CHARACTERIZATION OF MAGNETO-IMPEDANCE THIN FILM MICROSTRUCTURES

I. Giouroudi – Hans Hauser – G. Daalmans – G. Rieger – J. Wecker

   The magnetoimpedance (MI) effect is based on the magnetic field dependence of the transverse permeability �μ of an ac current carrying conductor. Both inductance and skin effect depend on �μ, thus causing a change of the impedance Z of amorphous wires, ribbons, and - at very high frequencies - also of thin films. In order to overcome this disadvantage, tri-layer structures of two 20, 50, and 100 nm thin amorphous CoFeB layers with a central 40, 100, and 200 nm thin Cu layer are rf sputtered onto a thermally oxidized Si wafer. 300 μm long strips of 3.20 μm width are structured by plasma etching and connected by ultrasonic bonding to a printed circuitboard. Magnetization curves - both of the plain film and of the structures - parallel and perpendicular with respect to the easy axis of uniaxial anisotropy are measured by the magnetooptical Kerr effect, showing an anisotropy field of 2 kA/m and low coercivity in the hard axis direction. During magnetization reversal, the domain structure in the strips was observed by Kerr microscopy, indicating possible magnetostatic coupling of the two 100 nm magnetic layers. Measurements of the frequency dependence of the complex permeability revealed ferromagnetic resonance at 1.5 GHz. The MI effect was measured by means of a network analyzer (NA), using the complex t/r ratio of the incoming (reference r) and the transmitted t wave through the sample. The NA was calibrated for linear frequency response of t at �μ₀H_s = 12 mT. The unstructured 100/200/100 trilayer (sample size 4 �~ 8 mm²) showed a flat MI maximum at 65 MHz, whereas the 6 �μm wide strip had a MI maximum of (Z - ZH_s)/ZH_s = 5.7% at 460 MHz with a field of �μ₀H =1.6 mT, applied by a Helmholtz coil pair, parallel to the long axis of the strip, which is oriented perpendicular to the easy axis. The MI maximum of the 50/100/50 structure strips is beyond 500 MHz. The reason of the low measured MI effect is the high contact resistance R_c =11 Ohm and the inductivity L = 3 nH of the 6.7 cm long leads to the strip. Measured by a four wire method, the dc resistance R_d = 4 of the 6 �μm wide 100/200/100 strip is in good aggreement with the theoretical resistance of the copper layer. If directly integrated in the electronic circuit, the MI effect is increased to about 37%.

Keywords: magnetic thin film, tri-layer structure, magnetooptical Kerr effect, complex permeability

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