  Spin resonance of single spin centers bears great potential for chemical
structure analysis, quantum sensing and quantum coherent manipulation.
Essential for these experiments is the presence of a two-level spin system
whose energy splitting can be chosen by applying a magnetic field. In recent
years, a combination of electron spin resonance (ESR) and scanning tunneling
microscopy (STM) has been demonstrated as a technique to detect magnetic
properties of single atoms on surfaces and to achieve sub-${\mu}$eV energy
resolution. Nevertheless, up to now the role of the required magnetic fields
has not been elucidated. Here, we perform single-atom ESR on individual Fe
atoms adsorbed on magnesium oxide (MgO), using a 2D vector magnetic field as
well as the local field of the magnetic STM tip in a commercially available
STM. We show how the ESR amplitude can be greatly improved by optimizing the
magnetic fields, revealing in particular an enhanced signal at large in-plane
magnetic fields. Moreover, we demonstrate that the stray field from the
magnetic STM tip is a versatile tool. We use it here to drive the electron spin
more efficiently and to perform ESR measurements at constant frequency by
employing tip-field sweeps. Lastly, we show that it is possible to perform ESR
using only the tip field, under zero external magnetic field, which promises to
make this technique available in many existing STM systems.
