Ferromagnetic excited-state Mn2+dimers in Zn1−xMnxSe quantum dots observed by time-resolved magnetophotoluminescence

Colloidal Mn${}^{2+}$-doped semiconductor nanocrystals are solution processable analogs of classic phosphor and diluted magnetic semiconductor materials with promising applications ranging from fluorescence microscopy to spintronic information processing. At doping levels of only a few cation mole percent, Mn${}^{2+}$ dimers form in appreciable concentration and cause shortened photoluminescence decay times and reduced luminescence circular polarization under applied magnetic fields. Here, we show that these differences allow the use of time-resolved magnetophotoluminescence measurements to investigate the magnetic properties of the luminescent dimer excited state in Zn${}_{1\ensuremath{-}x}$Mn${}_{x}$Se nanocrystals. These measurements reveal that Mn${}^{2+}$-Mn${}^{2+}$ dimers are coupled ferromagnetically in their luminescent excited state, in contrast with the antiferromagnetic coupling of their ground state. We find that Mn${}^{2+}$-Mn${}^{2+}$ dimers also luminesce with much purer circular polarization than Mn${}^{2+}$ monomers under applied magnetic fields. These results are explained well by perturbation theory and density functional theory analyses of the microscopic orbital exchange interactions within the photoexcited Mn${}^{2+}$-Mn${}^{2+}$ dimers. This discovery of photoswitchable dimer magnetism (from $S$ = 0 to $S$ = 4) with strong associated circularly polarized luminescence raises intriguing possibilities for optical spin manipulation in doped semiconductors.