Using a type of magnetic insulator material that normally doesn’t conductelectricity, scientists working at Stanford University and the Department ofEnergy’s SLAC National Accelerator Laboratory have shown that electric currentscan still be made to flow along the borders of the grains within the material. This latest researchnot only validates a long-held belief that magnetic insulators could be used toconduct electricity, but offers a more tantalizing possibility of creatinghighly-efficient magnetic memory devices.

"This physics is interesting and had been thought to exist, but it remainedelusive for the last 60 years," says Zhi-Xun Shen, the SLAC and Stanford professor who led the research. "Now it’s been seen directlyin this experiment."

The material being researched is a recently-developed amalgam of neodymium,iridium and oxygen (ND2Ir2O7) . Synthesizedin the laboratories of the University of Tokyo and the RIKEN researchinstitution in Japan, the substance is technically a magnetic material, whenviewed in the conventional sense, as it has large numbers of un-paired electronsstacked in an array that would normally give the material an overall magnetism.

Stanford postdoctoral researcher Yongtao Cui, left, and graduate student Eric Yue Ma with the microwave impedance microscopy (MIM) instrument Yongtao Cui/Stanford University

However, unlike recent experiments that created magnetic materials from non-magnetic ones, the molecular make up of this new material rendersit non-magnetic by having the neodymium's influence effectively cancelled out by the other ingredients in the mix. Whilst this may sound somewhatcounterintuitive, it simply means that scientists can take advantage of the material’sunusual molecular structure to observe various and unusual electron behaviorsnot possible in more conventional substances.

Also unable to conduct electricity in any conventional way, the latestresearch has shown that the material’s unusual mixture provides conductionpaths via the magnetic domains that exist along the boundaries of itsconstituent grains. At certain temperatures and with the application ofmicrowave energy, these regions form snaking, continuous paths for the movementof electrical current. Also able to be switched from conducting to insulatingby altering the temperature applied, this phenomenon holds hope forhighly-efficient magnetic memory applications.

"This can provide a more straightforward way to use magnetic materialas memory," says Eric Yue Ma, a graduate student in the laboratory of Zhi-XunShen. "Today you need toconvert magnetic information into electrical information when reading magneticmemory, usually via multiple layers of different materials. But if you haveboth types of information within the material itself, you can skip that step."

A microwave impedance microscopy (MIM) device sends microwaves down through the tip of a probe that is in direct contact with the material and collects the microwave signals that are reflected Eric Yue Ma/Stanford University

To observe the switchable conductive behaviour of the material, the Stanfordteam incorporated what is known as Microwave Impedance Microscopy, or MIM. Thismethod involves injecting microwave energy directly to the substance via a veryfine probe and measuring the strength of that energy on its return journey.Capable of testing areas as small as just 100 nanometers, this process alsoallows the researchers to measure the electrical resistance of the material.

Though the material being researched is only able to demonstrate this domainboundary conductivity at exceptionally low temperatures (around 4.7 K (-268.45 ºCor -451.21 ºF)), and so far only at the tested frequencies (approximatelyone Gigahertz) the SLAC team is quick to point out that other researchers have justrecently described comparable behavior in a similar material that may work at highertemperatures of around - 45 ºF (- 42 ºC).

The microwave impedance microscopy (MIM) instrument used to image tiny ribbons of electrical conductivity between magnetic regions Yongtao Cui/Stanford University

"It’s a beautiful experimental technique, to be able to make these measurements, and the technique could be applied to a huge range of materials. It’s remarkable how large the magnetic domains are. That’s great news; it means we can isolate a single domain and measure its properties," says Leon Balents, a professor of physics at the University of California, Santa Barbara (who wasn't actually involved in the study). "And now that they have a handle on being able to locate the domains, they can use those techniques to attack further questions. While this is a fundamental physics study, in the long term people are thinking toward many different types of applications."

The researchers also suggest that altering the organization of the magnetic domains in the material may offer unique ways to store information. That is, they believe, by changing the amount of domains present in a narrow area by heating or bending the material, for example, the insulating or conductive properties could be significantly altered at will. As such, these two opposite states could be taken as ones and zeroes and applied to the storage and retrieval of binary information.

According to the scientists, such memory devices could be more inherently stable than current FLASH memory, and may be able to be created at physical dimensions far smaller than those possible today.

The results of this research were recently published in the journal Science.

Source: SLAC