Effect of cell size in calculating frequencies of magnetic modes using micromagnetics: Special role of the uniform mode

Using the time evolution of the dynamic magnetization, we study the frequencies of spin excitations in small ferromagnetic bars with particular attention to the uniform mode. The calculation is done using micromagnetics, and the frequency of various modes is found as a function of the cell size used in the calculation. We find that the uniform mode and some standing spin-wave modes are relatively insensitive to changes in the cell size. In contrast, edge modes and corner modes show significant frequency changes as the cell size is changed in the micromagnetics calculation. These results justify the use of micromagnetics with larger cells to estimate the frequency of the uniform mode of micronsized magnetic bars. A comparison of experiment and theory shows that the micromagnetics estimate is significantly better than the usual Kittel formula with effective demagnetizing factors; however appreciable differences remain.