Magnetic properties of (Ga,Mn)As digital ferromagnetic heterostructures

Magnetic properties of (Ga,Mn)As digital ferromagnetic heterostructures have been investigated by polarized neutron reflectometry and magnetometry. T-c of three samples with 20, 50, and 100 ML GaAs spacers ranges from 30 to 40 K. The saturation magnetization of three samples exhibits a pronounced tail extending over 50 K above T-c in addition to a temperature-independent background. For the 50 ML sample, PNR measurements show a similar tail but no background. These behaviors can be explained by a two-step ordering process. In the tail region, two-dimensional islands first individually become ferromagnetic. Long-range order develops as the temperature is decreased below T-c. (C) 2004 American Institute of Physics.