# Cubic B20 helimagnets with quenched disorder in magnetic field

## TL;DR

This paper theoretically investigates how small concentrations of defect bonds in cubic B20 helimagnets affect their magnetic spiral structure and neutron scattering, revealing long-range distortions and disorder-induced diffuse scattering.

## Contribution

It introduces a theoretical model describing the impact of quenched disorder on the magnetic ordering in B20 helimagnets under magnetic fields, including corrections to spiral parameters and scattering signatures.

## Key findings

- Long-range spiral distortions due to defect bonds.
- Corrections to spiral vector and cone angle calculated.
- Diffuse neutron scattering with power-law tails observed.

## Abstract

We theoretically address the problem of cubic B20 helimagnets with small concentration ${c \ll 1}$ of defect bonds in external magnetic field $\bf H$, which is relevant to mixed B20 compounds at small dopant concentrations. We assume that Dzyaloshinskii-Moriya interaction and the exchange coupling constant are changed on imperfect bonds which leads to distortion of the conical spiral ordering. In one-impurity problem, we find that the distortion of the spiral pitch is long-ranged and it is governed by the Poisson equation for an electric dipole. The variation of the cone angle is described by the screened Poisson equation for two electric charges with the screening length being of the order of the spiral period. We calculate corrections to the spiral vector and to the cone angle at finite $c$. The correction to the spiral vector is shown to be independent of $H$. We demonstrate that diffuse neutron scattering caused by disorder appears in the elastic cross section as power-law decaying tails centered at magnetic Bragg peaks.