Reduced interface spin polarization by antiferromagnetically coupled Mn segregated to the $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$/GaAs (001) interface

We have investigated the interfacial structure and its correlation with the calculated spin polarization in $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$/GaAs(001) lateral spin valves. $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$ (CMS) films were grown on As-terminated c($4\times 4$) GaAs(100) by molecular beam epitaxy using different first atomic layers: MnSi, Co, and Mn. Atomically resolved $Z$-contrast scanning transmission electron microscopy (STEM) imaging and electron energy loss spectroscopy (EELS) were used to develop atomic structural models of the CMS/GaAs interfaces that were used as inputs for first-principles calculations to understand the magnetic and electronic properties of the interface. First-principles structures were relaxed and then validated by comparing experimental and simulated high-resolution STEM images. STEM-EELS results show that all three films have similar six atomic layer thick, Mn- and As-rich multilayer interfaces. However, the Co-initiated interface contains a $\mathrm{M}{\mathrm{n}}_2\mathrm{As}$-like layer, which is antiferromagnetic, and which is not present in the other two interfaces. Density functional theory calculations show a higher degree of interface spin polarization in the Mn- and MnSi-initiated cases, compared to the Co-initiated case, although none of the interfaces are half-metallic. The loss of half-metallicity is attributed, at least in part, to the segregation of Mn at the interface, which leads to the formation of interface states. The implications for the performance of lateral spin valves based on these interfaces are discussed briefly.

Figure 1

(a) Cross-sectional high-resolution HAADF-STEM image of CMS film on GaAs(001) surface taken along the $\left[1\overline{1}0\right]$ zone axis of a GaAs substrate and the line profile of Co and MnSi (inset figures), (b) and (c) shows the FFT of the region mentioned in (a), which indicates the $\mathrm{L}{2}_1$ and ${\mathrm{B}}_2$ phases, respectively.

Figure 2

(a) HAADF-STEM image of the CMS/GaAs interface for the MnSi initiated sample along the [110] direction (the red box outlines the location of the EELS maps). (b) ADF image taken in parallel with EELS map. (c)–(f) Atomic resolution EELS map of Mn (red), Co (blue), Ga (yellow), and As (green). (g) Model image after comparison of the EELS map and ADF image. Similarly, (h) HAADF-STEM image along the $\left[1\overline{1}0\right]$ direction and corresponding EELS maps, ADF images, and model figure are shown in (i), (j), (k), (l), (m), and (n), respectively.

Figure 3

(a) HAADF-STEM image of the CMS/GaAs interface for the Mn-initiated sample along the [110] direction (the red box outlines the location of the EELS maps). (b) ADF image taken in parallel with EELS map. (c)–(f) Atomic resolution EELS map of Mn (red), Co (blue), Ga (yellow), and As (green). (g) Model image after comparison of the EELS map and ADF image. Similarly, (h) HAADF-STEM image along the $\left[1\overline{1}0\right]$ direction and corresponding EELS maps, ADF images and model figure are shown in (i), (j), (k), (l), (m), and (n), respectively.

Figure 4

(a) HAADF-STEM image of the CMS/GaAs interface for the Co-initiated sample along the [110] direction (the red box outlines the location of the EELS maps). (b) ADF image taken in parallel with EELS map. (c)–(f) Atomic resolution EELS map of Mn (red), Co (blue), Ga (yellow), and As (green). (g) Model image after comparison of the EELS map and ADF image. Similarly, (h) HAADF-STEM image along the $\left[1\overline{1}0\right]$ direction and corresponding EELS maps, ADF images and model figure are shown in (i), (j), (k), (l), (m), and (n), respectively.

Figure 5

(a) and (d) Ball-and-stick representation of the DFT-relaxed interface structure for the MnSi initiated model with Si at L3 along the [110] and $\left[1\overline{1}0\right]$ directions, and (b) and (e) corresponding frozen-phonon multislice STEM simulations image. (c) and (f) High-resolution $Z$-contrast STEM images of the CMS/GaAs interface (MnSi initiated sample). Color scheme: Co - blue, Mn - red, Si - turquois, As - green, and Ga - yellow.

Figure 6

Ball-and-stick representation of the DFT-relaxed interface structure for the MnSi-initiated interface without Si at L3 along the [110] (a) and $\left[1\overline{1}0\right]$ (b) directions (left images), and corresponding frozen-phonon multislice STEM simulations image (right images), Color scheme: Co - blue, Mn - red, As - green, and Ga - yellow.

Figure 7

(a) and (d) Ball-and-stick representation of the DFT-relaxed interface structure for the Co-initiated model along the [110] and $\left[1\overline{1}0\right]$ directions, and (b) and (e) corresponding frozen-phonon multislice STEM simulations image. (c) and (f) High-resolution $Z$-contrast STEM images of the CMS/GaAs interface (Co-initiated sample).Color scheme: Co - blue, Mn - red, As - green, and Ga - yellow.

Figure 8

(a) Total DOS of the periodic supercell structure for the MnSi-initiated model with Si shown in Fig. 5. The total DOS for CMS/GaAs(001) with ideal abrupt SiMn/As termination is shown in the shaded portion. (b) Layer-by-layer LDOS at the interface for the same model. The DOS of L1-Ga are multiplied by a factor of 5 with respect to other DOS plot for comparison.

Figure 9

(a) Total DOS of the DFT-model for the MnSi-initiated model without Si at L3 shown in Fig. 6. The total DOS for ideal abrupt SiMn interface is shown in the shaded portion. (b) The layer-by-layer LDOS at the interface for the same model.

Figure 10

(a) Total DOS of the DFT-model for the Co-initiated interface model in Fig. 7. The total DOS for CMS/GaAs(001) with Co/As ideal interface is shown in the shaded portion. (b) Layer-by-layer LDOS at the interface for the same model.

Figure 11

(a) Forward current-voltage characteristics for the Co, MnSi, and Mn-initiated interfaces. The device structures are based on the graded Schottky barriers described in Ref. [28]. The same doping profile is used in all three samples. (b) Lateral spin valve signals for devices with the three types of interfaces obtained at 20 K with an injection current of 2.0 mA and a detector bias current of 0.5 mA. The nominal dimensions of the spin valve contacts were $5\mu \mathrm{m}\times 50\mu \mathrm{m}$, and the separation was 10 μm.