  We consider the Ginzburg-Landau mean field theory of ferromagnetic
superconductors with complex order parameter $\psi_i, i=1,2,3$. Below the
critical temperature superconductivity is generated by means of a tachyonic
term in the free energy $-\alpha |\psi_i|^2$ $(\alpha>0)$, stabilized by a
$\psi^4$-term. However, we show that when $\alpha$ becomes negative
superconductivity can also be generated spontaneously by the magnetic field
through the Zeeman like coupling between the field and the spin of the order
parameter, in analogy with what happens in W-meson condensation. We also show
that this mechanism leads to a vortex lattice which has anti-screening
currents, enhancing the magnetic field instead of counteracting it. The lattice
of vortices is a collective phenomenon where the individual elements do not
exist in isolated form. If the parameter $\alpha$ vanishes it is possible to
obtain an explicit form of the vortex lattice in terms of Weierstrass'
p-function.
