Structure And Magnetic Properties Of Gd-doped FeCo Thin Films

With the rapid development of electronic devices, the requirements for high frequency soft magnetic thin films are more demanding. The high-frequency application of soft magnetic thin film needs a higher saturation magnetization (Ms), permeability (μ), the resistivity (ρ) and appropriately magnetic anisotropy field (Hk), so that the material has a higher cut-off frequency. In order to meet the requirements of high-frequency magnetic thin film device, the materials with the resonance frequency in the GHz or above have become research hotspots in this field. FeCo soft magnetic thin films have a high magnetic permeability and high saturation magnetization, which become a new focus of research. Many studies have shown that the amount of doped rare earth elements can increase magnetic permeability and the resonant frequency. Therefore, it will increase the high frequency soft magnetic properties of the material.In this work, we prepared a series of Cu/FeCoGd/Ta films on glass substrates by changing the preparation process. The crystal structure of as-deposited FeCoGd thin films was investigated by X-ray diffraction. Energy dispersive spectroscopy (EDS) was used to determine the composition of the films. Surface topographies were observed with an atomic force microscope (AFM), and the surface domain structure was studied using magnetic force microscopy (MFM) with soft magnetic tips magnetized perpendicular to the sample plane. The static magnetic properties of FeCoGd films were measured by vibrating sample magnetometer (VSM) at various temperature. The high-frequency characteristics of the samples were obtained by using the shorted microstrip transmission-line perturbation method. The following conclusions were reached:(1) For the Cu/FeCoGd/Ta films with the same thickness, an increasing trend of grain size of FeCo and a declining trend of Gd content with the increasing sputtering power was shown. It indicated that Gd-doping can refine grain of FeCo. When sputtering power is below30W, the films exhibited obviously in-plane uniaxial magnetic anisotropy, and the in-plane magnetic anisotropy field Hk decreased with increasing deposition power. Futhermore, the in-plane uniaxial magnetic anisotropy of the sample gradually decrese and eventually become a perpendicular magnetic anisotropy when the deposition power is larger than30W. When the Gd content is very close to the magnetic compensation point of the FeCoGd film, the sample will have a very small saturation magnetization.(2) By studying the magnetization reversal mechanism of representative sample which has a good in-plane uniaxial anisotropy, the angular dependence of coercivity indicated that the magnetization reversal mechanism of FeCoGd film is domain-wall depinning and coherent rotation when the applied field is close to the easy axis and hard axis, respectively.(3) From soft magnetic thin film with large anisotropy field and high saturation magnetization can obtain a good high frequency performance. In this work, the FeCoGd films with in-plane uniaxial anisotropy have a large in-plane magnetic anisotropy field Hk and a high saturation magnetization, so that the films have a higher cut-off frequency. It may have potential for high frequency application.(4) The magnetic properties of FeCoGd films with different sputtering powers are different at low temperature. Only the FeCoGd film with sputtering power of1OW appears spin reorientation phenomenon from in-plane to out-of-plane anisotropy at low temperature. The reason is this sample is close to the magnetic compensation point, and its saturation magnetization is very small, corresponding to a low demagnetic field at the perpendicular direction of film of this sample. As the temperature decreases, the saturation magnetization and the demagnetizing field all decrease, so that the sample will appear spin reorientation phenomenon at low temperature.(5) The magnetic properties of FeCoGd films are strongly dependent upon the thickness of films. When the thickness is higher than a critical thickness tc, a stripe domain pattern will be observed due to the existence of weak perpendicular magnetic anisotropy. The occurrence of perpendicular anisotropy is suggested to originate from columnar structure with columns perpendicular to the film plane. On the contrary, reducing the thickness below tc, a well in-plane uniaxial magnetic anisotropy behavior and favorable soft magnetic properties are achieved.