Nanogranular Co-Fe-Al-O films with a high electrical resistivity for GHz magnetoelastic device applications

Co-Fe-Al-O nanogranular thin films are fabricated by RF-magnetron sputtering under an Ar + O₂ atmosphere. High resolution transmission electron microscopy reveals that the Co-Fe-Al-O films are composed of bcc (Co,Fe) nanograins finer than 5 nm and an Al-O amorphous phase. A very large electrical resistivity of 374 μm cm is obtained, together with a large uniaxial anisotropy field of 50 Oe, a hard axis coercivity of 1.25 Oe, and a saturation magnetization of 12.9 kG. The real part of the relative permeability is 260 at low frequencies and this value is maintained up to 1.3 GHz. The ferromagnetic resonance frequency is 2.24 GHz. A saturation magnetostriction of 50 ppm is achieved at a magnetic field as low as 30 Oe. The present Co-Fe-Al-O nanogranular thin films with a high electrical resistivity, high resonance frequency and good low-field-magnetostrictive response are considered to be suitable for GHz magnetoelastic device applications.