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The effect of swift heavy ion irradiation on ferromagnetic metallic glasses Fe 40 Ni 38 Mo 4 B 18 and Fe 78 Si 9 B 13 has been studied. The ion beams used are 100 MeV 127 I and 180 MeV 197 Au. The specimens were irradiated at fluences ranging from 3 × 10 12 to 1.5 × 10 14 ions/cm 2 . The irradiations have been carried out at temperatures 100 and 300 K. The magnetic moments are sensitive towards the irradiation conditions such as irradiation temperature and stopping power of incident ion beam. The irradiation-induced effects have been monitored, by using Mössbauer spectroscopy. The modifications in magnetic anisotropy and hyperfine magnetic field distributions, as an effect of different irradiation temperature as well as different stopping power have been discussed. After irradiation, all the samples remain amorphous and magnetic anisotropy considerably changes from its original in-plane direction. The results show enhancement in magnetic anisotropy in the specimen irradiated at 100 K, as compared to that of irradiated at 300 K. It is expected that at low temperature, the stresses produced in the material would remain un-annealed, compared to the samples irradiated at room temperature and therefore, the modification in magnetic anisotropy would be enhanced. A distribution of hyperfine magnetic field, of the samples irradiated at low temperature, show a small but distinct peak at ∼ 11 Tesla, indicating Fe-B pairing.Keywords Swift heavy ion irradiation · Ferromagnetic metallic glass · Magnetic anisotropy · Mössbauer spectroscopy PACS 61.80-x · 75.30.Gw · 75.50.Kj · 76.80+y

Heavy ion irradiation in the electronic stopping power region induces macroscopic dimensional change in metallic glasses and introduces magnetic anisotropy in some magnetic materials. The present work is on the irradiation study of ferromagnetic metallic glasses, where both dimensional change and modification of magnetic anisotropy are expected. Magnetic anisotropy was measured using Mössbauer spectroscopy of virgin and irradiated Fe 40 Ni 40 B 20 and Fe 40 Ni 38 Mo 4 B 18 metallic glass ribbons. 90 MeV 127 I beam was used for the irradiations. Irradiation doses were 5×10 13 and 7.5×10 13 ions/cm 2 . The relative intensity ratios D 23 of the second and third lines of the Mössbauer spectra were measured to determine the magnetic anisotropy. The virgin samples of both the materials display in-plane magnetic anisotropy, i.e., the spins are oriented parallel to the ribbon plane. Irradiation is found to cause reduction in magnetic anisotropy. Near-complete randomization of magnetic moments is observed at high irradiation doses. Correlation is found between the residual stresses introduced by ion irradiation and the change in magnetic anisotropy.

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