  Voltage control of ferromagnetism on the nanometer scale is highly appealing
for the development of novel electronic devices. Here a key challenge is to
implement and combine low power consumption, high operation speed, reliable
reversibility and compatibility with semiconductor technology. Hybrid
structures based on the assembly of ferromagnetic and semiconducting building
blocks are attractive candidates in that respect as such systems bring together
the properties of the isolated constituents: They are expected to show magnetic
order as a ferromagnet and to be electrically tunable as a semiconductor. Here
we demonstrate the electrical control of the exchange coupling in a hybrid
consisting of a ferromagnetic Co layer and a semiconductor CdTe quantum well,
separated by a thin non-magnetic (Cd,Mg)Te barrier. The effective magnetic
field of the exchange interaction reaches up to 2.5 Tesla and can be turned on
and off by application of 1 V bias across the heterostructure. The mechanism of
this electric field control is essentially different from the conventional
concept, in which wavefunctions are spatially redistributed to vary the
exchange interaction, requiring high field strengths. Here we address instead
control of the novel exchange mechanism that is mediated by elliptically
polarized phonons emitted from the ferromagnet, i.e. the phononic ac Stark
effect. An essential parameter of this coupling is the splitting between heavy
and light hole states in the quantum well which can be varied by the electric
field induced band bending. Thereby the splitting can be tuned with respect to
the magnon-phonon resonance energy in the ferromagnet, leading to maximum
coupling for flat band conditions. Our results demonstrate the feasibility of
electrically controlled exchange coupling in hybrid semiconductor
nanostructures at quite moderate electric field strengths.
