Microwave properties and anisotropy field distribution in nanogranular Fe-Co-Al-O films

In order to investigate and possibly exploit in real devices the interplay between anisotropy terms of different origin, the microwave scattering matrix S of nanogranular Co-Fe-Al-O films with an induced in-plane anisotropy under the additional influence of an external field H_ext, is measured. The induced anisotropy causes a well defined domain pattern, encompassing a few nanograins, as shown by AFM/MFM imaging. The low anisotropy field measured is due to the suppression of the magnetocrystalline anisotropy of individual grains. Microwave scattering results show that at zero applied field the resonance peaks occur at 4-10 GHz, much higher than the ferromagnetic resonance frequency f_fmr≈ 1-2 GHz estimated from vibrating sample magnetometer (VSM) measurements for the 550 nm and 125 nm thick films, respectively. The effect is mainly connected to the incomplete suppression of the magnetocrystalline anisotropy at low applied field.