Evidence of a Symmetry-Dependent Metallic Barrier in Fully Epitaxial MgO Based Magnetic Tunnel Junctions

We report on the experimental observation of tunneling across an ultrathin metallic Cr spacer layer that is inserted at the interface of a $\mathrm{Fe}/\mathrm{MgO}/\mathrm{Fe}(001)$ junction. We show how this remarkable behavior in a solid-state device reflects a quenching in the transmission of particular electronic states, as expected from the symmetry-filtering properties of the MgO barrier and the band structure of the bcc Cr(001) spacer in the epitaxial junction stack. This ultrathin Cr metallic barrier can promote quantum well states in an adjacent Fe layer.

Figure 1

(a) Band structures along the $\Gamma -H$ direction of electron propagation, of spin-polarized bcc Fe and bcc Cr, with ${\Delta }_1$ (${\Delta }_5$) bands in black (gray). Panels (b)–(g): Schematics of the two MTJ potential landscapes for ${\Delta }_1$ spin-$\uparrow$ electrons in the parallel (P) junction state at zero, negative, and positive bias.

Figure 2

Experimental and theoretical variations of normalized TMR with Cr thickness $x$.

Figure 3

$\mathrm{Fe}/\mathrm{Cr}(x\mathrm{ML})/\mathrm{MgO}(10\mathrm{ML})/\mathrm{Fe}$: (a) evolution of the $\mathrm{P}\uparrow$, $\mathrm{P}\downarrow$, $\mathrm{AP}\uparrow$, and $\mathrm{AP}\downarrow$ conductance channels with increasing Cr thickness $x$. Transmission probability of the dominant $\mathrm{P}\uparrow$ conductance channel as a function of $k_{\parallel }$ for (b) $x=0$ and (c) $x=6$.

Figure 4

Bias dependencies for $\mathrm{Fe}/\mathrm{Cr}(x\mathrm{ML})/\mathrm{MgO}/\mathrm{Fe}$ MTJs of (a) $\Delta G_{\mathrm{P}}(V)/G_{\mathrm{P}}(0)$ for $x=0$ and (b) $\Delta G_{\mathrm{P}}(V)/G_{\mathrm{P}}(0)$ and $\Delta G_{\mathrm{AP}}(V)/G_{\mathrm{AP}}(0)$ for $x=6$.

Figure 5

Relative variation of $G_{\mathrm{P}(\mathrm{AP})}(V)$ in standard $\mathrm{Fe}/\mathrm{MgO}/\mathrm{Fe}$ and Fe (7 ML) QW MTJs in the (a) P and (b) AP junction states. (c) $d^2{I}_{\mathrm{P}}/d{V}^2(V)$ data for $\mathrm{Fe}/\mathrm{MgO}/\mathrm{Fe}$ and $\mathrm{Fe}(d)$ QW MTJs.