In the near future, next-generation spintronic devices will augment or replace current electronics in memory chips, transistors and optical devices such as semiconductor lasers. This emerging field uses the intrinsic spin property of electrons and their magnetic moment to create solid-state storage devices, tiny transistors and other advances that will make electronics smaller, faster and more energy efficient.
Understanding and controlling magnetism is key to developing these new spintronic devices. However, at the nanoscale level, researchers have difficulty seeing what, exactly, is going on. That’s why engineers at the University of Wisconsin-Madison were part of an international collaboration to investigate a new type of x-ray diffraction to understand the magnetic properties of multilayer spintronic devices.
“There’s a very ambitious idea to use crystal magnetic materials in new ways. To do that you have to control the magnetism very precisely,” says Paul Evans, a professor of materials science and engineering at UW-Madison.
Evans and Jiamian Hu, an assistant professor of materials science and engineering, and graduate students Minyi Dai and Samuel Marks, are co-authors of a new paper on the research in the Oct. 2, 2020, issue of Science Advances.
“The devices people are making have nanoscale dimensions, which are mismatched to the tools we have for solving magnetic problems,” says Evans. “As you look at magnetism in smaller and smaller modern device dimensions, it’s really hard to know what’s going on and it’s hard to predict as you scale down these effects. Tiny differences start to be really important.”
In particular, determining the magnetic moment of very-small-volume devices has proven extremely difficult, but it’s key to taking spintronics in new directions, including caloritronics, in which magnetic structures exploit nanoscale magnetism to recycle waste heat into usable electricity using a process called the spin Seebeck effect.
A key ingredient in these experiments was the fabrication of extremely precise crystalline films with few structural defects, patterned into prototype device structures using lithography methods, by Stephan Geprägs at the Walther Meissner Institute in Munich, Germany.
To try to image the magnetic moment of the spintronic device, composed of layers of thin-film oxides and garnet, the team used the European Synchrotron Radiation Facility in Grenoble, France, an x-ray source based on a particle accelerator in which electrons orbit at velocities close to the speed of light. This creates intense x-ray beams millions of times brighter than those available to researchers in their individual laboratories.
Instead of imaging the spintronic device directly with the x-ray—similar to the way a broken arm is imaged—they used hard x-ray nanobeam diffraction techniques to probe all the layers of the spintronic device at once. The revelation of magnetic moments throughout the device clarified lingering uncertainties about the materials and will change how the devices are modeled and designed in the future.
The precise understanding of how the crystalline and magnetic structure are interrelated revealed in these x-ray experiments is crucial to optimizing these magnetic oxides for commercial devices exploiting the spin Seebeck effect.
“When Paul showed me the results, I was really impressed by the high resolution of the magnetic domain structures,” says Hu. “It’s not simple to image these structures at nanometer scale, you need really good tools to look at them.”
These newly developed measurement tools will be of great help to Hu, his students and the broader community of scientists constructing nanoscale models of magnetism. “We need both sides of this,” says Evans. “You can’t have good models without some relationship to reality because the state of the modeling still relies on good measurement to develop the parameters.”
Besides lifting the veil on the magnetic structure of the spintronic devices, Evans says the novel x-ray diffraction technique the team developed will likely become commonplace soon. New technology means high-powered x-ray sources all over the world are undergoing significant upgrades to make them about 100 times more powerful. “This type of experiment will go from a thing you do with a team of 20 with six months of planning and two years of interpretation, to something routine,” says Evans.
The international team assembled by Danny Mannix of the ESS in Sweden & CNRS in France, includes Stephan Geprägs, Maxim Dietlein and Rudolf Gross of the Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, in Germany; Yves Joly of the CNRS and Université Grenoble Alpes in France; Laurence Bouchenoire and Paul B. J. Thompson of the XMaS beamline of the European Synchrotron Radiation Facility and the University of Liverpool in the UK; Tobias U. Schülli and Marie-Ingrid Richard of the European Synchrotron Radiation Facility in France; and Dina Carbone of the MAX IV Laboratory in Sweden.
The team at the University of Wisconsin-Madison acknowledges support from the U. S. Department of Energy Office of Basic Energy Sciences through contract DE-FG02-04ER46147 and travel support from the U. S. National Science Foundation through the University of Wisconsin Materials Research Science and Engineering Center through grant number DMR-1720415.
Author: Jason Daley