  Ba(Fe1-xCox)2As2 is the most tunable of the Fe-based superconductors (FBS) in
terms of acceptance of high densities of self-assembled and artificially
introduced pinning centres which are effective in significantly increasing the
critical current density, Jc. Moreover, FBS are very sensitive to strain, which
induces an important enhancement in critical temperature, Tc, of the material.
In this paper we demonstrate that strain induced by the substrate can further
improve Jc of both single and multilayer films by more than that expected
simply due to the increase in Tc. The multilayer deposition of Ba(Fe1-xCox)2As2
on CaF2 increases the pinning force density Fp by more than 60% compared to a
single layer film, reaching a maximum of 84 GN/m^3 at 22.5T and 4.2 K, the
highest value ever reported in any 122 phase.
