  Europium, one of the rare earth elements, exhibits +2 and +3 valence states
and has been widely used for magnetic modification of materials. Based on
density functional theory calculations, we predict the 2D EuBr/graphene
heterojunctions to exhibit metallicity, huge intrinsic-ferromagnetism nearly
7.0 {\mu}B per Eu and the special monovalent Eu ions. Electron localization
function (ELF), difference charge densities and Bader charge analyses
demonstrate that there are cation-{\pi} interactions between the EuBr films and
graphene, which explains the stability of these unusual heterojunctions.
Graphene works as substrate to enable the stability of EuBr monolayer crystals
where EuBr plays an important role to yield ferromagnetism and enhance
metallicity in the heterojunctions. Monte Carlo simulations are used to
estimate a Curie temperature of about 7 K, which, together with magnetic
configurations, can be further modulated by external strains and charge-carrier
doping. In general, our theoretical work predicts the properties of the novel
2D ferromagnetic EuBr/graphene heterojunctions, suggests the possibility of
combining the 2D intrinsic-ferromagnetic metal halide crystals and graphene,
and opens up a new perspective in next-generation electronic, spintronic
devices and high-performance sensors.
