  In cubic helimagnets MnSi and Cu2OSeO3 with their nearly isotropic magnetic
properties, the magnetic structure undergoes helical deformation, which is
almost completely determined by the helicoid wavenumber k = D / J, where
magnetization field stiffness J is associated with isotropic spin exchange, and
D is a pseudoscalar value characterizing the antisymmetric
Dzyaloshinskii-Moriya (DM) interaction. While the wavenumber can be measured
directly in a diffraction experiment, the values of J and D can be calculated
from the constants of pair spin interactions, which enter as parameters into
the Heisenberg energy. However, the available analytical expression for D,
which is of the first order in the spin-orbit coupling (SOC), has significant
problems with accuracy. Here we show that hardly observable distortions of the
magnetic structure, namely the antiferromagnetic spin cantings, can
significantly change the constant D in the next approximation in SOC, thus
affecting the wavenumber of magnetic helicoids. The obtained analytical
expressions agree with the results of numerical simulation of magnetic helices
in Cu2OSeO3 to within a few percent.
