Objective Femtosecond laser-induced periodic surface structure (LIPSS) is the most common surface morphology on almost any material after irradiation by a linearly polarized laser, which has been widely studied by researchers. However, research on inducing LIPSS on magnetic thin films using high femtosecond lasers is still relatively limited. The authors used femtosecond laser with central wavelength of 1 030 nm, pulse width of 300 fs, and repetition rate of 100 kHz to induce the formation of LIPSS on 100 nm nickel film. The optical microscope and scanning electron microscope images show that affected by the thermal effect of the high repetition femtosecond laser, the measured period of LIPSS stripes is about 989 nm. Through the analysis of experimental data measured by the X-ray diffractometer and SQUID, it was proved that the material atomic recombination caused by the formation process of LIPSS did not change the particle size or composition of the nickel film. The saturation magnetization of the laser-induced magnetic material is also basically consistent with the reference sample, but the coercivity changes significantly. The authors consider that it is due to the pinning effect generated by the little amount of anti-ferromagnetic nickel oxide particles splashed out during the processing.

Methods This study used a galvanometer-based femtosecond laser processing system (Fig.1) to produce two grating samples with different periods of LIPSS (Fig.2-3). The period was measured based on the diagrams of the scanning electron microscope (Fig.4), and the X-ray diffraction spectra of the samples were measured by XRD (Fig.5). The hysteresis loops were measured by SQUID (Fig.6, Tab.1), and the calibrated saturation magnetization was calculated (Tab.2).

Results and Discussions The SEM measurement shows that the period of LIPSS is about 989 nm, which is caused by the thermal effect of high repetition femtosecond laser. The XRD results show that the particle size and composition of the grating samples is basically the same with the reference sample. The SQUID data show that the saturation magnetization of the grating samples with period of 40 μm and 24 μm is 97% and 91% of the reference sample after calibration, respectively. The sample with period of 24 μm has a little amount of damage, so the ratio of the calibrated saturation magnetization should be slightly higher than the value of 91%. Therefore, the authors consider that the recombination of the atoms in LIPSS did not significantly alter the saturation magnetization of the material. The coercivity of the grating sample has undergone significant changes, which may be due to the pinning effect of anti-ferromagnetic nickel oxide nanoparticles splashed out during the laser-induced processing on ferromagnetic nickel materials.

Conclusions This article used high repetition femtosecond laser to induce LIPSS on the surface of nickel film. Its period is close to the center wavelength of the laser pulse under the influence of thermal effect brought by the high repetition femtosecond laser. The average value measured by SEM is 989 nm. The grating samples with different periods were fabricated by a galvanometer-based femtosecond laser scanning system. Through the analysis of X-ray diffraction spectra and hysteresis loops, the authors consider that although the process of generating LIPSS is a process of material recombination, it has no significant impact on the particle size, composition, and saturation magnetization of the material. However, this process significantly changes the coercivity of the material. On the one hand, it is due to the atomic recombination of the material in the LIPSS zone, resulting in a change in internal stress. On the other hand, it is due to the pinning effect of anti-ferromagnetic nickel oxide nanoparticles splashed out during the laser-induced processing on ferromagnetic nickel particles. The experimental results show that high repetition femtosecond laser can be a powerful tool for rapid processing of micro patterns on magnetic thin film.