  Heterostructures of superconducting and ferromagnetic materials are of
fundamental interest because of the mutual interaction of antagonistic kinds of
ordering at the S-F interface. Normally, the superconducting transition
temperature Tc should be strongly suppressed at the S-F interface owing to the
penetration of Cooper pairs into the ferromagnetic side. Nevertheless,
constructive interactions between S and F orders have been suggested to occur
via the modification of ferromagnetic order by the superconducting state. This
may induce an inhomogeneous magnetic state, often called a cryptoferromagnetic
state, and the relevant domain wall effect, which will lead to a local decrease
of the pair-breaking parameter. However, the domain wall effect, even if it
exists, is quite subtle from the experimental view point and is normally
difficult to observe. Here we show that the defect-related d0 ferromagnetism in
MgO and the superconductivity in MgB2 do not antagonize, but rather enhance the
superconducting transition temperature Tc to any significant degree. We found
in superconducting MgB2-d0 ferromagnetic MgO composites that the
superconducting transition proceeds in two steps. The first at the S-F
interface, between 110-120 K, then in the rest of the bulk at 39 K, which is
the Tc of single phase MgB2 superconductor. Moreover, the additional transition
emerges at 60 K at the S-F interface especially in the ferromagnetic side,
showing a spin-glass-like magnetic state. Our findings reveal that the
proximity effect in the superconductor-d0 ferromagnet heterostructures will
provide the knowledge and basis to enhance the Tc value of the existing
superconductors.
