ORNL neutrons add enhanced polarization capabilities for measuring magnetic materials

Spherical neutron polarimetry is characterized by advanced superconducting technology in combination with an improved cooling system, adding new research opportunities in the United States, allowing the study of exotic magnetic phenomena in new dimensions. Courtesy: ORNL / Peter Jiang, Genevieve Martin.

Understanding magnetism at its most fundamental level is vital for developing more powerful electronics, but materials with more complex magnetic structures require more sophisticated tools to study them — powerful tools called simply “neutrons”.

The two most powerful sources in the world neutron scattering The Oak Ridge National Laboratory (ORNL) of the US Department of Energy (DOE) is undergoing modernization. Adding an advanced feature called spherical neutron polarimetry will allow researchers using the ORNL (HFIR) and Spallation Neutron Source (SNS) isotope reactors to measure materials with exotic magnetic structures and quantum states that were previously unavailable in the United States.

“Neutrons are ideal for studying magnetic phenomena,” said ORNL researcher Nicholas Silva. “They are electrically neutral or chargeless and have magnetic moments that make them look like tiny magnets.”

When neutrons pass through a material and scatter the magnetic fields created by the atoms of the material, they draw an atomic portrait or even a three-dimensional model of the atomic arrangement of the material and show how the atoms behave in the system.

Neutrons have a “rotation” or orientation, similar to the north and south poles of fridge magnets. In a typical neutron beam, the neutrons within the beam have spins arranged randomly. The measurement of some highly dynamic or complex magnetic systems, however, requires more uniformity, which is ensured by a polarized neutron beam, in which each neutron spin is aligned in parallel and with the same orientation.

“Neutron polarization filters allow us to see what we don’t want to see, which can spoil the signal of interest to us,” said a scientist from Barry Wynn. “Just like polarized lenses allow anglers to see fish floating below that would otherwise be blocked by water reflection.”

Neutrons will change their spins in a predictable way when they scatter. The use of a polarized beam allows researchers to better understand what is happening in the material by setting the neutron spin to and measuring the neutron spin after the beam hits the sample. For example, a neutron spin can be flipped in the opposite direction during scattering.

“In the USA, most of the measurements we have done with polarized neutrons so far have been based on whether the neutron scattered from the material or its a magnetic fieldrotates 180 degrees or maintains its orientation. We call it a coup, not a coup, ”said Wynn.

“But there is a problem with that. If we get any scatter in the sample other than a coup without a coup or a coup with a spin — or something other than 0 and 180 degrees — the strategy will explode in our face. ”

This strategy works well for conventional magnetic materials, such as ferromagnets and antiferromagnets, in which all magnetic atoms are directed either in the same direction or in alternative directions, but remain parallel to their neighbors. However, the strategy does not work for more complex magnetic structures.

For example, the technique is limited when it comes to the study of exotic particles, such as skyrmions, quasiparticles that exhibit chiral motion, or entangled vortices, or whirlpools of asymmetric field lines of force. Such particles provide exciting potential for materials used in modern data storage and quantum computing applications.

To solve this problem, polarization scientist Peter Jiang leads the ORNL team, which includes Wynne and Silva, as part of a research project aimed at developing spherical neutron polarimetry for several ORNL ray lines. The technology will allow neutron measurements of materials that do not correspond to traditional areas with spin flip and without flip, or, in other words, allow researchers to see the dynamic magnetic behavior that exists between them.

“Traditional methods are not complicated enough to study some complex magnetic systems,” Jiang said. “Now we are no longer limited to coups. This allows us to look at magnetic devices that we could not understand before. ”

Spherical neutron polarimetry was used in Europe, and now Jiang and the ORNL team are adapting the technology to SNS and HFIR instruments. They create technology based on ongoing research by Tianhao Wang, first a graduate student at Indiana University, Bloomington, and then a postdoctoral research on the ORNL team.

The basic technology includes additional optical devices installed both on the incoming beam, which hits the sample – incident beam, and on the output beam, scattering it, which allows measurements of scattered neutrons oriented in any direction. ORNL technology is based on previous prototype designs and will offer several innovations.

In ORNL spherical neutron polarimetry devices, the scattered beam path does not have to coincide with the incident beam, but instead can be angled around the sample.

“This means that if the neutron does not experience a complete flip, we can adjust the field at the other end or move the apparatus to detect neutron scattering in different directions,” Silva explained.

The team also developed two independent cooling systems to allow researchers to learn how magnetic structures change according to temperature. The first system cools two spherical components of the polarization of neutrons located on both sides of the sample to make them superconducting. The second system introduces an additional cryostat with liquid helium gas station an ability that allows researchers to more easily study materials in the temperature range without affecting the temperatures needed for superconductivity in the first system.

Finally, spherical neutron polarimetric devices are made of more efficient materials. While niobium was used for the superconducting sheets in previous designs, yttrium-barium-copper oxide (YBCO) was used in the new design, which was superconducted at a temperature of 93 Kelvin (-292 ° F), which is significantly higher than that of its niobium precursor . In addition, superconducting films are bonded to Mu metal yokes, which combine to shield all other magnetic fields and establish a zero field around the sample to study the spins of the materials in their natural state.

“To achieve superconductivity, a significant amount of cooling power is required. To maintain superconductivity, niobium must be cooled to a temperature below 10 K, therefore, European designs required extensive cooling systems, which often had to be filled with liquid helium manually, ”Jiang said.

“With YBCO high-temperature films, we can use a single-stage closed-cycle refrigerator to cool the film to a temperature well below its critical temperature, so we don’t worry about any loss in superconductivity. And with added liquid helium, an autofill system for the cryostat and a closed-loop cooling system, the device will be easier to use and more efficient. "

Moreover, the system is compact compared to previous systems – high-temperature superconductors, which eliminate the need for a large cooling system, make it mobile.

“In any case, there is evidence of how portable the device is. We moved it to nuclear reactor at the university of missourithen back to HFIR and from HFIR to SNS, ”said Silva. “I assembled it and disassembled it several times, and each time I found simpler ways to connect the parts – only a small quality of the changes in life that we make to increase its usefulness.”

The system was successfully tested, with full polarization measurements using several known materials, including silicon, manganese oxide, and bismuth-iron oxide.

The team plans to implement the system on a three-phase HFIR PTAX spectrometer and a GP-SANS diffractometer, which will be optimized for the stationary neutron beam of the reactor, with full functionality expected by the end of 2020.

Subsequently, the team will develop a similar spherical neutron A polarimetric device exclusively for the HYSPEC SNS device, making it the only device in the world that connects super mirror array and the possibility of a wide angle. The device will also benefit from the unique capabilities provided by the SNS pulse source accelerator.

“At the same time,” said Wynn, “we will have a workhorse at PTAX that will knock out our socks.”

The world's first observation of spin schemes using neutron transfer

ORNL neutrons add enhanced polarization capabilities for measuring magnetic materials (2020, March 16)
restored March 16, 2020
from https://phys.org/news/2020-03-ornl-neutrons-advanced-polarization-capability.html

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