Japanese scientists are proposing a new material for use in the permanent magnets of electric vehicle motors.

NIMS-TohokuGakuinU-SmFe-magnet

The incumbent technology – neodymium-iron-boron, sometimes with dysprosium – works well, but Nd and Dy are considered minerals or strategic importance by nations which mine them, and are mostly found in China where exports are restricted.

Working together, the Japanese National Institute for Materials Science (NIMS) and Tohoku Gakuin University have looked at samarium – already used in samarium-cobalt magnets, which were essentially the more-expensive predecessor to Ne-Fe-B magnets.

Sm is another rare metal, but according to NIMS, it has developed a material “containing only small amount of rare earth elements. The compound exhibited 1.2 tesla coercivity, sufficient for use in automotive electric motors, and superior magnetic properties to that of Nd-Fe-B based magnets even when processed into a thin film.”

The material is boron-doped anisotropic Sm(Fe0.8Co0.2)12, where iron replaces some of the also-expensive cobalt. It is part of a family of magnets described as samarium-iron-cobalt, or more loosely as ‘samarium-iron’.

“Among rare-earth elements, Nd and Sm have a more or less similar natural abundance. However, we want to use less rare-earth in the permanent magnet, which is the case for the Sm(Fe,Co)12-based compounds compare to Nd-Fe-B magnets,” NIMS researcher Hossein Sepehri-Amin told Electronics Weekly. “In addition, broadening the use of [different] rare earth elements is one of the strategies to avoid shortage of the rare-earth elements in the future considering ever-increasing demands for the current Nd-Fe-B based magnets.”

In 2017, NIMS confirmed that Sm(Fe0.8Co0.2)12 could beat neodymium magnets in magnetisation, magnetocrystalline anisotropy and Curie temperature, but fell too far behind in coercivity.

Key to this research was the observation that neodymium magnets with high coercivity have a hetrogeneous structure – they consist of aligned microscopic Nd2Fe14B crystals, close together but separated by ~3nm walls of an amorphous version of the same material.

This prompted the group to persuade Sm(Fe0.8Co0.2)12 to adopt the same structure, which they managed by doping it with boron. The boron does not distribute evenly: boron-rich Sm(Fe0.8Co0.2)12 tends to form amorphous walls, while boron-poor Sm(Fe0.8Co0.2)12 continues to form crystals, now separated by ~3nm walls.

“This compound has an anisotropic granular microstructure,” said NIMS, “enabling it to exhibit a remnant magnetisation greater than that exhibited by other SmFe12-based compounds with isotropic granular microstructures. As result, it exhibited a large coercivity of 1.2T combined with a large remanent magnetisation of 1.5T, much larger than the previously developed SmFe12-based magnetic compounds. It may serve as a novel magnet capable of outperforming neodymium magnets.”

The coercivity is also stable at high temperature, resisting magnetisation reversals.

“Sm(Fe,Co)12 based magnets which do not need heavy rare earth elements such as Dy show great thermal stability of coercivity, even superior to that of Nd-Fe-B and (Nd,Dy)-Fe-B magnets,” said Sepehri-Amin.

This is all still in thin films.

“We want the magnet to have superior performance than that of Nd-Fe-B magnets,” said Sepehri-Amin. “This work demonstrates these properties in model thin-film, proving the potential of Sm(Fe,Co)12-based magnets. Efforts are still required to realise these properties in the anisotropic bulk magnet for practical applications.