Feb 8, 2011
Ferromagnetic nanocrystals for room-temperature applications
Magnetic nanoparticles have enormous potential value in applications such as ultrahigh-density recording and medicine. Detrimental effects emerge when the particles in the nanosize regime lose their stable magnetic order and become superparamagnetic at room temperature because thermal fluctuations induce flipping of the magnetic moment with time. In this context, diluted magnetic semiconductor (DMS) materials are an exciting option where the magnetic order could be tuned at room temperature due to coupling of the dopant spins to the itinerant electrons and holes of the semiconductor host.
Researchers from IISER-K, India and BIU, Israel, have successfully synthesized 40–50 nm nanocrystals of cadmium telluride (a semiconductor) doped with manganese (a transition metal) and encapsulated by a 6–9 nm carbon shell. The extent of doping concentration was probed with electron paramagnetic resonance (EPR), a technique that can distinguish Mn2+ locations at the substitutional sites in the CdTe lattice, and other tendencies of manganese to remain at the surface of the nanocrystals and/or as clusters. In this case, the effective substitutional doping was observed up to 3.1 atomic % of Mn.
The magnetic ordering in such DMS systems is a controversial subject due to the existence of a variety of dopant–dopant interaction mechanisms. Ferromagnetism (parallel spins) can coexist with the antiferromagnetic (antiparallel) interactions. The researchers have demonstrated that when manganese concentration is low (0.2 %), Mn2+ ions remain isolated and lead to ferromagnetic coupling. At higher doping concentrations (2.4 and 3.1%), manganese clusters are formed leading to weaker coupling of the semiconductor carriers and dopant spins, in addition to the antiferromagnetic interactions.
The nanocrystals were synthesized by an advantageous one-step method under high pressures and temperatures, which does not require the use of solvent, and allows gram-scale synthesis. Unlike the lengthy methods to functionalize the nanocrystal surface, the biocompatible carbon coating on the toxic nanocrystal core was formed in situ. The Cd1–xMnxTe / C nanocrystals are easy to synthesize, and their room-temperature ferromagnetic behaviour opens up the possibility for their use in magnetic storage devices. In addition, the carbon shell on the nanocrystals makes them viable for intracellular magnetic labeling and in vivo magnetic separation.
You can find more information on ferromagnetic nanocystals in the journal Nanotechnology.
About the author
The research was carried out jointly by scientists from India and Israel. Dr Sayan Bhattacharyya, a materials chemist, is an assistant professor at the Department of Chemical Sciences, Indian Institute of Science Education & Research (IISER) Kolkata, India. Dr David Zitoun is senior lecturer and Prof. Aharon Gedanken is professor Emeritus, both at the Department of Chemistry, Bar-Ilan University, Israel.