  We study the inhomogeneous solitonic modulation of a chiral condensate within
the effective Nambu--Jona-Lasinio model when a constant external magnetic field
is present. The self-consistent Pauli-Villars regularization scheme is adopted
to manipulate the ultraviolet divergence encountered in the thermodynamic
quantities. In order to determine the chiral restoration lines efficiently, a
new kind of Ginzburg-Landau expansion approach is proposed here. At zero
temperature, we find that both the upper and lower boundaries of the solitonic
modulation oscillate with the magnetic field in the $\mu$--$B$ phase diagram
which is actually the de Hass-van Alphan (dHvA) oscillation. It is very
interesting to find out how the tricritical Lifshitz point $(T_L,\mu_L)$
evolves with the magnetic field: There are also dHvA oscillations in the
$T_L$--$B$ and $\mu_L$--$B$ curves, though the tricritical temperature $T_L$
increases monotonically with the magnetic field.
