A new heterostructure is developed in which fractional monolayers of m
agnetic ions are introduced ''digitally'' within a semiconductor quant
um well. Rearranging moments within two-dimensional (2D) planes provid
es additional control over their interactions, yielding tunable two-le
vel electronic systems in which spin splittings are significantly larg
er than inhomogeneous linewidths in modest magnetic fields. Femtosecon
d-resolved electronic and magnetic spectroscopies reveal spin-flip sca
ttering dependent only on the Zeeman energy but not the local magnetic
environment, and long-lived dynamic magnetizations.