Science & Technology

Researchers Pinpoint the Source of a Mysterious Magnetism

This picture is an artist’s conception of a titanium atom in a man-made oxide heterostructure revealing magnetic properties to the probing X-ray beam of the Stanford Synchrotron Radiation Lightsource – although below regular circumstances titanium shouldn’t be magnetic in any respect. Greg Stewart/SLAC

In a newly revealed research, researchers from SLAC Nationwide Accelerator Laboratory proved that the magnetism in the LaAlO3/SrTiO3 heterostructure comes from the titanium atom.

Scientists from SLAC and Stanford have used finely tuned X-rays at the Stanford Synchrotron Radiation Lightsource (SSRL) to pin down the supply of a mysterious magnetism that seems when two supplies are sandwiched collectively.

Why is that this mysterious?

Neither materials exhibits a trace of magnetism by itself.

Each supplies are perovskites, a class of mineral oxides whose distinctive digital properties are of nice curiosity to scientists. Perovskites have already got a selection of industrial makes use of, and researchers are busy looking for methods to remodel some of them – comparable to the high-temperature superconductors – into supplies that would remodel our power grid or assist create environmentally pleasant fuels.

These explicit perovskites are referred to as LAO (lanthanum aluminum oxide) and STO (strontium titanium oxide), and each are insulators. However when sandwiched collectively, the ensuing “heterostructure” can conduct electrical energy at the interface the place the supplies meet. In truth, when cooled to near-absolute zero this heterostructure turns into a superconductor, conducting electrical energy with none resistance. Much more puzzling, it shows magnetic qualities at the juncture the place LAO and STO meet – one thing neither materials does alone, even when doped with impurities to tune its properties.

Which explicit atoms purchase this new property? That’s what the researchers wished to be taught, and the results of their study appeared this week in Nature Materials.

The researchers studied pattern heterostructures, every one a particularly skinny layer of LAO on an STO substrate. The samples have been grown by the group of Harold Hwang of the Stanford Institute for Supplies and Power Sciences (SIMES), a joint SLAC-Stanford institute. Hwang is an skilled on this heterostructure; he’s been learning it for a decade.

SSRL Employees Scientist Jun-Sik Lee mentioned the group relied on one essential property of SSRL’s X-rays: They are often tuned to only the proper wavelength to probe the properties of a particular component. In different phrases, the researchers might use the X-rays to look solely at the titanium in the STO slice of the heterostructure, or solely at oxygen.

Their investigation pointed to 1 perpetrator. “We’ve proved the magnetism comes from the titanium atom,” mentioned Lee, although exactly what’s inflicting this variation in a basic property is unclear. What’s extra, the magnetism arises in what’s known as the “floor state” of the titanium atoms, when they’re at their lowest power. In truth, the researchers pursued magnetism in the titanium atoms all the approach all the way down to 10 kelvins, 10 levels above absolute zero.

That’s intriguing, mentioned Lee, as a result of the construction of this magnetic floor state in the titanium of STO is the identical construction required for it to have the ability to transition to a superconductor. This hints at the risk the LAO/STO interface might exhibit one other unconventional conduct: magnetism and superconductivity coexisting, a risk that, mentioned Lee, “is kind of uncommon in our standard understanding of physics.” One of the figuring out traits of a superconductor is the approach it repels magnetic fields.

Sadly, the researchers weren’t in a position to research the magnetic titanium at temperatures low sufficient to trigger the heterostructure to transition to superconductivity. Nonetheless, their analysis does assist the analysis of SIMES member Kathryn Moler, published in Nature Physics in 2011, by which her analysis workforce detected each superconductivity and magnetism present at the LAO/STO interface.

Hwang famous that even after virtually a decade of investigation by many teams worldwide, this seemingly easy interface continues to generate new surprises. “One of the desires of our area is the notion of ‘supplies by design,’” he mentioned. “We hope that these research of sudden emergent phenomena, comparable to the magnetism right here, can result in an understanding by which we are able to predictably engineer the properties of synthetic heterostructures.”

Chi-Chang Kao, director of SLAC and a co-author of this work, emphasised the shut collaboration between the SLAC and Stanford members of the group. “The collaboration began from a collection of conferences Harold organized to introduce his group’s analysis, and is a prime instance of the form of work suited to being tackled collectively, making use of SLAC’s distinctive sources – comparable to SSRL – to the scientific issues of the SLAC and Stanford neighborhood,” he mentioned. “There are numerous different examples, and I hope to foster much more of them going ahead.”

As for subsequent steps, Lee simply laughed. “There are a lot of subsequent steps,” he mentioned. “We’ve handed on a very massive query to everybody. We’ve mentioned, ‘OK, we obtained this outcome – now all of us have a lot of homework to do.’”

Publication: J.-S. Lee,et al., “Titanium dxy ferromagnetism at the LaAlO3/SrTiO3 interface,” Nature Supplies, (2013); doi:10.1038/nmat3674

Picture: Greg Stewart/SLAC

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