Phase diagrams near the Lifshitz point. III. Tetragonal crystals with an easy plane of magnetization

Phase diagrams near a Lifshitz point are considered for tetragonal crystals with an easy plane of magnetization. The modulated phase is shown to be sinusoidal or helicoidal, depending on whether a certain parameter $\ensuremath{\mu}$ measuring the strength of tetragonal anisotropy is larger or smaller than 1. The line ${T}_{H}(P)$ of first-order phase transitions between the ferromagnetic and modulated phases is shown to be tangent to the order-disorder transition line ${T}_{\ensuremath{\lambda}}(P)$ at the Lifshitz point. For $\ensuremath{\mu}<1$, the line ${T}_{H}(P)$ is shown to be displaced in the direction of the helicoidal phase with the growth of $\ensuremath{\mu}$. Such displacement of this line with the increase of crystal anisotropy is shown to be a general property of phase diagrams in the vicinity of the Lifshitz point.

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