Unusual Metallic Multiferroic Transitions in Electron-Doped PbTiO3

The coexistence of some materials which are normally in mutually exclusive states is attracting considerable attention as an intriguing means of obtaining nontrivial physical phenomena and unconventional multifunctional substances. Although single-phase materials endowed with integrated ferroelectric, magnetic, and optical multifunctions hold promise for new technological paradigms, the mutually exclusive mechanisms within ferroelectricity, conductivity, and magnetism hinder the discovery of conducting multiferroics. Here, a new path toward metallic multiferroics is provided by theoretically demonstrating the possible compatible nature of ferroelectric distortion, free carriers, and magnetism in electron-doped PbTiO3 using the hybrid Hartree-Fock density functional theories. Doping with electrons is found to induce metallic conductivity that coexists with and even enhances the ferroelectric distortion in PbTiO3, due to the unique lone-pair ferroelectricity in this material. The injected excess electrons, in spin-polarized states, interact with one another in the plane perpendicular to the polar direction, resulting in layer-arranged ferromagnetism and multiferroics with nonlinear magnetoelectric coupling. These results indicate a means of circumventing conventional restrictions, leading to new types of multifunctional materials in which unusual multiferroic and conductive characteristics are simultaneously present.