Physicists have long known that magnetic behaviour originates from the tiny magnetic moments or “spins” that are carried by electrons. Now they have discovered a new property of electrons known as “colour”.
The spins on electrons can be either up or down and, depending on the spin-spin interactions, materials can be ferromagnetic (i.e. all the spins pointing in the same direction) or antiferromagnetic (i.e. neighbouring spins pointing in opposite directions).
Lithium nickel oxide, however, is a very unusual material. Although it appears to be ferromagnetic, the spins fail to align completely. This behaviour has been an interesting but controversial issue for years. Recently, Anatole Stepanov at the University of Aix-Marseille in France and co-workers have applied an oscillating magnetic field to check the electron spins – a technique known as electron spin resonance. They found magnetic anomalies that imply that the spin-spin interaction changes from antiferromagnetic at high temperatures to ferromagnetic at about 300 K and back to antiferromagnetic again below 40 K (F Reynaud et al. 2001 Phys. Rev. Lett. 86 3638). The results indicate that the electrons have another property, in addition to spin, known as “colour”. This extra degree of freedom describes the orbit of the electron around the atom.
In the July issue of Physics World, Fu Chun Zhang of the University of Cincinnati, USA, explains how spin-colour coupling introduces new magnetic behaviour into a spin system.
