  In semiconductors, quantum confinement can greatly enhance the interaction
between band carriers (electrons and holes) and dopant atoms. One manifestation
of this enhancement is the increased stability of exciton magnetic polarons in
magnetically-doped nanostructures. In the limit of very strong 0D confinement
that is realized in colloidal semiconductor nanocrystals, a single exciton can
exert an effective exchange field $B_{\rm{ex}}$ on the embedded magnetic
dopants that exceeds several tesla. Here we use the very sensitive method of
resonant photoluminescence (PL) to directly measure the presence and properties
of exciton magnetic polarons in colloidal Cd$_{1-x}$Mn$_x$Se nanocrystals.
Despite small Mn$^{2+}$ concentrations ($x$=0.4-1.6\%), large polaron binding
energies up to $\sim$26~meV are observed at low temperatures via the
substantial Stokes shift between the pump laser and the resonant PL maximum,
indicating nearly complete alignment of all Mn$^{2+}$ spins by $B_{\rm{ex}}$.
Temperature and magnetic field-dependent studies reveal that $B_{\rm{ex}}
\approx$ 10~T in these nanocrystals, in good agreement with theoretical
estimates. Further, the emission linewidths provide direct insight into the
statistical fluctuations of the Mn$^{2+}$ spins. These resonant PL studies
provide detailed insight into collective magnetic phenomena, especially in
lightly-doped nanocrystals where conventional techniques such as nonresonant PL
or time-resolved PL provide ambiguous results.
