Man-made material shows surprisingly magnetic personality

Jun 10, 2013 by Lori Ann White
This image 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 - even though under normal circumstances titanium is not magnetic at all. Credit: Greg Stewart/SLAC

(Phys.org) —Scientists from SLAC and Stanford have used finely tuned X-rays at the Stanford Synchrotron Radiation Lightsource (SSRL) to pin down the source of a mysterious magnetism that appears when two materials are sandwiched together.

Why is this mysterious?

Neither material shows a hint of magnetism on its own.

Both materials are perovskites, a class of mineral oxides whose unique are of great interest to scientists. Perovskites already have a variety of industrial uses, and researchers are busy trying to find ways to transform some of them – such as the – into materials that could transform our or help create environmentally friendly fuels.

These particular perovskites are known as LAO ( ) and STO (strontium ), and both are . But when sandwiched together, the resulting "" can conduct electricity at the interface where the materials meet. In fact, when cooled to near-absolute zero this heterostructure becomes a superconductor, conducting electricity without any resistance. Even more puzzling, it displays qualities at the juncture where LAO and STO meet – something neither material does alone, even when doped with impurities to tune its properties.

Which particular atoms acquire this new property? That's what the researchers wanted to learn, and the results of their study appeared this week in Nature Materials.

The researchers studied sample heterostructures, each one an extremely thin layer of LAO on an STO substrate. The samples were grown by the group of Harold Hwang of the Stanford Institute for Materials and Energy Sciences (SIMES), a joint SLAC-Stanford institute. Hwang is an expert in this heterostructure; he's been studying it for a decade.

SSRL Staff Scientist Jun-Sik Lee said the group relied on one very important property of SSRL's X-rays: They can be tuned to just the right wavelength to probe the properties of a specific element. In other words, the researchers could use the X-rays to look only at the titanium in the STO slice of the heterostructure, or only at oxygen.

Their investigation pointed to one culprit. "We've proved the magnetism comes from the titanium atom," said Lee, though precisely what is causing this change in a fundamental property is unclear. What's more, the magnetism arises in what's called the "ground state" of the titanium atoms, when they are at their lowest energy. In fact, the researchers pursued magnetism in the titanium atoms all the way down to 10 kelvins, 10 degrees above .

That's intriguing, said Lee, because the structure of this magnetic ground state in the titanium of STO is the same structure required for it to be able to transition to a superconductor. This hints at the possibility the LAO/STO interface may exhibit another unconventional behavior: magnetism and superconductivity coexisting, a possibility that, said Lee, "is quite unusual in our conventional understanding of physics." One of the identifying characteristics of a superconductor is the way it repels magnetic fields.

Unfortunately, the researchers weren't able to study the magnetic titanium at temperatures low enough to cause the heterostructure to transition to superconductivity. However, their research does support the research of SIMES member Kathryn Moler, published in Nature Physics in 2011, in which her research team detected both superconductivity and magnetism existing at the LAO/STO interface.

Hwang noted that even after almost a decade of investigation by many groups worldwide, this seemingly simple interface continues to generate new surprises. "One of the dreams of our field is the notion of 'materials by design,'" he said. "We hope that these studies of unexpected emergent phenomena, such as the magnetism here, can lead to an understanding by which we can predictably engineer the properties of artificial heterostructures."

Chi-Chang Kao, director of SLAC and a co-author of this work, emphasized the close collaboration between the SLAC and Stanford members of the group. "The collaboration started from a series of meetings Harold organized to introduce his group's research, and is a prime example of the kind of work suited to being tackled together, applying SLAC's unique resources – such as SSRL – to the scientific problems of the SLAC and Stanford community," he said. "There are many other examples, and I hope to foster even more of them going forward."

As for next steps, Lee just laughed. "There are a lot of next steps," he said. "We've passed on a very big question to everyone. We've said, 'OK, we got this result – now we all have a lot of homework to do.'"

Explore further: Theorists find a new way to improve efficiency of solar cells by overcoming exciton 'traps'

More information: www.nature.com/nmat/journal/va… t/full/nmat3674.html

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MrGrynch
1 / 5 (8) Jun 10, 2013
Perhaps the reason why this sudden emergence of magnetism is unclear is because the nature of magnetism itself is unclear, as is the nature of the vacuum medium which all matter resides in. We should be focusing more on vacuum engineering, if we want to tailor-make matter, but to do so would require facing some unfortunate truths about our current understanding, and also force disparate notions of the nature of the vacuum, to reconcile..
Jeffhans1
1.4 / 5 (10) Jun 10, 2013
Nice find, now make a layercake of YBCO and Graphene. At every boundary between the two, it becomes superconducting at well above room temp. Current best guess is that the Graphene lets the electrons skip past a tiny amount of molecules by following the path of least resistance over and over just like you can do in checkers if your opponent is deployed all wrong.. The Graphene also just happens to reinforce the normally fragile magnet which becomes very sturdy and ready to form into disks that can be spun at a few thousand RPM without fear of shattering.. Feel free to verify but I have been calling the result a Faraday Field.
antialias_physorg
3.7 / 5 (6) Jun 10, 2013
Nice find, now make a layercake of YBCO and Graphene

Unfortunately that's not as easy as all that because the two have different spatial parameters (i.e. the atomic lattices don't fit very well with one another and so you don't get a regular interface but lots of folding, domain boundaries and the like - which is usually a problem for superconductivity).

At every boundary between the two, it becomes superconducting at well above room temp.

I highly doubt that. Why would you think that?
Jeffhans1
1 / 5 (8) Jun 10, 2013
The folding bumpy interface creating boundaries is the point. I think that because of results....
antialias_physorg
2.6 / 5 (5) Jun 10, 2013
Feel free to verify

Please show some calculations as to why you think that would push Tc above room temperature so we have something to verify.
ValeriaT
1 / 5 (7) Jun 10, 2013
As I explained here, the superconductivity arises, when the electrons are compressed against each other. In HT superconductors, the electrons are squeezed with their attraction to positively charged copper atoms in their trivalent state. But the strontium/lanthanum oxides don't contain any oxidized states, which could attract electrons from outside of material. But the strontium titanate is strongly piezoelectric and its covered with free electrons, when we deform it. So my hypothesis is, due the different lattice constants the strontium titanate layers is deformed and they expose the electrons from deformed areas to conductive band. This strain could explain the magnetic properties of this material too, because the ferromagnetism arises, when the unpaired electrons are forced to revolve outside of symmetry plane of atoms.
ValeriaT
1 / 5 (6) Jun 10, 2013
show some calculations as to why you think that would push Tc above room temperature so we have something to verify
This is a typical nonsense of formally thinking physicist, which just needs to justify his existence somehow. Fortunately the physics is experimental science and it doesn't CONFIRM the theories with calculations, but it FALSIFIES them with experiments. We already know, that the graphene system exhibits the traces of room temperature superconductivity, so no calculations are needed anyway. With the waiting for mathematical model we could delay the cold fusion research for years, despite we already have the robust experimental evidence. After all, the math model doesn't prove anything, as the proponents of epicycles already know: for example, if we should believe the Euler calculations of hollow Earth theory, we should already start with drilling...
ValeriaT
1 / 5 (6) Jun 10, 2013
BTW The original study is here. Note that from many orbitals types only two are ferromagnetic (they allow the electrons to revolve the atoms in eccentric way) - and just one of them has been found at the STO/LTO interface.
antialias_physorg
3.9 / 5 (7) Jun 10, 2013
As I explained here,...

That's just your ususal handwaving.
The stuff you post can be (and is constantly) reinterpreted by you to fit wahtever pleases you. But that's not how scoence works.

Let's have some serious math, here - so it can be checked against experiment. Let's have some hard numbers. Without those your 'theories' are as worthless as "god did it".

What Tc do you predict for a YBCO-graphene interface? Based on what formula? Be specific. Give values.