Magnetoresistance in amorphous oxide films CoFeHfO

This paper shows results on structural and magnetotransport measurements for reactively sputtered amorphous oxide films (Co₅₅Fe₂₀Hf₂₅)_{1 − x}O_x with x ⩽ 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, Co–Co(Fe) pair-correlated atom clusters dominate and the films are soft ferromagnetic with excellent uniaxial magnetic anisotropy. Small coercivity H_c ≈ 1–4 Oe, enhanced magnetisation M_s ≈ 420 emu/cm³ and low resistivity ρ ≈ 10^{− 4} Ωcm were measured for these films, showing a good potential for application in magnetoelectronics. For large x, however, since Hf-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 ≈ 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 Co-Co(Fe) clusters. The layered structure shows a small, but low-field magnetotunnelling effect.