Scientists discover crystal with exotic spiral magnetism

This “semi-metal” crystal is made up 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 of which are magnetic. Understanding the special magnetic properties of the material requires nine of these unit cells, represented by the larger block on the right (which has a single unit cell framed in red). This 3×3 block shows green “Weyl” electrons traveling diagonally across the tops of cells and affecting the orientation of the magnetic spin of Nd atoms. A special property of the Weyl electron is the locking of its spin direction, which is either parallel or antiparallel to the direction of its motion, as represented by the small arrows in Weyl electrons. As these electrons move along the four Nd gold atoms, the Nd spins reorient themselves into a “spin spiral” which can be imagined as successively pointing in the 12 o’clock direction (closest to the viewer with the red arrow pointing up), 4 o’clock clock (blue arrow), 8 o’clock (also in blue) and another 12 o’clock (furthest from viewer and again in red). Lines of Nd atoms extend through many layers of the crystal, providing many examples of this unusual magnetic pattern. Credit: N. Hanacek / NIST

An exotic form of magnetism has been discovered and linked to an equally exotic type of electrons, 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 the flowing electrons to behave like massless particles, the magnetism of which is related to the direction of their movement. In other materials, these Weyl electrons have given rise to new behaviors related to electrical conductivity. In this case, however, the electrons promote the spontaneous formation of a magnetic spiral.

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

The conclusions, which appear in Natural materials, reveal a complex relationship between the material, the electrons which pass through it in the form of current and the magnetism exhibited by the material.

In a fridge magnet, we sometimes imagine each of its iron atoms as having a bar magnet piercing it with its “north” pole pointing in a certain direction. This image refers to the spin orientations of 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 the unit cells of a crystal – the smallest building block of a crystal lattice – than it is on one side. ‘other. This arrangement stabilizes the electrons flowing through the crystal, which in turn results in unusual behavior in its magnetism.

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

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

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

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

“For neutron scattering, we have benefited greatly from the large amount of neutron diffraction beam time available to us at the NIST Center for Neutron Research,” said Jonathan Gaudet, one of the co-authors of the article. . “Without the beam time, we would have missed out on this great physical news.”

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

“Magnetic memory technology like hard drives typically 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 sense a magnetic field. Electrical reading and writing of information is faster and more robust.”

Understanding the effects Weyl electrons cause could also shed light on other materials that have left physicists in consternation.

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

Discovery of a new quantum material

More information:
Gaudet, J. et al. Weyl-mediated helical magnetism in NdAlSi. Nat. Mater. (2021). ,

Provided by the National Institute of Standards and Technology

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