  We have created superconductor-ferromagnet composite films in order to test
the enhancement of critical current density, Jc, due to magnetic pinning. We
co-sputter the type-II superconductor niobium (Nb) and the low-temperature
ferromagnet dysprosium (Dy) onto a heated substrate; the immiscibility of the
two materials leads to a phase-separated composite of magnetic regions within a
superconducting matrix. Over a range of compositions and substrate
temperatures, we achieve phase separation on scales from 5 nm to 1 micron. The
composite films exhibit simultaneous superconductivity and ferromagnetism.
Transport measurements show that while the self-field Jc is reduced in the
composites, the in-field Jc is greatly enhanced up to the 3 T saturation field
of Dy. In one instance, the phase separation orders into stripes, leading to
in-plane anisotropy in Jc.
