MI effect in amorphous CoFe thin film trilayers and demonstrator packaging

The magnetoimpedance (MI) effect found in amorphous soft ferromagnetic materials occurs at high frequencies and consists of large changes in the impedance with an external magnetic field. It is the dependence of the transverse permeability on an external magnetic field. A uniaxial magnetic anisotropy is induced having the easy axis transverse to the current direction. Inductance and skin effect depend on permeability thus causing a change of the impedance of amorphous wires, ribbons and of thin films at very high frequencies. Trilayer structures of two 20, 50 and 100 nm thin amorphous CoFeB layers with a central 40, 100 and 200 nm thin Cu layer respectively were sputtered onto a thermally oxidized Si wafer. 300 mum long strips of 3 -20 mum width are structured by plasma etching and connected by ultrasonic bonding to a printed circuit board. Magnetization curves, parallel to the easy axis and hard axis of uniaxial anisotropy, are measured by the magnetooptical Kerr effect exhibiting anisotropy fields of around 1.6 kA/m and low coercivity in the hard axis direction, depending on the film thickness. The domain structure in the strips was observed by Kerr microscopy, indicating possible magnetostatic coupling of the two magnetic layers. The MI effect was measured, before and after field annealing, by means of a Network Analyzer, using the complex ratio of the incoming wave and the transmitted wave through the sample, automatically calculated using a LabVIEW program. The Network Analyzer was calibrated for linear frequency response at 15 mT. The MI maximum of the 20/40/20 nm structure strips is beyond 500 MHz. The MI effect of the 50/100/50 nm structure strips has been improved after field annealing, exhibiting a maximum of up to 9% in a 4 mum wide strip.