Coexistence of ferromagnetism and charge density waves in monolayer LaBr 2 .

Charge density waves (CDWs), a common phenomenon of periodic lattice distortions, often suppress ferromagnetism in two-dimensional (2D) materials, hindering their magnetic applications. Here, we report a novel CDW that generates 2D ferromagnetism instead of suppressing it, through the formation of interstitial anionic electrons as the charge modulation mechanism. Via first-principles calculations and a low-energy effective model, we find that the highly symmetrical monolayer LaBr 2 undergoes a 2 × 1 CDW transition to a magnetic semiconducting T' phase. Concurrently, the delocalized 5d 1 electrons of La in LaBr 2 redistribute and accumulate within the interstitial space in the T' phase, forming anionic electrons, also known as 2D electride or electrene. The strongly localized nature of anionic electrons promotes a Mott insulating state and full spin-polarization, while the overlap of their extended tails yields ferromagnetic direct exchange between them. Such transition introduces a new magnetic form of CDWs, offering promising opportunities for exploring novel fundamental physics and advanced spintronics applications.