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Abstract
Exchange-coupled thin films are promising candidates for use in the next generation of magnetic storage and logic devices. In this work, we present a trilayer system with particular emphasis on its static and dynamical roles of magnetization. Magnetization and magneto-optical measurements, as well as domain and domain-wall observations, are used to investigate sputtered films of the trilayer system Co/Ru/Co, where the two Co layers are exchange-coupled through a variable thickness Ru layer. The sign and magnitude of the interlayer exchange are tuned by the thickness of the Ru interlayer. The analyses of the in-plane hysteresis loops show that both ferromagnetic (FM) and antiferromagnetic (AFM) couplings are present, with the ratio of FM to AFM contributions decreasing as the Ru thickness increases. A model assuming a distribution of exchange coupling quantitatively describes the hysteresis loops. When the exchange is predominantly AFM, the magnetic-force microscopy (MFM) images show a stable antiphase domain structure with fairly immobile domain-walls. The domain-wall width undergoes a shrinking process with an applied magnetic field. Micromagnetic model calculations yield domain wall whose thickness decreases with the increasing magnetic field, in agreement with the experiments. In addition to these static measurements, magnetization dynamic precession measurements were carried out via a femtosecond-laser-based pump-probe system with direct optical excitation. Two precessional modes ("acoustic" and "optic") are observed. The precession frequencies of the modes are measured over a wide range of the applied field strength and angle and their behaviors are explained by using a two-layer Landau-Lifshitz-Gilbert (LLG) based model.
