Source for Spin Polarized Electrons

Technical Field of the Invention

The present invention relates to the field of electron sources, emitting a beam of spin-polarized electrons by photoemission, comprising an insulating thin film deposited onto the surface of a magnetic material.

Background of the invention

Electron sources are used in various fields of applications, such as imaging (e.g. scanning electron microscopes or cathode-ray tubes), particle-physics experiments (electron accelerators), or in synchrotron storage rings. In many cases, it is important to have full control over all properties of the electron beam, including the temporal structure of the beam, i.e. its pulse shape, and the spin-polarization of the electrons.

An electron current has two spin components with respect to a given quantization axis. If the expectation value of the spin component parallel is larger than the component antiparallel to this axis, the current is spin polarized parallel to this axis, or vice versa. Due to the additional spin degree of freedom, polarized elec- tron currents carry more information than unpolarized currents, which is advantageous in many application. For example, polarized electron currents can be used for reading and writing spin information on quantum storage devices. The advent of spintronics will further fuel the need for reliable and highly brilliant spin polarized electron sources.

There are several known sources for spin-polarized electrons.

If the photoelectric effect is used to drive the source, and if there are different cross sections for different spins, selection rules may be used to extract a spin- polarized electron current. Currently the standard source for spin polarized electrons is the non-magnetic GaAs type source (U.S. Patent 3,968,376 issued on

July 6, 1976), which was significantly improved and optimized over time. The switching of the polarization, parallel or antiparallel to the surface normal, is obtained by changing the helicity of the light that promotes the electrons into the vacuum.

The main drawback of this widely used type of source is the inherent sensitivity to contamination by residual gases due to the chemical reactivity of the semiconductor surface and due to the treatment with alkali metals and oxygen, which is required for achieving high emission currents. As a consequence, the photo- electron yield and the spin-polarization of the GaAs source decrease significantly over a few days even when the source is operated with exceptionally low pressure of the residual gas.

Summary of the invention According to one aspect of the invention, there is provided a source for spin polarized electrons, comprising the features of claim 1.

According to another aspect of the invention, there is provided a method of producing spin polarized electrons, comprising the features of claim 12.

Advantageous embodiments are defined in the dependent claims.

The present invention provides a source of spin-polarized electrons which is chemically inert. It furthermore provides a source of spin-polarized electrons after a simple preparation cycle. It furthermore provides a source of spin-polarized electrons that yields a high degree of polarization and high brilliance when excited with light from standard laser sources.

Surprisingly it has been found that the above can be achieved by a photoemis- sion source comprising a magnetic material covered by an insulating thin film and a light beam irradiating on said insulating thin film.

In a preferred embodiment of the invention, the insulating thin film comprises a material with unoccupied electronic states below the vacuum level whereby those states, if populated, have a higher lifetime than the corresponding states in the bare magnetic material. In a preferred embodiment of the invention, the magnetic material is ferromagnetic, and preferably has a large geometrical ani- sotropy in order to achieve a high remanent magnetization.

In another preferred embodiment of the invention, the magnetic material comprises a material in single crystalline form.

In one embodiment of the invention, the magnetic material comprises nickel in single crystalline form. In another embodiment of the invention, the magnetic material comprises cobalt. Alternative magnetic materials include iron containing alloys, nickel containing alloys, cobalt containing alloys, or Permalloys. In a pre- ferred embodiment of the invention, the surface of the magnetic material covered by the insulating thin film comprises the (111)-surface of a nickel single crystal. In another embodiment of the invention, the insulating thin film comprises at least one monolayer of hexagonal boron nitride. In still another embodiment of the invention, the insulating thin film comprises a single layer of hexagonal boron ni- tride.

In another embodiment of the invention, a transparent base substrate is covered with a thin film of a magnetic material, and an insulating thin film is being deposited onto said magnetic thin film, and said insulating thin film is illuminated with a light beam through the base substrate.

In a preferred embodiment of the invention the light beam comprises a laser beam. Alternative light beams include light beams from gas discharge light sources, such a mercury or sodium vapor light sources. In order to achieve spin- selective photoemission, narrow-band photoexcitation of said insulating material

within an energy range not exceeding the vacuum threshold by more than once the work function is used.

In another preferred embodiment of the invention the laser beam is pulsed.

In another embodiment of the invention, the wavelength of the laser beam is in the range of about 750 nm to 870 nm.

In one embodiment of the invention the light beam comprises a pulsed laser beam and its frequency doubled second harmonic beam.

In still another embodiment of the invention, the light beam comprises the fundamental light of a standard titanium-doped sapphire laser tuned to a wavelength of about 800 nm and its frequency doubled second harmonic beam.

In another embodiment of the invention, spin-polarization is achieved by suitable magnetization of the said magnetic material.

In another embodiment of the invention, the surface of the magnetic material has a large geometrical shape anisotropy. In a preferred embodiment of the invention, the source comprises nickel in single crystalline form with a (111) surface covered by at least one monolayer of hexagonal boron nitride (h-BN) and a light beam of a standard titanium-doped sapphire laser tuned to a wavelength of about 800 nm and its frequency doubled second harmonic beam irradiating on said h-BN. This produces a resonance peak in the photocurrent, caused by resonant excitation and photoelectron extraction via the conduction band of boron nitride. Together with the low work function of this surface, this yields a high photoelectron current. Spin selectivity of the photoemission process may be obtained by taking advantage of the occupied spin-split initial states and the spin splitting in the conduction band, which serves as intermediate state.

Because of the outermost layer being composed of closed shell boron nitride in this embodiment, the photo-emitting surface is extremely stable against contamination as the sticking coefficients of usual residual gases encountered in standard vacuum systems are very low. In one embodiment of the invention it was shown that such sources may be operated over about 15Oh, or over about 200 h, or over about 300 h in a moderate ultra-high vacuum without showing a significant decrease in photoelectron yield and spin polarization. Moreover, this source may have a small pulse width in pulsed applications, since the excitation of the electrons occurs a) in a two-photon photoemission process, and b) localized on an atomically sharp interface. Moreover electrons may be emitted into a small solid angle of emission, which results in a high brightness.

In one embodiment of the invention, the polarization is tuned via the magnetization of the magnetic material. By controlling the orientation of the magnetization axis with respect to the final electron propagation direction any given spin orientation is obtained without deflection of the beam.

In a further embodiment of the invention, additional materials such as alkali metals may be deposited onto the insulating thin film in order to reduce the work function of the surface.

Brief description of the drawings

The above mentioned and other features and objects of this invention and the manner of achieving them will become more apparent and this invention itself will be better understood by reference to the following description of various embodiments of this invention taken in conjunction with the accompanying drawings, wherein;

FIG 1 shows a sectional view of an embodiment of the invention;

FIG 2 and FIG 3 show possible alternative geometries for illumination and magnetization of the emitter surface;

FIG 4 shows a possible simple model of the excitation processes leading to emission of a spin-polarized photocurrent;

FIG 5 shows the energy spectrum of the photocurrent (top panel) together with the measured spin asymmetry (bottom panel) for one embodiment; the integral spin polarization as measured from this sample is given.

Description of various and preferred embodiments of the invention Referring to FIG 1 , in a preferred embodiment the magnetic material 1 is a ferromagnetic material in single crystalline form, which keeps a remanent magnetization after a short pulse of direct current through the solenoid 3. In this pre- ferred embodiment, the ferromagnetic single crystal material comprises a nickel single crystal material with the electrons being photoemitted from the (111)- surface of said nickel single crystal material, and the axis of the solenoid and that of the surface in-plane magnetization direction coincide with the magnetic second easy axis [-1 ,0,1] of the nickel single crystal material.

After standard cleaning of the nickel single crystal material surface, an atomically thin layer of hexagonal boron nitride (h-BN) 2 is deposited onto the surface. For this purpose a chemical vapor deposition technique may be used which may include exposing the surface to h-BN containing precursors (e.g. borazine (HBNH)3 or trichloroborazine (CIBNH)3). In this embodiment the growth rate drops by two orders of magnitude after the completion of the first layer, after which no more active sites for cracking the precursor are available. Alternative deposition techniques include molecular beam epitaxy or laser ablation techniques.

Two pulses of 800 nm and 400 nm wavelength 4 may impinge onto the h-BN covered surface and concomitantly produce a high yield of photoelectrons 5. Due to the direction of the magnetic material magnetization, the spin polarization is transverse with respect to the propagation direction of the electrons in this case. The photoelectrons are accelerated by the electric field between the source (1 ,2) and an anode 6, which may take the form of a grounded or biased grid or a simple plate with an extraction hole. In the embodiment shown, the anode was defined by the entrance aperture of the electron analyzer. The wall 7 denotes the wall of the vacuum chamber of the ultra-high vacuum system with a base pres- sure of 10" ¹⁰ mbar.

The present source may be operated with commercial laser systems that are based on titanium-doped sapphire crystals as gain medium. This, in conjunction with a commercial non-linear crystal for frequency doubling like beta- bariumborate, provides the two energies necessary to excite the resonant state in h-BN. In FIG 1 , a fundamental laser pulse of about 800 nm 8 is split into two orthogonal polarization components by a waveplate retarder 9, whose function is described below. The light is focused by means of a lens 10 into a non-iinear frequency-doubling crystal 11 and, together with the so produced frequency doubled second harmonic light beam, focused by a second lens 12 through a vacuum viewport 13 onto the surface 2. Since light at different wavelength travels at different speed in material, this effective path-length difference for fundamental and frequency doubled second harmonic light beam between the frequency-doubling crystal and the emitting surface has to be compensated for. Moreover, the polarization vector of the fundamental light has to be parallel to the one of the frequency doubled second harmonic light beam and both may be parallel to the emitting surface normal for maximum efficiency. Since the polarization of the frequency doubled second harmonic light beam is perpendicular to the one of the fundamental beam, the latter may be split into two pulses of or- thogonal polarization. This and the compensation of the path-length difference explained above may be done by means of the wave retarder 9.

Referring now to FIG 2, in another preferred embodiment photoelectrons are produced by illuminating the photoemitting surface 2 from the back instead of illuminating the emitting surface. For this purpose, a magnetic material such as nickel is deposited as thin film onto a suitable transparent base substrate like sapphire 14 and covered by a monolayer of h-BN. The thickness of the magnetic material thin film may be optimized for highest possible photoelectron yield or for highest spin polarization. The surface magnetization can be obtained by using an external "horseshoe"-type magnet 15 with solenoid 3 or else a closed loop of the magnetic material thin film together with a current loop.

Yet another embodiment is shown in FIG 3, where a ferromagnetic single crystal material has the form of a stick 16 elongated along the magnetic easy axis and covered at its front end by a monolayer of h-BN 2. The axis of said ferromagnetic single crystal material coincides with the axis of the magnetizing current loop 3. In this case, the light beam 4 may be directed onto the front end of said ferromagnetic single crystal material, and the spin polarization of the photoelectrons will then be longitudinal.

FIG 4 illustrates, as a possible simple model of the excitation processes, the im- balance of spin states in the photocurrent as being the consequence of different excitation energies for minority and majority states. Two photons, one of fundamental frequency and one having the double frequency, arrive coincidently at the sample and promote an electron above the vacuum threshold. The magnetized sample provides the spin splitting of the conduction band which gives rise to an asymmetry of the photocurrent between the two spin channels. In the case of a nickel (111)-surface the highest yield may be obtained by using linearly polarized light with the polarization vector having a strong component parallel to the surface normal.

Choosing the appropriate wavelength, the user may tune the spin polarization, since for specific wavelength, one of the two excitation channels may greatly

dominate the excitation spectrum, as indicated by the use of solid and dashed lines. Due to the very nature of this two-photon process, the photoelectron yield depends on the product of the two intensities or amplitudes of the light at the wavelengths used. The relative intensities of fundamental and frequency doubled second harmonic light beam may be optimized for high yield and/or spin contrast in the photocurrent.

In FIG 5 the results are shown which were obtained from a h-BN/Ni(111 ) combination using linearly-polarized light of 794 and 397 nm wavelength. For the proof of principle a ferromagnetic nickel (111 ) single crystal material was used having the cross section like a picture frame, on which the magnetization may be switched along the second easy axis in the (111 ) plane. The spectrum in the top panel of FIG 5 is dominated by a large peak at a kinetic energy of about 850 meV, followed by a secondary tail at lower energy, which is caused by inelastic scattering of the photoelectrons within the surface. The width of the peak is about 250 meV and is rather independent on the photon energy employed. It determines the energy spread of the photoelectrons and is superior to any thermally activated cathode material. The total polarization calculated by integrating the product of spectral intensity and the spin asymmetry, given in the lower panel of FIG 5, over the whole energy spectrum, amounts to about 5%. This value was compared to the degree of remanent magnetization, which was 21 % of the saturation magnetization in this case. The spin character of the photoelectrons is in- plane majority state in this embodiment. It will be appreciated that by using thin films of magnetic material the surfaces are likely to become fully magnetized, which will accordingly produce beams of at least 25% of transverse spin polarization. Furthermore, it will be appreciated that by going toward higher wavelength or lower photon energies the spin polarization is likely to be enhanced. Furthermore, it will be appreciated that by going toward higher wavelength or lower photon energies the spin polarization is likely to be of minority character.

It will be appreciated that modifications to the embodiments described above are of course possible. Accordingly the present invention is not limited to the embodiments described above.