Magnetization distribution and coercivity in multigrain materials

The effect of the interface or defect region between two ferromagnetic grains or variants on the dynamics of the magnetization process has been analyzed analytically. The grains can have arbitrary crystalline orientations with respect to each other and to the defect region. The composition of the defect region can also vary with respect to the host material. Boundary conditions are imposed on the direction of magnetization both at the far edges of the grains and at the two interfaces of the defect region. The sum of the exchange, anisotropy, and Zeeman energies is minimized and the resulting Euler equations are solved. The magnetization direction distribution throughout the defect region is determined as a function of the external magnetic field. It is shown that the magnetization rotation across this region deviates further from the Bloch wall form as the applied magnetic field increases. The coercivity as a function of defect width and difference in anisotropy orientations has also been determined for various applied fields along one of the crystalline axes. It is found that, for given defect widths, as the difference in orientation between the two grains increases (from 0degrees to 90degrees), the coercivity increases. We find that the dominant effect for the coercivity is the difference in magnetic anisotropy orientations between the two grains. Only when this difference is small does the change in chemical composition at the interface assume importance. (C) 2002 American Institute of Physics.