  Intermodulation distortion (IMD) of microwave signals incident upon a
superconducting YBa2Cu3O7 thin film sample on a LaAlO3 substrate is shown to
relax as the sample magnetization also relaxes, revealing the signature of
fluxon decay in the microwave distortion. The sample contained a microchannel
of columnar defects that was introduced using heavy ion beam irradiation. After
exposure to and then removal of a low strength static magnetic field, IMD
relaxed due to thermal activation of fluxons over the surface barrier. This
process influenced second and third order IMD differently, visible in the
differing functional form of the IMD in time and the temperature dependence of
the relaxation function. The presence of columnar defects in the test sample
suppressed thermal activation by pinning of fluxons, notably with complete
suppression of relaxation in third order IMD at low temperatures. Higher order
IMD was found to relax with the same functional form as the second and third
order IMD of the same parity.
