New electron microscope method detects atomic-scale magnetism

Scientists can now detect magnetic behavior at the atomic level with a new electron microscopy technique developed by a team from the Department of Energy’s Oak Ridge National Laboratory and Uppsala University, Sweden. The researchers took a counterintuitive approach by taking advantage of optical distortions that they typically try to eliminate.

“It’s a new approach to measure magnetism at the atomic scale,” ORNL’s Juan Carlos Idrobo said.

“We will be able to study materials in a new way. Hard drives, for instance, are made by magnetic domains, and those magnetic domains are about 10 nanometers apart.” One nanometer is a billionth of a meter, and the researchers plan to refine their technique to collect magnetic signals from individual atoms that are ten times smaller than a nanometer. 

“If we can understand the interaction of those domains with atomic resolution, perhaps in the future we will able to decrease the size of magnetic hard drives,” Idrobo said. “We won’t know without looking at it.”

Researchers have traditionally used scanning transmission electron microscopes to determine where atoms are located within materials. This new technique allows scientists to collect more information about how the atoms behave.

“Magnetism has its origins at the atomic scale, but the techniques that we use to measure it usually have spatial resolutions that are way larger than one atom,” Idrobo said. “With an electron microscope, you can make the electron probe as small as possible and if you know how to control the probe, you can pick up a magnetic signature.”

The ORNL-Uppsala team developed the technique by rethinking a cornerstone of electron microscopy known as aberration correction. Researchers have spent decades working to eliminate different kinds of aberrations, which are distortions that arise in the electron-optical lens and blur the resulting images.

Instead of fully eliminating the aberrations in the electron microscope, the researchers purposely added a type of aberration, called four-fold astigmatism, to collect atomic level magnetic signals from a lanthanum manganese arsenic oxide material. The experimental study validates the team’s theoretical predictions presented in a 2014 Physical Review Lettersstudy.

“This is the first time someone has used aberrations to detect magnetic order in materials in electron microscopy,” Idrobo said. “Aberration correction allows you to make the electron probe small enough to do the measurement, but at the same time we needed to put in a specific aberration, which is opposite of what people usually do.”

Idrobo adds that new electron microscopy techniques can complement existing methods, such as x-ray spectroscopy and neutron scattering, that are the gold standard in studying magnetism but are limited in their spatial resolution.

The study is published as “Detecting magnetic ordering with atomic size electron probes,” in the journal of Advanced Structural and Chemical Imaging. Coauthors are ORNL’s Juan Carlos Idrobo, Michael McGuire, Christopher Symons, Ranga Raju Vatsavai, Claudia Cantoni and Andrew Lupini; and Uppsala University’s Ján Rusz and Jakob Spiegelberg.

The electron microscopy experiments were conducted at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL. The research was supported by DOE’s Office of Science.

ORNL is managed by UT-Battelle for the Department of Energy's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. DOE’s Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Nanotechnology World Association

Nanotechnologists at UT make orientation of magnetism adjustable in new materials

New material offers potential for data storage and spintronics applications

 

Nanotechnologists at the UT research institute MESA+ are now able to create materials in which they can influence and precisely control the orientation of the magnetism at will. An interlayer just 0.4 nanometres thick is the key to this success. 

The materials present a range of interesting possibilities, such as a new way of creating computer memory as well as spintronics applications – a new form of electronics that works on the basis of magnetism instead of electricity. The research was published today in the leading scientific journal Nature Materials. 

Nanotechnologists at the University of Twente are specialized in creating new materials. Thanks to the top-level facilities at the MESA+ NanoLab they are able to combine materials as they wish, with the ability to control the material composition down to atom level. In particular, they specialize in creating materials composed of extremely thin layers, sometimes just one atom thick. 

Computer memory

 

In research published today in the scientific journal Nature Materials, they show their ability to create new materials within which they can precisely and locally control the orientation of the magnetism. This opens the way to new possibilities of creating computer memory. Moreover, this method of creating materials is interesting for spintronics, a new form of electronics that does not utilize the movement of charges but instead the magnetic properties of a material. This not only makes electronics very fast and efficient, but also allows them to be produced in extremely small dimensions.  

Interlayer

 

In the course of this research the scientists stacked up various thin layers of Perovskite materials. By placing an extremely thin interlayer of just 0.4 nanometres between the layers (a nanometre is a million times smaller than a millimetre), it becomes possible to influence the orientation of the magnetism in the individual Perovskite layers as desired, whereby the orientation of the magnetism in the bottom layer, for instance, is perpendicular to that of the layer above. By varying the location where the interlayer is applied, it becomes possible to select the local orientation of the magnetism anywhere in the material. This is an essential property for new forms of computer memory and for spintronics applications. This effect was already known for much thicker layers, but never before had researchers demonstrated that the orientation of the magnetism can be controlled so precisely with extremely thin layers, too. 

Research

 

The research has been conducted by scientists of the MESA+ research groupInorganic Materials Science in collaboration with colleagues from other institutes, including the University of Antwerp (Belgium), the University of British Columbia (Canada) and TU Wien (Vienna, Austria). Within the research project, the Twente-based researchers were responsible for coordination and for creating the materials. The colleague researchers from Antwerp visualized the materials and were able to image even the smallest atoms in the material. The Canadian researchers created a magnetic cross-section of the material, while the Austrian researchers handled the theoretical calculations.

The research is published under the title ‘Controlled lateral anistropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling’ by Z. Liao, M. Huijben, Z. Zhong, N. Gauquelin, S. Macke, R. J. Green, S. Van Aert, J. Verbeeck, G. Van Tendeloo, K. Held, G. A. Sawatzky, G. Koster and G. Rijnders.

MESA+ Institute for Nanotechnology

Nanoplasmonics makes the impossible possible

Over a five-year period, Alexander Dmitriev and his research team at Chalmers will take on a task that until now has been deemed impossible: creating strong interaction between light and magnetic fields and determining ways to control light with magnetism on the nanoscale. The Harnessing light and spins through plasmons at the nanoscale project has received close to SEK 38 million from the Knut and Alice Wallenberg Foundation, and may eventually lead to more effective ways to process and store information with light and create different types of optical elements.

"The entire field is still fairly unknown, and we are one of only a few research teams in the world currently looking specifically into light as nanoplasmonic resonances combined with magnetic nanostructures," says Alexander Dmitriev, associate professor of physics at Chalmers.

For a long time it has been deemed impossible to combine light and magnetism because of a frequency gap where light moves 10,000 times faster than magnetism reacts, which means they do not feel each other and cannot integrate. By capturing the light in what are known as nanoantennas, which are built over a surface, it is possible for the two to interact on the nanoscale. There are nanoplasmons in this artificially created surface of nanoantennas – in other words small units of electrons that when exposed to visible light, move or oscillate collectively and thus create enhanced and localised electromagnetic fields that can then be connected with magnetic materials via different types of magneto-optical effects.  

"We want to attempt to force the light to become steerable using magnetism, and vice versa, and thus eliminate the frequency gap," says Alexander Dmitriev.

Steerable optical components

When the project ends in five years, the team hopes to have obtained a fundamental understanding of the field and be better equipped to build the specific nanostructures needed to achieve the desired properties. By bringing internationally leading research teams from Chalmers and the universities in Uppsala and Gothenburg together, it will be possible to utilise expertise within both theoretical and experimental physics in nanoplasmonics, nanomagnetism and spintronics. However, even if the project has a purely fundamental character, Alexander Dmitriev sees clear areas of application where it will hopefully be possible to use the methods in the future.

"This technology could enable steerable and adaptable optical components that are not easily controlled with electric current, for example three-dimensional holograms that move in real time. Thanks to the enhanced interaction we want to create between light and magnetism on the nanoscale, it will be possible to use low-intensity magnetic fields similar to those found in regular refrigerator magnets, and it will be quick, energy-efficient and easy to integrate with electronics.  

Chalmers University of Technology

At the edge of a quantum gas

JQI physicists observe skipping orbits in the quantum Hall regime 

JQI scientists have achieved a major milestone in simulating the dynamics of condensed-matter systems – such as the behavior of charged particles in semiconductors and other materials – through manipulation of carefully controlled quantum-mechanical models.

Going beyond their pioneering experiments in 2009 (the creation of “artificial magnetism”), the team has created a model system in which electrically neutral atoms are coaxed into performing just as electrons arrayed in a two-dimensional sheet do when they are exposed to a strong magnetic field.

The scientists then showed for the first time that it is possible to tune the model system such that the atoms (acting as electron surrogates) replicate the signature “edge state” behavior of real electrons in the quantum Hall effect (QHE), a phenomenon which forms the basis for the international standard of electrical resistance.* The researchers report their work in the 25 September issue of the journal Science.

“This whole line of research enables experiments in which charge-neutral particles behave as if they were charged particles in a magnetic field,” said JQI Fellow Ian Spielman, who heads the research team at NIST’s Physical Measurement Laboratory.

“To deepen our understanding of many-body physics or condensed-matter-like physics – where the electron has charge and many important phenomena depend on that charge – we explore experimental systems in which the components have no electrical charge, but act in ways that are functionally equivalent and can be described by the same equations,” Spielman said.

Such quantum simulators are increasingly important, as electronic components and related devices shrink to the point where their operation grows increasingly dependent on quantum-mechanical effects, and as researchers struggle to understand the fundamental physics of how charges travel through atomic lattices in superconductors or in materials of interest for eventual quantum information processing.

Quantum effects are extremely difficult to investigate at the fundamental level in conventional materials. Not only is it hard to control the numerous variables involved, but there are inevitably defects and irregularities in even carefully prepared experimental samples. Quantum simulators, however, offer precise control of system variables and yield insights into the performance of real materials as well as revealing new kinds of interacting quantum-mechanical systems.

“What we want to do is to realize systems that cannot be realized in a condensed-matter setting,” Spielman said. “There are potentially really interesting, many-body physical systems that can’t be deployed in other settings.”

To do so, the scientists created a Bose-Einstein condensate (BEC, in which ultracold atoms join in a single uniform, low-energy quantum state) of a few hundred thousand rubidium atoms and used two intersecting lasers to arrange the atoms into a lattice pattern. 

Then a second pair of lasers, each set to a slightly different wavelength, was trained on the lattice, creating "artificial magnetism" — that is, causing the electrically neutral atoms to mimic negatively charged electrons in a real applied magnetic field.

Depending on the tuning of the laser beams, the atoms were placed in one of three different quantum states representing electrons that were either in the middle of, or at opposite edges of, a two-dimensional lattice.

By adjusting the properties of the laser beams, the team produced dynamics characteristic of real materials exhibiting the QHE. Specifically, as would be expected of electrons, atoms in the bulk interior of the lattice behaved like insulators. But those at the lattice edges exhibited a distinctive "skipping"motion.

In a real QHE system, each individual electron responds to an applied magnetic field by revolving in a circular (cyclotron) orbit. In atoms near the center of the material, electrons complete their orbits uninterrupted. That blocks conduction in the system’s interior, making it a “bulk insulator.” But at the edges of a QHE system, the electrons can only complete part of an orbit before hitting the edge (which acts like a hard wall) and reflecting off. This causes electrons to skip along the edges, carrying current.

Remarkably, the simulation's electrically neutral rubidium atoms behaved in exactly the same way: localized edge states formed in the atomic lattice and atoms skipped along the edge. Moreover, the researchers showed that by tuning the laser beams – that is, modifying the artificial magnetic field – they could precisely control whether the largest concentration of atoms was on one edge, the opposite edge, or in the center of the lattice.

“Generating these sorts of dynamical effects was beyond our abilities back in 2009, when we published our first paper on artificial magnetism,” Spielman said. “The field strength turned out to be too weak. In the new work, we were able to approach the high-field limit, which greatly expands the range of effects that are possible to engineer new kinds of interactions relevant to condensed-matter physics.”

* The Hall effect occurs when a current traveling in a two-dimensional plane moves through a magnetic field applied perpendicular to the plane. As electrons interact with the field, each begins to revolve in a circular (cyclotron) orbit. That collective motion causes them to migrate and cluster on one edge of the plane, creating a concentration of negative charge. As a result, a voltage forms across the conductor, with an associated resistance from edge to edge.

Much closer, detailed examination reveals the quantum Hall effect (QHE): The resistance is exactly quantized across the plane; that is, it occurs only at specific discrete allowed values which are known to extreme precision. That precision makes QHE the international standard for resistance.

An additional distinctive property of QHE systems is that they are “bulk insulators” that allow current to travel only along their edges.

Joint quantum Institute

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