Highlights

Vector Field Electron Tomography

Scientific Achievement

A new experimental technique has been developed to measure, in three dimensions, the magnetic vector potential field in and around a nano-scale magnetized object.  The method relies on the acquisition of images using a transmission electron microscope operated in the Lorentz imaging mode, followed by a tomographic analysis of the data to reconstruct the vector potential. The vector potential can be considered to be the most fundamental quantity describing the magnetization state of an object; all other magnetostatic quantities can be derived directly from the magnetic vector potential.  The procedure involves the acquisition of four tomographic tilt series, with a three-image through-focus series for each tilt angle.  From the through-focus series, we can reconstruct the total phase shift of the electron wave.  By taking the difference between the phase shift for a given orientation and the phase shift for the sample turned upside down, the magnetic component of the phase shift can be obtained; this magnetic phase shift is the projection of a component of the magnetic vector potential, and by combining the projections along different directions, the complete 3-D magnetic vector potential can be reconstructed using a tomographic filtered back-projection method.

We have used the Tecnai TEM of the EMC to obtain the first couple of datasets for this type of reconstruction.  The sample consists of a series of patterned permalloy islands (squares and ellipses), and we have achieved what we believe to be the first ever 3-D reconstructions of the magnetic vector potential around both types of shapes. 

Significance

The ability to determine the magnetic nanostructure of objects in a quantitative three-dimensional way is likely to have a large impact on the microscopy community as well as on the larger community interested in the properties of arrays of nano-particles.  We anticipate that our work will lay the foundation for a new method that will rapidly become a standard approach. Our current and future work will involve the streamlining of the experimental and computational components of vector field electron tomography, and the application of spherical aberration correction methods to improve the spatial resolution of the technique.  The theoretical foundation of the method was published in Ultramicroscopy 108, 503 (2008), and further publications are in progress.

Performers

C. Phatak, M. De Graef (Carnegie Mellon U.); A. Petford-Long, M. Tanase (Argonne-MSD)