The semiconductors at the heart of all modern electronics provide an easy mechanism for controlling the flow of electrical current through the circuit. Similarly facile control utilizing the ‘spin’ polarization of electrons is expected to add a whole new dimension to the functionality and efficiency that could be achieved in microcircuits. The possibilities presented by such ‘spintronic’ devices have fuelled the search for suitable semiconductors — those that exhibit ferromagnetism at room temperature.

Most research into these materials over the past decades has concentrated on semiconductors and oxides that have been atomically ‘doped’ with a small amount of a magnetic metal, such as manganese or cobalt. One of these materials is tin oxide, which displays ferromagnetic properties when doped with a variety of transition metals. Surprisingly however, some reports suggest that tin oxide might be intrinsically ferromagnetic when its dimensionality is reduced. It is therefore difficult to determine whether the observed magnetic properties are due to the dopants or not.

Cen Wang, Mingyuan Ge and J. Z. Jiang at Zhejiang University in China1 have clarified this issue through a study of the magnetic properties of ultrathin sheets of pure tin oxide. The team measured the magnetization for several films of less than 13 nm in thickness and observed ferromagnetic behavior up to well above room temperature. They estimate that the Curie temperature — the temperature above which the material becomes paramagnetic — could be as high as 573 K.

Fig. 1: Transmission electron microscopy images showing the overall form (a) and atomic crystal structure (b) of ultrathin tin oxide nanosheets. © 2010 AIP

To understand the origin of such intrinsic ferromagnetic behavior, the team performed first-principles calculations. They found that if a nanosheet (Fig. 1) is terminated by oxygen atoms, the spins of the oxygen electrons are unpaired, giving rise to a net magnetization. The situation is different if the sheets are terminated by tin, in which case there is no net magnetization. The researchers also found that in oxygen-terminated sheets, the underlying tin atoms contribute only 5% to the total magnetization.

According to Jiang, this study will stimulate more investigations aimed at understanding the origin of room-temperature ferromagnetism in transition-metal-doped tin oxide and transition-metal-doped semiconductor oxides more generally, as well as further studies on the magnetic behavior of low-dimensional systems that are nonmagnetic in the bulk state.