US lab gets stronger pull from rare-earth magnets

Rare-earth magnets could be made more powerful, thanks to work at the US Argonne National Laboratory near Chicago.

Under certain circumstances, electrons orbiting rare-earth atoms get highly elliptical orbits. “There is a strong coupling between the orientation of the magnetic moments and the orientation of the electronic orbitals, we call this spin-orbit coupling,” Argonne physicist Dr Daniel Haskel told Electronics Weekly.

The orientation of these magnetism-producing orbits is determined by interaction with the electron clouds of atoms nearby in the crystal lattice.

Looking into the crystal structure of neodymium-iron-boron (Nd₂Fe₁₄B) magnets, the Argonne team found Nd ions at two sites in the lattice, with local conditions pushing the orbitals in different directions. This means half the Nd atoms produce magnetism in one direction, and the other half in another direction - resulting in less field than would be obtained with them all aligned.

There are several possible ways to get more magnetism, said Haskel. “[You could] manipulate the atomic positions around rare earth ions so that both atomic surroundings give the same orientation of magnetic moments. This is atomic engineering, not trivial to do.”

An alternative is to replace half the Nd ions with a different rare earth type that will prefer the same orientation as the remaining half. “Not trivial, but probably possible,” is Haskel’s opinion.

A third option is to swap half the Nd ions for a non-magnetic ion that will decrease overall magnetisation, said Haskel, but will enhance coercivity, or the resistance to demagnetising.

One more possibility is to use half gadolinium. “It won’t add magnetic hardness, but at least it won’t subtract from it. It also has a larger magnetic moment than Nd, so it will add net magnetisation,” said Haskel.

Overall, “if you could fix them by manipulating the atomic surroundings, you can gain a factor of two” in magnetic field. If half the Nd ions are swapped for another rare earth “you can perhaps do better than a factor of two”, he said.

www.anl.gov