Long spin-relaxation time in a single metal nanoparticle.

Spin-relaxation time is key to the performance of spin-based devices. Although the spin-relaxation times of semiconductor materials are typically approximately 100 ns (ref. 3), they are on the order of picoseconds in bulk metals due to the high density of scattering centres. In metallic nanoparticles, the spin-relaxation times can be strongly enhanced due to the quantum size effect, reaching 150 ns in cobalt nanoparticles. Here, we show that for extra electrons confined in a single ferromagnetic-metal MnAs nanoparticle embedded in a GaAs semiconductor matrix, the spin-relaxation time can reach 10 micros at 2 K, which is seven orders of magnitude longer than those of conventional metallic thin film or bulk systems, and the longest value ever reported for metallic nanoparticles. This long relaxation time is made possible by using epitaxially grown single-crystal devices with abrupt interfaces, and by avoiding surface contamination of the MnAs nanoparticle. Such a long spin-relaxation time can be very useful in nanoscale spintronic devices.