Van der Waals Spin Valves

We propose spin valves where a 2D nonmagnetic conductor is intercalated between two ferromagnetic insulating layers. In this setup, the relative orientation of the magnetizations of the insulating layers can have a strong impact on the in-plane conductivity of the 2D conductor. We first show this for a graphene bilayer, described with a tight-binding model, placed between two ferromagnetic insulators. In the antiparallel configuration, a band gap opens at the Dirac point, whereas in the parallel configuration, the graphene bilayer remains conducting. We then compute the electronic structure of graphene bilayer placed between two monolayers of the ferromagnetic insulator ${\mathrm{CrI}}_3$, using density functional theory. Consistent with the model, we find that a gap opens at the Dirac point only in the antiparallel configuration.

Figure 1

Van der Waals spin valve, where a conducting 2D crystal is sandwiched between two insulating ferromagnets. Lateral contacts can drive in-plane current. The magnetization of the bottom ferromagnetic layer is pinned, whereas the top layer can switch, resulting in two configurations, (a) antiparallel (AF) and (b) parallel (FM), with very different in-plane conductance.

Figure 2

Bands structure computed with model Hamiltonian for the FM (left) and AF (right) configurations, Red and blue denote different spin states. For the AF state, the bands have been projected in the upper and lower layer of the graphene bilayer, with the size of the dots proportional to the layer polarization, revealing that the top of the valence band and bottom of the conduction band are spin polarized in each of the layers.

Figure 3

(a) Momentum resolved density of states revealing chiral in-gap states, for a given valley, computed for the domain wall between two insulating antiferromagnetic domains with opposite spin orientation (b). The velocity of the bands changes sign in the opposite valley.

Figure 4

(a) Band structure, for the graphene monolayer on top of a ${\mathrm{CrI}}_3$ monolayer. (b) Scheme of energy bands for graphene monolayer on ${\mathrm{CrI}}_3$. (c),(d) Band structure for grahene bilayer placed between ferromagnetic monolayers of ${\mathrm{CrI}}_3$ with FM (c) and AF (d) relative alignment. The insets show a closeup around the Dirac point, showing a spin splitting in the FM alignment and a gap for the AF case, both taking the same value of 7 meV.