Magnetic materials are essential in every human being daily life. They played a vital role in the development of modern technology up to the point of being part of the massive generation of electricity in the 19th century. Therefore, it is interesting to take a look to the past and observe the history of such an important group of materials nowadays.
The history of permanent magnets start in Magnesia ad Sypilum, modern Manisa, in the west of Turkey. Magnesia ad Sypilum was an ancient Greek city in Ionia, which was named after the Magnetes, an ancient Greek tribe of that region who discovered mysterious stones that could attract or repel each other more than 3500 years ago. After reading the name of the tribe who made the discovery is easy to guess where magnetism, magnetite or magnet word came from, isnâ€™t it? Those lodestones were indeed magnetite, Fe3O4, with an energy product of only 1 kJ/m3.
Although much had been discovered about magnetism, it wasnâ€™t up to the 18th century when there was a change in the materials that magnets where used to be produced. Gowin Knight, an English entrepreneur, made a fortune by manufacturing magnets but he didnâ€™t publish his methods in life. After his death in the 1770â€™s his method was published and consisted of stirring a slurry of iron fillings to obtain a suspension of finely divided iron oxide. This was mixed with linseed oil to create a paste that was moulded into shape and baked. The resulting block was magnetized with an energy product of 2 kJ/m3.
During the 19th these magnetic steels were improved by adding tungsten and this combined with chromium, thus giving an energy product of 2.5 kJ/m3. But it was in 1917 when two Japanese scientist, K. Honda and T. Takai added cobalt to tungsten steel to increase the coercive force of permanent magnets improving industrial applications with an energy product of 7.6 kJ/m3.
Al-Ni-Fe & Alnico
In 1930, a new alloy of nickel, aluminium and iron was developed by Mishima in England with an energy product of 10 kJ/m3. After this discovery, a new generation of permanent magnets was developed based on Al-Ni-Fe by the addition of copper, cobalt, niobium and titanium, getting the name of Alnico and achieving energy product values up to 72 kJ/m3.
All this new alloys were cheaper than cobalt steels, presented higher magnetic properties and didnâ€™t required as much operations as the steel did. Therefore, Alnico rapidly displaced small electromagnets in motors, transformers, and loudspeakers, lowering the cost and simplifying the construction. Consequently, for the next twenty years Alnicos were extensively researched throughout the world and many companies competed to maximize its properties.
Ceramic magnets: ferrites
During the 1950â€™s, as a result of their work with soft ferrites, the Philips organisation discovered that hard magnetic ferrites could be produced. In general, these ferrites are based on barium (BaFe12O19) or strontium (SrFe12O19), and iron oxide, achieving energy products about 28 kJ/m3.
Although the magnetic properties of the ferrites are lower than Alnico magnets, the great advantage of ferrite magnets is their cost due to they have the lowest price per unit of energy product. Additionally, it is possible to produce flexible rubber magnets and plastic magnets by mixing ferrite powder with the rubber or plastic prior to its manufacture. These flexible ferrites have many applications such as fridge magnet, loudspeakers or magnetic recording tapes.
Rare earth magnets
In spite of rare earth magnets, a Nd-Fe alloy, were reported in 1935, even before than ceramic magnets, they were not developed until the 1960s, when a concerted research effort to identify new permanent magnets based on alloys of rare earth elements was carried out.
In 1967, Strnat et al. investigated phases of the type RCo5, where R means one rare earth element of the following: yttrium, cerium, praseodymium or samarium. This first generation of rare earth permanent magnets were produced by liquid phase sintering of magnetically aligned powders, being SmCo5 the reference amongst them with energy products about 190 kJ/m3. Immediately after their discovery, this magnets found a place in space and military industry due to their high energy product compared with those of previous materials.
After the development of SmCo5, a second generation of rare earth magnets emerged in the early 1970â€™s containing copper, cobalt and rare earth elements, and led to the development of the high energy product alloy Sm2Co17. These magnets evolved by the addition of iron to become Sm2(Co,Fe)17, which opened a door to the addition of more alloying elements such as niobium, vanadium or zirconium, that helped to increase the magnetic properties of the material, thus giving an energy product of about 240 kJ/m3.
The third generation of rare earth permanent magnets began in 1984 when General Motors in the US and Sumitomo in Japan simultaneously developed a new magnet, Nd2Fe14B. As it was explained in a previous post, Sumitomo magnets were made by sintering aligned powders and General Motorsâ€™ ones were produced by hot pressing of melt spun ribbons, with an energy product of 290 and 114 kJ/m3 respectively. This new generation of magnets rapidly displaced others in a number of applications such as electric motor, hard disk drives or loudspeakers. Currently, NdFeB-based magnets produced are able to give an energy product over 470 kJ/m3.
Other permanent magnets
Cu-Ni-Fe & Cu-Ni-Co
The Cu-Ni-Fe and Cu-Ni-Co alloys were developed in the 1930s and were used due to their high ductility, thus easing the manufacturing process. They offered an energy product of 11 kJ/m3, which was higher than the one given by Al-Ni-Fe but it was surpassed when the Alnicoâ€™s were developed.
A new kind of magnets was developed in the 1970â€™s, the FeCrCo magnets. They were a replacement for CuNiFe due to those are not commercially available anymore. In spite of giving an energy product of just 55 kJ/m3, comparable with to those of Alnico magnets, they are characterised by its ductility and high working temperature and they are used in tachometers, micro relays and nautical instruments amongst others.
The equiatomic Pt-Co alloy was developed in the 1950â€™s and was the most expensive permanent magnet in commercial production. With an energy product of 76 kJ/m3, they were better than Alnico alloys and due to their corrosion resistance they were used in biomedical applications. However, with the arrival of rare-earth-based permanent magnets that properties were achievable and they were inevitably superseded.
In the 1990â€™s a new magnetic compound was discovered by Coey and Sun consisting of Sm2Fe17N with an energy product about 400 kJ/m3. The development of this alloy is still ongoing and it is a promising new candidate for permanent magnet applications, although it is commercially available on its bonded form with an energy product of 112 kJ/m3; the highest up to date on this kind of magnets.
As it can be seen, permanent magnet materials are important components of consumer, transport, industrial, military and aerospace systems. There has been an increasing use of permanent magnet materials as the properties of these materials have been improving, which means how relevant are they in nowadaysâ€™ world.
Overshott, K.J., 1991. Magnetism: It is Permanent. IEE Proceedings A,138, p22-30.
Moosa, I.S., 2014. History and Development of Permanent Magnets. International Journal for Research & Development in Technology, 2, p18-26.