Combining magnetism with the ability to control the electronic properties of semiconductors could lead to the realization of devices with novel functionalities based on the control of electron spin. The search for semiconductors exhibiting ferromagnetism at room temperature is motivated by such applications in spintronics.

Ferromagnetism above room temperature has been observed several times in diluted magnetic semiconductors (DMSs) containing small amounts of magnetic impurities. However, it is still unclear whether the magnetism in DMSs is intrinsic, as would be necessary for spintronic applications, or due to spurious effects, such as the existence of clusters of magnetic impurities.

A clear problem in clarifying the source of ferromagnetism in DMSs is that most magnetic studies are conducted at the macroscopic scale, and it is therefore difficult to dismiss the presence of microscopic magnetic secondary phases of materials.

Quan Li and colleagues from the Chinese University of Hong Kong, in collaboration with scientists from China and Germany1, have studied both the structural and magnetic properties of the same nanoparticle at a microscopic level. Their results provide strong evidence for intrinsic ferromagnetism in zinc-oxide (ZnO) nanoparticles doped with cobalt (Co), an actively studied DMS.

Fig. 1: Schematic diagram of the structure of Co-doped ZnO.

Transmission electron microscopy revealed the absence of secondary phases. In addition, channeling enhanced microanalysis showed that cobalt had replaced zinc in the lattice (Fig. 1), rather than inserting between zinc and oxygen sites, which could lead to a local high concentration of magnetic atoms.

Ferromagnetism in the same nanoparticle was then confirmed by electron magnetic chiral dichroism (EMCD) analysis. Combining this observation with the structural analysis led the researchers to conclude that the ferromagnetism was intrinsic to the ZnO itself.

For the team, the results clarify the long-running argument concerning the magnetic properties of cobalt-doped ZnO. Their procedure could also be used for studying the properties of DMSs at the nanoscale.

“We will continue investigation of the magnetic properties of individual transition metal-doped semiconductors in a systematic manner, using the same methodology, aiming at deeper understanding of the mechanisms that determine the ferromagnetism in these DMS materials,” says Li.