A team of physicists at the Okinawa Institute of Science and Technology Graduate University (OIST) has predicted the existence of a new kind of spin liquid. A spin liquid is an exotic phenomenon that intrigues scientists: it is a magnetic material in which the magnetism of the atoms fluctuates continuously between different directions. Their theoretical discovery found confirmation through computer simulation. Notably, this mathematical description of a spin liquid shares important similarities with a gauge symmetry, which is a key element in the way physics describes the world. The researchers, all from OIST Theory of Quantum Matter Unit, published their results in Nature Communications.
Spin liquids are well known to physicists. The name ‘spin liquid’ is misleading, as a spin liquid is a solid that is liquid only from the point of view of the directions of its atoms’ magnetism. The direction of an atom’s magnetism is defined by the magnetism that originates from the rotation of the electrons around the atom’s nucleus. In a visual representation of an atom, the magnetism’s direction can be drawn as an arrow, which points in a specific direction of space.
At high temperature, the arrows inside a given material typically point in random directions: the magnetism of each atom is different. When the material is cooled down, the arrows usually arrange themselves in a repeating pattern. A spin liquid is a specific type of magnetic material in which the atoms’ directions keep fluctuating even at low temperatures.
Spin liquids are hard to pin down because they lack the regular repeating patterns of other magnets. Being able to predict the existence of a new kind of spin liquid is then a very important achievement. The new spin liquid theoretically discovered by the scientists is characterized by a unique internal structure describing how the magnetism of each atom relates to the magnetism of those around it.
The finding is significant because there is a strong relationship between the scientists’ mathematical description of the spin liquid and a gauge symmetry. Gauge symmetries are the way in which physicists understand the fundamental forces of nature, such as electromagnetism. Finding gauge symmetries in spin liquids is interesting because it reveals deep connections between different branches of physics. These connections have historically fostered new understandings of how we interpret reality from the physicists’ point of view.
Karl A. Gschneidner and fellow scientists at the U.S. Department of Energy’s Ames Laboratory have created a new magnetic alloy that is an alternative to traditional rare-earth permanent magnets.
The new alloy—a potential replacement for high-performance permanent magnets found in automobile engines and wind turbines–eliminates the use of one of the scarcest and costliest rare earth elements, dysprosium, and instead uses cerium, the most abundant rare earth.
The result, an alloy of neodymium, iron and boron co-doped with cerium and cobalt, is a less expensive material with properties that are competitive with traditional sintered magnets containing dysprosium.
Experiments performed at Ames Laboratory by post-doctoral researcher Arjun Pathak, and Mahmud Khan (now at Miami University) demonstrated that the cerium-containing alloy’s intrinsic coercivity—the ability of a magnetic material to resist demagnetization—far exceeds that of dysprosium-containing magnets at high temperatures. The materials are at least 20 to 40 percent cheaper than the dysprosium-containing magnets.
“This is quite exciting result; we found that this material works better than anything out there at temperatures above 150° C,” said Gschneidner. “It’s an important consideration for high-temperature applications.”
Previous attempts to use cerium in rare-earth magnets failed because it reduces the Curie temperature—the temperature above which an alloy loses its permanent magnet properties. But the research team discovered that co-doping with cobalt allowed them to substitute cerium for dysprosium without losing desired magnetic properties.
Finding a comparable substitute material is key to reducing manufacturing reliance on dysprosium; the current demand for it far outpaces mining and recycling sources for it.