Micromagnetic modeling of magnetization processes in FePt polytwin crystals

Magnetization processes in FePt polytwin crystals of $L10$ phase under external magnetic fields along different crystallographic directions are studied by using micromagnetic modeling. Computer simulations show highly anisotropic magnetic response of FePt polytwin crystals, and reveal strong effects of twin boundaries on magnetic domain structure evolutions. Due to high magnetocrystalline anisotropy of $L10$ phase, magnetic domain rotation is suppressed while magnetic domain wall motion plays a dominant role in magnetization processes. It is found that magnetostatic interaction between magnetic domains in neighboring twin crystals generates magnetic charges at the twin boundaries, which produce an internal magnetic field. The competition between the internal and external magnetic fields determines magnetic domain structures and their evolution paths, explaining the different characteristics of the magnetization curves under applied magnetic fields along different directions.