Soliton spin excitations and their perturbation in a generalized inhomogeneous Heisenberg ferromagnet

Abstract

We investigate the nonlinear spin dynamics of a generalized inhomogeneous anisotropic Heisenberg ferromagnetic spin chain with bilinear and biquadratic exchange, octupole-dipole, and weak interactions in the semiclassical limit by using Glauber's coherent state method in combination with Holstein-Primakoff bosonic representation of spin operators. The dynamics is found to be governed by a generalized nonlinear Schrödinger equation in the continuum limit. We have identified several completely integrable spin models with soliton spin excitations for specific parametric choices. Finally, we carry out a multiple scale perturbation analysis to find the effect of discreteness and inhomogeneity on the soliton excitations in a more general case. The results show that the discreteness effect introduces symmetric fluctuations in the localized region of the soliton without altering its amplitude and velocity. On the other hand, the inhomogeneity introduces asymmetric fluctuations in the localized region with a decrease in the amplitude and velocity of the soliton with time. Further, the inhomogeneity reverses the velocity of the soliton after some time. different orders of the lattice parameter and integrable spin models have been identified. In Sec. IV, for the more general nonintegrable case, we carry out a multiple scale perturbation analysis to understand the effects of discreteness and inhomogeneity. The results are presented in Sec. V. II. MODEL AND DYNAMICAL EQUATIONThe Heisenberg spin Hamiltonian for an anisotropic weak ferromagnetic spin system with site-dependent bilinear, biquadratic, and octupole-dipole interactions ͑a generalized Heisenberg spin model͒ is written aswhere J 1i Ј , J 2i Ј , and J 3i Ј represent site-dependent coefficients of bilinear, biquadratic, and octupole-dipole interactions, respectively, and the constants J 4 Ј and AЈ ͑and A 1 Ј͒ correspond to weak and crystal field anisotropy interactions. D is the constant Dzyaloshinskii-Moriya ͑DM͒ vector, and the coefficients b and c introduce exchange anisotropy in the spin system. In Eq. ͑1͒, S i = ͑S i x , S i While writing Eq. ͑2͒, we have expressed the site-dependent coefficients J 1i , J 2i , and J 3i as J 1i = J 10 + J 11 f i , J 2i = J 20 + J 21 g i , and J 3i = J 30 + J 31 h i , where f i , g i , and h i are time-independent site-dependent functions related to the bilinear, biquadratic, and octupole-dipole interactions, respectively, and J 10 , J 11 , J 20 , J 21 , J 30 , and J 31 are constant coefficients. Different models of one-dimensional spin-1/2 ordered ferromagnetic chains with exchange interaction were proven to be exactly solvable with a complete description of their energy spec-

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