Mineral Magnetism

In addition to holding holiday cards to refrigerators well into the new year, magnets are also used in everyday items such as cell phones, children’s toys, and shower curtains. Certain magnets garner the grandiose status of “strong magnets,” and although the computing, medical imaging, and manufacturing industries all rely on strong magnets, by far the largest consumers of strong magnets are green technologies like wind energy and electric vehicles.

Strong magnets draw their defining strength from rare-earth minerals, such as neodymium and samarium. While these aren’t exactly rare in nature, there is a scarcity in the United States stemming from limits on foreign sources, and each year, rare-earth minerals get harder to come by. Hence, the Laboratory is making a concerted effort to create rare-earth-free strong magnets.

A Los Alamos team led by materials physicist Joe Thompson wants to delineate what the crucial microscopic properties are that make a material magnetic. If the team can parse those out, it might be able to create new strongly magnetic materials without rare-earth elements.

To tackle this many-faceted problem, the team devised a two-pronged approach that combines quick computation with in-depth theoretical and experimental understanding. What it came away with was a set of rules to guide its search for rare-earth-free, strongly magnetic materials. The rules are very specific and have to do with ideal crystal structure, electron-electron interactions, and magnetic anisotropy. Anisotropy refers to a physical  property that is not identical in all directions; magnetic anisotropy, then, is a specific kind of anisotropy that indicates how easily a material can be magnetized along one axis while resisting magnetization along another axis.  Strong magnets have high magnetic anisotropy. Armed with the set of rules they devised, the scientists set about making and measuring materials they thought might pass muster.

To explore a material’s physical properties, it is best to work with a single crystal of the material, in which the constituent atoms are arranged in a single ordered lattice. Synthesis of candidate magnetic materials in the necessary single-crystal solids proved time and cost restrictive. But the team quickly devised a workaround: because the materials were magnetic, they could be synthesized as polycrystals (much easier and faster to produce than single crystals), ground into a fine powder, then aligned by magnetic field while being glued back together into a single-crystal-like solid. This aligned-powder approach shaved months off of the time for initial analysis and helped whittle down the pool of candidates for further examination.

he material yttrium pentacobalt (YCo⁵) is magnetic and free of rare-earth minerals, but it falls just short of the strength requirement to be classified as “very strong.” However, YCo⁵ is still a decent proxy for what the Los Alamos team is after. So, in concert with the aligned-powder effort, the team conducted a series of calculations and experiments aimed at a microscopic understanding of YCo⁵'s magnetism. By studying YCo⁵ in depth, the team achieved two things: first, it proved the validity of its design-guiding rules, and second, it established a benchmark to which it could compare new candidate materials.

Out of hundreds of candidates, a single compound containing iron, germanium, and tellurium emerged as the front runner. Now, the time and cost to synthesize a true single crystal was easily justified. And that crystal has withstood ever-more rigorous probing of its magnetic nature.

But the new material is not quite ready for the mainstream. Since it becomes a strong magnet only at subzero temperatures, its immediate applications would be quite limited. Thompson is confident, however, that with further experiments and calculations, and by examining closely related compounds, there is promise for raising the temperature while maintaining the magnetism.

“The project was motivated by the need to replace rare-earth magnets in green technologies like wind turbines and electric cars,” says Thompson. “We don’t want dependence on those minerals to impede progress.” Considering that a popular model of hybrid car uses more than two pounds of neodymium in each motor and that a typical wind turbine uses more than 100 pounds of it, the United States is consuming millions of pounds of neodymium per year. With green technologies gaining serious traction, the timing is perfect to find an alternative to endangered rare-earth magnets, allowing their use to go the way of whale oil and fade back into obscurity