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It is reported that focused-ion-beam (FIB) processing causes considerable grain growth in magnetic thin films, which may be a concern for the use of this technique in recording head pole trimming. Two magnetic thin films (NiFe and FeCoN) were used to examine FIB-induced grain growth. NiFe showed more significant grain growth with grain size increasing from ∼20 to ∼500 nm after FIB etching with a dose of 0.15 nC/μm2. The grain size in the FeCoN films increased from ∼20 to ∼100 nm at the same dose. It was found that the NiFe films fibbed with the same ion dose (0.15 nC/μm2) and different beam currents (99, 672, and 1500 pA) experienced similar grain growth, suggesting that the grain growth is driven by momentum transfer rather than local heating. Grains in the NiFe films often had an elongated shape rather than circular and many crystallographic twins were observed in the FIB images, and confirmed with transmission electron microscopy. These twins tended to be parallel to the fast axis of beam rastering, while the axis of elongation, if present, tended to be along slow axis of rastering. It was found that the ratio of pixel space to beam diameter controls this behavior. It was revealed that values of this ratio less than 0.5 resulted in elongated or “acicular” grains. For fibbed FeCoN films, no acicular grains or twins were observed. This is believed to be due to the difference in crystal structure and twinning planes of the face-centered-cubic and body-centered-cubic structures. Both fibbed films showed magnetic degradation in the form of reduced moment and/or increased coercivity, as measured by Kerr imaging. © 2002 American Institute of Physics.