Spin-dependent tunneling in epitaxial Fe/Cr/MgO/Fe magnetic tunnel junctions with an ultrathin Cr(001) spacer layer

We fabricated fully epitaxial Fe/Cr/MgO/Fe(001) magnetic tunnel junctions (MTJs) with an atomically flat ultrathin Cr(001) layer grown below the MgO barrier layer and studied the spin-dependent transport to clarify scattering process of tunneling electrons. Because Cr does not have Bloch states with ${\Delta }_1$ symmetry at the Fermi energy $\left(E_F\right)$, ${\Delta }_1$ evanescent states in MgO, which dominantly mediate the tunneling current, cannot couple with Cr Bloch states without a scattering process. The Fe/Cr/MgO/Fe(001) MTJs are therefore a model system for studying nonspecular scattering processes where the orbital symmetry of tunneling states is not conserved. The resistance-area (RA) product of the MTJs was found to not increase exponentially as a function of the Cr thickness $\left(t_{\text{Cr}}\right)$, indicating that the Cr layer does not act as a perfect tunnel barrier despite of the absence of ${\Delta }_1$ states at $E_F$. Moreover, the magnetoresistance ratio of the MTJs was seen to oscillate as a function of $t_{\text{Cr}}$ with a period of 2 monolayers, reflecting the layered antiferromagnetic structure of Cr(001). Surprisingly, the MR ratio showed local maxima at the odd numbers of Cr monolayers and local minima at the even numbers of Cr monolayers, indicating that the tunneling current is oppositely spin polarized with respect to the interface magnetization. These results suggest that nonspecular scatterings mediate the coupling between evanescent states in MgO and certain non-${\Delta }_1$ Bloch states in Cr that have negative spin polarization, thereby inducing nonspecular tunneling current even at a low temperature and a small bias voltage. We also investigated, as a reference sample, Fe/MgO/Cr/Fe MTJs with a less-oxidized Cr/MgO interface by growing the Cr(001) layer on the MgO barrier layer and found that their RA product increased much more rapidly with increasing $t_{\text{Cr}}$. This indicates that partial oxidation of interface Cr atoms in the Fe/Cr/MgO/Fe MTJs is one of the major origins of nonspecular scatterings. Both an increase in temperature and the application of bias voltage were found to greatly enhance the electron scattering that gives rise to tunneling conductance. While the temperature dependence of the antiparallel conductance due to magnon scatterings follows the Bloch $T^{3/2}$ law, the conductance due to nonspecular scatterings deviates from the Bloch $T^{3/2}$ law. The present experimental results give us important clues to a clearer understanding of the tunneling process in MgO-based MTJs.

Figure 1

(Color online) Dependence of MR ratio on Cr thickness $\left(t_{\text{Cr}}\right)$ at 50 K (open circles) and room temperature (solid circles) for Fe/Cr/Al-O/FeCo MTJs [data from Ref. 20].

Figure 2

(Color online) (a) Cross section of epitaxial Fe/Cr/MgO/Fe(001) MTJ film with wedge-shaped Cr(001) layer. [(b)–(d)] Top-view photographs of Fe/Cr/MgO/Fe MTJs prepared with highly precise microfabrication techniques: (b) overall view of all MTJs fabricated on identical substrate, (c) closeup of one MTJ with contact pads, and (d) optical microscopic image of MTJ with a junction area of $36\mu {\text{m}}^2$.

Figure 3

(Color online) (a) Cross section of epitaxial Fe/Cr/Fe(001) trilayer film with wedge-shaped Cr(001) layer grown at $120°\text{C}$ on a substrate and (b) its magnetic hysteresis loop at Cr thickness $\left(t_{\text{Cr}}\right)$ of 13.6 MLs where the interlayer exchange coupling between Fe layers is antiferromagnetic. Magnetic hysteresis loop was measured by longitudinal magneto-optical Kerr effect at room temperature. (c) Coupling fields ($H_1$ and $H_2$) of the upper Fe layer of the structure shown in (b) as a function of $t_{\text{Cr}}$.

Figure 4

(Color online) Cr thickness $\left(t_{\text{Cr}}\right)$ dependence of (a) MR ratio and (b) RA product in parallel magnetic state ($\mathrm{P}$ state) (open circles) and antiparallel magnetic state (AP state) (solid circles) at 20 K for epitaxial Fe/Cr/MgO/Fe(001) MTJs.

Figure 5

(Color online) Cr thickness $\left(t_{\text{Cr}}\right)$ dependence of (a) MR ratio and (b) normalized RA product in $\mathrm{P}$ state at 20 K for epitaxial Fe/MgO/Cr/Fe MTJs (open circles), in which a wedge-shaped Cr(001) layer was grown on the MgO(001) barrier layer. The inset in (a) schematically shows the cross section of the MTJs. RA product is normalized by the value at $t_{\text{Cr}}=0$ and the triangles in (b) show the normalized RA product for epitaxial Fe/Cr/MgO/Fe MTJs with the Cr layer grown under the MgO barrier layer [data from Fig. 4b].

Figure 6

(Color online) Dependence of RA product ($R$ measured at bias voltage of $-10\text{mV}$) on temperature $\left(T\right)$ in (a) $\mathrm{P}$ state and (b) AP state for epitaxial Fe/Cr/MgO/Fe(001) MTJs with various Cr thicknesses $\left(t_{\text{Cr}}\right)$. For each $t_{\text{Cr}}$, the RA product is normalized by that at 20 K.

Figure 7

(Color online) Cr thickness $\left(t_{\text{Cr}}\right)$ dependence of exponent $n$ in $\Delta G_{\text{AP}}\left(T\right)=C•T^n$. Error bars indicate standard deviation $\left(\sigma \right)$ in least-squares fitting.

Figure 8

(Color online) Bias voltage $\left(V\right)$ dependence of second-derivative conductance $\left(d^{2}I/d{V}^2\right)$ in $\mathrm{P}$ state (left-hand side) and AP state (right-hand side) for (a), (b) Fe/MgO/Fe MTJs and for (c), (d) Fe/Cr/MgO/Fe MTJs with $t_{\text{Cr}}=10$ ML and (e), and (f) Fe/MgO/Cr/Fe MTJs with $t_{\text{Cr}}=10$ ML. The vertical axis is the normalized second-derivative conductance $\left(d^{2}I/d{V}^2\right)/\left(dI/dV\right)$ in units of (A/V). In (a) and (b), the spectra were measured at 3 K with the conventional lock-in method and positive bias means that electrons tunnel from the top Fe layer to the bottom Fe layer. In [(c)–(f)], $d^{2}I/d{V}^2\text{−}V$ characteristics were derived mathematically from the data of the $I\text{−}V$ characteristics measured at every 20 mV measured at 20 K and positive bias means that electrons tunnel from the Fe layer to the Cr layer.