Erik Nielsen and Ravin Bhatt found that non-magnetic n-doped semiconductors at low dopant concentrations – whose electrons have strong antiferromagnetic interactions – can show significant ferromagnetism at the nanoscale if a small concentration of excess electrons is introduced. Interestingly, the researchers also saw that adding holes (removing electrons) did not lead to ferromagnetism at a similar scale. According to them, this effect can be understood through an analogy with doped magnetic semiconductors, such as
Ga1–xMnxAs, which are known to display macroscopic ferromagnetism. Moreover, it suggests that behaviour very different from that seen in bulk semiconductors is possible in nanoscale systems, like quantum dots.
Nielsen and Bhatt obtained their results using the Hubbard model, which can be applied to many electronic systems with strong electron–electron interactions. Examples include doped semiconductors and high-temperature superconductors. In particular, the researchers solved the Hubbard model using special "matrix diagonalization routines for sparse matrices" in their simulations.
"Such nanoscale-doped semiconductors may provide the first experimental evidence of a phenomenon first postulated over 40 years ago by physicist Yosuke Nagaoka," said Nielsen and Bhatt. "Additionally, the possibility of tailored high-spin ground states in nanoclusters, which have a much smaller spin-orbit coupling than other materials with similar high magnetic moment, may prove quite useful for applications like quantum computing," they told nanotechweb.org.
The team will now find out whether the ferromagnetic behaviour persists to larger length scales, or if it is a purely nanoscale effect. It will also study clusters based on various lattice structures to see how the spin state depends on geometry and size. "Given recent successes in fabricating made-to-order structures of phosphorus dopants in silicon by other research groups, such calculations are not simply theorists' models but are applicable to experimentally realizable structures and geometries," said the researchers.
The results were reported as a Rapid Communication in Physical Review B.