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Materials Science at Oregon State University

Investigations of magnetic topology and domain wall chirality using in-situ Lorentz TEM

Date: Thursday, May 25th
Presenter: Dr. Benjamin McMorran, University of Oregon


Chiral magnetization in thin films, such as magnetic domain walls (DWs) with controlled chirality and topological magnetic bubbles such as skyrmions, show promise for spintronics applications. We use transmission electron microscopy (TEM) to investigate these magnetic features at the nanoscale. We have developed a modified single image Transport of Intensity Equation (TIE) analysis to Lorentz TEM images that allows us to rapidly collect magnetic microscopy images under varying external conditions, such as during an applied field sweep. Contrast in Lorentz TEM is caused by the Lorentz force between the magnetic field of a sample and the electrons used to illuminate the sample. It is thus generally only sensitive to magnetic fields perpendicular to the electron beam’s direction of propagation. However, the chirality of a DW cannot be determined from knowledge of only the x- and y- components of the magnetization. The standard approach in LTEM for determining the 3rd component of the magnetization is to tilt the sample to some angle and record a second image. This presents a problem for any domain structures that are stabilized by an applied external magnetic field (e.g. skyrmions), because the standard Lorentz TEM setup does not allow independent control of the angle of an applied magnetic field, and sample tilt angle. From images recorded during an applied field sweep from negative saturation to positive saturation, the direction of the z- component of the magnetization can be easily determined. Using this history-dependent information and the standard in plane information gained from Lorentz TEM, we have mapped the chirality of DWs in an [Fe (0.34 nm)/ Gd(0.4 nm)/ Pt(0.25 angstroms)/Ir(0.75 angstroms)] × 80 multilayered film made by the Fullerton group at UCSD.

Professor McMorran joined the Physics Department at University of Oregon as an assistant professor in Fall 2011. He is a member of the Oregon Center for Optical, Molecular, and Quantum Sciences (OMQ). Working under the guidance of Prof. Alex Cronin, he earned his Ph.D. from the University of Arizona in 2009, with the thesis entitle "Electron Diffraction and Interferometry Using Nanostructures". Shortly thereafter, he joined the Electron Physics Group in the Center for Nanoscale Science And Technology (CNST) at NIST in Gaithersburg. There he worked with John Unguris and Jabez McClelland on magnetic electron microscopy. Since 2007, his research focuses on the physics and applications of sculpted electron wavefunctions such as vortex beams, as well as magnetic microscopy. In addition to microscopy technique development, the McMorran group uses the electron microscope as an “optics bench” to perform basic research in electron quantum optics, computer-generated (inverse) electron holography, and electron interferometry. He is a 2013 Department of Energy Early Career Award recipient.