Ferroelectric control of altermagnetism in momentum space has been studied widely, while the control of magnetism in real space of altermagnets are still rare. We present a design rule to identify multiferroicity in n=2 Ruddlesden-Popper halides. Our results show that a Jahn-Teller distortion can cooperate with oxygen octahedral rotations to break inversion symmetry, which we demonstrate in K3Cr2F7 and cation-ordered KAg2Cu2Cl7, and leads to a ferrielectric-to-ferroelectric phase transition in K3Cr2F7. Altermagnetic spin order in the ferrielectric phase of K3Cr2F7 transforms into a conventional antiferromagnetic order in the ferroelectric phase, at which strain/pressure engineered sizable changes of weak ferromagnetism can occur. Our study is not only conducive to realize strong magnetoelectric coupling in multiferroics, but also reveals more functionalities in altermagnetic materials.
@article{arxiv.2508.13952,
title = {Piezomagnetism-driven magnetoelectric coupling in altermagnetic multiferroic K3Cr2F7},
author = {Ying Zhou and Hui-Min Zhang and Cheng-Ao Ji and Hongjun Xiang and Shuai Dong and James M. Rondinelli and Xue-Zeng Lu},
journal= {arXiv preprint arXiv:2508.13952},
year = {2025}
}