Novel approaches for the characterization of electromagnetic fields using electron holography
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Novel approaches for the characterization of electromagnetic fields using electron holography Takeshi Kasama1,2, Yanna Antypas2, Ryan K.K. Chong2 and Rafal E. Dunin-Borkowski2,1 1 RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 2 Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, U.K. ABSTRACT Two recent developments related to the application of off-axis electron holography to the characterization of magnetic and electrostatic fields in nanoscale materials and devices are described. The first is based on the design and implementation of a three-contact electrical biasing specimen holder that allows electron holograms to be recorded from samples as they are tilted to angles of up to ±70° with voltages applied to them in situ in the electron microscope. The second relates to the prospect of characterizing magnetic vector fields in materials in three dimensions using electron holography. INTRODUCTION Off-axis electron holography is increasingly used to characterize magnetic and electrostatic fields in materials in the transmission electron microscope (TEM) at a spatial resolution that can approach the nanometer scale. The technique [1] involves applying a positive voltage to an electron biprism in order to overlap a coherent electron wave that has passed through a sample with a part of the same electron wave that has passed only through vacuum. Analysis of the resulting interference pattern allows the phase shift of the specimen wave
φ(x) = CE ∫ V(x,z) dz −
e ∫∫ B⊥ (x,z) dxdz h
(1)
to be recovered quantitatively and non-invasively. In Eq. 1, x is a direction in the plane of the sample, z is the electron beam direction, CE takes a value of 6.53×106 rad V-1m-1 at 300 kV, V is € the electrostatic potential and B⊥ is the component of magnetic induction perpendicular to x and z. Here, we illustrate the use of an ultra-high-tilt cartridge-based TEM specimen holder designed by E.A. Fischione Instruments, Inc. to acquire electron holograms from samples that have up to three electrical contacts applied to them. We also describe recent developments in the acquisition and analysis of electron holograms that may lead to the use of the technique to characterize magnetic vector fields inside materials in three dimensions, rather than simply in projection. EXPERIMENTAL DETAILS The off-axis electron holograms that are presented below were all acquired at an accelerating voltage of 300 kV using a Philips CM300ST field emission gun TEM equipped with a 'Lorentz' lens, an electron biprism, a GatanTM imaging filter and a 2048 pixel charge-coupleddevice camera. The holograms were acquired with the conventional microscope objective lens switched off and the sample located in magnetic-field-free conditions. Reference holograms were used to remove distortions associated with the imaging and recording system of the microscope.
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EXPERIMENTAL RESULTS Ultra-high-tilt three-contact electrical biasing specimen holder The concept of a 'laboratory in an electron microscope', which allows severa
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