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Oct 15, 2024

Inventing the world’s strongest magnet

Produced by A ferrofluid — made up of particles of iron oxide suspended in a liquid such as oil — is induced to form a shape by strong magnetic fields from a neodymium magnet. Credit: Oliver

Produced by

A ferrofluid — made up of particles of iron oxide suspended in a liquid such as oil — is induced to form a shape by strong magnetic fields from a neodymium magnet. Credit: Oliver Hoffmann/Shutterstock

In 1982, two researchers working independently of each other developed the neodymium–iron–boron magnet, which was stronger than any known magnet. It revolutionized the low-cost mass production of permanent magnets and enabled smaller motors and hard disk drives to be produced, thus playing a vital role in shaping modern society. Even today, it remains the most powerful permanent magnet in the world. The two researchers — Japanese engineer Masato Sagawa and American metallurgist John Croat — have been awarded the 44th Honda Prize for their work.

New materials

Before the discovery of the neodymium magnet, the strongest permanent magnet was the samarium–cobalt magnet, which was based on the Sm2Co17 intermetallic phase. However, samarium is expensive, and much of the world’s cobalt is found in geopolitically unstable regions, making its price susceptible to fluctuations.

Sagawa began working on permanent magnets when his employer Fujitsu tasked him with enhancing the mechanical strength of the samarium–cobalt magnet. At the same time, he believed there was a need for new permanent magnets that combined rare-earth elements and iron. “Iron is more abundant, cheaper, and has a stronger magnetic moment than cobalt, but it lacked a sufficient coercivity — a measure of a magnet’s resistance to other strong magnetic fields,” explains Sagawa. “However, I knew that if we could produce magnets using something as abundant as iron, I had to pursue it.”

Meanwhile, Croat, who was at General Motors Research Laboratories at the time, was looking to develop inexpensive, high-performance permanent magnets for automotive components such as blower motors and seat-adjusting motors.

“At that time, all these motors used ferrite magnets, which are low-performance magnets,” he says. “Samarium–cobalt magnets would have been wonderful for automotive use, but they were too expensive.” To develop an economically viable permanent magnet, Croat knew he needed to produce it from a light rare-earth element, such as lanthanum, cerium, praseodymium or neodymium, which together account for more than 97% of the elements found in rare-earth deposits.

The basic problem confronted by both researchers was the lack of suitable intermetallic phases that formed in the rare-earth–iron alloy systems, like those that formed in the rare-earth–cobalt binary systems.

The problem seemed unsolvable since an intermetallic compound is a basic requirement of any rare-earth magnet. Although the intermetallic phase Nd2Fe17 did form, its properties were unsuitable for developing a permanent magnet. Knowing that they had to have a suitable intermetallic phase, both researchers set about trying to produce one.

Left: Masato Sagawa, formerly at Fujitsu, Japan. Right: John Croat, formerly at General Motors Research Laboratories, USA.

Two approaches

Sagawa tried to improve the properties of the existing Nd2Fe17 intermetallic phase by adding an element with a small atomic size in order to expand the iron-to-iron atomic distance. On discovering that boron does exactly that, Sagawa developed the neodymium magnet through sintering — a process that involves making powders, compacting them through pressing, and then baking them in a vacuum.

Croat, on the other hand, began to investigate producing a suitable intermetallic phase by annealing rapidly solidified alloys. He produced the rapidly solidified alloys by the melt spinning technique, which involved directing a stream of molten alloy onto the edge of a rapidly spinning disk.

During the course of their research, both Croat and Sagawa discovered the same ternary Nd–Fe–B intermetallic phase (Nd2Fe14B) that had all of the physical requirements needed to produce a permanent magnet. Remarkably, they appear to have discovered this phase within weeks of each other in early 1982. Coincidentally, both researchers announced their findings at the Magnetism and Magnetic Materials Conference in Pittsburgh, USA, in November 1983.

Two processing methods

During the course of their research, the researchers developed two families of neodymium–iron–boron magnets having quite different properties. Sagawa developed a family of sintered magnets. Croat, on the other hand, developed a family of bonded magnets using the melt spinning technique to produce a rapidly solidified ribbon, which is ground into a powder, blended with a binder, and pressed into a bonded magnet.

Sagawa’s sintering process made for stronger magnets, with twice the strength of the samarium–cobalt magnets. In contrast, Croat’s melt spinning method allowed for a cheaper manufacturing process and produced a wider range of magnet shapes, notably thin-walled ring magnets that proved ideal for use in small motors.

The development of neodymium–iron–boron permanent magnets has allowed the miniaturization of motors and hard disk drives, drastically reducing the size of electronics that benefitted from smaller size or increased mobility.

Today, the sintered neodymium magnet has achieved greater economic success due to a larger market that encompasses a wider range of applications, including cars, air conditioners, vacuum cleaners, medical imaging equipment, and wind turbine generators. On the other hand, the easily shapeable bonded neodymium magnets made from the melt spinning process have proved particularly useful in small devices that require compact yet high-performance magnets, such as small automobile motors and sensors and mobile-phone speakers.

Sustainable surge

The neodymium magnet now accounts for 95% of permanent magnets used in generators for wind turbines, as well as most magnets made for electric and hybrid vehicles. With the green revolution taking off, the demand for neodymium magnets is predicted to surge. Sagawa believes that robots are another area where the technology will shine: neodymium magnets will be essential for robots that require compact yet powerful limbs, he says.

Considering these emerging applications, further enhancements to neodymium magnets are imperative to ensure their future sustainable production. Croat no longer conducts any R&D, but still follows the field closely, whereas Sagawa continues to work on improving neodymium magnets. One area of focus is reducing the amount of dysprosium, a scarce and expensive rare-earth element used to improve heat resistance in neodymium magnets.

Sagawa is also striving to improve manufacturing methods. An ongoing challenge in manufacturing is the inability to produce small magnets, such as found in smartphones, from scratch, he says. Currently, these magnets are sliced from larger sheets or blocks, resulting in significant waste during the cutting process.

“Up to half of the magnet is turned into scrap and goes unused,” says Sagawa. “Although the scraps can be dissolved and recovered using acids, this process generates pollutants. Future recycling methods must be more environmentally friendly for the magnets to truly contribute to sustainability.”

The Honda Foundation was established in 1977 with contributions from Soichiro Honda, founder of Honda Motor, and his brother Benjiro. The foundation established the Honda Prize in 1980 to acknowledge achievements by scientists whose efforts advance ‘ecotechnology’, the discoveries and inventions that contribute to complex human needs while working in harmony with the natural environment. Thus far, the foundation has recognized the achievements of 43 individuals or groups. The 2023 ceremony took place on 16 November at the Imperial Hotel in Tokyo, Japan.