Spin-Injection Enhancements in van der Waals Magnetic Tunnel Junctions through Barrier Engineering

The weak interlayer coupling in two-dimensional materials enables the formation of sharp crystalline magnetic tunnel junctions without the epitaxial constraints found in the bulk. Amid the large number of heterostructures that can be formed using these layered materials, a means to guide the experimental design of systems with enhanced responses is desired. Here, we attain meaningful improvements in spin injection by tailoring the tunneling barriers through the choice of the metal electrodes. Owing to the weak coupling, the barrier engineering can be rationalized from properties of bulk components from first-principles calculations, leading to superior spin injection and magnetoresistance. Analysis of ${\mathrm{Cr}\mathrm{I}}_3$ junctions formed with transition-metal dichalcogenide electrodes shows that junction conductivities increase by nearly 3 orders of magnitude with respect to those experimentally demonstrated with graphite leads. Moreover, we find that tunneling magnetoresistance significantly augments with low-work-function electrodes when carriers are injected near the ${\mathrm{Cr}\mathrm{I}}_3$ conduction-band edge. The predictive approach employed in this work shows good agreement with detailed quantum transport calculations and can potentially accelerate the design of tunnel junctions based on two-dimensional materials.

Figure 1

(a) The CBS for bulk ${\mathrm{Cr}\mathrm{I}}_3$ along different symmetry lines (from left to right): $\mathrm{\Gamma }$-$A$, $K$-$H$, and $M$-$L$. The blue (red) lines represent majority (minority) evanescent states produced in the FM configuration, while the gray lines correspond to states in the AFM configuration. (b) Approximate transmission probabilities ${\overline{T}}_s^m(E)$ and (c) the corresponding $\overline{\mathrm{TMR}}$ results obtained from CBS for ${\mathrm{Cr}\mathrm{I}}_3$ channels with bilayer (2L) and tetralayer (4L) thicknesses. The energies are referenced to the FM ${\mathrm{Cr}\mathrm{I}}_3$ spin-majority midgap and the shaded areas denote the location of the valence and conduction bands.

Figure 2

(a) Color maps of the most stable metallic TMD monolayer work functions, where transition metals are arranged following their location in the periodic table; each row corresponds to a different chalcogen element: $\mathrm{S}$ (top), $\mathrm{Se}$ (middle), and $\mathrm{Te}$ (bottom); the columns denote $1T$ (left) and $2H$ (right) phase. (b) The DOS for monolayer TMDs: $1T$-${\mathrm{Ta}\mathrm{Se}}_2$ (blue), $2H$-${\mathrm{Nb}\mathrm{Se}}_2$ (green), and $2H$-${\mathrm{Ta}\mathrm{S}}_2$ (red). For comparison, graphene is also shown. (c) The 2D-BZ filling factor $\nu (E)$ for TMD bulk electrodes with $2\times 2$ epitaxy. (d)–(f) The electron modes of bulk electrodes at various energy levels: (d) $1T$-${\mathrm{Ta}\mathrm{Se}}_2$, (e) $2H$-${\mathrm{Nb}\mathrm{Se}}_2$, and (f) $2H$-${\mathrm{Ta}\mathrm{S}}_2$. Hexagons denote the 2D-BZ edges.

Figure 3

(a) A schematic of a ($2\times 2$)-TMD/($1\times 1$)-${\mathrm{Cr}\mathrm{I}}_3$ magnetic tunnel junction, where solid lines mark the supercell of the structure. (b)–(d) Layer-resolved PDOS for the 2L-${\mathrm{Cr}\mathrm{I}}_3$ channel and different metal electrodes (from top to bottom): (b) $1T$-${\mathrm{Ta}\mathrm{Se}}_2$, (c) $2H$-${\mathrm{Nb}\mathrm{Se}}_2$, and (d) $2H$-${\mathrm{Ta}\mathrm{S}}_2$. Spin-resolved projections onto localized atomic orbitals on one of the ${\mathrm{Cr}\mathrm{I}}_3$ layers and its adjacent TMD layer are plotted in the right and left column, respectively.

Figure 4

Transmissions $T_s^m$ (top) and the corresponding TMR (bottom) for 2L- and 4L-thick ${\mathrm{Cr}\mathrm{I}}_3$ MTJs with different electrodes: (a) $1T$-${\mathrm{Ta}\mathrm{Se}}_2$, (b) $2H$-${\mathrm{Na}\mathrm{Se}}_2$, and (c) $2H$-${\mathrm{Ta}\mathrm{S}}_2$. For the FM configuration, we plot the spin-majority (blue) and -minority (red) contributions; while for the AFM configuration we average the spin channels (black). The shaded areas denote ${\mathrm{Cr}\mathrm{I}}_3$ spin-majority band edges. Curves for bilayer (tetralayer) junctions are indicated by dotted (solid) lines. Black (green) portions of the TMR curves denote ${\sigma }_{\min }={\sigma }_{\mathrm{AFM}}$ (${\sigma }_{\min }={\sigma }_{\mathrm{FM}}$).