Patent Number: 041538447
Section: summary

This invention relates to apparatus for determining the degree of spin polarization of an electron beam by the backscattering of electrons by a crystal having a surface perpendicular to the beam direction which surface is formed in a plane of the crystal structure chosen for the scattering of electrons. In such equipment, the backscattering is measured by devices provided to respond to the intensity of electrons backscattered by the monocrystal at respectively complementary angles to the electron beam direction. BACKGROUND AND PRIOR ART An electron may have not only energy and momentum, but also spin. The direction of spin relative to a specified direction represents an important piece of information that can be determined by means of the interaction of the electron with other particles, for example with atoms in a surface layer of a solid. For carrying out such an investigation, apparatus for measuring the degree of spin polarization of electron beams is necessary. Such equipment is already known that operates on the basis of the dependence upon the electron spin of the scattering of electrons by free atoms, the so-called Mott scattering. In such equipment, determination of the degree of polarization of the incident electron beams is made from the comparison of intensity measurements at scattering angles that are equal in amount but of opposite sign. There is a disadvantage in this case, however, that appreciable intensity differences resulting from the electron spin direction occur only at low overall intensity. The sensitivity of the known equipment is therefore very limited. This is all the more significant a disadvantage, because strong polarization effects are limited to only small scattering angle ranges. That has the consequence that only a very small part (.ltorsim.10.sup.-4) of the aggregate quantity of scattered electrons can be used for measurement. The known measuring equipments for this purpose are based on the application of two different methods. In the first method, an atom beam, preferably of Hg, is used, against which the electrons are scattered with a few keV of energy. Since it is difficult, however, to produce atom rays in high density, difficulties occur in this kind of method regarding the intensity of the radiation. In the second method, thin foils are used for scattering the electrons. In this case, however, with the advantage of high atom density, there is the unavoidable disadvantage of multiple scattering of the electrons and their absorption in the foil. In order to cope with these difficulties, the measurement has therefore been carried out with electrons accelerated to high energy (100 to 150 keV). In that case, there is still the disadvantage that the effective cross-section for the scattering is very small and therefore only a small scattering intensity and measurement sensitivity is obtained. The application of high electric voltages, in addition, makes the known equipment very large and cumbersome on account of the necessary safety precautions. It has therefore already been recommended that the degree of polarization of an electron beam should be measured by means of the spin-dependent intensity of an electron beam specularly reflected by the surface lattice of a monocrystal. The differential effective cross-section for the refraction of slow electrons dependent upon the orientation of the electron beam is in this case higher by several orders of magnitude than in the case of the Mott scattering. The sensitivity is also correspondingly higher. Since in the case of the measurement of the degree of polarization the scattering must always be carried out at two complementary and as nearly as equal as possible angles in the incidence plane of the beam, it has been favored to measure the two scatter beams by tipping the crystal alternately by equal angles, first to one and then to the other side, and measuring the scatter intensity after each change (see in this regard R. Feder, Surf. Sci. 51, 297, 1975). Much time and expense, and therefore disadvantage, is involved, however, because the mechanical movement of the crystal must be carried out with high precision and reproducibility, very often, and with sufficient rapidity and, moreover, in ultrahigh vacuum (10.sup.-11 mbar). The time consumption required for carrying out such a double measurement is more than twice as high as required for a single measurement. THE PRESENT INVENTION It is an object of the present invention to provide equipment for measuring the degree of spin polarization of an electron beam with higher sensitivity than the equipment heretofore known and that permits, moreover, measurement with an electron beam independent of the primary energy of the electrons and measurement that can be carried out quickly with high accuracy and with equipment that lends itself to compact construction. Briefly, the electron beam is first passed through means for accelerating or decelerating the electrons constituted of electrostatic or magnetic lenses of respectively tubular or diaphragm form. Such devices are known as lenses because they have a focussing effect on the electrons, and a d.c. voltage is supplied so that they at the same time will accelerate or slow down the electrons of the beam, as the case may be. In this case the device adjusts the electron velocity to suit the function of a monochromator through which the beam next passes for the function of reducing the energy scatter of the electrons to an extent suitable for the measurement, after which the electron beam is then again accelerated or decelerated by electrostatic or electromagnetic lenses and, at the same time, focussed on the surface of the monocrystal oriented as already above mentioned. The second velocity adjustment is made to optimize backscatter measurement by the particular monocrystal reflection. The intensity of electrons backscattered by the monocrystal is measured by means comprising at least two detectors provided for measuring the intensity of electrons backscattered from the surface of the monocrystal at complementary angles, and between the measuring or detecting means and the surface of the monocrystal means are provided for separating or deviating the low-energy portion of the electrons that is backscattered by inelastic interaction with the monocrystal. In one embodiment, this is done by the formation of an opposing counter electric field and in another embodiment, this is done by providing an electrostatic or magnetic field for deviating the slow electrons in front of each of the detectors. The measuring arrangement preferably uses adjustable detectors arranged in pairs in planes that are as far as possible perpendicular to each other and intersecting in the direction of the incident electron beam. It is convenient to provide the measuring means in the form of collector plates distributed in the hemispherical space above the monocrystal facing the incident beam, and preferably electron multiplier channel plates are provided in front of the respective collector plates. The first velocity-adjusting electron lens system is so designed that when the electron beam has passed through the monochromator and the second electron lens system and is incident on the monocrystal, it will have exactly the energy scatter .DELTA.E, which is tolerable for the scattering process at the monocrystal surface. The permissible energy scatter .DELTA.E lies in the range between 0.7 and 5 eV. The acceleration of deceleration of the electrons in the first electron lens system makes it possible, in combination with the monochromator and the second velocity-adjusting electron lens system, the setting of the desired energy level designed for the scattering of the electrons at the monocrystal surface such that in addition to the specularly reflected scatter beam, corresponding to the Bragg scatter condition, there arise also scatter beams of higher orders having an angle to the normal that is sufficiently large to make possible detection of the scatter beam by the detectors and exhibiting strong polarization effects at an intensity that is as high as possible. The desired energy level in practice is in the range between 10 and 300 eV, according to the selection of the monocrystal, preferably however about 100 eV. The monochromator operating as an energy filter separates out the electrons that differ by more than the predetermined energy amount from the mean energy. The influence of the deflecting field on the orientation of the polarization vector of the electron beam is negligible. In the scattering of electrons by the monocrystal, the influence of the transversal components of the electron spin on the scattering of the electrons is measured. For the case in which the electrons in the primary beam already are transversally aligned to the beam direction, no further treatment is necessary. For the case in which the electrons in the primary beam are aligned longitudinally, the monochromator is so designed that the electron beam is deflected through an angle of 90.degree.. The electron spin previously aligned longitudinally in the primary beam is then aligned transversally in the resulting beam. In the second lens system for accelerating or decelerating the electrons, the electrons are brought to the energy level required or designed for the scattering by the crystal lattice. The electrons impinge with this energy on the monocrystal, which so far as possible, consists of a material having heavy atoms, such as tungsten, gold or platinum. An advantageous further development of the apparatus of the invention consists in constituting the arrangement for measuring the backscatter intensity of four adjustable detectors of backscattered electron beams, disposed in pairs in two planes that are so far as possible perpendicular to each other and that intersect each other in a line having the direction of the incident electron beam. The result is thereby obtained that the transversal polarization vector of the incident beam is then fully determined, even if it is not exactly perpendicular to the plane that is determined by the normals to the crystal surface and the line connecting two detectors of a pair that intersects them. The detectors can, for example, consist of so-called channeltrons. A tungsten monocrystal that has scattering planes that are perpendicular to each other is, for example, usable as the monocrystal for measurements carried out in this manner. A further feature of the apparatus of the invention is the arrangement of the backscattering intensity measuring devices as collector plates in the hemispherical space above the monocrystal facing the incident electron beam. The size of the plates is so determined that a refraction beam or a portion thereof is detected by each collector. This advantageous arrangement of the detectors dispenses with the adjustment of detectors for the measurement of predicted backscattered electron beams, since all of the backscattered electrons in the entire hemispherical space will be detected by the collector plates, and it is merely necessary to switch in for the measurement the collector plates corresponding to the backscattered beams as they are found or calculated. This arrangement also makes it unnecessary to readjust the position of the detectors when another monocrystal is selected. There is further the advantage that in carrying out the measurement, the symmetry of the apparatus can be checked by measuring beams that have no polarization effect. A still further feature of the apparatus of the invention is the provision of electron multiplier channel plates ahead of the collector plates. The measurement is then performable with high accuracy, even for primary electron beams of relatively low intensity. The provision of means for deviating or trapping a low-energy portion of the electrons produced by scattering at the monocrystal, in which there are provided above the crystal, one or more plane or curved grids next to each other, or one behind the other, with voltage applied either between the crystal and the grid or grids, or only between the grids, has the effect that only electrons with energy greater than a selected threshold energy pass through the grid. For the case in which individual detectors are used, the slow electron trapping system conveniently utilizes devices for producing an electrostatic or magnetic field disposed in front of the individual detectors. By a suitable choice of the field strength and the disposition of the deflection devices, the result is obtained that only electrons having an energy above a particular threshold energy reach the detector.