Composite magnetic thin films consisting of layers of different magnetic materials in close molecular contact with one another are studied. Because of ferromagnetic exchange coupling between spins, the magnetization in the neighborhood of the interface of the layers is expected to rotate continuously as in Bloch walls. The experimental magnetization curve of an extremely soft magnetic layer on an extremely hard magnetic layer agrees well with the theoretical curve based on such rotation models. Drastic deformations in critical switching curves from the asteroid of conventional uniaxial films, improvement in nondestructive reading characteristics, and existence of a new switching mode which is dependent upon the sequence of external magnetic fields are also expected from a theory based on a lumped-constant approximation to the continuous rotation model. All of these theoretical predictions have been confirmed by experiments using electroplated composite films.

A mathematical framework is presented for solving the Landau-Lifshitz-Gilbert equation expressed in Cartesian components of magnetization according to the backward difference method without conflicting with the constraint of constant magnetization. Test calculation shows that the method allows the use of a large time step almost independent of spatial mesh size and damping constant. The derived program is used to calculate the magnetization structure of a crosstie wall in a Permalloy film yielding calculated structures which closely resemble the electron-holography image of an actual cross-tie wall. It is also used to investigate magnetization reversal mechanisms in fine ferromagnetic particles by pursuing time dependent changes in magnetization structures. The paper gives detailed descriptions of the reversal mechanisms which differ depending on the size of the particle.

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It is confirmed that the calculation of the demagnetizing field in micromagnetic simulations can be accelerated significantly by using the discrete convolution theorem and the fast Fourier transform (FFT). When the magnetization distribution is periodic, application of the theorem to the demagnetizing field calculation is straightforward. Unlike the previously reported FFT method which is based on the continuous Fourier transform of the demagnetizing field, the method can also be used in the case of non-periodic magnetization structures. It is also confirmed that the result obtained using the new FFT method coincides with that of the conventional direct method, as expected. The principle of calculation and the results of one- and two-dimensional calculations which show the validity and effectiveness of the developed method are presented.

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