Professor Hanington's Speaking of Science: Stronger magnets

Professor Hanington's Speaking of Science: Stronger magnets

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Anyone who has played around with a set of strong magnets knows the pushback you get when trying to force two like poles together. It is almost magical the way a North Pole detests being brought near a second one of its kind. Somehow the invisible force we call a magnetic field crosses the distance. The closer the magnets come to each other the larger the repulsive force. In the end it is the charges within each structure that cause this repelling force. Electrons fighting other electrons, if you will.

The only way a magnetic field can be produced is to have a moving charge. In an electro-mechanical device such as an old-style doorbell, pushing the button sends a current flowing through a coil of wire which, since charges are moving, creates a magnetic field and attracts a piece of soft iron that strikes a chime. You cannot have a magnetic field without moving electrons because, at this time, no one has ever found the elusive magnetic monopole particle. So for now, electrons in motion are the only way to make magnets.

But what about a solid bar magnet? Nothing seems to be going on there, at least not on the macroscopic level. To see the moving electrons we have to look a little deeper down into the individual atoms themselves. Here, electrons orbiting around the nucleus do create magnetic fields but these are usually canceled out by other electrons traveling in opposite directions with the result that most elements are not magnetic at all.

Wood, mostly carbon, is not magnetic, neither is a rod of copper. There are however, five special elements on the periodic chart that are what physicists call ferromagnetic. If we were to examine those atoms, we would see an unbalance in the movements of charge and when many of these atoms line up together a stronger magnetic field is created. Iron atoms working in concert form the magnetic field of a bar magnet.

For hundreds of years, bar magnets have been made of ordinary steel that had large regions within its structure called “domains” where the atoms more or less were aligned spatially. They were easy to make by rapidly cooling some molten iron in the presence of an external magnetic field (from another larger magnet). If the iron quickly solidified, the alignment of the atoms was “frozen” in, so to speak, and this produced a strong bar magnet.

Over the years many different alloys of iron were investigated to see what formulation could produce the best field. In 1931 T. Mishima in Japan discovered that an alloy of iron that used nickel, aluminum and cobalt that had a coercivity of up to 400 Oersteds (a magnet factor that measures the resistance to becoming demagnetized), roughly double that of the best magnet steels of the time. This was used extensively throughout the 1950s in the electronics industry and was the best you could get at that time. As a kid I remember taking apart an old black and white television set my neighbor threw out to scavenge the alnico magnets around the picture tube. They were exceedingly strong. It was almost impossible to push two like poles together.

In the 1960s, researchers Strnat and Ray of Wright-Patterson and University of Ohio developed a formulation that included the rare-earth element samarium alloyed with cobalt. This offered a coercivity up to 700, a vast improvement. In the mid 1980s, both General Motors and Hitachi discovered an alloy using the rare metal neodymium that gave even higher strengths.

At the present Hitachi holds more than 600 patents related to the manufacture of neodymium based magnets but it doesn’t help them too much. Unfortunately, Chinese manufacturers have become a dominant force in neodymium magnet production and based on their control of much of the world’s sources of rare earth mines they have limited their supplies to Japan for the last five years. The United States has only one rare earth element mine and that is at Mountain Pass, California but that mine closed in 2002 in response to environmental restrictions that drove the mining company to bankruptcy.

Last week, scientists at the U.S. Department of Energy’s Ames Laboratory, working with the Department of Physics at the University of Nebraska, discovered a potential tool to enhance magnetization and improve the performance of samarium-cobalt magnets. Their work, published in Physical Review, describes how the team sought to test the limits of substituting iron for some of the cobalt in the alloy, attempting to make a Sm-Co magnet comparable in strength to neodymium types.

Durga Paudyal, Ames Laboratory scientist and project leader for Predicting Magnetic Anisotropy, said that substitutions of iron could range as high as 20 percent, keeping the coercivity of the magnet intact. Their modeling results (atom placement diagram shown) revealed that the electronic structure of the samarium in the material may violate Hund’s rule, which if you remember from your first-year chemistry “Aufabau” principle, predicts how electrons occupy available orbitals in the atomic structure.

Thankfully, the U.S. has a good supply of samarium!

Gary Hanington is Professor Emeritus of physical science at Great Basin College and chief scientist at AHV. He can be reached at garyh@ahv.com or gary.hanington@gbcnv.edu.

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