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A magneto-optic photometer system has been developed to study the magnetic characteristics of thin-film magnetic recording heads at frequencies up to 50 MHz, on spot sizes as small as 0.5 μm in diameter. The photometer system uses an argon laser as the light source to a Leitz MPV3 polarizing microscope. The changing magnetization of the head is detected by using the longitudinal Kerr magneto-optic effect. The signal is recovered with a photomultiplier tube and a lock-in amplifier detection scheme. Completed IBM 3380 thin-film heads have been analyzed by using the photometer system. The local frequency responses on the head have been measured while the head is being driven by its own coils. Different responses have been observed on different positions on the head. The differences in response are interpreted in terms of the domain structure of the yoke.
Re et al. (1984) studied this question.