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Lensless Imaging of Magnetic Nanostructures Print

Magnetism is useful for many devices and techniques, from electric motors and computer hard drives to magnetic resonance imaging used in medicine. By studying the basics of magnetism, scientists aim to better understand the fundamental physical principles that govern magnetic systems, perhaps leading to important new technologies. The high brightness and coherence of the ALS's soft x-rays have enabled scientists to apply lensless x-ray imaging for the first time to nanometer-scale magnetic structures in an alloy.

Many Ways To See

You open your eyes and detect the light rays streaming through your bedroom window (transmission), illuminating your socks on the floor (scattering). You put on your glasses (refraction) to detect the state of your image in the mirror (reflection). If you are an ALS scientist, perhaps you go to work and shine some x-ray light on a crystal to detect the arrangement of the atoms in the crystal (diffraction). Now, thanks to Turner et al., you can also shine some x-ray light on a magnetic sample to detect the arrangement of its electron spins through a method known as lensless imaging. This last example is an equally valid way to "see," but instead of using windows, lenses, or mirrors to manipulate light and construct an image, mathematical formulas are used to describe the effects that particles and fields in the sample have on the light.

These formulas have always contained terms that relate to the electron spin of magnetic atoms, but they were previously ignored. Using the full formula allows for the determination of not only crystal structure, but magnetic spin distribution and orientation as well, with a spatial resolution limited only by the wavelength of x-rays used. This promising method can be used at any coherent light source, including modern x-ray free-electron lasers, where ultrashort pulses would freeze-frame magnetic changes, offering the potential for imaging in unprecedented detail the structure and motion of boundaries between regions with different magnetic orientation.

An image of magnetic domains (red and green) in a thin magnetic film of a cobalt–terbium alloy. The colors represent the electron spin direction, i.e. pointing into or out of the page, while the brightness represents the amplitude of the magnetization.

Coherent diffraction imaging (CDI), also known as x-ray diffraction microscopy (XDM), is a method that was developed to "see" tiny structures without using a lens to produce a magnified image of the structure. This is particularly important in the x-ray regime, because x-ray optics technology cannot presently deliver the smallest theoretically possible spatial resolution (i.e., resolution at the "diffraction limit"). Images produced using CDI have the potential to reach this physical limit through diffraction methods and iterative mathematical algorithms. Recently, this technique has been especially popular in imaging such complex structures as aerogels and yeast cells.

When applying the CDI technique to a magnetic system, the same algorithms are used, but the interpretation is different. Traditionally, a material's structure is inferred from x-ray scattering off of its charge-density distribution (i.e., Bragg diffraction). In coherent magnetic x-ray scattering, however, the x-rays that interact with the magnetic atoms can be used to image a system's magnetic features that are otherwise undetectable.

Analysis of the x-rays' interaction with the electron spins in a magnetic atom can reveal where the boundaries are between domains of "up" and "down" spins. Above are two examples of how the spins between two oppositely oriented domains (red and green) might form either a "Neel" (top) or a "Bloch" (bottom) wall state.

A group of scientists working at ALS Beamline 12.0.2 have now demonstrated that the CDI technique works for producing nanoscale images of the magnetic structure of materials. They scattered coherent soft x-rays from an ultrathin amorphous alloy film of the two metals cobalt and terbium. These are systems in which the interaction of the localized rare-earth electrons in terbium and the itinerant transition-metal electrons in cobalt are interwoven. Measurements were performed at the cobalt resonance energy at the L-edge. The spatial resolution of the resulting image was about 75 nm. While the actual measurement was nothing new, the full magnetic-domain structure was discerned for the first time from the scattered magnetic x-ray data, through the use of iterative CDI algorithms, and it yielded some surprises.

For instance, linearly polarized light was used to fully recover the magnetic domain patterns. Usually, linear polarization cannot be used because the magnetic information is lost when the scattered intensity is measured. Instead, magnetic imaging is typically done with circularly polarized light using the x-ray magnetic circular dichroism (XMCD) effect. The data demonstrate, however, that magnetic information can be obtained by using the algorithms to recover the magnetically sensitive phase of the scattered x-rays. The result yields more information than the typical absorption contrast used in other x-ray magnetic imaging techniques, effectively giving two pieces of information for each pixel of the image, both amplitude and phase. The amplitude is sensitive to the distribution of magnetic domains in the material because it is proportional to the overall absolute value of the magnetic spin states. The phase image yields no information about the walls in this case, but demonstrates the polarity of the domains, into or out of the sample plane in this experiment.

Close-up images showing the phase and amplitude separately enhanced (from the sample shown at top).

In the future, increasing the resolution past the 10-nm scale would enable us to observe much finer features such as the orientation of a small number of spins inside domain walls. In addition, the ability to use linearly polarized light enables us to dispense with complex methods of creating circularly polarized x-rays for many magnetic studies. This new method, applied to the field of magnetism, will also allow snapshot images with unlimited resolution at x-ray free-electron laser facilities, a prerequisite first step to recording ultrafast movies of magnetism in action.

 

Read the ALS Science Brief X-Ray Diffraction Microscopy of Magnetic Structures.


 

Research conducted by: J.J. Turner (SLAC National Accelerator Laboratory and Stony Brook University), X. Huang (University College London), O. Krupin (European XFEL and SLAC), K.A. Seu and S. Roy (ALS), D. Parks and S. Kevan (University of Oregon), E. Lima and K. Kisslinger (Brookhaven National Laboratory), I. McNulty (Argonne National Laboratory), R. Gambino (Stony Brook University), S. Mangin (Université de Lorraine and the French National Centre for Scientific Research), and P. Fischer (Berkeley Lab).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES). Operation of the ALS is supported by DOE BES.

Publication about this research: J.J. Turner, X. Huang, O. Krupin, K.A. Seu, D. Parks, S. Kevan, E. Lima, K. Kisslinger, I. McNulty, R. Gambino, S. Mangin, S. Roy, and P. Fischer, "X-ray diffraction microscopy of magnetic structures," Phys. Rev. Lett. 107, 033904 (2011).

ALS Science Highlight #244

 

ALSNews Vol. 329