value. For stronger planar fields the $y$–components finally vanish, leading to a ferromagnetic ordering of the planar spin components.

The effect, at $T=0$, of a nondiagonal coupling between spin components can be discussed for the tetragonal ferromagnetic model. For small absolute values of the nondiagonal exchange constant (be it $J^1_{xz}$ or $J^2_{xz}$, see Equations (5) and (6)), the ground state is given by a ferromagnetic phase where the spins have both axial and planar components. Consider first the two–spin exchange with the coupling constant $J^1_{xz}$. Increasing $\left| J^1_{xz} \right|$ does not change the energy of the ferromagnetic phase, whereas the energy of a second phase, consisting of ferromagnetically coupled layers with a stripped pattern in every layer, is decreased, until, for $\left| J^1_{xz} \right| > \sqrt{2} J +\frac{\sqrt{2}}{4} H_x$, the stripped phase has, at $T=0$, a lower energy than the ferromagnetic phase, thus forming the ground state. Note that the appearance of a new ground state at the threshold value does not depend on the sign of $J^1_{xz}$. If the second spin exchange with negative coupling constants $J^2_{xz}$ is considered, the energy of the ferromagnetic phase is increased when $J^2_{xz}$ decreases. For couplings $J^2_{xz} < - \frac{D + H_z + 