Spin-polarized tunneling spectroscopy of fully epitaxial magnetic tunnel junctions using ${\text{Co}}_2{\text{FeAl}}_{0.5}{\text{Si}}_{0.5}$ Heusler alloy electrodes

Spin-dependent tunneling spectroscopy has been studied in fully epitaxial magnetic tunnel junctions with full-Heusler ${\text{Co}}_2{\text{FeAl}}_{0.5}{\text{Si}}_{0.5}$ (CFAS) alloys. We fabricated CFAS/MgO/CFAS structures with $L{2}_1$- and $B2$-ordered CFAS layers and measured the bias voltage dependence of differential conductance $G$. We found for $L{2}_1\text{-CFAS}/\text{MgO}/L{2}_1\text{-CFAS}$ structure symmetrical conductance curves with respect to polarity of the bias voltage for parallel (P) and antiparallel (AP) magnetization configurations and two characteristic crossovers in $G$ between P and AP accompanied with a flat feature within $\pm 0.6\text{V}$ in $G$ (P). On the other hand, only one crossover was observed at a negative-bias voltage for $L{2}_1\text{-CFAS}/\text{MgO}/B2\text{-CFAS}$ structure. The direct tunneling that reflects the specific spin-dependent density of states of the half-metallic $L{2}_1\text{-CFAS}$ is proposed as a possible transport mechanism leading to the notable crossovers.

Figure 1

Cross-sectional high-resolution transmission electron microscopy image of a $L{2}_1\text{-CFAS}/\text{MgO}/L{2}_1\text{-CFAS}$ magnetic tunnel junction along the [110] direction of the CFAS layers. Structural imperfections are circled.

Figure 2

(Color online) TMR curves and schematic illustrations of magnetic tunnel junctions with (a) $L{2}_1\text{-CFAS}/\text{MgO}\left(1.1\text{nm}\right)/L{2}_1\text{-CFAS}$ and (b) $L{2}_1\text{-CFAS}/\text{MgO}\left(1.4\text{nm}\right)/B2\text{-CFAS}$ structures. Bias voltages of 1 mV (7 K) and 5 mV (RT) were applied for the measurement.

Figure 3

(Color online) Bias voltage dependences of (a) TMR ratio, (b) resistance area (RA) product of antiparallel state, and (c) RA of parallel state for $L{2}_1\text{-CFAS}/\text{MgO}\left(1.1\text{nm}\right)/L{2}_1\text{-CFAS}$ structure. Inset in (a) indicates closeup near 1 V for the TMR curve measured at 7 K.

Figure 4

(Color online) $dI/dV$ spectra of MTJs with (a) $L{2}_1\text{-CFAS}/\text{MgO}\left(1.1\text{nm}\right)/L{2}_1\text{-CFAS}$ and (b) $L{2}_1\text{-CFAS}/\text{MgO}\left(1.4\text{nm}\right)/B2\text{-CFAS}$ structures at 7 K. The dotted lines (solid lines) represent the parallel (antiparallel) magnetization configuration. The arrows represent the points where the parallel and the antiparallel lines cross each other.

Figure 5

(Color online) (a) Spin-resolved DOS of $L{2}_1{\text{-Co}}_2{\text{FeAl}}_{0.5}{\text{Si}}_{0.5}$ obtained from the generalized gradient approximation $\left(\text{GGA}\right)+U$ theorem. (b) Calculated $dI/dV$ spectra based on Fig. 5a and Eq. (2) for parallel (dotted line) and antiparallel (solid line) configurations.

Figure 6

(Color online) Schematic illustrations of the DOS of the CFAS electrodes.