  Electron-spin resonance carried out with scanning tunneling microscopes
(ESR-STM) is a recently developed experimental technique that is attracting
enormous interest on account of its potential to carry out single-spin
on-surface resonance with subatomic resolution. Here we carry out a theoretical
study of the role of tip-adatom interactions and provide guidelines for
choosing the experimental parameters in order to optimize spin resonance
measurements. We consider the case of the Fe adatom on a MgO surface and its
interaction with the spin-polarized STM tip. We address three problems: first,
how to optimize the tip-sample distance to cancel the effective magnetic field
created by the tip on the surface spin, in order to carry out proper magnetic
field sensing. Second, how to reduce the voltage dependence of the surface-spin
resonant frequency, in order to minimize tip-induced decoherence due to voltage
noise. Third, we propose an experimental protocol to infer the detuning angle
between the applied field and the tip magnetization, which plays a crucial role
in the modeling of the experimental results.
