Site disorder in ${\text{Co}}_2\text{Fe}\left(\text{Al},\text{Si}\right)$ Heusler alloys and its influence on junction tunnel magnetoresistance

The local structures around Co atoms in ${\text{Co}}_2{\text{FeSi}}_{1-x}{\text{Al}}_x$ ($x=0$, 0.5, and 1.0) Heusler alloys have been investigated using ${}^{59}\text{C}\text{o}$ NMR spin-echo method and their effect on the tunneling magnetoresistance for the junctions using ${\text{Co}}_2{\text{FeSi}}_{0.5}{\text{Al}}_{0.5}$ electrodes has been discussed. The ${}^{59}\text{C}\text{o}$ NMR spectra of the Heusler alloys are extremely sensitive to the site disorder and clearly distinguish among the $A2$, $B2$, and $L{2}_1$ structures providing quantitative information on the amount of disorder. The tunnel magnetoresistance for the junctions is demonstrated to be sensitive to the site disorder in ${\text{Co}}_2{\text{FeSi}}_{0.5}{\text{Al}}_{0.5}$ electrodes.

Figure 1

(Color online) ${}^{59}\text{C}\text{o}$ NMR spectrum from bulk ${\text{Co}}_2\text{FeSi}$ with $L{2}_1$ structure; inset is the local environment around a Co atom in $L{2}_1\text{−}{\text{Co}}_2\text{FeSi}$ (Al).

Figure 2

(Color online) ${}^{59}\text{C}\text{o}$ NMR spectrum from bulk ${\text{Co}}_2\text{FeAl}$ with $L{2}_1$ structure; the full circles are experimental spectrum and the continuous lines are theoretical fitted spectrum, assuming $L{2}_1$ structure with the disorder consisting of Fe and Al mixing corresponding to a single $c=0.16$ (thin line) and to two concentrations $c=0.1$ and $c=0.21$ (thick line) of Fe (Al) antisites.

Figure 3

(Color online) Summary of ${}^{59}\text{C}\text{o}$ NMR main line and the satellite frequency positions as a function of NN shell composition in the spectra for ${\text{Co}}_2\text{FeSi}$, ${\text{Co}}_2\text{FeAl}$, and ${\text{Co}}_2{\text{FeSi}}_{0.5}{\text{Al}}_{0.5}$ Heusler bulks and thin films with $L{2}_1$ structure.

Figure 4

${}^{59}\text{C}\text{o}$ NMR spectrum for ${\text{Co}}_2\text{FeAl}$ films sputtered on a MgO (100) substrate at RT and $500°\text{C}$ within $A2$ (a) and $B2$ (b) structures (according to XRD data), respectively.

Figure 5

(Color online) (a) ${}^{59}\text{C}\text{o}$ NMR spectrum for $L{2}_1\text{−}{\text{Co}}_2\text{FeSi}/\text{MgO}\left(100\right)$ thin film (TF) grown at $200°\text{C}$ (full circles) compared to the reference spectrum from bulk $L{2}_1\text{−}{\text{Co}}_2\text{FeSi}$ (open square) and the theoretical fitted spectrum assuming $L{2}_1$ structure with 10% of Fe antisites on Si sublattice (continuous line); (b) ${}^{59}\text{C}\text{o}$ NMR spectrum for a $L{2}_1\text{−}{\text{Co}}_2{\text{FeSi}}_{0.5}{\text{Al}}_{0.5}/\text{MgO}\left(100\right)$ thin film grown at RT and post-annealed at $T_a=600°\text{C}$ (full circles) compared to the reference spectrum from bulk $L{2}_1\text{−}{\text{Co}}_2\text{FeAl}$ (open square) and the theoretical fitted spectrum assuming $L{2}_1$ structure with the disorder 16% of Fe (Al) antisites on Al (Fe) sublattice (continuous line).

Figure 6

(Color online) TMR at RT as a function of post-annealing temperature for junctions using ${\text{Co}}_2{\text{FeSi}}_{0.5}{\text{Al}}_{0.5}$ films for top and bottom electrodes.

Figure 7

(Color online) XRD for (a) out-of-plane and (b) $\varphi$ scans for (111) plane for the ${\text{Co}}_2{\text{FeSi}}_{0.5}{\text{Al}}_{0.5}$ film on a Cr-buffered MgO(100) with different postannealing temperatures $T_a$.

Figure 8

(Color online) ${}^{59}\text{C}\text{o}$ NMR spectra for ${\text{Co}}_2{\text{FeSi}}_{0.5}{\text{Al}}_{0.5}$ films on Cr-buffered MgO(001) for $T_{\text{anneal}}=400°\text{C}$ $\left(B2\right)$ and $500°\text{C}\left(L{2}_1\right)$.