Spiral magnets with Dzyaloshinskii-Moriya interaction containing defect bonds

TL;DR

This paper develops a theory for spiral magnets with Dzyaloshinskii-Moriya interaction affected by bond disorder, revealing long-range defect effects, changes in spiral vector, and impurity-induced neutron scattering features.

Contribution

It introduces a model describing how bond disorder influences spiral magnetic order and its observable properties, including defect-induced long-range distortions and neutron scattering signatures.

Findings

Defects cause long-range spiral distortions decaying as 1/r^2.

Impurities can reverse the sign of spin chirality at low defect concentrations.

Impurities induce power-law tails in neutron scattering and affect magnon properties.

Abstract

We present a theory describing spiral magnets with Dzyaloshinskii-Moriya interaction (DMI) subject to bond disorder at small concentration $c$ of defects. It is assumed that both DMI and exchange coupling are changed on imperfect bonds. Qualitatively the same physical picture is obtained in two models which are considered in detail: B20 cubic helimagnets and layered magnets in which DMI leads to a long-period spiral ordering perpendicular to layers. We find that the distortion of the spiral magnetic ordering around a single imperfect bond is long-range: values of additional turns of spins decay with the distance $r$ to the defect as $1/r^2$ being governed by the Poisson's equation for electric dipole. At finite concentration of randomly distributed imperfect bonds, we calculate correction to the spiral vector. We show that this correction can change the sign of spin chirality even at $c\ll1$ if defects are strong enough. It is demonstrated that impurities lead to a diffuse elastic neutron scattering which has power-law singularities at magnetic Bragg peaks positions. Then, each Bragg peak acquires power-law decaying tails. Corrections are calculated to the magnon energy and to its damping caused by scattering on impurities.