Clustering of Cr in GaN nanotubes and the onset of ferrimagnetic order

Abstract

A comprehensive theoretical study of the structure and magnetic properties of Cr-doped GaN nanotubes yields some interesting and unexpected results: (1) A single wall GaN nanotube constructed from the GaN wurtzite crystal relaxes to a carbon-like zigzag SWNT structure and remains stable at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, while a multiwall GaN nanotube retains its original wurtzite form. (2) Cr atoms prefer to form clusters and the underlying magnetism depends on the degree of clustering. (3) The coupling between two Cr atoms mediated by the neighboring N is ferromagnetic, but changes to ferrimagnetic as the cluster grows. These results are based on spin polarized density functional theory with exchange and correlation potential approximated by both the generalized gradient approximation and the local spin density approximation $+U$ methods.