Manufactured multilayer magnetism

Ferromagnets are often described as a collection of atomic dipole moments all pointing in the same direction. But itinerant magnets—like iron, nickel, and cobalt—whose magnetic properties originate in the conduction electrons require a more sophisticated explanation. Nearly 80 years ago, Edmund Stoner showed that the conduction electron band in those systems splits into two subbands—one each for spin-up and spin-down electrons—that are shifted in energy relative to each other. Further, the splitting can occur only if the dimensionless product of the electronic density of states and the strength of the magnetic exchange between electrons is greater than one. The Stoner criterion, as it’s come to be known, explains why Fe, Ni, and Co are the only elements that order magnetically at room temperature. Now Oscar Céspedes of the University of Leeds and his collaborators have skirted the Stoner criterion by stacking layers of copper and fullerene molecules. Although Cu and fullerene are both nonmagnetic, magnetization measurements on the multilayers display the hallmark of ferromagnetism: a hysteresis loop. The figure shows one such loop for a sample with five sets of alternating 15-nm-thick fullerene and 2-nm-thick Cu layers. The group’s density-functional-theory calculations show that electron transfer from the Cu to fullerene molecules at their interface (blue layer in the upper left inset) dramatically strengthens the magnetic exchange and alters the Cu’s electronic structure. Placing an aluminum spacer layer between Cu and fullerene prevents the electron transfer and suppresses magnetic order. Céspedes and his colleagues also found similar results for fullerene–manganese multilayers; their results may provide a blueprint for making other novel hybrid magnetic systems. (F. Al Ma’Mari et al., Nature 524, 69, 2015. )