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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 Mn2+ ions. Injected electrons activate new ferromagnetic Mn2+–Mn2+ 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–Mn2+ exchange energies, because of competing electron-mediated ferromagnetic interactions involving distant Mn2+ ions in the same nanocrystal.

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Figure 1: Structural characterization and charge-induced magnetic effects at 298 K and 2 K.
Figure 2: Reversibility and Mn2+ concentration dependence of charge-controlled magnetism.
Figure 3: Influence of Jfar on dimer magnetization.
Figure 4: Density functional theory results.
Figure 5: Colloidal analogues to bound magnetic polarons.

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Acknowledgements

This work was supported by the US National Science Foundation (CHE 0628252-CRC to D.G. and X.L., DGE-0504573 (IGERT) to K.W.). Additional support from Gaussian Inc, the Research Corporation (Cottrell Scholar, D.G.), the Dreyfus Foundation (Teacher/Scholar, D.G.) and the University of Washington is gratefully acknowledged. E.B. thanks the Center for Nanotechnology at the University of Washington for UIF fellowship support. S.O. was supported by a fellowship for prospective researchers by the Swiss National Science Foundation (contract no. PBBE2-115064). EPR instrumentation support from the Center for Ecogenetics and Environmental Health UW Center grant no. P30 ES07033 from the National Institutes of Environmental Health Sciences, NIH, is gratefully acknowledged. TEM data were collected at Environmental Molecular Science Laboratory (PNNL), a national scientific user facility sponsored by the Department of Energy, and C. Wang (PNNL) is thanked for valuable assistance with this instrument.

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S.O., K.W. and W.L. performed the experiments. Y.F. and E.B. performed the DFT calculations. All authors discussed the experimental and computational results and analysis. S.O, Y.F., K.W., E.B., W.L., X.L. and D.G. co-wrote the paper.

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Correspondence to Daniel R. Gamelin.

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Ochsenbein, S., Feng, Y., Whitaker, K. et al. Charge-controlled magnetism in colloidal doped semiconductor nanocrystals. Nature Nanotech 4, 681–687 (2009). https://doi.org/10.1038/nnano.2009.221

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