Scientists Discover a New Crystal That Exhibits an Exotic Form of Magnetism

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This is not your grandmother’s fridge magnet.

An exotic form of magnetism has been discovered and linked to an equally exotic type of electron, according to scientists who analyzed a new crystal in which they appear at the National Institute of Standards and Technology (NIST). Magnetism is created and protected by the crystal’s unique electronic structure, providing a mechanism that could be exploited for fast and robust information storage devices.

The newly invented material has an unusual structure that conducts electricity but causes circulating electrons to behave like massless particles, whose magnetism is tied to the direction of their motion. In other materials, such Weyl electrons

have elicited new behaviors related to electrical conductivity. In this case, however, the electrons favor the spontaneous formation of a magnetic spiral.

“Our research shows a rare example of these particles driving collective magnetism,” said Collin Broholm, a physicist at Johns Hopkins University who led the experimental work at the NIST Center for Neutron Research (NCNR). “Our experiment illustrates a unique form of magnetism that may arise from Weyl electrons.”

The conclusions, which appear in Natural materialsreveal a complex relationship between the material, the electrons flowing through it as a current, and the magnetism the material exhibits.

Semi-metallic crystal

This “semi-metallic” crystal consists of repeating unit cells such as the one on the left, which has a square top and rectangular sides. The spheres represent atoms of silicon (purple), aluminum (turquoise) and, in gold, neodymium (Nd), the latter being magnetic. Understanding the material’s special magnetic properties requires nine such unit cells, represented by the largest block on the right (which has a single unit cell circled in red). This 3×3 block shows “Weyl” green electrons traveling diagonally across the top of the cells and affecting the magnetic spin orientation of the Nd atoms. A special property of the Weyl electron is the locking of its spin direction, which points parallel or antiparallel to the direction of its motion, as shown by the small arrows in Weyl electrons. As these electrons travel along the four Nd gold atoms, the Nd spins reorient themselves into a “spin spiral” which can be imagined as pointing successively in the 12 o’clock direction (closest to the viewer with a red arrow pointing up), 4 o’clock clock (blue arrow), 8 o’clock (also in blue) and 12 o’clock again (farthest from the viewer and again in red). Lines of Nd atoms stretch through many layers of the crystal, providing many examples of this unusual magnetic pattern. Credit: N. Hanacek/NIST

In a fridge magnet, we sometimes imagine each of its iron atoms as being pierced by a bar-shaped magnet with its “north” pole pointing in a certain direction. This image refers to the spin orientations of the atoms, which line up in parallel. The material studied by the team is different. It is a “semi-metal” composed of silicon and the metals aluminum and neodymium. Together, these three elements form a crystal, which implies that its component atoms are arranged in a regular repeating pattern. However, it is a crystal that breaks inversion symmetry, which means that the repeating pattern is different on one side of a crystal’s unit cells – the smallest building block of a crystal lattice – than on the other. ‘other. This arrangement stabilizes the electrons flowing through the crystal, which in turn results in unusual behavior in its magnetism.

The stability of electrons results in a uniformity in the direction of their spins. In most electrically conductive materials, such as copper wire, electrons flowing through the wire have spins that point in random directions. This is not the case in the semi-metal, whose broken symmetry transforms the circulating electrons into Weyl electrons whose spins are oriented either in the direction in which the electron is moving or in the exact opposite direction. It is this locking of Weyl electron spins to their direction of motion – their momentum – that causes the semimetal’s rare magnetic behavior.

“Our experiment illustrates a unique form of magnetism that may arise from Weyl electrons.” — Collin Broholm, physicist at Johns Hopkins University

The three types of atoms in the material all conduct electricity, providing springboards for electrons as they jump from atom to the atom. However, only neodymium (Nd) atoms exhibit magnetism. They are sensitive to the influence of Weyl electrons, which push the spins of the Nd atom in a curious way. Look along any row of Nd atoms that runs diagonally across the semimetal, and you’ll see what the research team calls a “rotating spiral.”

“A simplified way to imagine it is that the first Nd atom points at 12 o’clock, then the next at 4 o’clock, then the third at 8 o’clock,” Broholm said. “Then the pattern repeats itself. This beautiful spin “texture” is driven by Weyl electrons as they visit nearby Nd atoms.

It took collaboration between many groups within the Institute of Quantum Matter at Johns Hopkins University to reveal the particular magnetism that occurs in the crystal. It included groups working on crystal synthesis, sophisticated numerical calculations and neutron scattering experiments.

“For neutron scattering, we benefited greatly from the large amount of neutron diffraction beam time we had available at the NIST Neutron Research Center,” said Jonathan Gaudet, one of the co-authors of the item. “If it weren’t for the beam time, we would have missed this beautiful physical news.”

Each loop of the spin spiral is about 150 nanometers long, and the spirals only appear at cold temperatures below 7 K. Broholm said there are materials with similar physical properties that work at room temperature and that they could be harnessed to create an effective magnetic field. memory devices.

“Magnetic memory technology like hard drives usually requires you to create a magnetic field for them to work,” he said. “With this class of materials, you can store information without the need to apply or detect a magnetic field. Reading and writing information electrically is faster and more robust.

Understanding the effects induced by Weyl electrons could also shed light on other materials that have caused consternation among physicists.

“Basically, we might be able to create a variety of materials that have different internal rotation characteristics — and maybe we already have that,” Broholm said. “As a community, we have created many magnetic structures that we don’t immediately understand. Having seen the special character of Weyl-mediated magnetism, we might finally be able to understand and use such exotic magnetic structures.

Reference: “Weyl-mediated helical magnetism in NdAlSi” by Jonathan Gaudet, Hung-Yu Yang, Santu Baidya, Baozhu Lu, Guangyong Xu, Yang Zhao, Jose A. Rodriguez-Rivera, Christina M. Hoffmann, David E. Graf, Darius H. Torchinsky, Predrag Nikolić, David Vanderbilt, Fazel Tafti and Collin L. Broholm, August 19, 2021, Natural materials.
DOI: 10.1038/s41563-021-01062-8

Data for the study was obtained in part with the Multi-Axis Crystal Spectrometer (MACS) instrument, part of the Center for High Resolution Neutron Scattering (CHRNS), a national user facility jointly funded by the NCNR and the National Science Foundation (NSF).

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