Abstract—The decrease in the dimensions of the core of a magnetic head with increasing recording density leads to the necessity for the allowance of the changes in the micromagnetic structure of the magnetic core to be taken into account when calculating the writing field. A micromagnetic simulation of the process of changes in the magnetic structure under the action of an external magnetic field has been performed on the example of a rectangular magnetic core. The initial equilibrium periodic domain structure contains uncharged domain walls. When an external magnetic field acts along the longer side of the core, the structure changes so that the domain walls remain uncharged; i.e., no additional sources of stray field are generated, which simplifies the cal- culation of the domain structures of magnetic cores of a more complex shape. The thus-obtained parameters of the peripheral zone of the magnetic core make it possible to calculate the writing field and to choose optimum dimensions of the heads designed.