We discuss theoretically a ferromagnetic monolayer with an interfacial Dzyaloshinskii-Moriya interaction (iDMI) and a small axial anisotropy. It is shown that the system has a long-period cycloid magnetic order slightly distorted by the anisotropy whose modulation vector $\mathbf{k}$ can have several orientations. We find that due to iDMI-induced umklapp terms in the Hamiltonian, the spectrum of long-wavelength magnons is essentially anisotropic: it is linear and quadratic for momenta directed along and perpendicular to $\mathbf{k}$, respectively. Due to such a quasi-1D spectrum, the temperature correction to the mean spin value has a power-law singularity hampering the magnetic ordering at $T\ne 0$. We demonstrate that the umklapps lead to a peculiar band structure of the magnon spectrum similar to electronic bands in solids. We discuss also the effect of vacancies and defect bonds on system properties at $T=0$. Owing to the quasi-1D character of the spectrum, the distortion of the cycloid structure by a single defect bond is described at long distances by the field of a 1D electric dipole. As a result, even infinitesimal concentrations of defects $c$ destroy the long-range order and establish a short-range order whose correlation length shows a power-law dependence on $1/c$. Our findings should also be applicable to ultrathin films with strong enough iDMI.

• Received 9 August 2023
• Revised 11 October 2023
• Accepted 30 October 2023
• Corrected 29 November 2023

DOI:https://doi.org/10.1103/PhysRevB.108.174414