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Revealing giant internal magnetic fields due to spin fluctuations in magnetically doped colloidal nanocrystals


Strong quantum confinement in semiconductors can compress the wavefunctions of band electrons and holes to nanometre-scale volumes, significantly enhancing interactions between themselves and individual dopants. In magnetically doped semiconductors, where paramagnetic dopants (such as Mn2+, Co2+ and so on) couple to band carriers via strong sp–d spin exchange1,2, giant magneto-optical effects can therefore be realized in confined geometries using few3,4,5,6,7 or even single8,9 impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit10. In nanoscale volumes, the statistical fluctuations of N spins are expected to generate giant effective magnetic fields Beff, which should dramatically impact carrier spin dynamics, even in the absence of any applied field. Here we directly and unambiguously reveal the large Beff that exist in Mn2+-doped CdSe colloidal nanocrystals using ultrafast optical spectroscopy. At zero applied magnetic field, extremely rapid (300–600 GHz) spin precession of photoinjected electrons is observed, indicating Beff 15 −30 T for electrons. Precession frequencies exceed 2 THz in applied magnetic fields. These signals arise from electron precession about the random fields due to statistically incomplete cancellation of the embedded Mn2+ moments, thereby revealing the initial coherent dynamics of magnetic polaron formation, and highlighting the importance of magnetization fluctuations on carrier spin dynamics in nanomaterials.

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Figure 1: Quantifying the large sp–d spin interactions in Mn2+-doped CdSe nanocrystals using MCD spectroscopy.
Figure 2: Ultrafast electron spin dynamics and precession at B = 0 in Mn2+-doped CdSe nanocrystals.
Figure 3: Modeling magnetization fluctuations and ultrafast electron spin dynamics in Mn2+-doped NC ensembles at B = 0.
Figure 4: Ultrafast electron spin dynamics in Mn2+-doped NCs in applied transverse magnetic fields.
Figure 5: Pump-induced coherent transfer of angular momentum to the Mn2+ spins.

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We gratefully thank D. R. Yakovlev and D. L. Smith for helpful discussions and insight. W.D.R. acknowledges support from the Los Alamos LDRD programme. W.L., T.A.B., and V.I.K. are supported by the Office of Chemical Sciences, Biosciences, and Geosciences of the Department of Energy Office of Basic Energy Sciences. All optical measurements were performed at the National High Magnetic Field Laboratory, which is supported by NSF DMR-1157490.

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S.A.C. and V.I.K. conceived and directed the experiments. W.D.R. built and performed the optical experiments, and designed the numerical simulations. W.L. synthesized the nanocrystals, and T.A.B. made the films. N.S. provided theoretical insights. S.A.C. and W.D.R. analyzed the data and wrote the paper in close consultation with all authors.

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Correspondence to Victor I. Klimov or Scott A. Crooker.

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The authors declare no competing financial interests.

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Rice, W., Liu, W., Baker, T. et al. Revealing giant internal magnetic fields due to spin fluctuations in magnetically doped colloidal nanocrystals. Nature Nanotech 11, 137–142 (2016).

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