  Differential force measurements between spheres coated with either nickel or
gold and rotating disks with periodic distributions of nickel and gold are
reported. The rotating samples are covered by a thin layer of titanium and a
layer of gold. While titanium is used for fabrication purposes, the gold layer
(nominal thicknesses of 21, 37, 47 and 87 nm) provides an isoelectronic
environment, and is used to nullify the electrostatic contribution but allow
the passage of long wavelength Casimir photons. A direct comparison between the
experimental results and predictions from Drude and plasma models for the
electrical permittivity is carried out. In the models the magnetic permeability
of nickel is allowed to change to investigate its effects. Possible sources of
errors, both in the experimental and theoretical sides, are taken into account.
It is found that a Drude response with magnetic properties of nickel taken into
account is unequivocally ruled out. The full analysis of the data indicates
that a dielectric plasma response with magnetic properties of Ni included shows
good agreement with the data. Neither a Drude nor a plasma dielectric response
provide a satisfactory description if the magnetic properties of nickel are
disregarded.
