Demonstration of Half-Metallicity in Fermi-Level-Tuned Heusler Alloy ${\mathrm{Co}}_2{\mathrm{FeAl}}_{0.5}{\mathrm{Si}}_{0.5}$ at Room Temperature

Fermi level tuning has been successfully demonstrated in Co-based full-Heusler alloy ${\mathrm{Co}}_2{\mathrm{FeAl}}_{0.5}{\mathrm{Si}}_{0.5}$ (CFAS). The half-metallic band gap of CFAS was proved by the behavior of differential conductance of $\mathrm{CFAS}/({\mathrm{MgAl}}_2){\mathrm{O}}_x/\mathrm{CoFe}$ magnetic tunneling junctions with an unexplored crystalline $({\mathrm{MgAl}}_2){\mathrm{O}}_x$ barrier. CFAS exhibits the highest effective spin polarization ($P_{\mathrm{eff}}$) at 300 K and the weakest temperature dependence of $P_{\mathrm{eff}}$ among all known half metals. Further study shows that $P_{\mathrm{eff}}$ of CFAS decays with increasing temperature ($T$) following $T^{3/2}$ law perfectly, which indicates that the depolarization of CFAS is determined by spin wave excitation only.

Figure 1

(a) Magnetoresistance curves of $\mathrm{CFAS}/({\mathrm{MgAl}}_2){\mathrm{O}}_x/\mathrm{CoFe}$ MTJs at 7, 26, and 300 K. (b) Dependence of differential conductance on bias voltage ($dI/dV\mathrm{\text{−}}V$) of $\mathrm{CFAS}/({\mathrm{MgAl}}_2){\mathrm{O}}_x/\mathrm{CoFe}$ MTJs for parallel and antiparallel configurations at 7 and 300 K. Dashed lines A are corresponding to the critical bias voltages ($V_c$), at which $dI/dV$ curves show the minimum values for parallel configuration at 7 and 300 K; Dashed line B points to the bias voltage of 0 mV. The dependences of differential conductance of $\mathrm{CFAS}/({\mathrm{MgAl}}_2){\mathrm{O}}_x/\mathrm{CoFe}$ MTJs and $\mathrm{CoFe}/({\mathrm{MgAl}}_2){\mathrm{O}}_x/\mathrm{CoFe}$ MTJs for parallel configuration at 300 K on bias voltage (from 0 to 500 mV) are shown in the inset. (c) Schematic diagram of tunneling process of $\mathrm{CFAS}/({\mathrm{MgAl}}_2){\mathrm{O}}_x/\mathrm{CoFe}$ MTJs at a finite bias voltage for parallel configurations.

Figure 2

(a) High resolution TEM image of an epitaxial $({\mathrm{MgAl}}_2){\mathrm{O}}_x$ barrier on $B2\mathrm{\text{−}}\mathrm{CFAS}$. $({\mathrm{MgAl}}_2){\mathrm{O}}_x[100]$ axis is parallel with CFAS[110] axis in horizontal direction because of the small lattice mismatch between them. Both [001] axes of CFAS and $({\mathrm{MgAl}}_2){\mathrm{O}}_x$ are along the vertical direction. (b) Diffraction pattern taken in CFAS. (c) Diffraction pattern taken in barrier, $\{022\}$ planes of spinel structure are indicated by arrows. (d) HAADF-STEM image. (e) EELS map of Al. (f) EELS map of Co. Mg map is absent because of the undistinguishing Mg region in the image.

Figure 3

(a) Temperature dependence of TMR ratio for $\mathrm{CFAS}/({\mathrm{MgAl}}_2){\mathrm{O}}_x/\mathrm{CoFe}$ MTJs and $\mathrm{CoFe}/({\mathrm{MgAl}}_2){\mathrm{O}}_x/\mathrm{CoFe}$ MTJs and the fitting curves by Bloch $T^{3/2}$ law. The inset shows the dependence of normalized magnetization on temperature from 10 to 300 K for CFAS (30 nm) single layer and the fitting curve by Bloch $T^{3/2}$ law. (b) Temperature dependence of $G_P$, $G_{Ap}$ and $G_T$ for $\mathrm{CFAS}/({\mathrm{MgAl}}_2){\mathrm{O}}_x/\mathrm{CoFe}$ MTJs and the fitting curve of $G_T$, where $G_P$ and $G_{Ap}$ are the conductance of MTJs for parallel and antiparallel configurations, respectively; $G_T$ is the prefactor for direct spin-dependent elastic tunneling.