Magnetoresistance in Co/Cu Multilayers

Co/Cu multilayer films were prepared by rf sputtering, both with and with-out a substrate bias voltage V
_B
=-30 V. X-ray diffraction results indicated that the layer boundaries in -30 V biased films are sharper, and the orientation of (111) planes are better, than in films prepared without a bias voltage. In films with zero bias, the MR ratio increases monotonically with the Cu layer thickness d
_Cu
up to about 40 Å, and then decreases. On the other hand, in films prepared with a -30 V bias the MR ratio oscillates with d
_Cu
, and peaks appeared at d
_Cu
= 23, 36 and 48Å. The maximum MR ratio was obtained for the structure [Co(15.1 Å)/Cu(24.3 Å)]
₃₀
/Cu(50 Å), with a value of 13.1% at room temperature and 17.0% at 110K. In this film, the interlayer coupling strength J was very small, 0.008 erg/cm
²
. The sharpness of the MR change, ΔR/R(H
_s
)·ΔH, was a very large 2.2 kOe
^-1
The different d
_Cu
dependences of films prepared with a -30 V bias and with zero bias is thought to originate in the different qualities of the layer boundaries. The extraordinary Hall effect and the planar Hall effect are discussed.