  We investigate theoretically the effect of nearby As (arsenic) vacancies on
the magnetic properties of substitutional Mn (manganese) impurities on the GaAs
(110) surface, using a microscopic tight-binding model which captures the
salient features of the electronic structure of both types of defects in GaAs.
The calculations show that the binding energy of the Mn-acceptor is essentially
unaffected by the presence of a neutral As vacancy, even at the shortest
possible ${\rm V}_{\rm As}$--Mn separation. On the other hand, in contrast to a
simple tip-induced-band-bending theory and in agreement with experiment, for a
positively charged As vacancy the Mn-acceptor binding energy is significantly
reduced as the As vacancy is brought closer to the Mn impurity. For two Mn
impurities aligned ferromagnetically, we find that nearby charged As vacancies
enhance the energy level splitting of the associated coupled acceptor levels,
leading to an increase of the effective exchange interaction. Neutral vacancies
leave the exchange splitting unchanged. Since it is experimentally possible to
switch reversibly between the two charge states of the vacancy, such a local
electric manipulation of the magnetic dopants could result in an efficient
real-time control of their exchange interaction.
