  We demonstrated the control of ferromagnetism in a surface quantum well
containing a 5-nm-thick n-type ferromagnetic semiconductor (In,Fe)As layer
sandwiched between two InAs layers, by manipulating the carrier wavefunction.
The Curie temperature (Tc) of the (In,Fe)As layer was effectively changed by up
to 12 K ({\Delta}Tc/Tc = 55%). Our calculation using the mean-field Zener
theory reveals an unexpectedly large s-d exchange interaction in (In,Fe)As. Our
results establish an effective way to control the ferromagnetism in quantum
heterostructures of n-type FMSs, as well as require reconsideration on the
current understanding of the s-d exchange interaction in narrow gap FMSs.
