We have developed a new femtosecond-resolved optical technique with wh
ich one may examine magnetic spin dynamics in near-atomic scale struct
ures by employing a highly sensitive, ultrafast measurement of the Far
aday rotation. We apply this spectroscopy to novel band-gap-engineered
II-VI diluted magnetic semiconductor heterostructures of two types to
demonstrate the ability to simultaneously monitor electronic and magn
etic interactions in a quantum geometry. The experiments show that the
se dynamics evolve on widely different time scales, and reveal the ons
et and decay of magnetization due to carrier spin scattering with femt
osecond temporal resolution (300 fs). Although photoexcited carriers r
ecombine within several hundred picoseconds, they leave behind a magne
tic ''footprint'' that persists considerably longer and relaxes throug
h an entirely different spin-lattice mechanism. Time-resolved magnetic
measurements reveal the unusual dynamical properties of low-dimension
al systems incorporating magnetic spins as compared to traditional sem
iconductor heterostructures.