Electromagnetic field mapping

Phase contrast techniques such as off-axis electron holography can be used to measure functional properties of nanoscale materials, such as magnetic induction and electrostatic potential. These measurements can be used to obtain information about local variations in magnetization and charge density.

The Electromagnetic Field Mapping group applies off-axis electron holography to a wide variety of materials and develops new methodologies based on phase shifting, including concepts based on sculpted electron beams.

Experimental work is performed using the Cs corrected FEI Titan Holo and Cc and Cs corrected FEI Titan PICO microscopes. A highly sensitive and fast Gatan K2 IS direct electron detection camera is installed on the FEI Titan Holo microscope, while a Gatan OneView camera is installed on the FEI Titan PICO microscope.

In situ experiments are performed to study switching processes as a function of external stimuli such as temperature, magnetic field and electrical bias.

Quantitative high-resolution mapping of the magnetic properties of Bloch-type skyrmions in B20-type FeGe. Skyrmions are nanoscale vortex-like spin structures that are stabilized in chiral magnets, magnetic thin films and Heusler alloys. The images show the magnetic phase shift (M) of a skyrmion lattice measured using off-axis electron holography in the presence of externally applied magnetic field of 100 mT at 200 K. By adding contours and colours, a magnetic induction map (Bxy) with a contour spacing of 2π/64=0.098 radians can be created. The experimental phase image was used to calculate the projected in-plane magnetization (Mxy) using an iterative model-based reconstruction algorithm. Mxy is shown in units of kA/m and in the form of a colour-coded vector map of the recovered projected magnetization a single Bloch-type skyrmion.

Current research topics include the study of:

• Projected electrostatic potentials and charge densities in needle-shaped specimens;
• Magnetic fields of skyrmions in chiral magnets and thin films;
• Complementary measurements of in-plane and out-of-plane magnetic signals using electron magnetic circular dichroism (EMCD) and off-axis electron holography;
• Magnetic microstructures and domain wall widths in soft and hard magnets and magnetic high entropy alloys;
• Ultrafast electron tomographic holography;
• Double-exposure electron holography;
• Microwave-assisted high frequency dynamical processes;
• Strain measurements in functional oxides using dark-field electron holography;
• Tunable phase plates and electron vortex beam generators;
• Orbital angular momentum of the electron wavefront.

Contact

Dr. András Kovacs
Phone: +49 2461 61-9276
e-Mail: a.kovacs@fz-juelich.de