Spiral magnets with Dzyaloshinskii-Moriya interaction containing defect bonds

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 ≪ 1 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.

Cubic B20 helimagnets with quenched disorder in magnetic field

“…This equations can be solved self-consistently as in Ref. 11 . Denoting ρ R1 −ρ R0 = γ, the solution of Poisson equation (39) reads as…”

“…In the first order in c, it is related with the double Fourier transforms of ρ R1 ρ R2 , ρ R1 ρ R2 , and ρ R1 ρ R2 from a single defect, where the line denotes averaging over disorder realizations (see Ref. 11 for more details) and we also neglect terms containing products of more than two ρ. Due to the long-ranged character of the dipole field, the contribution from ρ R1 ρ R2 is the most singular one.…”

“…At x 1 and x ≈ 1, dopant ions can be naturally considered as defects inside an ordered matrix, and the system can be described by a model of B20 helimagnet with small amount of defect bonds 24 . In our recent paper 11 we discussed in detail the spiral phase at zero magnetic field in this model. We found a remarkable electrostatic analogy in this problem: the distortion of the spiral ordering by a single defect is given by a Poisson equation for an electric dipole.…”

Antiferromagnets with random vacancies and substitutional spins on the triangular lattice

“…Point defects are much less studied in non-collinear spin systems mainly because any impurity disturbs the local magnetic order by producing a long-range spin texture. [12][13][14][15][16] It was difficult to apply the well-developed T -matrix formalism to non-collinear systems with defects because it was unclear how to take into account the long-range textures from different randomly distributed impurities and their interference. For instance, investigation 12 of dynamical properties of 2D Heisenberg AF in magnetic field using the T -matrix technique relies on numerical results for spin textures.…”

“…It is quite an old but seemingly well forgotten idea by Villain 17,18 that the impurity-induced spin textures in Heisenberg magnets is described by electrostatic equations. This idea was particularly useful in considerations of frustrated defects in Heisenberg AF on simple square lattice 19,20 and in recent study 16 of 3D spiral magnets. It was shown in Ref.…”