Observation of the magnetic soft mode in (Cd,Mn)Te quantum wells using spin-flip Raman scattering

We report the experimental observation of a magnetic soft mode of collective spin excitations in (Cd,Mn)Te quantum wells, containing a 2D hole gas interacting with magnetic ions. We found that with increasing hole concentration the eigenfrequency of the paramagnetic spin resonance decreases, which is predicted to occur prior to the ferromagnetic phase transition. Such a magnetic soft mode is detected in spin-flip Raman-scattering experiments as a reduction in the Mn g factor. The results are described in terms of collective dynamics of strongly coupled spin systems with positive feedback. Based on this approach the evolution of such excitations with hole concentration is calculated.