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). The magnetism is created and protected by the crystal’s unique electronic structure, offering a mechanism that might be exploited for fast, robust information storage devices.
The newly invented material has an unusual structure that conducts electricity but makes the flowing electrons behave as massless particles, whose magnetism is linked 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 promote 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 can arise from Weyl electrons.”
The findings, which appear in Nature Materials, reveal a complex relationship among the material, the electrons flowing through it as current and the magnetism the material exhibits.
This “semimetal” crystal consists of repeating unit cells such as the one to the left, which has a square top and rectangular sides. The spheres represent silicon (violet), aluminum (turquoise), and — in gold — neodymium (Nd) atoms, the last of which are magnetic. Understanding the special magnetic properties of the material requires nine of these unit cells, shown as the larger block to the right (which has a unit single cell outlined in red). This 3×3 block shows green “Weyl” 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 either points parallel or antiparallel to the direction of its motion, as represented by the small arrows in the Weyl electrons. As these electrons travel along the four gold Nd 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 viewer with red arrow pointing upward), 4 o’clock (blue arrow), 8 o’clock (also in blue) and again 12 o’clock (farthest from viewer and again in red). Lines of Nd atoms stretch through many layers of the crystal, offering many instances of this unusual magnetic pattern. Credit: N. Hanacek/NIST
In a refrigerator 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 atoms’ spin orientations, which line up in parallel. The material the team studied is different. It is a “semimetal” made 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, meaning that the repeating pattern is different on one side of a crystal’s unit cells — the smallest building block of a crystal lattice — than the other. This arrangement stabilizes the electrons flowing through the crystal, which in turn drive unusual behavior in its magnetism.
The electrons’ stability shows itself as a uniformity in the direction of their spins. In most materials that conduct electricity, such as copper wire, the electrons that flow through the wire have spins that point in random directions. Not so in the semimetal, whose broken symmetry transforms the flowing electrons into Weyl electrons whose spins are oriented either in the direction the electron travels or in the exact opposite direction. It is this locking of the Weyl electrons’ spins to their direction of motion — their momentum — that causes the semimetal’s rare magnetic behavior.
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