Charge-controlled magnetism in colloidal doped semiconductor nanocrystals

Abstract

Electrical control over the magnetic states of doped semiconductor nanostructures could enable new spin-based information processing technologies. To this end, extensive research has recently been devoted to examination of carrier-mediated magnetic ordering effects in substrate-supported quantum dots at cryogenic temperatures, with carriers introduced transiently by photon absorption. The relatively weak interactions found between dopants and charge carriers have suggested that gated magnetism in quantum dots will be limited to cryogenic temperatures. Here, we report the observation of a large, reversible, room-temperature magnetic response to charge state in free-standing colloidal ZnO nanocrystals doped with Mn(2+) ions. Injected electrons activate new ferromagnetic Mn(2+)-Mn(2+) interactions that are strong enough to overcome antiferromagnetic coupling between nearest-neighbour dopants, making the full magnetic moments of all dopants observable. Analysis shows that this large effect occurs in spite of small pairwise electron-Mn(2+) exchange energies, because of competing electron-mediated ferromagnetic interactions involving distant Mn(2+) ions in the same nanocrystal.

Orbital pathways for Mn2+-carriersp−dexchange in diluted magnetic semiconductor quantum dots

“…4(b) and related text). 48 This conclusion contrasts sharply with the trends predicted from the k·p approach 36,37 and claimed in some experimental reports, 36,37,[41][42][43][44][45] but is consistent with the trends predicted from tight-binding calculations, 46 previous DFT calculations, 31,51 and other experimental results. 47 In summary, these DFT and perturbation theory results predict that bulk-like kinetic s-s exchange dominates over kinetic s-d exchange in Cd 1−x Mn x Se, even in QDs that are more strongly quantum confined than can be achieved experimentally.…”

“…[24][25][26] DMSs quantum dots may enable versatile control of magnetism by means unavailable in their bulk counterparts, for example through lattice deformations (piezomagnetism), 27 exchange interactions involving closed-shell QD configurations, 28 and optical or electrical gating. [29][30][31] All of the potential spin-electronic and spin-photonic applications of DMSs are ultimately determined by their microscopic dopant-carrier (sp-d) magnetic exchange interactions. Most analyses of dopant-carrier magnetic exchange in quantum-confined DMSs have relied on methods developed to describe the corresponding bulk materials.…”

Recent Advances in Magnetic Ion-Doped Semiconductor Quantum Dots

“…For example, Zheng and Strouse 60 have shown carrier-mediated ferromagnetic interaction in Mn-doped CdSe QDs arising from photoexcited carriers from surface defect states of ultra small (<3 nm) QDs. Similarly, conduction band electron generated during photoexcitation is also shown to exhibit ferromagnetic exchange interactions in Mn-doped ZnO QDs under anaerobic conditions 61 54 . These results confirm conduction band electron-dopant ferromagnetic exchange interaction, which can lead to magneto-electric and magneto-plasmonic properties.…”

Electrical control of magnetic quantum wells: Monte Carlo simulations

“…[52][53][54][55][56][57][58][59] Analogous to our considerations of changing magnetization at a constant total carrier density, in magnetic quantum dots it may also be possible to realize gate-controlled change of magnetization at a fixed number of carriers.…”