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

It is well-known that in a 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). The existence of a stripe phase was proposed with elongated domains of alternating direction of the 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, a two-dimensional Heisenberg ferromagnet with strong easy-axis anisotropy and dipolar forces. The noncollinear (stripe) phase is discussed using the technique of Bose-Einstein condensation of magnons. Some previously unknown results are observed concerning the stripe phase. Evolution of the spin arrangement in the noncollinear phase is established upon field rotation within the plane. We find a rapid decrease of the period of the stripe structure as the field decreases. We demonstrate that spin components perpendicular to the film form a sinusoid in the noncollinear phase at H≈H(c) that transforms to a step-like profile with decreasing field 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.