The »super magnets« have competition: an unusual bond between rare earths makes a newly created molecule three times more magnetic than the strongest magnetic substance known to date. A working group led by Colin A. Gould from the University of California at Berkeley constructed a class of substances with two atoms of a rare earth element such as dysprosium or terbium, between which three iodine atoms are arranged in a triangle. As the team reports in Science, the two metal atoms are also linked by a direct bond that runs right through the center of the iodine triangle. According to the team, this bond is not only responsible for the extremely strong magnetism, but also the first direct bond between two rare earths in a molecule.
A substance is particularly magnetic if, on the one hand, it contains many unpaired electrons whose magnetic moment is not directly neutralized by an oppositely oriented partner electron – and, on the other hand, these electrons are all aligned in the same way. Rare earths make very good magnets, such as the very strong neodymium supermagnets, because they contain many unpaired electrons aligned together through a bond to a metal like iron.
Theoretically, much stronger magnets are possible if you simply use another rare earth element instead of iron as a binding partner. But so far there has been no substance with rare earths directly bonded to one another. There is such a bond in the new molecule, but it is so weak that the two metal atoms have to be held together by three iodine atoms in order for it to form. In this bond there is also an unpaired electron right in the middle between the rare earths.
This aligns all other unpaired electrons of both metal atoms in the same way, so that the entire molecule becomes extremely magnetic. The working group measured the strength of molecular magnetism using the so-called coercive field strength, among other things. This indicates how strong an external magnetic field must be in order to overcome the internal magnetism of a substance. With two terbium atoms in the molecule and at a temperature of about 60 Kelvin, the coercivity was over 25 Tesla, according to the team.