The generation of harmonics in a laser ablated YBCO film deposited on
a [100] MgO substrate is reported. Higher odd harmonics appeared when
the film was subjected to an ac field. The presence of a de field indu
ces only the second harmonic with a small value of slope of V-2-H-dc c
urve (delta V-2/delta H-dc) compared to bulk YBCO. The variation of th
e amplitude of third harmonic (V-3) with H-ac and temperature was stud
ied. These results are explained in terms of a critical state model. T
he observation of only a small amplitude of second harmonic (V/(2)) wi
th a small delta V-2/delta H-dc is explained in terms of a special kin
d of clean grain boundary present in YBCO laser ablated films on [100]
MgO.