1.Background to research and development
Our study has revealed that an electron source combining a mechanism generating spin-polarized electrons by exciting valence electrons of a GaAs semiconductor to a conduction band through absorption of circularly polarized light and a mechanism taking out the polarized electrons from the NEA surface having negative electron affinity into vacuum, possesses excellent potential capacity. Cooperation of accelerator physicists (Nakanishi et al.) and semiconductor physicists (Takeda et al.) showed that high polarization of 90% by changing a semiconductor from bulk GaAs to a strained super lattice film, and high current density over 10 A/cm2 by a method narrowing the band bending area on the NEA surface could be obtained. In addition, degradation of the NEA surface was mitigated by the extreme high vacuum environment on the order of 10-10Pa, enabling continuous use for several weeks.
On the other hand, a low energy electrons microscope (LEEM) allows real time observation of dynamic change of metal surface morphology in a visual field diameter of a few to several 10 micrometers with 10 nm resolution. In Japan, Koshikawa et al. have been engaged in the study and made an effort for its popularization.
SPLEEM (spin-polarized low energy electron microscope) using a polarized electron beam in LEEM was put to practical use late in the 1990s enabling observation of a magnetic domain structure, however, a large defect remained present in terms of performance. Since the beam intensity of a conventional polarized electron source was 1/1000 or less compared with an unpolarized LaB6 electron source and at least 4 seconds was needed to acquire even a single image, real time observation was impossible. Research and development aiming to overcome this was started as the Development of Advanced Measurement and Analysis Systems (component technology) Project of JST (Japan Science and Technology Agency) 7 years ago.
We made 2 developments to create a high-intensity 20keV polarization electron gun. (1) As a method minimizing the laser spot for high intensity, by abandoning the conventional type in which laser enters through the electron emission surface, we developed a system in which the laser enters from the rear surface of the cathode crystal and converges on the supperlattice structure. Specifically, we obtained breakthrough improvement by achieving the intensity of 2.0 × 107A/(cm2steradian) which was 10,000 times or more of the conventional and one order of magnitude larger than the LaB6 electron source by converging a parallel laser beam of 3 mmφ to 1/1000 or less, a spot of 1.3 μmφ. (2) Along with this, we developed a new photo cathode to maintain polarization 90% of the conventional one. To begin with, we replaced a conventional GaAs substrate with a GaP substrate transmitting a laser beam having a wavelength of 780 nm, and formed a GaAsP mitigation layer and a GaAs-GaAsP strained super lattice film on it, however polarization was up to 60%. This problem was solved by inserting a 0.5 μm GaAs layer between the GaP substrate and the mitigation layer and thus 90% polarization could be attained.
Fig. 1 shows a picture of the new electron gun using the high-intensity / high polarization electron source loaded on the LEEM microscope.
Fig. 1 Entire configuration diagram of SPLEEM microscope
The first experiment of real time observation of the SPLEEM image of the magnetic domain forming process on the Co surface vapor-deposited on the W (110) substrate showed that the image acquisition time was reduced to 20 milliseconds, 1/200 of the conventional one, allowing video recording of 50 frames / second, and thereby the original target was achieved.
Since the direction of the spin of the polarized electron beam can be controlled in any direction, the omnidirectional components of magnetization of a sample can be observed. For example, investigation of in-plane and perpendicular magnetization components of [CONi2] y/W (110) film proposed as a candidate for spin injection (STT) magnetic memory showed that the stable perpendicular magnetization component can be obtained after y=4 stratification. Additionally an investigation of in-plane and perpendicular magnetization components of Au vapor-deposited [CONi2] /W (110) film showed that the single atomic layer of vapor-deposited Au changed its initial in-plane magnetization to perpendicular magnetization as shown in Fig. 2.
Fig. 2 Observation of Au vapor-deposition on [CONi2]/W(110) film
SPLEEM images with the spin of a polarized electron beam directed perpendicular (top) and in-plane (bottom), and Au vapor-deposited in 0 (zero) to one-atom layer
Magnetic domain structure formation in various samples will be investigated in the future, and it is expected that the results will be able to contribute to the development of next-generation magnetic memory.
3.Meaning of the achievements
Power of the high-intensity / high polarization electron gun developed in cooperation of the researchers in 3 different fields of accelerator, semiconductor and electron microscope has already been internationally recognized in the LEEM and surface-related fields, and there are some order requests placed to them. The commercial production by a company is expected and the JST's device development project is in progress at present. As for research, it is expected that SPLEEM which is capable of real time observation will advance detailed study of the surface magnetic domain structure forming process that will contribute to the development of new magnetic storage devices. It is also expected that effectiveness of this polarization electron gun as a measurement means for spin property such as transmission electron microscopy and inverse photoemission spectroscopy will be confirmed and popularized in the future.