Magnetoresistance in amorphous oxide films

This paper shows results on structural and magnetotransport measurements for reactively sputtered amorphous oxide films (${\rm Co}_{55}{\rm Fe}_{20}{\rm Hf}_{25})_{1-{x}}{\rm O}_{x}$ with $x \leq 0.35$ and their layered structures. A strong x-dependency was found to be related to microstructural changes caused by oxygen-induced chemical phase separation. For low x, \chem{Co\tx{-}Co(Fe)} pair-correlated atom clusters dominate and the films are soft ferromagnetic with excellent uniaxial magnetic anisotropy. Small coercivity H_{c} \approx 1\tx{--}4 Oe, enhanced magnetisation $M_{s} \approx 420$ emu/cm3 and low resistivity $\rho \approx 10^{-4}$ Ωcm were measured for these films, showing a good potential for application in magnetoelectronics. For large x, however, since \chem{Hf}-\chem{O} pair-correlated atom clusters are dominant, the films become granular-like superparamagnetic with a saturation field $H_{s} > 20$ kOe and a remanence $M_{r}/M_{s} \approx 0$. ρ is extremely high presumably due to Coulomb blockade effect. A large magnetoresistance was observed, being 30% (70 kOe) at 5 K and 10% at room temperature. This effect can be attributed to spin-dependent tunnelling among lateral separated \chem{Co}-\chem{Co(Fe)} clusters. The layered structure shows a small, but low-field magnetotunnelling effect