The molecular beam epitaxy system used to make an axion insulator device whose resistance varies greatly with applied magnetic field. (Image: RIKEN Center for Emergent Matter Science)Quantum effects usually manifest themselves on an atomic or molecular scale, but they are observable on much larger scales in a special class of materials known as quantum materials. One quantum material that has been generating a lot of excitement recently is topological insulators, which have the curious property of conducting electricity on their surfaces but not in their interiors. Devices based on topological insulators will be more robust against internal defects and external conditions than conventional devices. Topological insulators also promise an array of exotic new carrier phenomena when symmetry rules are relaxed.
One such phenomenon is the topological magnetoelectric effect, in which the interior of a topological insulator is magnetized by a quantized current circulating on its surface. This effect has been difficult to observe experimentally because it has been challenging to find a suitable material system.
Masataka Mogi at the University of Tokyo and his collaborators had previously made a suitable system known as an axion insulator, but they observed axion insulator behavior only in a narrow range of magnetic fields.
Now, Mogi, with colleagues at the RIKEN Center for Emergent Matter Science and other institutions in Japan, has succeeded in making the axion insulator state more robust and useful by doping one magnetic ion into the bottom surface of their device and a different one into the top. This allowed them to individually reverse one surface magnetization at a time and observe how the axion insulator responds to a wider range of magnetic fields, electric fields and temperatures.
As a result, the researchers were able to observe a whopping change in the resistance of the system—over 10,000,000 per cent—with a small change in the applied magnetic field. While effects like this have been observed in other topological materials, they have usually required a much larger swing in the magnetic field.
This novel behavior may lead to new kinds of electronic devices, Mogi says. “Using contemporary spintronic techniques may enable us to construct dissipationless electronic circuits in which the edge current runs in parallel magnetization regions, but not in anti-parallel regions.”
Making practical devices will require increasing operating temperatures, which are currently capped below 1 kelvin. Mogi is confident that this can be accomplished by growing higher quality films.