| With the development of high frequency micro-electronic devices, such as micro-inductors, micro-transformers and the core material of write elements in modern recording heads, the high frequency soft magnetic thin films which were used in these devices become a hot issue. The high-frequency application of soft magnetic thin film needs a high saturation magnetization (4πMs) and an appropriately large anisotropy field (HK) to achieve the high cut-off frequency, which is the highest workable frequency of soft magnetic materials and usually proportional to the square root of (47πMs+HK)HK. Recently, the amorphous magnetic thin films comprising of rare-earth elements and 3d-transition metals with large in-plane anisotropy constant are interested in the high frequency application. These magnetic films with amorphous state exhibite an in-plane uniaxial anisotropy with the easy axis parallel to the direction of the applied field during deposition. Anisotropy in these materials is closely related to their preparation conditions and must be due to deviation from isotropy in the local arrangements of atoms and the interaction between an orbital moment of the localized 4f-electron of rare-earth elements and the related 3d-electron of transition metals via spin-orbit coupling. In this work, we investigate the dependence of the static and dynamic magnetic properties and microstructures of FeCoSm thin films on Sm content (x), the thickness (t), and annealing temperature (Ta). The main results are shown as fellows.(1)) The samples have different Sm content with the same thickness are investigated. The structure of the sample with different Sm content from x=0 to 25 at % changes from bcc to amorphous. The grain size decreased with the Sm content (x) increase and when x>13.2 (at%) the FeCoSm samples become amorphous and have the best soft magnetic properties, for example, the small coercivity and lage anisotropy field, which could be explained by the exchange coupling model. The origin of the large in-plane uniaxial anisotropy field observed in FeCo based alloy films may be explained by the interaction between an orbital moment of the localized 4f electrons of Sm and related 3d electrons of transition metals (Fe&Co) via spin-orbit coupling, since the magneto-crystalline anisotropy can be neglected due to the amorphous state of the as prepared FeCoSm film.(2) The thickness dependence of the microstructure and soft magnetic properties of the FeCoSm films was investigated. As the thickness increase, the microstructure changes from amorphous to nanocrystalline, while the coercivity increases and the anisotropy field decreases, indicating the degradation of the soft magnetic properties, which can be ascribed to the thermal effect since there is no cooling system for substrate during deposition. On the contrary, when the film thickness is thiner than tc, the sample is amorphous and with good soft magnetic properties.(3) In order to study the thermal stability, a 5 minutes vacuum magnetic annealing treatment was executed with different temperatures and a 1500 Oe magnetic field, which was in the same direction of the induced applied magnetic field. When Ta<350℃, the samples are amorphous and have good soft magnetic properties, when Ta≥350℃, the samples became partial crystallized, which results in the coercivity increase and the anisotropy decrease with the annealing temperature increase. The anisotropy disappears when Ta≥500℃. The dynamic magnetic properties of FeCoSm films have been investigated in the frequency range of 0.1-9 GHz, through the one terminal micro-strip transmission-line perturbation method and analyzed by Landau-Lifshitz-Gilbert equation. The anisotropy field and the resonance frequency of FeCoSm films can be tuned in the range of 50-1200 Oe and 1.8-12.1 GHz, respectively, by controlling the film thickness or annealing temperature, indicating that FeCoSm films have much potential on high frequency application.
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