Manufacturing Bits: Feb. 20

Hedgehog spin-vortex crystals; magic spectroscopy.


Hedgehog spin-vortex crystals
The U.S. Department of Energy’s Ames Laboratory has discovered a missing piece to enable novel superconductor devices–the hedgehog spin-vortex crystal phase.

Superconductors are devices that have zero electrical resistance, making them attractive for a range of applications. But superconductors must be cooled down to temperatures at or near absolute zero on the Kelvin scale to work.

Then, there is a class of unconventional iron-based superconductivity devices where magnetism plays a key role. According to Ames Laboratory, based in Ames, Iowa, there are three types of magnetic orders in these devices–stripe-type spin-density wave (SSDW), spin-charge-density wave (SCDW) and spin-vortex crystal (SVC).

SSDW and SCDW have been observed in the lab, but SVC has not. But in a possible breakthrough, Ames Lab has observed a magnetic phase consistent with the hedgehog variation in the SVC order in nickel- and cobalt-doped CaKFe4As4.

The iron arsenide CaKFe4As4 has strong superconducting characteristics. The nickel- and cobalt-doped materials alter the magnetic order, but it retains its superconducting properties.

Image of hedgehog spin-vortex crystals in action (Source: Ames Lab)

The technology could enable higher temperature superconductive devices. “In the research of quantum materials, it’s long been theorized that there are three types of magnetism associated with superconductivity. One type is very commonly found, another type is very limited and only found in rare situations, and this third type was unknown, until our discovery,” said Paul Canfield, a senior scientist at Ames Laboratory and a Distinguished Professor and the Robert Allen Wright Professor of Physics and Astronomy at Iowa State University.

“The resources of the national laboratories were essential for providing for the diversity of techniques needed to reveal this new magnetic state,” said Canfield. “We’ve been able to stabilize it. It’s robust, and now we’re able study it. We think by understanding the three different types of magnetism that can give birth to iron-based superconductors, we’ll have a better sense of the necessary ingredients for this kind of superconductivity.”

Magic spectroscopy
In a separate effort, Ames Laboratory has received funding to study materials using a new technique in solid-state nuclear magnetic resonance (NMR) spectroscopy.

The technique is called ultrafast magic-angle spinning (UFMAS). NMR is sometimes confused with magnetic resonance imaging (MRI). MRI is used in medicine.

In contrast, NMR or nuclear magnetic resonance probes the nuclei of atoms. The nuclei of atoms are probed as they absorb and re-emit radio waves when they are moved into a magnetic field. Those nuclei resonate at measurable radio-frequencies.

For this work, Ames Laboratory uses an in-house DNP-NMR spectrometer. “DNP” stands for dynamic nuclear polarization. This method uses microwaves. DNP-NMR enables high sensitivity and faster measurements. It is possible to measure the distances in between atoms within a trillionth of a meter.

In its latest efforts, Ames will use DNP-NMR with a new technique called UFMAS. UFMAS relies on spinning a sample at high frequencies (>6 million RPM), according to researchers.

UFMAS improves NMR technology. “Many new materials have been developed in the past decade to address needs for energy conversion and storage,” said Aaron Rossini, a scientist at Ames Laboratory, and a professor of chemistry at Iowa State University. “However, there is still a lot we don’t know about how these materials function. We want to change that and bring new information to the table that will be used to optimize these materials.

“Our work could have far-reaching impact on a lot of fields, in electronics, lighting, solar cells, nanoparticle design, materials with a variety of energy applications,” said Rossini. “If we are able to explain how structure and function are related, we can help direct intelligent materials design.”

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