Instability of the collinear phase in two-dimensional ferromagnet in strong in-plane magnetic field

It is well-known that in thin ferromagnetic film with a net magnetization perpendicular to the film the collinear arrangement of spins is unstable in an in-plane field $H$ smaller than its saturation value $H_c$. Existence of a stripe phase was proposed with elongated domains of alternating direction of magnetization component perpendicular to the film. We consider in the present paper the strong-field regime $H<H_c$ and discuss the minimal microscopic model describing this phenomenon, two-dimensional Heisenberg ferromagnet with strong easy-axis anisotropy and dipolar forces. The noncollinear (stripe) phase is discussed using the technique of the Bose-Einstein condensation of magnons. Some previously unknown results are observed concerning the stripe phase. Evolution is established of the spin arrangement in the noncollinear phase upon the field rotation within the plane. We find a rapid decreasing of the period of the stripe structure as the field decreases. We demonstrate that spins components perpendicular to the film form a sinusoid in the noncollinear phase at $H\approx H_c$ that transforms to a step-like profile upon the field decreasing so that the domain wall density decreases from unity to a value much smaller than unity. The spin-wave spectrum in the noncollinear phase is discussed.