A strain induced polar metal phase in a ferromagnetic Fe₃GeTe₂ monolayer

The electronic structure and magnetic properties of the ferromagnetic Fe₃GeTe₂ monolayer have been extensively studied in recent years. Experimentally, external strain can be produced inevitably during the growth on the substrate. However, the impact of strain on the structural, electronic, and magnetic properties remains largely underexplored. Herein, by using density functional theory, we systematically investigate the crystalline configuration and electronic structure of the Fe₃GeTe₂ monolayer in the presence of external strain. We find that a moderate compressive strain could break the structural vertical symmetry, leading to a sizable out-of-plane dipole moment, while the ferromagnetism can be retained. Surprisingly, strain-induced polarization in the off-center Fe and Ge atoms barely contributes to the energy states at the Fermi level. The efficient decoupling of the conductivity and polarization in the strained Fe₃GeTe₂ monolayer results in an extremely rare phase with the coexistence of polarization, metallicity, and ferromagnetism, i.e., magnetic polar metals for potential applications in magnetoelectricity and spintronics.