Physics. Chameleon magnets.

The spin of an electron can serve as a magnetic messenger. Permanent magnetism, or ferromagnetism, comes from the spontaneous alignment of the electron spins and their associated magnetic moments in metals such as iron and cobalt, which results in their or magnetization. Ferromagnetism plays an important role in information storage, not only to keep refrigerator magnets in place (and notes held by them) but to store data in computer hard drives (1, 2). A more common effect is paramagnetism—a material becomes magnetic only when an external magnetic field causes its spins to align. Silicon is paramagnetic, but its semiconductor properties, not its magnetism, make silicon useful in logic circuits. On page 1065 of this issue, Yamada et al. (3) report a breakthrough that brings together these two different worlds of ferromagnetic metals and paramagnetic semi conductors and may better integrate logic and memory. By adding cobalt impurities to nonmetallic and nonmagnetic titanium dioxide, they created an intriguing material (Ti,Co)O2, which, like a chameleon, can reversibly transform from a paramagnet to a ferromagnet at room temperature.