Magnetic properties and spin wave spectra of a ferromagnetic monolayer with 2D tetragonal structure: an application for Co2S2 monolayer

•Significant role of thermal spin fluctuations on magnetic properties of FM monolayer.•Exchange interaction between magnetic and non-magnetic atoms is given in the model.•Presence of the non-magnetic atoms causes an increase of the thermal fluctuations.•The model is applied for Co2S2 monolayer with 2D tetragonal structure.