Impurity-Assisted Interlayer Exchange Coupling across a Tunnel Barrier

Localized impurity or defect states in the insulating barrier layer separating two ferromagnetic films affect dramatically the interlayer exchange coupling (IEC), making it significantly stronger compared to perfect barriers. We demonstrate that the impurity-assisted IEC becomes antiferromagnetic if the energy of the impurity states matches the Fermi energy and that the coupling strength decreases with temperature. These results explain available experimental data on the IEC across tunnel barriers.

Figure 1

IEC versus impurity energy $E_i$ (given with respect to $E_F$) for different impurity positions $r_i$ within the barrier of thickness $d=8\AA$.

Figure 2

Energy-resolved contributions to the spin current from majority and minority spin electrons incident from the left ferromagnet [(a),(b)] and local densities of states (LDOS) at the impurity position ${\mathbf{r}}_i$ [(c),(d)] for $E_i=-0.3\mathrm{e}\mathrm{V}$ [(a),(c)] and $E_i=0$ [(b),(d)]. Shadow regions show filled states which contribute to the spin current at $T=0$, $d=8\AA$, and $r_i=3\AA$.

Figure 3

IEC averaged over impurity position versus (a) impurity energy for different barrier thicknesses and (b) barrier thicknesses for different impurity energies.

Figure 4

IEC averaged over impurity position as a function of temperature for different barrier layer thicknesses and $E_i=0$.