  In this work, we demonstrate the use of frequency-tunable superconducting
NbTiN coplanar waveguide microresonators for multi-frequency pulsed electron
spin resonance (ESR) experiments. By applying a bias current to the center pin,
the resonance frequency ($\sim$7.6 GHz) can be continuously tuned by as much as
95 MHz in 270 ns without a change in the quality factor of 3000 at 2K. We
demonstrate the ESR performance of our resonators by measuring donor spin
ensembles in silicon and show that adiabatic pulses can be used to overcome
magnetic field inhomogeneities and microwave power limitations due to the
applied bias current. We take advantage of the rapid tunability of these
resonators to manipulate both phosphorus and arsenic spins in a single pulse
sequence, demonstrating pulsed double electron-electron resonance (DEER). Our
NbTiN resonator design is useful for multi-frequency pulsed ESR and should also
have applications in experiments where spin ensembles are used as quantum
memories.
