|
Principle of Operation
SPLEED (Spin Polarized Low Energy Electron Diffraction) is one of the most efficient techniques of analysing the Spin
polarization
of an electron beam (1-3). Its underlying principle is based on the fact that every scattering process of an electron beam from a heavy (i.e. high Z) target is spin dependent. The differential cross section describes the number of electrons scattered into a certain solid angle interval. It depends on the projection of the spin polarization vector of the electron beam onto the direction normal to the scattering plane.
These basic properties of single-atom scattering (4) are quantitatively modified but not principally altered in the electron diffraction process at a solid high-Z material. As a consequence, a
left-right asymmetry of the backscattered electrons arises, which is a direct measure of the transversal spin polarization components.
The great advantage of exploiting the patented* SPLEED-technique, i.e. diffraction from a single-crystal surface is that the
total scattering intensity is concentrated into a few well-defined diffraction spots. Furthermore, the asymmetry function is the highest
of all spin polarization detectors being based on left-right asymmetries. This reduces apparatus-related artefacts to a minimum. The degree of polarization can be derived from the intensity asymmetry between two corresponding diffraction spots of the LEED pattern.
The Focus SPLEED detector is based on this principle. It employs low energy electron diffraction by a W(100) single crystal. The
LEED spots exhibit intensity variations and asymmetries that depend on the energy and degree of spin polarization of the incident electrons. The figure shows experimental data analysis of
corresponding á2,0ñ LEED-spots for the reflectivity (top) and spin polarization (bottom), that equals the asymmetry function (Sherman function) S for polarization analysis.
 
Experimental data for the reflectivity R, the figure of merit (a) and the polarization of the detector beam and its
sensitivity with respect to the angle of incidence q (b). The "working point" at 104.5 eV is indicated. (from /3/ )
(c) schematic LEED pattern
To minimize the error of the measured polarization the figure of merit S2×I/I0 (shown in Fig. 3.1 as P2×R) has to be maximal. This is the fact for a scattering energy of 104.5 eV. The
geometry of the Focus SPLEED detector is optimized for that scattering energy. In the case of vanishing apparatus-related asymmetries the polarization can be calculated as
 ,
where N1 and N2 are the intensities (countrates) of the corresponding [ 2,0] and [ -2,0]
LEED spots and S is the asymmetry function (Sherman function). The asymmetry occurs in the LEED spots that lie in the plane perpendicular to the spin direction. Employing four detectors (i.e. 2pairs)
both transversal components of the spinpolarisation vector can be measured simultaneously. With the electron spectrometer CSA300 that empoys a 90
degree deflection of the electrons coming from the sample one in plane and on out of plane component of the spinpolarisation vector can be measured simultaneously. The third (in
plane) component is accessible by a 90 degree azimuthal sample rotation.
Literature:
(1) J. Kirschner and R. Feder: Phys. Rev. Lett. 42, 1008 (1979)
(2) J. Kirschner: Polarized Electrons at Surfaces, Springer Tracts in Modern Physics 106, Springer Berlin,
Heidelberg (1985)
(3) J. Kirschner: Sources and Detectors for Polarized Electrons in: R. Feder (ed.) Polarized Electron in Surface
Physics, World Scientific Singapore
(4) J. Kessler: Polarized Electrons, Springer Tracts on Atoms and Plasmas, Springer Heidelberg (1985)
*Patent number: DE 264639402 for Germany and 4153844 for USA
Construction/Operation
The spin detector is a self-contained unit with a mounting flange NW 100CF, comprising:
An electron entrance lens for focusing and adjustment of the incident electron beam, a
tungsten (100) single crystal oriented with an accuracy of 0.25° which has been cleaned by 150 cycles of heating in oxygen atmosphere to reduce its carbon content, a stage for fine
adjustment of the crystal containing an electron beam heating facility and two respectivly four channeltrons that are mounted with retarding grids to suppress secondary electrons. A low
noise signal coupling facilitates count rates up to 13 million counts per second.
Transfer lens from analyser
The transfer lens images the exit slit of the analyser (e.g. CSA300) onto the SPLEED crystal; the configuration of the lens element and deflectors are optimised for the analyser used.
Electron optics and the main parts of the SPLEED detector that are described in the next sections.
The deflector plate voltages are primarily used to adjust the countrates of the two counters of
one spinchannel when no polarization is present. The equality of two counters of one spinchannel also depends on the focusing of the whole electron optics, that is the sample
position, analyser lens, correction elements of the analyser (e.g. K1/K2 at CSA300) and the transfer lens to the SPLEED detector.
The spectra of counters that lie in the analyser's direction of dispersion (i.e. in the dispersion
plane) can have a slight energetic shift, depending on the lens settings. This is due to energy spread across the exit slit and the different focusing onto the SPLEED crystal of electrons
that originate at different points on the exit slit. This spread should be proportional to the passenergy of the analyser, but can be neglected except for high resolution measurements.
The shift is omitted when measurements with a polarization reversal are carried out .
Retard grids
The suppressor grids in front of the channeltrons are used to repel non-elastic electrons
(essentially true secondaries). A lower suppressor voltage yields a higher countrate (preferably used to optimise intensity) whereas a higher suppressor voltage results in a
higher asymmetry and a higher figure of merit of the detector. For operation, the grids should be set to about 5V with respect to the kinetic energy potential of the electrons.
Electron detection
For electron detection the SPLEED detector employs 4 Channeltrons, two for each spin orientation.
|
