  We present non-conventional electron spin resonance (ESR) experiments using
microfabricated superconducting waveguides. We show that a very broad frequency
range, from 0.5 to 40 GHz, becomes accessible with a high frequency accuracy of
less than 1 MHz at low temperatures down to 1.6 K and in fields up to 1.4 T.
This allows an accurate inspection of the ESR absorption position in the
frequency domain, in contrast to the more common observation in the field
domain. We demonstrate the applicability of frequency-swept ESR on Cr3+ atoms
in ruby as well as on organic radicals of the Nitronyl-nitroxide family (using
the 2-(4'-methoxyphenyl)-4,4,5,5-tetra-methylimidazoline-1-oxyl-3-oxide, a.k.a.
NitPhOMe). Measurements between 1.6 and 30 K reveal a small frequency shift of
the ESR and a resonance broadening below the critical temperature, which we
both attribute to a modification of the magnetic field configuration due to the
appearance of shielding currents.
