Instability of the collinear phase in a two-dimensional ferromagnet in a strong in-planemagnetic field

Abstract

It is well-known that in a thin ferromagnetic film with a net magnetization perpendicularto the film the collinear arrangement of spins is unstable in an in-plane fieldH smaller than itssaturation value Hc. The existence of a stripe phase was proposed with elongated domainsof alternating direction of the magnetization component perpendicularto the film. We consider in the present paper the strong-field regimeH<Hc and discuss the minimal microscopic model describing this phenomenon, a two-dimensionalHeisenberg ferromagnet with strong easy-axis anisotropy and dipolar forces. Thenoncollinear (stripe) phase is discussed using the technique of Bose–Einsteincondensation of magnons. Some previously unknown results are observed concerningthe stripe phase. Evolution of the spin arrangement in the noncollinear phase isestablished upon field rotation within the plane. We find a rapid decrease of theperiod of the stripe structure as the field decreases. We demonstrate that spincomponents perpendicular to the film form a sinusoid in the noncollinear phase atH≈Hc that transforms to a step-like profile with decreasing field so that the domain wall densitydecreases from unity to a value much smaller than unity. The spin-wave spectrum in thenoncollinear phase is discussed.