Mighty magnets with minuscule structure

INEEL researchers have discovered a way to make magnets used in computer hard drives and motors more powerful and durable, while also slashing their manufacturing costs.

Materials scientist Dan Branagan found that tweaking the standard formula for these high-end magnets produces stronger magnets that can withstand high manufacturing temperatures. Branagan, of the Department of Energy’s Idaho National Engineering and Environmental Laboratory, worked in collaboration with researchers from Ames Laboratory in Iowa and Brookhaven National Laboratory in New York.

High heat usually transforms rare earth magnets into worthless hunks of metal. But Branagan and his colleagues found that adding extra elements to the mix improves the temperature resistance and magnetic field strength. Surprisingly, the extra elements improve the magnet by forming non-metallic compounds. The researchers also added an unusual and important step – creating a metallic glass.

‘The improved manufacturing ability and increase in magnetic strength is a great improvement,’ says Branagan. He and his colleagues describe their discovery in the cover story of the July 15, 2000 issue of the Journal of Materials Science.

Known as rare earth magnets (from their mix of rare earth elements), the formula developed by Branagan and his colleagues includes the standard rare earth mix of neodymium, iron, and boron, with titanium and carbon as extras (among others). Magnets like those developed at the INEEL have household uses, such as in cordless power tools and miniature speakers.

Magnets are actually a composite of thousands of miniature magnetic fields. Tiny crystals, or grains of metal, form a magnet much like a rock is made of distinct minerals. Each grain has one or more randomly aligned magnetic fields called domains. The ideal case is one domain per grain, but typically several grains cram into a single grain. The domain alignment affects the strength of the magnet. If researchers can get the domains all pointing in the same direction, then the magnet has an extremely strong magnetic field.

At the INEEL, Branagan forms magnets composed of grains so tiny that each one is too small to host more than one direction of magnetic field. The key is the unique nanocomposite structure, composed of nanoscale amorphous blobs less than one hundredth of a millionth of a meter across – 500 times smaller than a red blood cell.

Having only one magnetic field direction in each grain minimizes the chance of defects, says Branagan. ‘These defects are like a dam with a crack in it,’ he says. ‘Like a dam burst begins with a hairline crack, one magnetic domain pointing the wrong way creates a cascading effect, causing other domains to randomly realign.’ Since the tiny grains in the nanocomposite magnets have only one domain per grain, defective domains have less influence on their neighbors.

The researchers can use two different methods to make metallic glass. To coax the mix of elements into microscopic form, Branagan and his colleagues combine the elements into a melt and cool the liquid into a metal ingot. The ingot is then re-melted and fed into a pressurized stream that shoots the molten metal onto a copper wheel spinning at 100 miles per hour, a process known as melt-spinning. Once it hits the wheel, the metal cools almost instantly into a shiny metallic glass ribbon, as fine and flexible as Christmas tree tinsel.

They can also form an ash-like powder of metallic glass through gas-atomization: skipping the copper wheel step and injecting the molten metal into a pressurized stream of gas. Similar to the processes in a volcanic eruption, the gas stream instantly cools the melt into thousands of tiny glass particles.

Heating the glass in an oven at ‘low’ temperature (about 600 degrees Celsius) morphs the glass blobs into metallic and non-metallic crystals. This transformation is the group’s sophisticated trick. Normally, heating metallic glass causes the blobs to grow together into large crystals, but non-metallic crystals of titanium carbide plug the spaces between the larger crystals of neodymium-iron-boride, preventing them from growing bigger.

Until now, manufacturing rare earth magnets was extremely dependent on temperature – an error of 10 degrees will destroy the magnetic properties by making the grains too large. Since the extra elements in the alloy control grain size, there is more room for error. ‘That means lower costs and less stringent controls,’ says Branagan.

Another difference is that the new formula permits gas-atomization. The standard manufacturing process of melt-spinning generates angular particles, while gas-atomization produces spherical particles. ‘Spherical powders flow more easily, making it easier to inject the powders,’ says co-author R. William McCallum of Ames Laboratory. The spherical particles also pack together better than flakes. Bonding the nanocomposite powder into a magnet produces a smoother, more accurate shape than a flaky powder.

Branagan’s research at the INEEL was funded by the DOE’s Basic Energy Science Program. The purpose of the program is to develop basic scientific knowledge to help meet future national energy needs.