  The Casimir pressure is investigated in three-layer systems where the
intervening stratum possesses magnetic properties. This subject is gaining in
importance in connection with ferrofluids and their use in various
microelectromechanical devices. We present general formalism of the Lifshitz
theory adapted to the case of ferrofluid sandwiched between two dielectric
walls. The Casimir pressure is computed for the cases of kerosene- and
water-based ferrofluids containing a 5% fraction of magnetite nanoparticles
with different diameters between silica glass walls. For this purpose, we have
found the dielectric permittivities of magnetite and kerosene along the
imaginary frequency axis employing the available optical data and used the
familiar dielectric properties of silica glass and water, as well as the
magnetic properties of magnetite. We have also computed the relative difference
in the magnitudes of the Casimir pressure which arises on addition of magnetite
nanoparticles to pure carrier liquids. It is shown that for nanoparticles of 20
nm diameter at 2 micrometer separation between the walls this relative
difference exceeds 140% and 25% for kerosene- and water-based ferrofluids,
respectively. An interesting effect is found that at a fixed separation between
the walls an addition of magnetite nanoparticles with some definite diameter
makes no impact on the Casimir pressure. The physical explanation for this
effect is provided. Possible applications of the obtained results are
discussed.
