Positive spin polarization in $\mathrm{Co}∕{\mathrm{Al}}_2{\mathrm{O}}_3∕\mathrm{Co}$ tunnel junctions driven by oxygen adsorption

Using a first-principles Green’s function technique, we study spin-dependent tunneling in two model realizations of (111) fcc $\mathrm{Co}∕{\mathrm{Al}}_2{\mathrm{O}}_3∕\mathrm{Co}$ tunnel junctions assuming O-terminated crystalline epitaxy in the corundum structure. For the first model, which includes 3 O atoms at the interface, the tunneling current is polarized negatively, just as for the clean Co surface. The second model contains additional oxygen atoms inside large pores at each interface. Located at the three-fold hollow adsorption sites, these O atoms bind very strongly to Co. This bonding creates an interface band in the majority-spin channel which strongly enhances the tunneling current in this channel. As a result, the spin polarization changes sign and becomes positive, similar to that for the oxidized Co surface studied previously. These results show that the common argument of “mostly $s$-electron tunneling,” which is often used to explain the positive spin polarization in $\mathrm{Co}∕{\mathrm{Al}}_2{\mathrm{O}}_3∕\mathrm{Co}$ junctions is quantitatively incorrect. In reality, the spin polarization in these junctions is controlled by the interfacial structure and bonding. Moreover, interfacial adsorption of oxygen may be a prerequisite for achieving the positive spin polarization.

Figure 1

Interfacial structure for model 1 (a), (b) and model 2 (c), (d). Panels (a), (c) show “front” views from a direction normal to the three-fold axis; panels (b), (d) show “top” views along the three-fold axis. There are two types of Co and O atoms at the interface for model 2: three O(I) atoms, one O(II) atom, one Co(I) atom, and three Co(II) atoms per unit cell.

Figure 2

${\mathbf{k}}_{\Vert }$-resolved transmission in units of $e^2∕h$ (logarithmic scale) in $\mathrm{Co}∕{\mathrm{Al}}_2{\mathrm{O}}_3∕\mathrm{Co}$ tunnel junctions with different interface structures. (a), (b) model 1; (c), (d) model 2. (a), (c) Majority spin; (b), (d) minority spin.

Figure 3

${\mathbf{k}}_{\Vert }$-resolved majority-spin densities of states at the Fermi level for different atoms at the $\mathrm{Co}∕{\mathrm{Al}}_2{\mathrm{O}}_3$ interface for model 2. (a) Co(I), (b) Co(II), (c) O(II), and (d) O(I). All values are given per atom of the given type. The units are arbitrary.

Figure 4

Partial densities of states for model 2. Energy is in electron volts, DOS is in ${\mathrm{electron\; volts}}^{-1}$ per atom. Co(I), Co(II), O(I), and O(II) atoms are at the $\mathrm{Co}∕{\mathrm{Al}}_2{\mathrm{O}}_3$ interface (see Fig. 1). Layer numbers for other atoms increase into the barrier. For example, Layer-II Al is just in the middle of the barrier. Layer-I Al DOS is the average of those for two inequivalent Al atoms in this layer.

Figure 5

Quality factor (see text) for model 1 plotted in the surface Brillouin zone. $Q=1$ corresponds to factorizing transmission. Gray shading shows the region where $0.85<Q<1.15$.

Figure 6

Five smallest imaginary parts of the eigenstate wave vectors of ${\mathrm{Al}}_2{\mathrm{O}}_3$ strained to the epitaxy of model 1.