P17S

23BG007

BL-7

Preparation of Pt Thin Film on MgO (001)and Observation of Its Electronic Structure
T. Asanoa, K. Sumidab, K. Kunitomoa, T. Okudab, and K. Miyamotob
aGraduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japanb
Hiroshima Synchrotron radiation Center (HSRC), Hiroshima University 2-313 Kagamiyama,
Higashi-Hiroshima 739-0046, Japan

Keywords: Pt thin film, Spin Hall effect, Photoemission spectroscopy

Spin Hall effect is an important physical phenomenon that generates spin-polarized currents and isessential for the development of spintronics devices. The spin Hall effect in metals has attracted attention
because conductance mismatches are markedly suppressed at an interface with ferromagnet compared to thesemiconductors and allows the use of spin-polarized current supplied by the ferromagnetic metal [1]. Among
many metals, elemental Pt is extensively investigated both experimentally and theoretically [2,3]. Kimura etal. reported that the Pt wire exhibits largest spin Hall conductivity as large as 2.4×104 S/m at room
temperature, which is 4 orders of magnitude larger than that of typical semiconductors, such as GaAs [2]. In
order to inject spin-polarized current into ferromagnets more efficiently, the orientation of spin current must
be precisely controlled, i.e., it is necessary to observe its spin orientation. In previous studies, the rough spinorientation of spin currents induced by spin Hall effects has been obtained by using the magneto-optical Kerr
effect [4]. However, the experimental method to precisely detect the spin orientation induced by the spin
Hall effect has not been established yet. One solution is operando spin-resolved photoemission spectroscopy(PES) measurements under an external electric field.
In this work, as a first step towards the development of the experimental method, we fabricated Pt thinfilms and investigated the electronic structure by PES measurements. The Pt thin films were prepared byusing electron beam evaporation source with a carbon crucible on a MgO(001) substrate at room temperature.
The MgO substrate was annealed in oxygen atmosphere (~10-1 Pa) at ~200℃ for 60 min., and then, annealed
at 600℃ for 240 min. in vacuum prior to the Pt deposition.
To observe the electronic structure of the fabricated Pt/MgO film, we performed the PES measurements
at BL-7 of Hiroshima Synchrotron Radiation Center utilizing various incident photon energies from 30 to
160 eV. Figure 1 shows Pt 4f core level spectrum taken at hν = 155 eV. Two peaks derived from the spinorbit split Pt 4f5/2 and 4f7/2 are clearly seen at binding energy (EB) of 74.4 and 71.1 eV. To obtain more detailed
information, the experimental result was fitted by Doniac-Šunjić function with Shirley background function[see blue curve in Fig. 1]. From this fitting result, asymmetric parameters were determined to be 0.018 for
4f5/2 and 0.043 for 4f7/2. These values are very small as a metal, suggesting that the density of states (DOS)near the Femi level (EF) is small.
The red curve in Fig. 2 shows the experimentally observed valence band PES spectrum taken at hν = 70eV. We can see broad peak structures around EB = 1.6 and 4.5 eV. Moreover, a dip structure is recognized at
EB = 3.2 eV. To compare the valence band spectrum, we carried out the first-principles calculation for theface-centered-cubic Pt using WIEN2k program including the spin-orbit coupling. Based on the calculation,
it was found that the d orbital component is dominant in the valence band rather than the s and p orbitals.
Moreover, photo-onization cross-section of d orbital is dominated in the vacuum ultraviolet region.
Therefore, in Fig. 2, we compare the PES spectrum with the Pt d partial DOS. Here, the calculated Pt dpartial DOS was multiplied by the Fermi-Dirac distribution function at 300 K and convoluted by a Gaussianfunction (experimental resolution). Comparing the experimental and theoretical results, we notice that thelocations of the peak and dip structures and the band width are qualitatively reproduced. However, a very

sharp peak predicted by the calculation in the vicinity of EF was not observed in the experiment. This suggeststhat the interface and/or surface structure of the fabricated Pt film on MgO substrate makes the electronicstructure differ from the electronic structure in the bulk Pt.

FIGURE 1. Pt 4f core level spectrum (red circles) taken at hν = 155 eV. The blue curve represents the fitting
result using the Doniach-Šunjić function and active Shirley background (green curve).

FIGURE 2. Observed photoemission spectrum taken at hν = 70 eV (red) and the calculated Pt d partial DOS(blue). The calculated DOS is multiplied by the Fermi-Dirac distribution function and convoluted by aGaussian function.
In conclusion, we are successful to prepare Pt thin film on MgO(001) using electron beam evaporate
source with a carbon crucible. Around EF, the observed electronic structure on Pt thin film is differrent from
calculated spectrum based on the bulk Pt. In general, the performance of the spin Hall effect is closely related
to the electronic structure, so this finding of discrepancy may be important information for improving theperformance of the spin Hall effect for Pt thin film on MgO(001).

REFERENCES
1. G. Schmidt, et al, Phys. Rev. B 62, R4790(R) (2000).
2. T. Kimura et al., Phys. Rev. Lett. 98, 156601 (2007).
3. G. Y. Guo, et al., Phys. Rev. Lett 100, 096401(2008).
4. Y. K. Kato, et al, Science 306, 5703 (2004).