Soft magnetic materials application in the RF range

Owing to high resistivity and high inplane uniaxial anisotropy, granular films are known to exhibit high permeability at radio frequencies. But this granular resistivity is not high enough to suppress eddy current when they are used at gigahertz frequencies. Recent developments to solve this problem are described. One is patterning the granular films. It is effective to control the inplane anisotropy and to suppress inplane eddy current. Another is deposition of granular films by evaporation. A peculiar fiber structure is formed for which very high resistivity and anisotropy are realized. Finally, a study of using these films as near-field electromagnetic noise absorbers is described, and it is demonstrated that they have high potential as micrometer-scale noise absorbing elements in the gigahertz frequency range.

Granular thin films with high RF permeability

High-frequency signal processing using magnetic layered structures

FeTaN thin films of 100nm with in-plane uniaxial anisotropy were fabricated by rf magnetron sputtering and patterned into strips with width varied from 30to500μm along both the easy and hard magnetization axes of the unpatterned films. The values of effective magnetic anisotropy of the patterned films increase as the strip widths decrease. The ferromagnetic resonance frequency showed a corresponding increment from 1.38to2.58GHz as the strip width varied. It indicates that this method is effective to tune the ferromagnetic resonance frequency of magnetic thin films. The resultant permeability spectra were understood according to the well-known Kittel formula where an averaged shape anisotropy arisen from the patterning was taken into account for the strips patterned along the easy axis. For the strips patterned along the hard axis, the increase in resonance frequency in narrower strips is attributed to the strengthened internal domain pinning effect predicted from the domain patterns numerically calculated.

Magnetic anisotropy and resonance frequency of patterned soft magnetic strips

A CoAlO thin film was found to have good soft magnetic properties and gigahertz frequency response when it had a thickness of tens of nanometres. Increasing the thickness by more than 100?nm rapidly increased the coercivity and seriously degraded the high frequency response. A multilayering technique (laminating the magnetic layer with oxide spacers) was exploited to improve the magnetic properties in thick films. AlOx and SiOx were used as the oxide spacing layers in this experiment. For the 105?nm CoAlO films, the easy and hard axis coercivities were reduced by more than 8 times by introducing 2?nm AlOx or 5?nm SiOx spacers. With our optimum lamination [CoAlO (52.5?nm)/AlOx (2?nm)]7, good soft magnetic properties (easy and hard axis coercivities: 21 and 14?Oe and effective anisotropy field: 54?Oe) and high frequency response (real permeability: >200 up to ~2?GHz) were successfully obtained in a multilayer film at a thickness of 380?nm. The changes in the microstructures and film stress with the introduction of the oxide spacers are discussed in this paper.

http://iopscience.iop.org/article/10.1088/0022-3727/40/11/005/pdf

Soft magnetic properties and high frequency permeability in [CoAlO/oxide] multilayer films

We demonstrated the radio-frequency (rf) noise suppressor using soft magnetic films on a coplanar transmission line from 0.1 to 20 GHz. The coplanar transmission line is composed of magnetic film/polyimide/Cu transmission line/seed layer (Cu/Ti)/glass substrate with the dimension of 50 μm width of the signal line and 3 μm thickness (characteristic impedance: 50 Ω). The magnetic films (CoPdAlO, CoZrO, and CoNbZr) as a noise suppressor are prepared by rf sputtering. The saturation magnetization of each magnetic film is about 10 kG. The magnetic anisotropy field and the ferromagnetic resonance frequency are 230, 89, and 6 Oe and 4.2, 2.5, and 0.7 GHz, respectively. The power loss of the coplanar line with magnetic films is significantly larger than without magnetic and nonmagnetic films due to ferromagnetic resonance losses.

Hideaki Nagura

Papers

Effect of radio-frequency noise suppression on the coplanar transmission line using soft magnetic thin films

Granular thin films with high RF permeability

Database of Effective Permeability of Magnetic Film with Parallel Pattern.

Rf noise suppression on coplanar transmission line using loss generation of the soft magnetic films