  Magnetic media remain a key in information storage and processing. The
continuous increase of storage densities and the desire for quantum memories
and computers pushes the limits of magnetic characterisation techniques.
Ultimately, a tool which is capable of coherently manipulating and detecting
individual quantum spins is needed. The scanning tunnelling microscope (STM) is
the only technique which unites the prerequisites of high spatial and energy
resolution, low temperature and high magnetic fields to achieve this goal.
Limitations in the available frequency range for electron spin resonance STM
(ESR-STM) mean that many instruments operate in the thermal noise regime. We
resolve challenges in signal delivery to extend the operational frequency range
of ESR-STM by more than a factor of two and up to 100GHz, making the Zeeman
energy the dominant energy scale at achievable cryogenic temperatures of a few
hundred millikelvin. We present a general method for augmenting existing
instruments into ESR-STMs to investigate spin dynamics in the high-field limit.
We demonstrate the performance of the instrument by analysing inelastic
tunnelling in a junction driven by a microwave signal and provide proof of
principle measurements for ESR-STM.
