| Recently, the high-frequency magnetic properties of soft magnetic films have attracted much attention for their applications in electromagnetic devices such as microwave absorbers, micro-inductor and magnetic head. In these high-frequency applications, the operating frequencies have approached gigahertz (GHz) range, which requires the magnetic materials to possess high ferromagnetic resonance frequency fr as well as high initial permeability (ui). In previous widely studied Fe-and Co-based soft magnetic films, although they have in-plane uniaxial anisotropy (Hu) and high saturation magnetization (Ms), their out-of-plane anisotropy fields only come from the demagnetization field. Therefore, their (μi-1)fr will be restricted by Acher’s limit, hence the increasing of fr inevitably results in the decrease of μi. If the negative magnetocrystalline anisotropy (Kg) is introduced, the natural resonance frequency of films can be improved without affecting the initial permeability.In this work, in order to understand the orign of negative uniaxial magnetocrystalline anisotropy of the hcp-Co based soft magnetic materials, we have investigated their magnetocrystalline anisotropy by first-principles calculation. Then we have prepared the c-axis oriented hcp-CoIr soft magnetic films with negative magnetocrystalline anisotropy through sputtering, and investigated the component, thickness, temperature and sputtering condition dependence of their high-frequency magnetic properties. The main results are listed as follows:(1) First-principles calculations indicate that the spin reorientation happened in both Ni or Ir doped hcp-Co alloys:the easy axis changes from c-axis to c-plane and then truns back with increasing Ni or Ir concent. Compared to CoNi alloy, the CoIr alloy has quite large negative magnetocrystalline anisotropy (MA) energy in a large range of Ir content, hence it is a good object to study high-frequency magnetic properties.(2) Effects of an external electric field on the MA in Co2Si2multilayers are investigated by first-principles calculation. The results demonstrate that, the MA energy can be effectively modificated by external electric field, and the negative MA energy is obtaind with the electric field ranges from0.3Ry to0.6Ry. The hybridization between metal-d electrons and Si-p electrons results in near-rigid-band behavior under an external electric field, which makes it convenient to manipulate the related properties. This observation opens a way to control the MA energy of hcp-Co1-xMx/Si multilayers.(3) The orientation of hcp-CoIr films is largely denpendent on the sputtering conditions of seed-layer; the hcp-Ru seed-layer with good c-axis oriented is necessary to fabricate c-axis oriented hcp-Colr soft magnetic films.(4) The out-of-plane anisotropy field can be significantly enhanced by the introduction of negative magnetocrystalline anisotropy, which is effectively in improving the the ferromagnetic resonance frequency without affecting the initial permeability, hence the excellent high-frequency magnetic properties. Besides, the higher Kg is more conducive to enhance the out-of-plane anisotropy field.(5) Heating the substrate in sputtering process is an effective way to promote the formation of regular columnar crystal structure film, which in turn improves the Ms and total out-of-plane anisotropy field. Hence, the high-frequency magnetic parameter (μi-1)fr is two times lager than that of the film fabracted at room temperature. In addition, the damping constant decreases quit a lot with increasing temperature.(6) In the range of50nm to270nm for CoIr thickness, good high frequency soft magnetic properties are achieved. And with increasing CoIr thickness, the Ms and the total out-of-plane anisotropy field change little; the in-plane uniaxial anisotropy field increases first and then keeps; μi decreases slightly; the natural resonance frequency increases at first and then changes little after165nm, and the maximum of fr is5.46GHz; the damping constant decreases gradually. |
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