Optical and magneto-optical properties and magnetoresistance of amorphous CoFeNiSiB alloys

The optical and magneto-optical properties of amorphous CoFeSiB ribbons were investigated for incident photons in the energy range from 0.05 to 2.2 eV. The diagonal and off-diagonal optical conductivities were determined from the results of spectroscopic ellipsometry and magneto-optical Kerr effect measurements. The Kerr rotation spectra for the amorphous films have a shape in the 0.6-2.2 eV region that is quite similar to that of bulk Co. The optical conductivity of the amorphous CoFe(Ni)SiB alloys increases slowly as a function of frequency (proportional to ω+1/3$/) in the IR region. Such a σ approx. ω+1/3$/ dependence in the IR region suggests an additional localisation of the electron states. A large room-temperature magnetoresistance effect has been demonstrated in thin Co₇₁Fe₇Si₁₂B₁₀ and Co₅₉Fe₅Ni₁₀Si₁₁B₁₅ amorphous ribbons for AC at applied fields, H < 80 Oe. The peak value of the magnetoresistance ratio is very sensitive to the Co and Fe concentration in the alloys. The dependence of the magnetoresistance for amorphous alloys is consistent with a theoretical model in which the resistivity arises from the scattering of the conduction electrons by a domain wall in the weakly localised regime.