1, The Ohio State University, Columbus, Ohio, United States
The development of novel materials and heterostructures relies heavily on the ability to correlate structural and functional properties across length scales, especially down to the nanometer or atomic scale. Advances in aberration corrected electron microscopy has allowed for real space imaging, diffraction, and spectroscopy at the atomic scale; moreover, it has improved the capabilities of in situ experiments in the electron microscope. One such example is imaging magnetic domain structures as a function of applied magnetic field and temperature using aberration corrected Lorentz transmission electron microscopy (LTEM) and, more recently, differential phase contrast scanning TEM (DPC-STEM). Both techniques operate with the specimen in variable applied magnetic field by shutting off or reducing the strength of the objective lens, which surrounds the specimen in most microscopes. With the specimen in field-free or low field conditions, a wide range of important and interesting magnetic structures can be studied in a large swath of field/temperature space.
Over the last few years, the magnetic skyrmion phase has piqued scientific interest for potential applications in next-generation electronic devices such as racetrack memory due to their robustness to defects and extremely low pinning energy, which can be orders of magnitude lower than that of domain wall motion. Skyrmions are chiral magnetic spin textures that can form either in isolation or in extended lattices, with sizes ranging from the micron to single nanometer scale. The most well-studied skyrmion form is the Bloch skyrmion which is circular like a vortex; however, in thin and ultra-thin film heterostructures, a different kind of skyrmion can form with radial symmetry called a Néel skyrmion.
In our work, perovskite bilayers of SrIO3/SrRuO3 are grown on (001) SrTiO3 via off-axis magnetron sputtering. In this system, skyrmions have been reported to be sub-10nm in size and Néel-type, making them difficult to image using LTEM without large specimen tilts; however, using DPC-STEM, overall spatial resolution in the image, as well as sensitivity to the gradient of in-plane magnetism should be improved. Imaging skyrmions in these materials as a function of applied magnetic field and temperature, as well as investigating the effect of varying SrRuO3 and SrIrO3 layer thickness gives direct insight into the stability and properties of skyrmions in this type of system and is only possible through such advanced imaging techniques.