Half-metallic transport and spin-polarized tunneling through the van der Waals ferromagnet Fe_4GeTe_2

The recent emergence of van der Waals (vdW) ferromagnets has opened new opportunities for designing spintronic devices. We theoretically investigate the coherent spin-dependent transport properties of the vdW ferromagnet Fe_4GeTe_2, by using density functional theory combined with the non-equilibrium Green's functions method. We find that the conductance in the direction perpendicular to the layers is half-metallic, namely it is entirely spin-polarized, as a result of the material's electronic structure. This characteristic persists from bulk to single layer, even under significant bias voltages, and it is little affected by spin-orbit coupling and electron correlation. Motivated by this observation, we then investigate the tunnel magnetoresistance (TMR) effect in an magnetic tunnel junction, which comprises two Fe_4GeTe_2 layers separated by the vdW gap acting as insulating barrier. We predict a TMR ratio of almost 500%, which can be further boosted by increasing the number of Fe_4GeTe_2 layers in the junction.