People have been fascinated with the phenomenon of magnetism since ~600 BC. An ancient Greek philosopher observed that lodestone (magnetite) can attract iron. The ancient Chinese were credited with inventing the compass, which had great impact on human history and global exploration. The first written reference to compasses used in Chinese navigation dates to 1086, and it was later used by European mariners. However, the compass was used centuries earlier for spiritual and religious purposes. The ancient compasses were built using lodestone, a natural permanent magnet, which aligns itself with the Earth’s magnetic field.
In the last 100 years, various permanent magnets have been developed and have achieved great commercial success. From ferrite, Alnico, SmCo5, Sm2Co17 to Nd2Fe14B magnets, the maximum energy product, (BH)max, has been increased from a few MGOe to over 50 MGOe, as is shown in the following figure.
Historical information about the maximum energy product of permanent magnets
Following is a list of basic requirements for a good permanent magnet:
High saturation magnetization
High anisotropy field
High Curie temperature
High resistance to demagnetization
In the 1930s, MK steel magnets were developed, which are precipitation hardened alloys containing iron, nickel, and aluminum. Further research of MK steel and the addition of cobalt, copper and titanium led to the development of Alnico and its commercial success in 1950s. Alnico magnets have very high residual induction, excellent thermal stability and corrosion resistance, but its intrinsic coercivity (up to 2 kOe) is relatively low as compared to other magnets developed in later years. The maximum energy product of Alnico can be as high as 9 MGOe. For more information about Alnico magnets, please visit the Alnico Magnets page.
Hard ferrite magnets were also developed in 1950s. Hard ferrite is often referred to as ceramic magnet or ferrite magnet, which significantly expanded the magnet applications due to the low cost of raw materials and production process as well as excellent corrosion resistance. Commercial ceramic magnets today have a maximum energy product up to 4 MGOe and higher intrinsic coercivity (up to 4 kOe) as compared to Alnico. Ceramic magnets are still widely used today due to the low cost. For more information on Ceramic magnets, please visit the Ceramic Magnets page.
In 1960s, the U.S. Air Force Materials Research Laboratory proved that SmCo5 has excellent magnetic properties, which became the first generation of rare earth magnets. SmCo5 magnets became commercially available in 1970s. The maximum energy product of today’s SmCo5 magnets can reach at least 20 MGOe, which is much higher than that of Alnico and ceramic magnets. An even more powerful Sm2Co17 magnet was developed and commercialized in the 1970s and became the second generation of rare earth magnets. Modern Sm2Co17 magnets have a maximum energy product up to 33 MGOe. Both SmCo5 and Sm2Co17 magnets have excellent thermal stability due to high Curie temperature and extremely high intrinsic coercivity. Sm-Co magnets have superior corrosion resistance. No surface coating is needed for most applications. For more information about Sm-Co magnets, please visit the Samarium Cobalt Magnets page.
The third generation of rare earth magnets, Nd2Fe14B, was invented in the 1980s, which is still the most powerful magnet today with a maximum energy product of more than 50 MGOe. Nd2Fe14B has very high residual induction but slightly lower intrinsic coercivity than SmCo magnets. Heavy rare earth elements, such as dysprosium (Dy), are often added to increase the intrinsic coercivity for higher temperature applications. Surface coating, such as nickel plating, is recommended to protect Nd2Fe14B magnets from corrosion. For more information about Nd2Fe14B magnets, please visit the Neodymium Iron Boron Magnets page.
