Ferromagnetic monolayer with interfacial Dzyaloshinskii-Moriya interaction: Magnon spectrum and effect of quenched disorder

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 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 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 not equal 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.