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## Cubic B20 helimagnets with quenched disorder in magnetic field

We theoretically address the problem of cubic B20 helimagnets with a small concentration *c* <<1 of defect

bonds in external magnetic field **H**, which is relevant to mixed B20 compounds at small dopant concentrations.

We assume that the Dzyaloshinskii-Moriya interaction and the exchange coupling constant are changed on

imperfect bonds which leads to distortion of the conical spiral ordering. In a 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.