Device applications based on control of the magnetic properties of materials would benefit from the use of light-element magnets, in which magnetism originates from the properties of lower atomic levels (s and p), in place of conventional metallic elements.

Interestingly, magnetism has been observed in several forms of carbon-based materials. Now, Zhuhua Zhang and Wanlin Guo at the Nanjing University of Aeronautics and Astronautics in China1 demonstrate that under specific conditions of fluorine adsorption, boron-nitride (BN) nanotubes exhibit long-range ferromagnetic order and high spin polarization.

Previous studies have confirmed the presence of magnetic moments localized around adsorbed fluorine atoms. First-principles calculations carried out by Zhang and Guo, however, showed that in fluorine-doped BN nanotubes (F-BNNTs) with sufficiently small diameter, electronic states around the fluorine defects could extend far enough for the electronic wavefunctions of adjacent sites to overlap. The nature of these defects is such that the associated electron states are always over half-filled, which gives rise to coupling between defects. The combination of these two factors leads to long-range ferromagnetic order.

The magnetic order was found to become weaker with increasing tube diameter, and to be absent in a planar BN material. This behavior was attributed to a reduction in electron binding energy, and consequently the magnetic moment, in larger-diameter tubes.

Fig. 1: Model of a distorted boron-nitride nanotube with adsorbed fluorine atoms.

The dependence of magnetic order on the local curvature of the nanotubes could allow the ferromagnetism to be modulated by distorting or 'squeezing' the nanotubes (Fig. 1). This squeezing effect is enhanced in large-diameter nanotubes. “Larger F-BNNTs exhibit more remarkable magnetic modulation by radial compression, which has practical importance, such as for piezo-magnetic devices,” says Guo. The ability to tune the magnetic properties of BN nanotubes, combined with the high stability of this material, may lead to the development of efficient nanotube-based spintronic devices.