Effect of interface states on spin-dependent tunneling in $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}∕\mathrm{Fe}$ tunnel junctions

The electronic structure and spin-dependent tunneling in epitaxial $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}∕\mathrm{Fe}\left(001\right)$ tunnel junctions are studied using first-principles calculations. For small MgO barrier thickness the minority-spin resonant bands at the two interfaces make a significant contribution to the tunneling conductance for the antiparallel magnetization, whereas these bands are, in practice, mismatched by disorder and/or small applied bias for the parallel magnetization. This explains the experimentally observed decrease in tunneling magnetoresistance (TMR) for thin MgO barriers. We predict that a monolayer of Ag epitaxially deposited at the interface between Fe and MgO suppresses tunneling through the interface band and may thus be used to enhance the TMR for thin barriers.

Figure 1

Normalized minority-spin ${\mathbf{k}}_{\Vert }$-resolved DOS at the interfacial Fe layer in $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}∕\mathrm{Fe}\left(001\right)$ MTJ for three values of energy: (a) $0.02\mathrm{eV}$ below $E_F$, (b) at $E_F$, and (c) $0.02\mathrm{eV}$ above $E_F$. The scale is logarithmic. Interface states and resonances are marked as IS and IR in panel (b).

Figure 2

Transmission probability as a function of ${\mathbf{k}}_{\Vert }$. (a)–(c), $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}∕\mathrm{Fe}$ junction with 4 MgO ML; (d)–(f), same junction with Ag interlayers; (a) and (d), majority spins; (b) and (e), minority spins; (c) and (f), antiparallel configuration (per spin).

Figure 3

Conductance (left axis) and TMR (right axis) vs barrier thickness for $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}∕\mathrm{Fe}$ junctions (open symbols). Triangles, majority spin, parallel configuration; diamonds, minority spin, parallel configuration; squares, each spin, antiparallel configuration; circles, TMR ratio $R$, calculated disregarding minority spin in the parallel configuration (see text). Solid symbols: conductance and TMR for a $\mathrm{Fe}∕\mathrm{Ag}∕\mathrm{Mg}\mathrm{O}∕\mathrm{Ag}∕\mathrm{Fe}$ junction.