Magnetic striction revealed

The basic Heisenberg model of magnetic materials invokes spin exchange, an interaction in which a pair of ions (a dimer) share electrons. Many materials, however, manifest higher-order interactions whose physical origins are subject to some debate. Now, scientists in Switzerland have studied a model magnetic system and found strong evidence that coupled ions reduce their separation to gain magnetic energy at the expense of elastic energy in the material. To see the effect, called magnetic exchange striction, the team exposed a manganese compound to neutrons—first, to simultaneously observe several different dimer excitations and thereby obtain magnetic energies as a function of temperature and pressure; and second, to directly probe the ions’ separations in the relatively soft lattice. The results were consistent with striction being the dominant higher-order interaction. Mechanical deformations caused by magnetic fields are fairly common: Submarine telephony uses the effect; striction also explains the 120-Hz hum of a transformer’s iron core. (Th. Strässle et al., Phys. Rev. Lett. 92, 257202, 2004 http://dx.doi.org/10.1103/PhysRevLett.92.257202 .)