Co 2 MnSi thin film preparation and characterization

Co 2 MnSi samples were prepared and investigated completely in situ in an ultrahigh vacuum cluster consisting of sputtering chambers, an molecular beam epitaxy (MBE) chamber, and a SRUPS chamber equipped with a He gas discharge lamp (hν=21.2 eV) and a hemispherical energy analyzer with multi-channel spin filter27 (energy resolution ≃400 meV, Sherman function S=0.42±0.05 (ref. 28)28). First, an epitaxial buffer layer of the Heusler compound Co 2 MnGa (30 nm) was grown on the MgO(100) substrate by radio frequency (RF)-sputtering at room temperature. By an optimized additional annealing process at 550 °C L2 1 order is obtained as shown by high energy electron diffraction (RHEED) and X-ray diffraction (XRD). Co 2 MnSi (70 nm) was RF-sputtered on top at room temperature. Induced by the buffer layer the Co 2 MnSi thin films show already some degree of L2 1 surface order as deposited. By additional annealing the order is improved as demonstrated for the film surface by RHEED (Fig. 1). Already a low annealing temperature of T a =300°C results in a significantly increased intensity of the characteristic RHEED L2 1 superstructure peaks. However, by XRD no (111) peak, which is indicative for L2 1 order, is observed for T a <400 °C. This suggests that L2 1 order is present at the film surface, but not in the bulk of the thin film. For T a ≥400 °C the (111) peak appears in XRD. For T a ≥500 °C some Ga from the buffer layer is observed by core-level HAXPES to have diffused to the Co 2 MnSi surface. The magnetic moments of all samples amount to ≃5 μ B per formula unit at 4 K and is reduced by ≃3% at room temperature, in agreement with theoretical predictions and experimental values measured on bulk samples29.

Figure 1: RHEED intensities of Co 2 MnSi (100) thin films. The intensities for samples with different annealing temperatures T a are evaluated from the area of the RHEED image indicated by the yellow rectangle in the inset. The inset shows the 0th order region of the RHEED image of a Co 2 MnSi thin film annealed at 450 °C. The low intensity maxima indicate the doubling of the unit cell (compared with the B2 cell) generated by L2 1 order. Full size image

Photoemission spectroscopy experiments

Figure 2 shows in situ UPS spectra of Co 2 MnSi thin films annealed at different temperatures T a without spin analysis. The large acceptance angle of the spectrometer (±10°) and applied sample bias voltage of −10 V result in k|| values which cover the complete Brillouin zone. The spectra of all samples are almost identical, only the broad hump at E−E F =−2,900 meV vanishes and the peak at E−E F =−1,150 meV is slightly broadened for the deposited and the T a =550 °C sample.

Figure 2: In situ spin-integrated UPS of Co 2 MnSi thin films. The samples were deposited at room temperature and annealed at different temperatures T a . Curves are offset for clarity. Energy resolution ΔE=400 meV. Full size image

However, by spin analysis clear differences between the samples are revealed. Figure 3 shows the spin polarization of MgO/Co 2 MnGa(30 nm)/Co 2 MnSi(70 nm) thin films annealed at different temperatures T a as measured by SRUPS. A huge room temperature spin polarization of 90–93% at the Fermi energy at room temperature was obtained for samples annealed between 300 °C and 450 °C. In combination with the UPS calculations discussed below, these exceptionally high values are the first direct observation of half-metallicity in the surface region of any Heusler compound, which provide strong evidence for 100% spin polarization in the bulk of the thin films. With lower annealing temperatures the spin polarization is reduced at E F and slightly increased at higher binding energies, which can be explained by an energy broadening of the electronic states owing to reduced structural order. With higher annealing temperatures the spin polarization is reduced as a result of interdiffusion with the buffer layer.

Figure 3: Spin polarization determined by in situ spin resolved UPS. The Co 2 MnSi thin films were deposited at room temperature and annealed at different temperatures T a . The error of the Sherman function S=0.42±0.05 (ref. 28)28 results in a relative error of 12% of the spin polarization. Full size image

UPS is a surface sensitive method and thus the results cannot be directly associated with electronic bulk band structure properties. However, as will be shown below, band structure based calculations of photoemission spectra provide this link. As additional experimental input for such calculations, a comparison of spin-integrated ex situ HAXPES with a photon energy of 6 keV of AlO x capped (oxidation protection) Co 2 MnSi thin films and spin-integrated in situ UPS (uncapped films) was carried out. Owing to the increased information depth of HAXPES, true surface states are typically not observed by this method.

As shown in Fig. 4, the in situ spin-integrated UPS and the HAXPES results fundamentally agree although the information depth of both experiments varies from 2 nm to 20 nm. This provides evidence that true surface states like Shockley or Tamm states, which are mainly located at the first atomic layer30, do not contribute to the UPS data.

Figure 4: Comparison of spin-integrated in situ UPS and ex situ HAXPES. The Co 2 MnSi thin film for the ex situ HAXPES was capped by 2 nm AlO x (sample for in situ UPS uncapped). Please note the different energy resolutions of the UPS (ΔE=400 meV) and HAXPES (ΔE=200 meV) experiments. Additionally, the spin and k-space integrated DOS as calculated using the SPR-KKR package with Perdew–Burke–Ernzerhof functional and dynamical mean field theory (U Mn =3.0 eV, U Co =1.5 eV) is shown, which provides the basis for the UPS/HAXPES intensities calculated within a one-step model of photoemission (see text). The systematic deviations between calculations and measurements at higher binding energies are presumably associated with the energy-dependent detector efficiencies of the spectrometers. Full size image

Bulk DOS and photoemission calculations

We calculated the spin resolved bulk DOS of Co 2 MnSi using the spin polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) Green function method implemented in the Munich SPR-KKR band structure programme package employing the Perdew–Burke–Ernzerhof functional31. 100% spin polarization was obtained with the gap edges at ≈−200/+200 meV. Adding correlation effects by means of dynamical mean field theory to Perdew–Burke–Ernzerhof functional32 shifts the upper edge to higher energies, but leaves the lower edge almost unchanged. For a comparison of our experimental data with UPS- and HAXPES calculations this electronic structure provides the basis for a one-step model of photoemission, which includes all matrix-element effects, multiple scattering in the initial and final states33, and all surface-related effects in the excitation process. We used a recently developed relativistic generalization for excitation energies ranging from about 10 eV to more than 10 keV (ref. 34) realized in the full spin-density matrix formulation for the photocurrent35.

In Fig. 4 the calculations and the experimental spin-integrated UPS and HAXPES results are compared. Nearly quantitative agreement for both, UV and hard X-ray photon energies, is obtained. Particularly with regard to the small DOS just below the Fermi energy the agreement of the calculations with the high UPS and HAXPES intensities in this energy range is remarkable and is traced back to a bulk-like surface resonance as will be discussed below.

The obtained agreement between the spin-integrated UPS/HAXPES experiments and calculations based on a half metallic bulk band structure represents already evidence for half-metallicity. Additional strong evidence is provided by the analysis of the SRUPS data. For the surface region we can estimate the position of the lower band edge of the minority gap directly from the experimental data by taking the maximum of the derivative of the minority spin intensity with respect to the energy, which is found at E−E F ≃ −500 meV. From previous surface sensitive X-ray magnetic circular dichroism experiments we estimated the position of the upper band edge to be at E−E F ≃+400 meV (ref. 36).

In Fig. 5 the highest experimentally obtained spin polarization is shown together with the spin polarization derived directly from the calculated DOS, the calculated photoemission asymmetry including all broadening effects considering bulk contributions only, and the calculated photoemission asymmetry including surface-related effects. The correspondence between the DOS and calculated pure bulk-like UPS spectrum becomes clear, if the influence of intrinsic life time broadening owing to electronic correlations and included experimental energy resolution (ΔE=400 meV) is considered. It is obvious that these broadening effects within the bulk calculations reduce the expected UPS spin polarization although the DOS is half-metallic. However, including surface effects in the calculation changes the results clearly.

Figure 5: Calculated and experimental spin polarizations. Comparison of the spin polarization obtained by in situ SRUPS on a Co 2 MnSi thin film with the calculated DOS-derived spin polarization, the calculated UPS spin polarization including broadening effects and considering only bulk states, and the calculated total UPS spin polarization including broadening effects with additional surface state contributions. The theoretical curves were obtained using the SPR-KKR package with Perdew–Burke–Ernzerhof functional and dynamical mean field theory. The error of the Sherman function S=0.42±0.05 (ref. 28)28 results in a relative error of 12% of the spin polarization. Full size image