Permanent Magnets

A permanent magnet is magnetic material that when magnetized by an external magnetic field, continues to produce a magnetic field.

Magnetism occurs when there are unpaired electrons. Iron has 5 HOMO's (highest occupied molecular orbitals) of equal energy level, each with one electron, which makes it strongly magnetic. In addition, iron crystallizes with miniature crystals, and all the single electrons in the micro crystal become aligned, which increases the magnetism. When iron is in an electrostatic field, the micro crystal electronic structure aligns with the field and remains in alignment even after the electrostatic field is removed, forming a permanent magnet.

Doping a material can create multiple low energy orbitals which can each have one electron in it. A doubly occupied orbital (therefore non-magnetic) can be replaced with two singly occupied orbitals creating magnetic properties.

Permanent Magnet History

An excellent history of the invention, evolution and development of permanent magnets prior to 1900 is found in:

Edward Neville Da Costa Andrade, “The Early History of the Permanent Magnet,” Endeavour, Volume 17, Number 65, January 1958 (Reformatted and updated references, August 2009)

Neodymium Iron Boron Magnets

The invention of Neodymium Iron Boron magnets was the result of the efforts of two teams, one led by John Croat of the General Motors Physics Laboratory, and the other by Masato Sagawa of Sumitomo Special Metals of Japan.
Sumitomo developed full density sintered Nd₂Fe₁₄B magnets. An ingot is pulverized into powder. This powder is magnetically aligned and formed into dense blocks that are shaped, treated and magnetized.

Masato Sagawa, Setsuo Fujimura, Norio Togawa, Hitoshi Yamamoto, and Yutaka Matsuura; “New Material for Permanent Magnets on a Base of Nd and Fe,” Journal of Applied Physics, Volume 55, Number 6, 15 March 1984, pp. 2083-2087, doi:10.1063/1.333572 (Reformatted, color illustrations added, and updated references, August 2009)

General Motors focused on the development of melt-spun nanocrystalline Nd₂Fe₁₄B magnets. A thin ribbon is formed containing nano-scale grains that is than pulverized into particles, mixed with a polymer and formed into bonded magnets. While they offer less flux than sintered magnets, they can be formed into intricate parts.

John J. Croat, Jan F. Herbst, Robert W. Lee, and Frederick E. Pinkerton; “Pr-Fe and Nd-Fe-based Materials: A New Class of High Performance Permanent Magnets,” Journal of Applied Physics, Volume 55, Number 6, 15 March 1984, pp. 2078-2081, doi:10.1063/1.333571 (Reformatted, color illustrations added, and updated references, August 2009)

Both groups wrote papers examining in greater depth, the physical structures of Nd₂Fe₁₄B magnets.

By Masato Sagawa, Setsuo Fujimura, Hitoshi Yamamoto, Yutaka Matsuura, and Kenji Hiraga; “Permanent Magnet Materials Based on the Rare Earth-Iron-Boron Tetragonal Compounds,” IEEE Transactions on Magnetics, Volume 20, Issue 5, September 1984, pp. 1584-1589, doi:10.1109/TMAG.1984.1063214 (Reformatted, color illustrations added, and updated references, August 2009)

By Jan F. Herbst, John J. Croat, and Frederick E. Pinkerton; “Relationships Between Crystal Structure and Magnetic Properties in Nd₂Fe₁₄B,” Physical Review B, volume 29, Number 1, 1 April 1984, doi: 10.1103/PhysRevB.29.4176 (Reformatted, color illustrations added, and updated references, August 2009)

Permanent Magnet Review Papers

In his 1966 paper, Fred Luborsky describes the permanent magnets that were in use at that time (Alnico magnets), and their applications:

Fred Luborsky, “Permanent Magnets in Use Today,” Journal of Applied Physics, Volume 37, Number 3, March 1966, pp. 1091-1094, doi:10.1063/1.1708348 (Reformatted, color illustrations added, and updated references, August 2009)

Rahman and Slemon in their 1985 paper provide an excellent comparison of the properties of various magnet materials versus Neodymium Iron Boron magnets.

M.A. Rahman and Gordon R. Slemon; “Promising Applications of Neodymium Boron Iron Magnets in Electrical Machines,” IEEE Transactions on Magnetics, Volume 21, n. 5, September 1985, pp. 1712-1716, doi:10.1109/TMAG.1985.1064113 (Reformatted, color illustrations added, and updated references, August 2009)

Another paper that provides an overall perspective on the evolution of permanent magnets from their origins to the 1990s is provided by:

K.J. Overshott, “Magnetism: It is Permanent,” IEE Proceedings A, Volume 138, Number 1, January 1991, pp. 22-30 (Reformatted, color illustrations added, and updated references, August 2009)

Two years before his death in 1992, Karl Strnat wrote the following review article on permanent magnet applications:

Karl Strnat, "Modern Permanent Magnets for Applications in Electro-Technology," Proceedings of the IEEE, Volume 78, Number 6, June 1990, pp. 923, doi:10.1109/5.56908 (Reformatted, color illustrations added, and updated references, September 2009)