Implementation of differential phase contrast Lorentz microscopy on a conventional transmission electron microscope

Abstract

Differential phase contrast (DPC) Lorentz microscopy is a powerful technique for investigating magnetization distributions within magnetic thin films. Previously this imaging technique has only been used on microscopes with nonstandard electron optical configurations because of two conflicting requirements: (1) The electron beam must be focused to a very fine probe on the specimen to obtain a high spatial resolution. (2) The specimen cannot be in the magnetic field of the objective lens, as this would destroy the magnetic domain structures of interest. This paper will describe the implementation of the technique on a conventional microscope, a JEOL JEM 2000 FX fitted with an ASID 20 scanning attachment, which satisfies these two requirements. We have modified the controls for the lenses and deflection coils in the microscope to allow an electron probe of less than 40 nm to be formed using the three condenser lenses, with the objective lens off. A position sensitive quadrant detector, required for DPC imaging, is mounted on a sliding O-ring below the projector lens and can be moved in for DPC work or out for standard microscope operation. Attached to the microscope is a digital image acquisition system based on an IBM PC AT. This system is used to acquire signals from the four quadrants of the detector simultaneously. The signals are then mixed digitally to produce the DPC images and an incoherent bright-field image. These images can be further processed to produce vector maps of the in-plane magnetization distributions within the sample. The spatial resolution of 40 nm is less than that of the VG HB501 STEM vacuum generators (VG) HB501 scanning transmission electron microscope (STEM) previously used for DPC imaging. This is due to the larger emitting area of the LaB6 filament compared to the field emission gun of the VG. The LaB6 has a much lower brightness than the FEG and this necessitates long image acquisition times. DPC images and the corresponding vector map from a NiFe film will be shown to demonstrate the technique.