Domain walls connected with spontaneous deformation of an antiferromagnetic chain

A 1D model of an antiferromagnet (AFM) is analyzed, in which the Frenkel-Kontorova model is generalized to the system of two fields: the atomic displacement field and the field of atomic spin orientations. It is assumed that the equilibrium ordering of atomic masses associated with mechanical interactions between atoms corresponds to a completely frustrated spin ordering. However, magnetoelastic interaction generates a spontaneous uniform deformation leading to either a doubly degenerate, or nondegenerate ground state of the AFM. In both cases, magnetic domains separated by a domain wall (DW) (which is the soliton solution of the nonlinear equation for the displacement field) can be formed. It is shown that DW can be of two types. A domain wall of the first type preserves the uniformity of spin distribution in a chain and is determined only by the type of kink (large or small) in the displacement field. Such a DW includes either a vacancy, or a crowdion, viz., inelastic deformation of the displacement field. A domain wall of the second type is a 2π-kink in the spin orientation field and causes deformation of the displacement field without violating the atomic ordering in the chain.

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