G. Schönhense (Institut für Physik)
In cooperation with:
The spin-resolved HAXPES experiment has been implemented at the Japanese Synchrotron Radiation source Spring8 by a combination of a Scienta R-4000-10keV hemispherical analyzer with a spin detector based on spin polarized LEED at a W(100) surface . Our new design was adapted for high voltage operation. The relatively large inelastic mean free path (up to 20 nm) of fast photoelectrons allows to extend the HAXPES technique with electron-spin polarimetry and to develop spin-analysis for buried magnetic multilayers and interfaces. The first results are published in .
For the direct spin-resolved measurements in HAXPES mode, samples of exchange-biased Co2FeAl0.5Si0.5 (CFAS) film were prepared (Fig. 1). The 15 nm value for the thickness of CFAS layer was selected to get it about the probing depth in the used HAXPES mode but still exchange-biased in bottom-pinned configuration. Courtesy K. Inomata and M. Yamamoto for the fabrication of these samples.
Fig. 1: Multilayer stack with Co2FeAl0.5Si0.5 ferromagnetic layer (FM) exchange-biased in on-top configuration with antiferromagnetic IrMn (AFM) pinning layer. The 3 nm thick MgO/AlOx layer is used as a protective cap.
Fig. 2 shows the spin resolved spectra of the Fe 2p3/2 state upon excitation with p-polarized 5.95 keV photons. A maximum polarization of about 50% is obtained after background subtraction. The structure of the Fe 2p3/2 mj multiplet (shown as bars) was taken from the band-structure calculation of Co2FeAl0.5Si0.5 performed by the all-electron first-principle relativistic KKR Green's function method (SPR-KKR) . The bars topped with filled and hollow triangle marks depict the states with negative and positive spin projection ms values; the larger and smaller bars correspond to the |mj|=3/2 and |mj|=1/2 states, respectively.
The sub-states of 2p3/2 electrons with well defined spin up (mj=3/2; ms=+1/2) and spin down (mj=-3/2; ms=-1/2) are clearly reproduced with maxima in the spin-resolved spectra (Fig. 2, upper frame). The electron spin (ms) is not well defined for 2p3/2 states with mj=±1/2 since the respective mj values (mj=ml+ms) may be obtained with different ml and ms combination. The 2p3/2 state with mj=-1/2 is, however, revealed as a shoulder in the spin-down component. Thus, the features of Fe 2p3/2 core states are well revealed in the measured spin-resolved spectra showing a good agreement with the one-step model of photoelectron emission. One should, however, note the broadening of the measured spin-resolved spectra. Part of this broadening originates from a relatively poor resolution of spin-resolved measurements, but this is not the only reason. After electron escape from the Fe 2p orbital, the coupling of the 2p5 (2P3/2) core electrons with 3d5 (6S5/2) valence electrons results in a variety of ionic 7,5P|L+S|≥J≥|L-S| configurations. The difference in energy levels of the ionic state implies a broad kinetic energy distribution for the emitted electrons and broadening of the photoemission spectra, particularly at the side of higher binding energies.
Fig 2: First spin-resolved HAXPES experiment, showind countrates in the spin detector channels (a), spin polarization (b) and spin-resolved spectra of Fe 2p3/2 photoelectrons (c) from the buried Co2FeAl0.5Si0.5 layer. The energy resolution interval (1 eV) is marked by the vertical slit.
Fig. 3 shows the correlation of the spin-resolved spectra with the magnetic circular and linear dichroism. A maximal MCDAD value of 45% has been reached for the Fe 2p3/2 photoelectrons from the CFAS layer. The spin-resolved HAXPES experiment proves that a spin polarization of about 50% is retained during the transmission of the photoelectrons through a 3 mm thick oxide capping layer. The reported spin-resolved spectra correlate well with the features of magnetic circular and linear dichroism in photoelectron emission. For details, see .
The direct detection of electron spin employing single-channel electron scattering implies an extremely strong reduction of detection efficiency at the spin-discriminating stage by 4 orders of magnitude. To overcome this drawback, a novel approach of two dimensional multichannel spin-detection  will be implemented in the next stage of the experiment. The multichannel spin-polarimeter provides a uniquely high efficiency that is orders of magnitude enlarged in comparison to the performance of single-channel electron spin detectors . This will give way to spinresolved valence band spectroscopy in the HAXPES range, which is up to now only possible in the VUV range .
Funding by DFG (FE633/6-1) and JST on the Japanese side is gratefully acknowledged.
 J. Kirscher and R. Feder, Phys. Rev. Lett. 42, 1008 (1979)
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