity, indicating that the behavior is owing to an exchange–geometry instability rather than charge or orbital ordering. The observation shows that the thermally induced changes in Mn–O–Te–O–Mn pathway rebalance competing exchange interactions. A further substitution investigation shows that the chemical pressure can shift the incommensurate offset and the CM–ICM boundary. The crystal structure, magnetic structure and magnetic phase transition of HMTO will be discussed, demonstrate a benchmark system where geometric tuning of super-superexchange governs a CM→ICM transition, and provides implications for designing controllable helices in double perovskites.

Keywords: Magnetic structure, NPD, Double perovskite, High pressure synthesis