  We propose a reversible mechanism for switching Heisenberg-type exchange
interactions between deposited transition metal adatoms from ferromagnetic to
antiferromagnetic. Using first-principles calculations, we show that this
mechanism can be realized for cobalt atoms on the surface of black phosphorus
by making use of electrically-controlled orbital repopulation, as recently
demonstrated by scanning probe techniques [Nat. Commun. 9, 3904 (2018)]. We
find that field-induced repopulation not only affects the spin state, but also
causes considerable modification of exchange interaction between adatoms,
including its sign. Our model analysis demonstrates that variable
adatom-substrate hybridization is a key factor responsible for this
modification. We perform quantum simulations of inelastic tunneling
characteristics and discuss possible ways to verify the proposed mechanism
experimentally.
