Tunable magnetism in two-dimensional ferroelectric Janus NbOXY (X, Y = Cl, Br, I; X ≠ Y) by hole doping

Abstract

Two-dimensional (2D) ferroelectric materials exhibit spontaneous polarization, offering significant potential for applications in low-dimensional electronics and data storage. In this study, we predict the ferroelectric properties of Janus NbOXY (X, Y = Cl, Br, I; X ≠ Y) monolayers using first-principles calculations. Our results reveal intrinsic ferroelectric polarization values of 1.959, 2.736 and 2.559 × 10⁻¹⁰ C m⁻¹ for NbOClBr, NbOClI and NbOBrI, respectively. Notably, the transitions from ferroelectric to antiferroelectric (FE–AFE) is energetically more favorable than the conventional ferroelectric-to-paraelectric (FE–PE) transition, with energy barriers of 23.794, 16.660, and 15.756 meV per f.u.⁻¹ for NbOClBr, NbOClI, and NbOBrI, respectively. Ab initio molecular dynamics (AIMD) stimulations further demonstrate that the NbOClBr monolayer exhibits room-temperature ferroelectricity. Moreover, applying strain along the x-axis significantly modulates both the polarization intensity and the phase transition barrier, yielding a maximum change in spontaneous polarization of over 60%. Importantly, hole doping provides a pathway for introducing magnetism, while the ferroelectric phase transitions are more likely to induce changes in the spin state, facilitating the strong coupling of ferroelectricity and ferromagnetism in NbOXY monolayers. Our findings not only expand the family of 2D ferroelectrics but also pave the way for the development of innovative multiferroic technologies.