The High-Field, High-Frequency Permeability Of Narrow, Thin-Film Magnetic Stripes

We have constructed a high-frequency permeameter to measure the magnetic response of magnetic thin-film stripes to sinusoidally oscillating rf magnetic fields of up to 5 Oe over the frequency range from 300 kHz to 30 MHz. These fields simulate those used to drive magnetic heads allowing the magnetic dynamics during recording of the magnetic thin-films in a recording head to be studied in a much simpler geometry. Measurements were performed on single and multi-layer permalloy thin-films, photolithographically patterned into arrays of 3-mu-m to 1000-mu-m wide rectangles, 1 cm long. The low field-amplitude permeability of narrow unlaminated permalloy stripes is frequency independent except for eddy current losses as expected for magnetization rotation. At higher fields the magnetic response increases at frequencies below 10 MHz with increasing drive amplitude as domain wall motion becomes significant (as is seen in heads). Our results are consistent with the expected domain wall mobility (around 2 x 10(3) cm/(s-Oc) for 2-mu-m thick films). At still higher fields the permeability drops as the film saturates above H(c). Because of the high density of walls in microscopic structures, wall motion is the dominant flux conduction mechanism below 30 MHz at high drive fields. This means that wall oscillations (seen in dynamic-Kerr images of recording heads) determine the head response below 30 MHz during writing.