Non-linear dynamics and two-dimensional solitons for spin $ S=1$ ferromagnets with biquadratic exchange

TL;DR

This paper develops a semiclassical theory for S=1 ferromagnets with biquadratic exchange, revealing novel longitudinal spin oscillations, non-linear waves, and solitons absent in traditional models, with potential excitation via ultrafast lasers.

Contribution

It introduces a new semiclassical framework for S=1 ferromagnets with biquadratic exchange, uncovering longitudinal excitations and solitons not described by standard models.

Findings

Discovery of longitudinal spin oscillations where spin length varies.

Identification of non-linear uniform waves and localized solitons.

Discussion of ultrafast laser pulses exciting these novel excitations.

Abstract

We develop a consistent semiclassical theory of spin dynamics for an isotropic ferromagnet with a spin $ S=1$ taking into consideration both bilinear and biquadratic over spin operators exchange interaction. For such non-Heisenberg magnets, a peculiar class of spin oscillations and waves, for which the quantum spin expectation value $ {\rm {\bf m}}=<{\rm {\bf S}}>$ does not change it direction, but changes in length, is presented. Such ``longitudinal'' excitations do not exist in regular magnets, dynamics of which are described in terms of the Landau-Lifshitz equation or by means of the spin Heisenberg Hamiltonian. We demonstrate the presence of non-linear uniform oscillations and waves, as well as self-localized dynamical excitations (solitons) with finite energy. A possibility of excitation of such oscillations by ultrafast laser pulse is discussed.