  We used electron spin resonance (ESR) combined with scanning tunneling
microscopy (STM) to measure hydrogenated Ti (spin-1/2) atoms at low-symmetry
binding sites on MgO in vector magnetic fields. We found strongly anisotropic
g-values in all three spatial directions. Interestingly, the amplitude and
lineshape of the ESR signals are also strongly dependent on the angle of the
field. We conclude that the Ti spin is aligned along the magnetic field, while
the tip spin follows its strong magnetic anisotropy. Our results show the
interplay between the tip and surface spins in determining the ESR signals and
highlight the precision of ESR-STM to identify the single atom's spin states.
