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The magnetic properties of NiFe and FeCoN thin films were examined using Kerr microscopy after Ga+ focused-ion-beam (FIB) irradiation. Magneto-optic characterizations of 30 μm × 30 μm irradiated surface areas showed that the magnetization was reduced in approximately linear proportion to the Ga+ ion dose. Above an ion dose of approximately 0.6 nC/μm2 (3.8 × 1017 ions/cm2), little sign of the Kerr rotation could be found. The effect of the irradiation on the coercive force was also examined. Little effect of the beam on the coercivity of NiFe was seen, but an increase of coercivity was seen in FeCoN. The reduction of the Kerr rotation is attributed to magnetic moment loss caused by Ga implantation into the magnetic thin films. This loss has been quantified and is relevant to pole trimming for high-density magnetic recording heads.