  An external magnetic field, $H$, applied parallel to a quasi two-dimensional
system modifies quantitatively and qualitatively the density of states. Using a
self-consistent numerical approach, we study how this affects the entropy, $S$,
the free energy, $F$, and the magnetization, $M$, for different sheet carrier
concentrations, $N_s$. As a prototype system we employ III-V double quantum
wells. We find that although $M$ is mainly in the opposite direction of $H$,
the system is not linear. Surprisingly $\partial M / \partial H$ swings between
negative and positive values, i.e., we predict an entirely orbital diamagnetic
to paramagnetic fluctuation. This phenomenon is important compared to the ideal
de Haas-van Alphen effect i.e. the corresponding phenomenon under perpendicular
magnetic field.
