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

TL;DR

This paper investigates the instability of the collinear phase in a two-dimensional ferromagnet under strong in-plane magnetic fields, revealing the evolution of stripe phases and spin configurations using magnon Bose-Einstein condensation techniques.

Contribution

It introduces a microscopic model for stripe phase formation in 2D ferromagnets with anisotropy and dipolar forces, analyzing the spin structure evolution under varying magnetic fields.

Findings

Stripe period decreases rapidly as magnetic field decreases.

Spin components form sinusoidal profiles near saturation, transforming into step-like profiles at lower fields.

Spin-wave spectrum characteristics in the noncollinear phase are discussed.

Abstract

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.