The magnetic domain structures of NiFe films in thin film heads driven by dc and ac currents were investigated using spin-polarized SEM, in order to study the magnetization process under high-frequency magnetic fields. The image contrast under a high-frequency magnetic field was calculated as the time-averaged component of the magnetization along the detection direction due to magnetization rotation and to reversible domain wall movement. From the relation between the observed domain images and the calculated distribution of the average magnetization about the domain wall, it was found that irreversible domain wall movement occurs throughout the magnetic core under high-frequency excitations. Domain walls move slightly and reversibly until the direction of the magnetic field is reversed under high-frequency excitation.
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