Magnetization-Ripple Internal Fields in Thin Magnetic Films Measured by Ferromagnetic Resonance

The internal fields presumably due to magnetization ripple are measured using parallel ferromagnetic resonance techniques. At these frequencies (>400 MHz) the ripple does not have time to follow the average magnetization, hence its full internal field can be detected. In low-frequency techniques (<10 KHz) only the pinned ripple walls significantly contribute. To properly measure the resonance condition, the point at which the phase angle between the average magnetization and the rf field is 90° is detected. The resulting internal fields have the sign and power dependences which agree with Harte's nonlinear ripple theory. It was assumed that the internal field Hi is the following function of the uniform effective field on the average magnetization, Hu; Hi=K/Hun. From film to film, the measure of n's ranged from 0.4 to 1.6. Therefore, according to Harte's model, the physical size of the inhomogeneities causing magnetization ripple can range from the size of the crystallites to many microns. The measurements also indicate that the total effective field on the average magnetization, Heff=Hu+Hi, versus Hu has a minimum. This is contrary to Harte's non-linear theory. The discrepancy is attributed to severe pinning of the ripple walls, which occurs for small values of Hu. When this happens, the equilibrium conditions on which Harte bases has calculations are no longer true.