Interaction between crystallographic and magnetic microstructure. A) Nanobeam diffraction maps of Fcir, the flipping ratio using circular light. B) Crystallographic tilt toward the vertical direction. C) Integrated diffracted x-ray intensity at the 008 Bragg reflection. Credit: the Authors
Spintronic devices exploit the combined electric charge and spin properties of the electron in wafer thin, nanoscale layers of magnetic materials. New advances in this field have led to the discovery of spin caloritronic materials, which couple spin, charge, and heat currents to directly convert thermal energy to electrical power. Research from the journal Science Advances describes a novel method to reveal magnetic micro and nano-structure buried deep in the layers of nanocrystalline gadolinium iron garnet (GdIG), and how it is influenced by crystallographic distortions. Characterization of this phenomenon is a step towards controlling magnetism in devices by tuning magnetic domain size in nanoscale layers.
At its core, the study explores the magnetic structure of GdIG (Gd3Fe5O12) in relation to thermoelectric power generation in prototype device structures. “This research can help us determine how to tailor and control its magnetic structure in order to increase the efficiency of heat harvesting,” said Dina Carbone, Scientist at MAX IV Laboratory. “We now know that these devices perform better in presence of large magnetic domains, which develop and stabilise at low temperatures. Revealing the small spin structure that dominates at room temperature and understand how it can be stabilised by structural distortions or strain is a first step toward the development of future devices.”
In caloritronic devices a spin current, generated inside a magnetic insulator from a temperature gradient, is injected into a metal layer where is transformed in electric current. This is known as the Spin Seebeck Effect (SSE). Stabilisation of large magnetic domains in the absence of an external magnetic field is a key element for the optimisation of SSE technologies. This requires an understanding of the magnetic microstructure and its effect on voltage generation.
Current: LINXS webinar series CoWork with Dina Carbone. The series is dedicated to the exploitation of the coherence properties of X-rays for advanced materials characterization, with a special focus on inverse microscopy techniques, such as Coherent Diffraction Imaging (CDI), Ptychography and Holography.
The project initiated through a collaboration between Carbone, who works with coherent diffraction imaging, and Danny Mannix, an X-ray magnetic scattering expert and HEIMDAL lead instrument scientist at the European Spallation Source ERIC (ESS) in Lund. Groundwork for the study was carried out at NanoMAX imaging beamline at MAX IV and included expert commissioning with colleagues from the Paul Scherrer Institute (PSI) in Switzerland.
The first attempts to observe magnetic domains with coherent light enabled the researchers to learn about beam intensity and signal strength, and plan future coherent imaging experiment at 4th generation synchrotron sources such as MAX IV, the first facility in operation. Measurements of test magnetic structures with forward ptychography also led to the installation of a diamond phase plate—a tool used to convert polarized light from horizontal to circular orientation—which is now available to other users at NanoMAX.
Interest in their research questions grew from there, and so, the team of researchers involved. “The whole project builds on cross-disciplinary expertise of an international team, from sample fabrication to data analysis, modelling, and optimisation of experimental setup, all essential to our success,” explained Carbone.
At the European Synchrotron Radiation Facility (ESRF) in France, the research group, led by principal investigator Danny Mannix, used scanning nanodiffraction at the hard X-ray nano beamline ID01 to investigate magnetic domains of the classic SSE material, garnet (Gd3Fe5O12) thin film. The sample was grown by Stephen Geprägs at Walther-Meißner Institute in Germany. Measurements at 500-nanometer resolution revealed an image map of the magnetic domains in zero-magnetic field, and magnetic domain walls that align with crystallographic orientations and interact with structural strain from the substrate. A cryostat provided the low temperature environment necessary to measure larger magnetic domains with a more intense diffraction signal.
“This is a very exciting development because it provides a new way for us to study nanoscale phenomena in magnetic materials. It allows us in particular to think of ways of understanding how to use the distortion of the crystal lattice to control the magnetic configuration within tiny volumes,” said Paul G. Evans, Materials Science and Engineering Professor at the University of Wisconsin-Madison in the United States and collaborating scientist in the study.
3-dimensional with NanoMAX
Fast forward after successful characterization of the magnetic domains in garnet, the researchers have returned to NanoMAX to study the samples with 3D visualizations. The 100-nanometer small beam has thus far produced measurements at room temperature. Through comparison of previous measurements of large domain size at low temperature, and the typically small magnetic domains present at high temperatures in caloritronic devices, the researchers aim to elucidate what parameters create stability in domains.
“During our recent beamtime at NanoMAX, we obtained coherent X-ray diffraction data of excellent quality, adequate for a 3D description of the structure of the same sample. This represents a clear progress from the initial studies,” said Mannix. “The next phase of the project will be focused on experiments towards imaging the magnetic structure using high resolution Bragg ptychography, which will bring us closer to our original goal of implementing new science methodologies.”
In summary, the revelations that magnetic domains link with crystalline structure brings the research one step closer to enabling control of magnetism in SSE devices.
Resonant nanodiffraction x-ray imaging reveals role of magnetic domains in complex oxide spin caloritronics. Paul G. Evans, Samuel D. Marks, Stephan Geprägs, Maxim Dietlein, Yves Joly, Minyi Dai, Jiamian Hu, Laurence Bouchenoire, Paul B. J. Thompson, Tobias U. Schülli, Marie-Ingrid Richard, Rudolf Gross, Dina Carbone, Danny Mannix. Science Advances 02 Oct 2020: Vol. 6, no. 40, eaba9351. DOI: 10.1126/sciadv.aba9351
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