DE102005045622B4 - Methods and arrangements for detecting electron spin polarization - Google Patents

Abstract

method to the spatially resolved Analysis of electron spin polarization in the beam path of a parallel imaging electron microscope, comprising a polarization-sensitive scattering target (1) and an arrangement (2, 3) for electron optical beam guidance, the lateral distribution the degree of spin polarization of the electrons in the expanded beam analyzed simultaneously by the polarization-sensitive scattering target (1) and by an electron optical round lens (3) on the image detector (4) of the microscope is made visible.

Description

The The present invention relates to a method and a corresponding one Arrangements for analyzing the electron spin polarization in a parallel imaging Electron microscope and each a method and an arrangement for the analysis of electron spin polarization in an electron spectrometer. The respective arrangement comprises a polarization-sensitive scattering target in an electron-optical beam path and a spatially resolving detector or an array of multiple detectors to detect the scattered Electrons.

Known are different methods for analyzing the spin polarization of Electron beams, including the spin-dependent Mottstreuung, the low-energy electron diffraction or scattering and the exchange scattering on ferromagnetic targets. An overview J. Kessler ["Polarized Electrons", 1st Edition, Springer Publisher, Berlin (1976)]. Important applications for the analysis of electron spin polarization exist in the field of electron spectroscopy, see, for. B. J. Kirschner ["Polarized Electrons at Surfaces ", Springer Tracts in Mod. Phys., Vol. 106, Berlin (1985)] and the Electron microscopy on magnetic samples [H. Hopster and H. P. Oepen (ed.) "Magnetic Microscopy of Nanostructures ", Springer, Berlin (2005)]. Well-known examples are scanning electron microscopy with polarization analysis (SEMPA) or spin-resolved photoelectron spectroscopy. For magnetic samples the spin polarization of the electron reaching to prove important information about the magnetization structure of the examined sample. The height of the spin polarization is a measure of the associated sample magnetization. About that There is also a direct correlation between the direction of Spin polarization vector of the electrons with the direction of the local Magnetization of the sample. A spin-resolved image is therefore suitable the magnetic domain structure directly from ferromagnetic samples or dynamic effects, such as As the behavior of the magnetization in ultrafast excitation processes, in time disbanded to pursue.

The references US 4,153,844 . DE 198 42 476 C1 and the US 2002/0003213 A1 are all concerned with the application of a polarimeter in the scanning electron microscope - called SEMPA (Secondary Electron Microscopy with Polarization Analysis). In this case, an image is created by screening; The spin analysis for the individual pixels is performed sequentially.

Also at the DE 103 39 404 A1 a spin-filtered image is generated. Here is a spin filter foil in the beam path ( 3 of the DE 103 39 404 A1 ). With this slide, part of the image will be filtered by a spin filter. That in the literature US 2002/0003213 A1 described polarimeter is also a special design for use in SEMPA, in which case the accelerating electrodes produce a ball field.

From the JP 59 79948 A the use of a spin polarimeter (mott detector) behind a photoemission electron microscope is known. The detector analyzes the electron beam passing through a hole in the phosphor screen. This is followed by a sequential analysis of the polarization distribution by scanning. However, a sequential analysis for image acquisition is very time-consuming, so the efficiency of the polarization analysis is very low.

A fundamental problem of all previous arrangements for analysis of spin polarization is that the exploited spin-dependent scattering process is characterized by a significant loss of electron intensity. In the most widely used methods, mottling and low energy electron diffraction, the intensity loss is about three orders of magnitude. In addition, there is an analysis strength (asymmetry function A) that is significantly lower than 100%. Typical values are around 25%, so that the so-called quality function A ² I is only typically 10 ^-4 . This results in very large measuring times; In many cases, the low signal strength prohibits a spin-resolved measurement.

The present invention solves by the features of the methods according to claims 1 and 8 the task, the Efficiency of polarization analysis in microscopy and spectroscopy experiments increase by simultaneous detection of the polarization distribution. Thereby becomes a spin-resolved parallel image allows. Ie. the lateral spatial distribution of the spin polarization is in the analyzed electron beam detectable. In typical microscopy experiments, the increase is Measuring efficiency more than two orders of magnitude, in spectroscopy experiments more than an order of magnitude. The procedure is as well as High energy electron beams (kinetic energy> 10 keV) as well as low energy electron beams (kinetic energy a few eV to several 100 eV) suitable. In the former Case, the spin polarization analysis works according to the known principles the Mottstreuung or Moeller scattering, while in the second case the low-energy electron diffraction or the exchange scattering at ferromagnetic targets is exploited. Advantages of the method according to the invention across from previous approaches are the possibilities for direct parallel imaging of a spin resolved (i.e., spin filtered) Image in electron microscopy and for simultaneous acquisition from spin-resolved Angular or energy distributions of scattered electrons or Photoelectrons in spectroscopy.

Various embodiments and use cases The invention will be exemplified below with reference to the drawings explained in more detail.

1 Figure 12 is a schematic representation of a device for spatially resolved analysis of electron spin polarization based on Mott scattering or low energy electron diffraction on a scattering target in the column of an electron microscope in a non-symmetric geometry;

2a Figure-b shows a schematic representation of a device for spatially resolved analysis of electron spin polarization based on Mott scattering or low energy electron diffraction (a) or exchange scattering (b) on a scattering target in the column of an electron microscope in a symmetrical 45 ° geometry;

3a -D shows schematic sketches for the definition of the angular and energy resolution;

4a FIG. 1b shows a schematic representation of the arrangement according to FIG 1 in the column of an electron microscope using a magnetic sector field in an orthogonal geometry (a) and a non-orthogonal geometry (b);

5a Figure-c shows a schematic representation of the arrangement for spatially resolved analysis of the electron spin polarization behind the beam exit of an electron spectrometer for the simultaneous detection of a 1D energy range (a), a 1D angular range or location range (b) or a 2D energy and angular range (c );

6a Figure -c shows preferred types of arrangement for spatially resolved analysis of electron spin polarization behind the beam exit of an electron spectrometer in which the first transfer lens (a) or both transfer lenses (b) can be dispensed with or in which the scattering target is segmented, thus a larger one To be able to simultaneously detect the energy or angular range of the electrons (c).

A preferred embodiment of the spatially resolving spin polarimeter according to the invention makes use of the fact that the spin-dependent scattering on a target with a homogeneous surface makes it possible to spin-filter the entire beam, ie in particular the lateral distribution of the electrons in the beam. Homogeneous target surfaces can be achieved by suitable dissection on single crystals with well-defined crystallographic orientation. A suitable example is the surface of a tungsten monocrystal with (100) orientation. Alternatively, ordered (epitaxial) layers of high atomic number materials can be evaporated on suitable substrates. For the Mottstreuung or the diffuse Niederenergieugung polycrystalline layers with sufficiently homogeneous fine-crystalline structure can be used. 1 shows a schematic representation of this embodiment in the electron-optical column of a parallel imaging electron microscope in a non-symmetrical geometry. The incident beam is transmitted via the first transfer lens ( 2 ) on the scattering target ( 1 ) and after the scattering or diffraction by the second transfer lens ( 3 ) on the screen or an intermediate image plane ( 4 ). The 1 and the figures below are simplified schematically. In practice, the transfer lenses may consist of several lens elements and behind the intermediate image plane ( 4 ) lenses (projective) may be appropriate for further magnification. The beam cross sections are shown greatly enlarged for the sake of clarity.

In the illustrated geometry, the component of the spin polarization vector marked P is analyzed perpendicular to the plane of the drawing. In the case of a monocrystalline scattering target, the diffraction angle is energy-dependent. If the scattering energy is correct, for the selected diffraction reflex 7 ) limited incident beam so that it covers the diaphragm ( 9 ) can happen. This case is due to the central ray ( 6 ) and the marginal rays ( 8th ) illustrated. Electrons with too high or too low scattering energy as well as inelastically or diffusely scattered electrons are passed through the aperture ( 9 ) and do not contribute to the image formation in the image plane ( 4 ) at. This is due to the marginal rays ( 10 ), which correspond to a lower energy. This arrangement is particularly advantageous when the energy width of the incident electron beam is too large for spin analysis by means of low energy diffraction. A practical example of this geometry is the diffraction at the (100) surface of tungsten at a scattering energy of 104.5 eV and use of the polarization sensitivity of the (2.0) beam. The analysis strength is about 27% [J. Kirschner, "Polarized Electrons at Surfaces", Chapter 3.1.1].

With diffuse scattering on a polycrystalline layer or Mott scattering at higher energies, no diffraction reflections occur and the effect of the energy dispersion does not exist. In this case, the aperture ( 9 ) for selecting the scattering angle range used for imaging and for discriminating inelastically scattered electrons. The ratio of the beam diameter on the scattering target to the image width of the transfer lens ( 2 ) is chosen so that the depth of focus of the intermediate image on the obliquely arranged scattering target is sufficiently large. The transfer lens ( 3 ) is in this case arranged perpendicular to the surface of the scattering target. To fine adjustment, the litter target ( 1 ) and around its surface normal ( 12 ) to be turned around.

2a shows a schematic representation in a symmetrical geometry that satisfies the condition entrance angle = exit angle. In this case, the mirror beam, also called the (0,0) beam, arrives at the picture. There is no energy dispersion. The surface normal ( 12 ) coincides with a low indexed direction of the target crystal. A practical example of this geometry is the diffraction at the (100) surface of tungsten at a scattering energy of about 30 eV or 7 eV and use of the polarization sensitivity of the (0,0) mirror beam. The aperture ( 11 ) is used to limit the scattering angle range used for imaging. In the case of diffraction on a single crystal or an epitaxial layer, the target crystal acts on the (0,0) beam like a mirror. In the case of diffuse scattering (polycrystalline layer or Mottstreuung), the depth of focus of both transfer lenses ( 2 ) and ( 3 ) be sufficiently large with respect to the obliquely arranged scattering target.

Alternatively, a ferromagnetic layer can be used as a spinsensitive scattering target. The spin-filtering effect is based on the exchange scattering [R. Bertacco, D. Onofrio, F. Ciccaci, Rev. Sci. Instrum. 70 (1999) 3572]. 2 B) shows a schematic representation of a ferromagnetic litter target ( 1 ), which on a magnetic yoke ( 13 ) and by means of a current through a magnetizing coil ( 14 ) can be magnetized. In addition, a second coil can be mounted perpendicular to the plane of the drawing. This allows the orientation of the magnetization along the directions parallel or perpendicular to the plane of the drawing (M _p or M _s ). These vectors define the quantization directions for spin polarization analysis.

3a -C define the parameters relevant for a quantitative estimation of the achievable energy and angular resolution. The through the contrast aperture of the electron microscope ( 15 ) and their distance from the litter target ( 1 ) defined angular range Δα in 3a has values <1 mrad for emission electron microscopes (typically <0.2 mrad for high resolution). An energy deviation ΔE from the target energy E _o leads to an angle change Δε. For Δε = Δα, the two emission cones begin to overlap. This limits the achievable energy resolution, in the case of 104.5 eV scattering energy to ΔE <500 meV (in the case of high resolution <100 meV).

3b shows the case of diffuse low energy scattering or mottling at higher energies. The aperture ( 11 ) defines the range of scattering angles to be imaged. Their diameter determines the acceptance angle of each point on the litter target ( 1 ) outgoing beam cone. Due to the chromatic aberration of the transfer lens ( 3 ) the aperture ( 11 ) also as an energy filter.

3c shows by the divergence of the marginal rays ( 7 ) caused angular variation on the litter target. The angular divergence Δθ may not exceed a maximum of about 35 mrad in the single-crystal diffraction, since the asymmetry function has a strong angular dependence [see J. Kirschner, "Polarized Electrons at Surfaces", chapter 3.1.1]. 2 ) must therefore be sufficiently large in relation to the beam diameter D on the litter target. In the case of diffuse low-energy scattering and mottling at higher energies, the angular interval may be considerably larger, since the angular dependence is weaker [for Mott scattering see J. Kessler, "Polarized Electrons", chapter 3.5.1] .There are values up to Δθ> 200 mrad possible.

3d shows the example of a telescopic beam path. The diaphragm (not shown) limits both the opening angle and the scattering angle here.

A special embodiment uses a magnetic sector field for beam guidance ( 16 ). Two possible geometrical arrangements are in 4a and b. Similar sector fields are used in low-energy electron microscopy [E. Farmer "LEEM-Basics", Surf. Rev. Lett. 5 (1998) 1275]. An advantage of this embodiment is that the incident beam ( 7 ) and the scattered beam ( 8th ) both parallel or nearly parallel to the surface normal ( 12 ). The transfer lens ( 17 ) can be operated as an immersion lens to allow very small scattering energies. A practical example of this geometry is the diffraction at the (0001) surface of cobalt at a scattering energy around 2 eV and use of the polarization sensitivity of the mirror beam. The arrangement in 4a Due to the two-fold 90 ° deflection, the advantage is that the linear column of the electron microscope is preserved overall.

The spatially resolving Spin polarimeter can also be used in energy-filtered electron microscopes become. The energy selection can be done by dispersive Energy filter as well as by time-of-flight filtering done.

The inventive arrangement used for spin polarization analysis in an electron spectrometer the spatial Separation of electrons with different entrance energies or Entry angles in the plane of the exit slit of the spectrometer.

5 shows the Anord invention tion in conjunction with an electrostatic hemisphere analyzer, consisting of outer sphere ( 19 ), Inner ball ( 20 ), Entrance slit ( 21 ) and exit slit ( 22 ). The scattering target ( 1 ) is located in the beam path behind the analyzer. In the illustrated embodiment, the beam is transmitted through the transfer lens (FIG. 2 ) on the scattering target ( 1 ) and through the transfer lens ( 3 ) on the spatially resolving electron detector ( 18 ). The aperture ( 11 ) serves to limit the scattering angle interval and to discriminate inelastically scattered electrons.

The multichannel polarization analysis of an energy spectrum is in 5a represented (viewing direction perpendicular to the dispersive plane). The energetically fanned electron beam ( 23 ) is transmitted through the first transfer lens ( 2 ) on the scattering target ( 1 ). After scattering, the spin-filtered beam is transmitted through the second transfer lens (FIG. 3 ) on the spatially resolving electron detector ( 18 ). Different locations on the detector correspond to different energies. Thus, a spin-filtered energy spectrum is recorded simultaneously. Typical energy intervals range from the order of one eV (for high-resolution spectroscopy) to the order of several 10 eV. For an energy interval of 2 eV and a desired energy resolution of 25 meV, the gain in measurement efficiency due to multichannel detection is ideally a factor of 80. In practice, the non-perfect imaging (chromatic aberration) of the transfer lenses ( 2 ) and ( 3 ) limit the number of resolved points on the detector. For large energy intervals, the principle of mottling at higher energies offers advantages, since the low energy diffraction shows a strong energy dependence. Energy intervals of up to about 3 eV can be analyzed simultaneously by low-energy electron diffraction.

The multichannel polarization analysis of an angular distribution is in 5b represented (viewing direction in the dispersive plane). The electron beam fanned out with respect to the entrance angle ( 24 ) is passed through the transfer lens ( 2 ) on the scattering target ( 1 ) and then through the transfer lens ( 3 ) outside the drawing plane on the spatially resolving detector ( 18 ). In this case, different locations of the electrons on the detector correspond to different angles. Thus, a spin-filtered angular distribution is recorded simultaneously. Typical angular intervals for hemisphere analyzers are in the range of ± 180 mrad around the central beam. At a desired angular resolution of 9 mrad, the gain in measurement efficiency due to the multi-channel detection in this case is determined by the factor 40. Alternatively, the entrance lens of energy analyzers can also be operated such that instead of an emission angle range, a strip-shaped local area on the sample is analyzed simultaneously.

5c shows that the plane of the energy expansion and the plane of the fan-out are perpendicular to each other. Therefore, the two aforementioned modes can be combined into a two-dimensional multi-channel detection. The gain in measurement efficiency can in principle exceed three orders of magnitude; however, there are practical limits to the usable angular and energy acceptance of the scattering target and to the aberrations of the transfer lenses. For the sake of clarity, the beam of the energy interval ( 23 ) in 5 (a) and the beam of the angular interval ( 24 ) in 5b only marked with 5 rays each. Furthermore, the electron trajectories are only drawn as dashes, but in practice the beams in the analyzer have a finite extent.

According to a particularly simple embodiment, the scattering target can be dispensed with without the transfer lens (FIG. 2 ) just after the spectrometer exit slit ( 22 ). This embodiment of the invention is in 6a shown. The scattering energy in this case is equal to or nearly equal to the kinetic energy (pass energy) of the electrons in the spectrometer. A practical example of this embodiment is the diffraction at the (100) surface of tungsten at a scattering energy around 30 eV or around 7 eV and using the polarization sensitivity of the mirror beam. If the spatially resolving detector ( 18 ) very close to the litter target ( 1 ) can also be placed on the transfer lens ( 3 ) are waived. This embodiment of the invention is in 6b shown. In this case, the energy selection through the aperture ( 11 ). Therefore, before the spatially resolving detector ( 18 ) means for suppressing the inelastically scattered electrons, such. B. an arrangement of retardation networks ( 25 ).

In practice, by means of low-energy diffraction, an energy interval of up to about 3 eV can be detected simultaneously. To extend the usable energy interval, a segmented scattering target ( 26 ) are used; this embodiment is in 6c outlined. The individual segments of ( 26 ) can be operated at different voltages to compensate for the energy variation of the incident beam. The uppermost segment is at a voltage U1, the lowest at U2, the segments in between are at voltages between these boundary values according to a linear voltage divider. By a suitable choice of U1 and U2 it can be achieved that all the electrons in the energetically fanned-out beam ( 23 ) strike the segmented litter target with the same energy. A segmentation with electrical insulation of the individual segments can be achieved by using an epitaxial layer as a scattering target. An example of a suitable epitakti Tungsten layer is tungsten in (110) orientation on a sapphire substrate surface in (1120) orientation. All electron optical beam paths in the 1 - 6 are simplified schematically.

1

scattering target for analysis of spin polarization

2

First Electron-optical transfer lens

3

Second Electron-optical transfer lens

4

screen or intermediate image plane

5

electron optical Axis of the incident beam

6

electron optical Axis of the scattered beam

7

marginal rays of the picture

8th

ray tracing for the desired Energy (energy of interest)

9

cover for energy selection

10

ray tracing for one lower energy

11

cover for limiting the scattering angle range

12

surface normal

13

magnetic yoke

14

magnetizing coil

15

contrast trim of the microscope

16

magnetic Sector field for beam deflection

17

electron microscopy transfer lens

18

Spatial resolution detector for the scattered electrons

19

outdoor ball of the energy analyzer

20

innerball of the energy analyzer

21

entrance slit of the energy analyzer

22

exit slit of the energy analyzer

23

Beam of the parallel analyzed energy interval

24

Beam of the parallel analyzed angular interval

25

Retardierungsnetze for suppression the inelastically scattered electrons

26

segmented scattering target

Δα θ

angular divergence of the incident central ray

Δθ

angular divergence the marginal rays of the incident electron beam

D

Beam diameter on the scattering target

e

electron energy

Φ

angle in the electron spectrometer

P

analyzed Component of the polarization vector

M _p

parallel Component of magnetization of a ferromagnetic scattering target

M _s

vertical Component of magnetization of a ferromagnetic scattering target

B

magnetic field perpendicular to the drawing plane

Claims (22)

Method for the spatially resolved analysis of electron spin polarization in the beam path of a parallel imaging electron microscope, comprising a polarization-sensitive scattering target ( 1 ) and an arrangement ( 2 . 3 ) for electron optical beam guidance, the lateral distribution of the degree of spin polarization of the electrons in the expanded beam being determined by the polarization-sensitive scattering target (US Pat. 1 ) and simultaneously analyzed by an electron-optical round lens ( 3 ) on the image detector ( 4 ) of the microscope is visualized. Method according to claim 1, characterized in that the scattering target ( 1 ) and the beam guide are positioned and adjusted so that on the image detector ( 4 ) of the microscope a spin-filtered electron-optical image of the sample comes to the image. Method according to claim 1, characterized in that the scattering target ( 1 ) and the beam guide are positioned and adjusted so that on the image detector ( 4 ) of the microscope a spin-filtered electron optical diffraction pattern, ie the spin filtered impulse distribution of the emitted electrons from the sample, comes to the image. Method according to one of the preceding claims, characterized characterized in that an energy or time-of-flight filtered electron distribution laterally resolved is analyzed. Method for the spatially resolved analysis of the electron spin polarization in the beam path behind the beam exit of an electron spectrometer, comprising a polarization-sensitive scattering target ( 1 ) and a spatially resolving detector ( 18 ), whereby the lateral distribution of the degree of spin polarization of the electrons behind the beam exit of the spectrometer is analyzed simultaneously by the polarization-sensitive scattering target and on the spatially resolving detector ( 18 ) is made visible. Method according to Claim 5, characterized in that the electrons are conveyed through a transfer lens ( 2 ) are imaged behind the beam exit of the electron spectrometer on the scattering target. Method according to one of Claims 5 and 6, characterized in that the electrons are conveyed through a transfer lens ( 3 ) behind the scattering target on the spatially resolving detector ( 18 ). Method according to one of claims 5-7, characterized that the spin polarization distribution within an energy interval is analyzed simultaneously in the manner of one-dimensional multi-channel detection. Method according to one of claims 5-7, characterized in that the spin polarizations distribution within an angular interval in the manner of a one-dimensional multi-channel detection is analyzed simultaneously. Method according to one of claims 5-7, characterized that the spin polarization distribution within a stripe-shaped spatial interval the sample in the manner of a one-dimensional multi-channel detection is analyzed simultaneously. Method according to one of claims 5-7, characterized that the spin polarization distribution is within an energy and angular (or spatial) interval in the manner of a two-dimensional multi-channel detection simultaneously is analyzed. Method according to one of claims 1-11, characterized in that the spatially resolving detector ( 18 ) detects the scattered electrons as single counting events, as a current or as a brightness distribution on a fluorescent screen. Method according to one of claims 1-12, characterized in that the scattering asymmetry utilized for the polarization analysis is based on the known principle of mottling on a scattering target ( 1 ) based on high atomic number. Method according to one of Claims 1-12, characterized in that the scattering asymmetry utilized for the polarization analysis is based on the known principle of spin-dependent low-energy electron diffraction or scattering on a monocrystalline or polycrystalline scattering target ( 1 ) or a thin layer. Method according to one of Claims 1-12, characterized in that the scattering asymmetry utilized for the polarization analysis is based on the known principle of exchange scattering on a ferromagnetic scattering target ( 1 ) or a thin ferromagnetic layer. Method according to claim 15, characterized in that the direction of magnetization of the ferromagnetic scattering target ( 1 ) or the layer by a magnetization device ( 13 ) is reversed or rotated 90 ° so that two mutually orthogonal components of the spin polarization vector can be analyzed. Arrangement for electron-optical beam guidance in a parallel-imaging electron microscope for spatially resolving analysis of the electron spin polarization, comprising a polarization-sensitive scattering target ( 1 ), as well as an electron-optical transfer lens ( 3 ) downstream of the scattering target for beam expansion and imaging. Arrangement according to claim 17, characterized in that the arrangement for beam guidance a magnetic or electric sector field for beam deflection ( 16 ) includes. Arrangement according to claim 17 or 18, characterized in that the scattering target ( 1 ) is positioned in or near an intermediate image plane, a diffraction plane or in a region of parallel beam guidance. Arrangement for the spatially resolved analysis of the electron spin polarization in the beam path behind the beam exit of an electron spectrometer, comprising a polarization-sensitive scattering target ( 1 ) and a spatially resolving detector ( 18 ), whereby the polarization-sensitive scattering target ( 1 ) the lateral distribution of the degree of spin polarization of the electrons is analyzed simultaneously behind the beam exit of the spectrometer, and on the spatially resolving detector ( 18 ) is made visible. Arrangement according to claim 20, characterized in that a diaphragm ( 11 ) in the beam path of the transfer lens ( 3 ) behind the scattering target ( 1 ) is inserted to limit the angular and energy intervals of the scattered electrons. Arrangement according to claim 20, characterized in that the scattering target ( 1 ) has a plurality of segments at different electrical potentials, so that a larger energy and angular interval can be analyzed simultaneously by means of low-energy diffraction on a monocrystalline scattering target.