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We have generated intense electromagnetic transients by femtosecond laser pulse illumination of ferromagnet/metal (F/M) nanobilayers, in the presence of an external magnetic field. Fourier analysis revealed that the frequency content of these transients extended up to ~5 THz. We have also observed that upon the increase of the magnetic field, the entire THz transient shifts towards earlier times by up to 110 fs. We ascribe this magnetically tunable onset-time shift to extra acceleration of photoelectrons induced due to the Lorenz force.

The investigation of magnetization dynamics in magnetic film is currently attracting significant attention [1]. This interest is driven by the requirement for understanding the physics and application of magnetic microwave devices. There has been an increasing research for the soft magnetic film with ultra high ferromagnetic resonance (FMR) frequencies in recent years [2], and the films in these results are mainly resonated based on the traditional FMR, which belongs to the acoustic mode. In the acoustic mode, the magnetizations of films show the in-plane uniform precession, and the resonance frequency of the film is determined by the in-plane anisotropy field, which can be included by the preparation methods [3]. However, the results of the optic mode FMR are rarely investigated. This is due to the low permeability of optic mode FMR, and it only observes in some particular structure films. The two observable FMR modes have mainly discovered in two kinds of magnetic thin films: exchange-coupling-biased multilayer films [4] and thin films with stripe domains [5]. This attributes to the moments of the optical mode for two systems precess an out-of-phase when compared with those of the acoustic mode [6, 7]. Significantly, FeNi film will arise from both the in-plane and out-of-plane magnetizations when the film thickness is above a critical film thickness, and an array of oscillating “up and down” magnetic domains forms, which are known as the stripe domain [7]. FeNi stripe domain structure film has been widely studied for its dynamic microwave magnetic properties [8–10], and the magnetic spectra exhibit multiple resonance modes, which are interpreted in terms of the magnetization configuration of stripe domains [8, 9]. In this work, FeNi films with different phases structure were prepared by electrodeposition method, the phase structures of films were controlled by the composition of the film. We focused the discussion on the influence of the phase structure on the acoustic and optical mode FMR of FeNi stripe domain films. The results indicated that the domain structures of films were changed by the phase structure of film, which depended on the composition of the film. The acoustic and optical mode FMR of films shows the similar laws in FeNi alloys but not in the pure Fe and Ni films. In addition, the magnetization dynamics of FeNi stripe domain films are further disscussed.