  Free electron laser-powered pulsed electron paramagnetic resonance (EPR)
experiments performed at 240 GHz / 8.56 T on the crystalline organic radical
BDPA reveal a tip-angle dependent resonant frequency. Frequency shifts as large
as 11 MHz (45 ppm) are observed during a single Rabi oscillation. We attribute
the frequency shifts to a "dressing" of the nutation by spin-spin interactions.
A nonlinear semi-classical model which includes a temperature- and
sample-geometry-dependent demagnetizing field reproduces experimental results.
Because experiments are performed without a cavity, radiation damping -- the
most common nonlinear interaction in magnetic resonance -- is negligible in our
experiments.
