Micro-theoretical Investigation On Layered Magnetic Materials

The magnetic film is a hard magnetic material with its thickness smaller than 1μm. Such materials provide a new development stage of low dimension for magnetic devices. It has been found that experimentally when the thickness of magnetic film is nano sized, the material behaves a strong effect of size. Currently, various counterpart films of magnetic bulk materials are outstanding and unusual in magnetism. The artificial superlattice, sandwiched films and tunneling magneto-resistance have come into being. Therefore, the theoretical study on multi-layer magnetic films has attracted more and more interest of scholars, including Kaneyoshi T, who has great achievement in investigating the magnetic characters of bilayer systems, but the study on layered spin system thirsts for further perfection.By use of effective-field theory with correlations, differential operator technique and decoupling approximation, this dissertation has systematically studied some physical properties of four-sublattice bilayer systems and a three layer superlattice.The magnetization and susceptibility of the four-sublattice bilayer systems consisting of AB-CD with spins 1/2-1/2, 1/2-3/2, 1-1 and 1-3/2 are studied by applying a transverse or longitudinal magnetic field. Formulas for internal energy are given. The phase diagrams for spin 1/2-1/2 system and 1/2-3/2 system are depicted. The results show that the longitudinal magnetic field increases the saturation magnetization of the system and makes the system hard to demagnetize, while the transverse magnetic filed decreases the saturation magnetization and critical temperature of the system.A three layer superlattice is studied. The system in use is consisting of bilayer AA, BB and CC with spins 1/2, 3/2 and 1/2. Formulas for magnetization and internal energy are given. It can be seen from the numerical results that the longitudinal magnetic field changes the saturation magnetization and decreases the peak of susceptibility. When the crystal field is fixed at D/J_α=-3.5, the BB sublayer is in±1/2 state, instead of in±3/2 state.