Electron Phase Microscopy of Magnetic Fields in Ferromagnets and Superconductors
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Electron Phase Microscopy of Magnetic Fields in Ferromagnets and Superconductors
Akira Tonomura Advanced Science Institute, RIKEN, Wako, Saitama 351-0198, Japan Advanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-0395, Japan Initial Research Project, Okinawa Institute of Science and Technology, Kunigami, Okinawa 904-0411, Japan E-mail: [email protected]
ABSTRACT Highly sensitive electron phase microscopy based on the Aharonov-Bohm (AB) effect principle has been used to observe microscopic distributions of magnetic fields in ferromagnets and superconductors. The observation examples include the unconventional behaviors of interlayer Josephson vortices in anisotropic layered high-Tc superconducting YBa2Cu3O7- (YBCO) thin films, which are produced when the applied magnetic field is greatly tilted to the layer plane, and the magnetic-field distributions of tiny magnetic heads for perpendicular recording and of colossal magnetoresistance (CMR) materials. INTRODUCTION Electrons behave as waves as long as they are not detected. Thanks to recent developments of brighter field-emission electron beams, it is now practical to use the wave nature of electrons to directly image the quantum world by using the phase information of electrons [1]. Magnetic lines of force can be quantitatively observed as phase contours of transmitted electrons by holographic interference microscopy [2] since a phase shift of 2 π is produced between two electron beams enclosing a magnetic flux of h/e due to the Aharonov-Bohm (AB) effect [3], and quantized vortices in superconducting thin films can be dynamically observed by Lorentz microscopy [4]. This paper describes the challenges we faced in our efforts to investigate the phenomena related to Josephson vortices in layered high-Tc superconductors, the magnetic characteristics of tiny magnetic heads used for high-density perpendicular recording, and the nucleation
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mechanism of ferromagnetic domains in CMR materials.
EXPERIMENTAL METHOD We carried out experiments on superconductors using a 1-MV field-emission transmission electron microscope and ones on magnetic heads and CMR materials using 300-kV fieldemission electron microscopes. We observed vortices in superconducting thin film as Lorentz micrographs by defocusing the electron microscope images (figure 1). The phase distribution of the transmitted electrons through magnetic flux is transformed into the corresponding intensity variations by image defocusing. When vortices penetrated a thin film parallel to the film surface as interlayer Josephson vortices, they could be observed without tilting the film (figure 1(a)). However, when the vortices were perpendicular to the film, the film had to be tilted so that the incident electrons would interact with a perpendicular component of the vortex magnetic field (figure 1(b)).
Figure 1. Experimental arrangements for observing Josephson vortices in layered superconductors by Lorentz microscopy. (a) Vortices in ab-plane film. (b) Vortices in ac-plane film. Electrons ar
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