New Approach to Decoding Data Could Lead to Faster, Smaller Digital Technology | Nebraska today

Most scientists would whitewash being called a “spin doctor”. But when it comes to Evgeny Tsymbal, Ding-Fu Shao and their colleagues, the lab coat fits.

Physicists at the University of Nebraska-Lincoln have pioneered spintronics, a next-generation data storage and processing class poised to complement the digital electronics that has dominated the field of high technology for decades.

In the face of this future, however, nanoscale obstacles loom, the size of which contradicts their difficulty. With the wind of a $ 20 million National Science Foundation grant behind their backs, physicists may be on the verge of overcoming a particularly tricky problem: finding order amid clutter and data in the sky. in the midst of apparent disarray. Beyond that hurdle, there are two prices, density and speed, that could make modern devices look greedy and lazy in hindsight.

number spinners

Electronics read and speak binary language – 1 and 0 – by measuring the charge of electrons flowing in their circuits. Spintronics differs by measuring the spin of an electron: a property related to magnetism that essentially points up or down. Devices that are fluent in both dialects of the binary can store and process much more data, at much faster speeds, with much less power than their electronic-only counterparts.

To date, most electronic and spintronic memories have relied on ferro-magnets, the type with a permanent magnetic field probably best known for pinning photos on refrigerators. In ferromagnets, the spin of each atom points in the same direction, a direction that can be switched by applying an external magnetic field.

These characteristics make them popular in so-called tunnel junctions, in which two ferromagnets are sandwiched around an insulating barrier, with electrons passing through this barrier to travel between the ferromagnets. If the spin of an electron matches the spin orientation of a ferromagnetic, the electron encounters little resistance, which increases its probability of tunneling. When those rotations don’t match, the odds collapse, dramatically reducing the overall flow of electric current. The difference between these two states, known as the magnetoresistance effect, can be read as 1 versus 0.

Because just as ferro-magnets work, their cousins ​​- the antiferromagnets – are even more promising. Antiferromagnetics are home to alternating columns of atoms whose spins point in opposite directions, meaning they generate virtually no sharp magnetic fields. No magnetic field means no chance for a tunnel junction to interfere with a neighbor’s magnetic state, allowing engineers to pack more data storage elements into a device without worrying about them corrupting them. each other’s data.

And if next-gen devices feel the need for speed, antiferromagnetics are the choice again, Tsymbal said. The spins of a ferromagnetic can be switched in nanoseconds. It seems fast until you realize that semiconductors can operate over a range of picoseconds – a picosecond is a second as a second is 31,710 years – or about 1,000 times faster than a second. ‘a ferromagnetic cannot switch. Antiferromagnetics, on the other hand, can keep pace, preparing them for a place in much faster devices.

Just a quick glitch: Encoding or decoding data into antiferromagnets can be a lot like trying to write with a dried out pen or deciphering a toddler’s scribbles.

“The difficulty – and this is a significant difficulty – is knowing how to write and read information,” said Tsymbal, professor of physics and astronomy at George Holmes University.

The same antiferromagnetic property that acts like a pro in a setting – the lack of a sharp magnetic field preventing data corruption – becomes a downside when it comes to actually saving data, Tsymbal said. Writing a 1 or 0 in a ferromagnetic is simply to reverse its spin orientation, or magnetization, through another magnetic field. This is not possible in an antiferromagnetic.

And while reading the spin state of a ferromagnetic is just as simple, it is not easy to distinguish between the spin states of an antiferromagnetic – high-low and low-high – because no does produce a sharp magnetization that would produce noticeable differences in the flow of electrons. Together, these facts have hampered efforts to develop antiferromagnetic tunnel junctions with practical use in real devices.

“So that’s one of the problems,” Tsymbal said. “But I think we’ve come up with a very, very good way to solve this problem.”

Say up and down

In principle, an antiferromagnetic tunnel junction should function much like a ferromagnetic junction. Rather than changing the overall magnetization of a ferromagnetic to regulate the flow of electrons, an antiferromagnetic version relies on modifying the so-called Néel vector: the axis along which the spins point in one direction or the other. .

But only specific types of antiferromagnets are suitable for detecting spin-related differences in electron flow, which are caused by a shift between the Néel vectors at each end of the tunnel junction. The secret of these antiferromagnetics? Momentum specific channels through which spin-up or spin-down electrons will primarily flow.

Tsymbal, Shao, and their colleagues identified ruthenium oxide as such an antiferromagnetic. They identified another material, titanium dioxide, as a barrier through which electrons can tunnel. Critically, the atoms of the two respective oxides form the same crystal structure, resulting in a transparent match that allows electrons to maintain their momentum – and their momentum-dependent spin – as they move between materials.

By taking these moments into account in the analyzes of the resulting electric current, the Husker team showed that it is possible to distinguish the channels and, therefore, their responses to different Néel vectors. According to the team’s calculations, this channel-specific magnetoresistance effect is of a similar magnitude to those produced by ferromagnetic tunnel junctions, making it a particularly promising way to write spintronic data that can also be read.

As the theorist has done in the past, Tsymbal is collaborating with Chang-Beom Eom at the University of Wisconsin-Madison and other experimenters who can fabricate and test the antiferromagnetic tunnel junction. He and other members of the Nebraska Center for Materials and Nanoscience are also busy considering other materials that share the unusual but not unique characteristics of ruthenium oxide.

“It’s not so many antiferromagnetics that have this property, but there are some,” Tsymbal said. “And we will be looking at these materials in the future as well.”

The team reported on their findings in the journal Nature Communication. Tsymbal, Shao and Eom are the authors of the study with Ming Li, doctoral student from Tsymbal research group, and Shu-Hui Zhang from Beijing University of Chemical Technology. The researchers received support from the we National Science Foundation, the we Bureau of Naval Research and National Science Foundation of China.


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