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

Effective magnetic fields as high as 30 T can arise in magnetically doped nanocrystals due to spin fluctuations alone, and can now be observed using ultrafast optical spectroscopy. 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 Mn 2+ , Co 2+ and so on) couple to band carriers via strong sp–d spin exchange 1 , 2 , giant magneto-optical effects can therefore be realized in confined geometries using few 3 , 4 , 5 , 6 , 7 or even single 8 , 9 impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit 10 . In nanoscale volumes, the statistical fluctuations of N spins are expected to generate giant effective magnetic fields B eff , which should dramatically impact carrier spin dynamics, even in the absence of any applied field. Here we directly and unambiguously reveal the large B eff that exist in Mn 2+ -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 B eff ∼ 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 Mn 2+ moments, thereby revealing the initial coherent dynamics of magnetic polaron formation, and highlighting the importance of magnetization fluctuations on carrier spin dynamics in nanomaterials.