Micromagnetic Theory of Phase Transitions in Inhomogeneous Ferromagnets III. Non‐Local Landau‐Ginzburg Theory

In amorphous ferromagnets spatially fluctuating exchange interactions give rise to a critical behaviour substantially different from crystalline magnets. Within the framework of a molecular field theoretical approach, a non‐local Landau‐Ginzburg equation is formulated for phase transitions in amorphous ferromagnets taking into account cooperative spin fluctuations. This is done by introducing a correlated molecular field, which can only vary on larger scale than the spin correlation length. Therefore, the manner how the structural disorder affects the critical behaviour changes with temperature. To some extent, the theory interpolates between the so‐called homogeneous and inhomogeneous versions of the molecular field theory so far used in the literature. The theory is capable of reproducing satisfactorily the experimentally observed curvatures of the Arrott plots and the temperature dependence of the paramagnetic susceptibility in disordered ferromagnets.