  The nature of spin density wave (SDW) and charge density wave (CDW) in the
mixed state of high T_c superconductors (HTS) is investigated by using the
self-consistent Bogoliubov-de Gennes equations and an effective model
Hamiltonian with competing SDW and d-wave superconductivity interactions. We
show that there exists a critical onsite Coulomb interaction U_c. For optimally
doped sample, a two dimensional SDW and CDW modulations are induced for U < U_c
while SDW and CDW orders become antiferromagnetic (AF) and charge stripes for U
> U_c. These stripe orders are stabilized and enhanced near the vortex cores.
The wavelengths of the AF stripes and charge stripes are found respectively to
be 8 a and 4 a with a as the lattice constant. We show that our results could
be applied to understand several recent experiments on HTS.
