  Coherent control of individual atomic and molecular spins on surfaces has
recently been demonstrated by using electron spin resonance (ESR) in a scanning
tunneling microscope (STM). Here we present a combined experimental and
modeling study of the ESR of a single hydrogenated Ti atom that is
exchange-coupled to a Fe adatom located in 0.6-0.8 nm away. Continuous wave and
pulsed ESR of the Ti spin showed a Rabi rate with two contributions, one from
the tip and the other from the Fe, whose spin interactions with Ti were
modulated by the radio-frequency electric field. The Fe contribution is
comparable to the tip, as revealed from its dominance when the tip was
retracted, and tunable using a vector magnetic field. Our new ESR scheme allows
on-surface individual spins to be addressed and coherently controlled without
the need for magnetic interaction with a tip. This study establishes a feasible
implementation of spin-based multi-qubit systems on surfaces.
