Direct observation of high spin polarization in Co2FeAl thin films

We have studied the Co2FeAl thin films with different thicknesses epitaxially grown on GaAs (001) by molecular beam epitaxy. The magnetic properties and spin polarization of the films were investigated by in-situ magneto-optic Kerr effect (MOKE) measurement and spin-resolved angle-resolved photoemission spectroscopy (spin-ARPES) at 300 K, respectively. High spin polarization of 58% (±7%) was observed for the film with thickness of 21 unit cells (uc), for the first time. However, when the thickness decreases to 2.5 uc, the spin polarization falls to 29% (±2%) only. This change is also accompanied by a magnetic transition at 4 uc characterized by the MOKE intensity. Above it, the film’s magnetization reaches the bulk value of 1000 emu/cm3. Our findings set a lower limit on the thickness of Co2FeAl films, which possesses both high spin polarization and large magnetization.

patterns of an as-grown Co 2 FeAl film with a thickness of 20 uc. The sharp streaky lines indicate a flat surface morphology, thus the growth is smoothly pseudomorphic. The epitaxial relationship is Co 2 FeAl (001)[110] // GaAs (001) [110]. Figure 1d exhibits X-ray 2θ-ω diffraction pattern of a Co 2 FeAl film with a thickness of 13 uc. Though the film is ultra-thin, Co 2 FeAl (200) and (400) peaks can still be clearly observed, in addition to the peaks of GaAs substrate. From the diffraction peaks, the lattice constant of the film can be estimated as 5.70 Å, slightly smaller than the theoretical value of 5.727 Å. This suggests that our film is still under compressive strain induced by the GaAs substrate.
In theory, a perfect chemically and structurally ordered Co 2 FeAl crystal is L2 1 phase. As exhibited in Fig. 1c, Co atoms (golden balls) sit at the eight vertexes of the cubic, while Fe (red balls) and Al atoms (purple balls) occupy the body center place alternately. It would be characterized by the peaks of superlattice reflections like (111) and (311) 17,18 . As the Fe and Al atoms mix with each other, B2 phase appears. In our case, the presence of both (200) and (400) peak indicates that our Co 2 FeAl films are in the B2 phase 19 . Besides the two main peaks, the absence of extra peaks suggests that our films possess a single crystal structure.
In-situ longitudinal MOKE measurements. The magnetic properties of the Co 2 FeAl films were probed in-situ by MOKE measurements at room temperature. We have measured the longitudinal MOKE along [ It is interesting to notice that the magnetization of our Co 2 FeAl films exhibit a combination of uniaxial and cubic anisotropy. As the film thickness decreases, the uniaxial anisotropy becomes more pronounced with the easy axis along [1 1 0] direction and hard axis at [1 1 0] (Fig. S1). This uniaxial anisotropy may be induced by the Co 2 FeAl/GaAs interface, as the dangling bonds at the GaAs surface are all along [1 1 0]. Along the easy axis, the field dependent MOKE signals (θ K ) of various thicknesses are presented in Fig. 2a. Square hysteresis loops can be observed with thickness down to 1 uc. This strong remanence for 1 uc suggests that the long range ordering has formed between the Co and Fe nanoclusters through the ferromagnetic double exchange couplings 20 at the early growth stage. As the film thickness increases, both θ K and the coercivity increases. The coercivity saturates after the film thickness is beyond 4 uc.
The thickness dependent saturated Kerr rotation and coercive field extracted from Fig. 2a are presented in Fig. 2b,c, respectively. When the thickness increases from 0 to 4 uc, the saturated Kerr rotation increases linearly. And more interestingly, it passes through zero (blue solid line in Fig. 2b), suggesting that there are no magnetic dead layers, and the entire Co 2 FeAl film is ferromagnetic at room temperature. At the same time, the coercivity also increases linearly with the thicknesses, as indicated by the blue solid line in Fig. 2c. For thicker films (t > 4 uc), the Kerr rotation is also linearly dependent on the film thicknesses, with a smaller slope as fitted by the orange solid line in Fig. 2b. This is because the film thickness is still thinner than the detection depth of MOKE measurement, which is usually 10~50 nm 16,21 . Thus, the thicker the film is, the stronger the MOKE signal is. On the other hand, the coercivities stay constant (Fig. 2c), which are equal to the bulk value 22,23 . The magnetization and coercivity imply that the films thicker than 4 uc are bulk-like, while films thinner than 4 uc are mostly affected by the interface 16 . High spin polarization at the Fermi level. To investigate the spin polarization of the Co 2 FeAl films, samples were transferred under ultra-high vacuum to the ARPES chamber upon completing the film growth. This in-situ ARPES set-up prevents the contamination from ambient environment, thus it gives us a chance to observe the real spin polarization at the fresh Co 2 FeAl surface. Prior to the measurements, the magnetization direction was pulled to the easy axis along [1 1 0] direction, with an external magnetic field of 500 Oe. During the measurements, no out-of-plane spin polarization was observed, which confirms that the magnetization of our Co 2 FeAl films is in plane, as confirmed by our MOKE measurements. Figure 3 exhibits the representative spin-resolved photoemission spectra and the corresponding spin polarization at room temperature. A broad peak at ~1.0 eV in Fig. 3a comes from the combination of Co and Fe's 3d electronic states, which is similar to the spectra of Co 2 MnSi films obtained in previous report 21 . The polarization of a free-electron beam can be determined by a spin-sensitive technique that involves scattering measurements from metals with strong spin orbit coupling 14 . Thus the spin polarization can be defined as: I I I I eff I + and Irepresent the intensity spectra for majority and minority spins, respectively. S eff means the Sherman function, representing the analyzing power or spin sensitivity of the polarimeter, which is equal to 0.16 ± 0.01 in our case 14,24 . The magnitude of spin polarization of ferromagnetic materials is a key property for their application in spintronic devices, especially at room temperature. As shown in Fig. 3b-e, the spin polarization of Co 2 FeAl films exhibits a peak at the Fermi energy (E F ), then decreases slowly with the binding energy increasing, and reaches zero beyond 1 eV. For the film of 2.5 uc, the peak value is much lower than the thicker films, and also the spin polarization goes to a negative value at higher binding energies, suggesting the swap of spin direction of the majority and the minority band. From the theoretical calculation 25 , the Fermi level crosses the majority band, touches the top of minority valence band, indicating the highest spin polarization at the Fermi level, which is in good agreement with our experimental observation. The thickness dependent spin polarization at the Fermi surface is exhibited in Fig. 3f. We find that the spin polarization decreases slowly as the film thickness is reduced from 21 uc to 6 uc, and drops to 29% (±2%) when the film thickness is reduced to 2.5 uc.

Discussion
The spin polarization of 58% (±7%) for the Co 2 FeAl films with thickness of 21 uc is the highest value detected directly up to now for this materials system. However, it is still smaller than the expected 100% for half-metallic ferromagnets 21 . The plausible reasons may be local atomic disorder of Fe and Al atoms as demonstrated by XRD measurements or nonstoichiometric phase at the surface 26,27 . We have to point out that the measured spin polarization is at the interface between the Co 2 FeAl bulk and the vacuum, which may not be equal to the spin polarization at the interface between the Co 2 FeAl and MgO in a real TMR device. That interface could alter the spin polarization dramatically, as we can see in Fig. 3f. With film thickness decreasing to 2.5 uc, the strong attenuation of the spin polarization happens at E F , which may be due to the interface bonding or the site disorder, resulting in a spin polarization that is much less than the bulk.
In conclusion, we have grown single crystalline Co 2 FeAl films with B2 structure by MBE. The films exhibit a combination of uniaxial and cubic anisotropy. As the first direct observation of spin polarization for the Co 2 FeAl system, a high spin polarization of 58% (±7%) at the Fermi edge at room temperature was obtained by in-situ spin-resolved ARPES for a 21 uc-thick Co 2 FeAl film. In-situ MOKE measurements indicate that the thickness of the Co 2 FeAl film must reach at least 4 uc to achieve both a bulk magnetization and a high surface spin polarization with only a weak thickness dependence. Our work paves the way for the design and application of spintronic devices based on Co 2 FeAl films.

Methods
Epitaxial growth. To prepare the samples, we have used highly insulating GaAs (001) epi-ready wafers whose lattice constant is very close to Co 2 FeAl (001) 28 , and the Co 2 FeAl thin films were grown in an ultra-high vacuum MBE system with the base pressure below 3 × 10 −9 mbar. Before the growth, GaAs (001) substrates were annealed at 580 °C to remove Gallium oxide. We used two e-beam evaporators for Co and Fe and a Knudsen cell for Al with the substrate sitting at 250 °C. The deposition rate of ~1 uc per min was measured by a quartz microbalance, which was calibrated by thickness measurements using atomic force microscopy (AFM). Structural characterization. The crystal structure was examined by a high resolution single crystal X-ray diffractometer (Bruker D8 Discover). The incident X-ray is from Cu-Kα emission and has a wavelength of 1.5418 Å. The scan mode is θ-2θ.
In-situ MOKE Characterization. The MOKE loops were collected during growth in the longitudinal geometry using an electromagnet with a maximum field of 500 Oe, and an intensity stabilized HeNe laser (633 nm) at 300 K. The MOKE signal is proportional to the Kerr effect, the angle between the polarizer and the analyzer, and the intensity of the light. During the in-situ MOKE measurement, cares were taken not to move any optical components in order to keep the laser intensity constant 16,21 . Spin-arpes measurements. In-situ Spin-ARPES measurements were performed using a lab-based Spin-ARPES system consisting of a SPECS PHOIBOS 150 hemisphere analyzer with 3D Micro-Mott detector and UVS 300 helium lamp (21.2 eV). The 3D Micro-Mott detector is equipped with 4 channels which allows us to measure both in-plane and out-of-plane spin components with an energy resolution of 150 meV at room temperature. We operated the Mott detector at a scattering energy of 25 keV, and an inelastic energy window equal to 800 eV, which leads to a Sherman function of 0.16 ± 0.01. The spectrometer was fixed at a large acceptance angle (±15°), which covered the complete Brillouin zone. The base pressure in Spin-ARPES chamber is better than 3 × 10 −10 mbar, and the samples were measured at 300 K.