Focus: The World’s Worst Compass Needle
Permanent magnets are commonly known for picking up objects, telling north from south, and even storing information, but a newly discovered magnet can do none of those things. The magnet, described in the 17 September print issue of PRL, is an alloy whose electrons’ spin magnetism perfectly cancels its atomic-orbital magnetism. The result is a material whose atoms and electron spins are aligned like those in a permanent magnet but produces no magnetic field. Researchers believe that the new material could have important applications in spintronics–electronic devices that use electron spins to carry information.
A solid’s magnetism arises from two separate phenomena: Individual electron spins create a tiny magnetic field, while the motion of electrons in their orbitals about the nucleus gives each atom a magnetic moment. The magnetic fields created by the spin and orbital moments are usually unbalanced, but they can be nearly identical in certain rare-earth elements.
Hiromichi Adachi and his colleagues at KEK, the High Energy Accelerator Research Organization in Tsukuba, Japan, and other Japanese institutions, employed one such rare-earth element to create a non-magnetic magnet. The team used samarium, an element whose spin and orbital moments are opposite and vary with temperature. At temperatures just above and below 70 K, their alloy could be weakly magnetized by a magnetic field and maintain its magnetization with the applied field turned off–just like an ordinary bar magnet. But at 70 K the team found that the spin and orbital moments canceled perfectly. The result was a material whose atoms were aligned like those in a permanent magnet, but which had no net magnetization.
Adachi and his colleagues detected the alignment of electrons with the first successful use of a recently developed technique involving circularly polarized x rays. By comparing the scattering of right- and left-circularly-polarized x rays through the material, they determined the degree of electron spin alignment, or “spin polarization.”
”I’m pleased to see that these results have come out,” says Andrew Stewart, a researcher at the Australian National University in Canberra who predicted zero-magnetism in samarium almost thirty years ago . Stewart says that because the electron spins are permanently aligned, this material could be used to study spintronics. Adachi believes that this new material could have another important application. No other spin-polarized material is known to exist without the presence of a magnetic field, Adachi explains. So could someday be used in spintronic applications that require a zero-magnetic field, such as a spin-polarized scanning tunneling microscope.
- A. M. Stewart, J. Phys. F 2, L44 (1972)