Harnessing insulation: Magnetic clue

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  • Published: Apr 15, 2015
  • Author: David Bradley
  • Channels: NMR Knowledge Base
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On the surface

High-resolution image of the vanadium-doped sample grown (gold and brown stripes) on an etched silicon substrate (bottom-left, brown region). Another capping layer (top-right, yellow region) is mainly composed of amorphous tellurium protection layers. The upper right inset is a reflection of the high-energy electron diffraction image showing the high crystalline quality of the custom-grown film.

A subatomic magnetic phenomenon predicted more than sixty years ago has been measured for the first time. The demonstration of van Vleck magnetism could open the door to harnessing topological insulators - hybrid materials that are both conducting and insulating - and may well lead to new approaches to quantum computing, spintronics applications, and much-improved semiconductors.

Researchers Jing Tao, Lijun Wu and Yimei Zhu of Brookhaven National Laboratory together with Mingda Li, Cui-Zu Chang, Jagadeesh Moodera and Ju Li of Massachusetts Institute of Technology and Weiwei Zhao and Moses H. W. Chan of Pennsylvania State University, have used transmission electron microscopy measure an elusive form of quantum magnetism. "Synthesis and characterization techniques have finally caught up to seminal theoretical work, and we are thrilled to have performed this groundbreaking research," explains Li. "Our experiment is the first to show conclusive evidence of van Vleck magnetism, which mediates the magnetic properties of topological insulators."

Classical materials tend to be conductors or insulators, copper wires as opposed to rubber bungs for instance, other materials lie in the realm of semiconductors, conducting under certain conditions but insulating under others. Then there are the topological insulators, materials that are, in the bulk, insulating, but the surface of which is highly conductive. The relationship between these competing qualities introduces strange phenomena, especially in the electrons at the surface.

Layers

"The surface electrons - Dirac electrons - exhibit the light-like mobility and extreme stability that enables so many exciting potential applications," Li explains. "But these electrons cannot be controlled directly. That’s where van Vleck magnetism comes in, to induce and harness Dirac electrons." The electronic behaviour of magnetic semiconductors is like an endless game of volleyball between equally matched opponents where the players are magnetic ions and the ball is a free electron. Interrupting the game or shifting the behaviour of the free electron is relatively easy and can be used to control properties and make semiconductors so useful in a wide range of electromagnetic applications.

For topological insulators, it could not be any different: forget volleyball, the game never gets off the ground. The magnetic action is held within a single crystal structure, there is no back and forth, and no ball, no free electron, with which to play. This intra-atomic magnetism is more akin to a lone player throwing their ball in the air and catching it repeatedly, throw the player a ball, a free electron, and it would ruin their solo game. "The all-important outer electrons can only be influenced through the topological insulator'" core electrons," Li explains. "The outer electrons can feel the effect of energy or magnetic fields on the core. That conversation between core and shell is mediated by van Vleck magnetism." John Hasbrouck van Vleck, 1997 Nobel physics laureate, is considered to be the father of modern magnetism, couching the nineteenth century science of magnetism in quantum theory. His groundbreaking work included predicting the internal magnetism discussed by Li, which was almost impossible to detect, until now.

Topological tales

Topological insulators are usually fabricated one atomic layer at a time through molecular beam epitaxy (MBE). The current work used an antimony-tellurium topological insulator doped with vanadium ions. "Vanadium-doping increased the signal of electrons changing energy levels within the material," explains team member Yimei Zhu of BNL. "Layering these materials in three-dimensional crystals is one of the most exciting frontiers in materials science - tiny changes to the composition or arrangement can radically impact performance."

The team blasted their topological insulator with the TEM electron beam focused to within one atom, which had the effect of exciting a core electron, which then raises the energy in the outer Dirac shell. They then used electron energy loss spectroscopy (EELS) to measure the difference in energy between the incident electron beam and the electrons that scatter from the sample. The energy loss betrayed the van Vleck effect in action.

"We needed extraordinary spatial and energy resolution," Li adds. "But the real challenge was achieving that precision at extremely low temperatures - that's what distinguishes Brookhaven's instrument and expertise." The elusive magnetic effect emerges at below 70 Kelvin, but in this experiment, atomic scale precision was possible only if the system was cooled to 10 Kelvin. "Elemental doping, like we did with vanadium here, is [only] one way to induce van Vleck magnetism," Li adds. "But we haven’t investigated the proximity effect, where adjacent elements influence the core electrons. We also expect to find new phases of matter at the interface between topological insulators and other materials. It’s a very exciting time to be exploring these materials."

Related Links

Phys Rev Lett 2015, 114, 146802: "Experimental Verification of the van Vleck Nature of Long-Range Ferromagnetic Order in the Vanadium-Doped Three-Dimensional Topological Insulator Sb2Te3"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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