The First Principle Study for Magnetic Properties of d0 Nano-materials

Abstract

It is common knowledge that the magnetism is originated from narrowing d-band in 3d transition metals. This can be accounted from band and Stoner's theory. Narrow d-band causes the high density of states at Fermi level and then, this leads the spontaneous spin polarized state in materials. For example, Fe, Ni, and Co have the ground state for ferromagnetic state. However, late transition metals, such as 4d and 5d metal, do not show ferromagnetic state. This can be well understood from Stoner's theory.

As mention above, one can observe magnetism only Fe, Ni, and Co in bulk structure, except 4f materials. However, many experimental researches have reported that pure oxide materials show magnetic signal in recent. HfO2 is the most representative material showing the magnetic state without any 3d transition metal dopant. Of course, magnetism can be observed in other pure oxide materials, such ZnO, TiO2, and CaO. The origin of pure oxide material is considered as cation vacancy defect, and many results have been reported. These phenomena also observed in surface state, and we have called this to d0 magnetism.

In this thesis, we have investigated the origin of d0 magnetism in 2p materials with nano-structure. We have used the art-of-state full potential linearized augmented plane wave (FLAPW) method. The d0 magnetism have two origin, atomic vacancy defect and surface effect.

In section 5.1 and 5.2, we have consider atomic defect in TiO2 and ZnO bulk systems. We have obtained that both TiO2 and ZnO show magnetic state with O and Zn atomic defect, respectively. Also, we can observe that the lattice distortion is very important factor, because both materials do not show magnetic state if we do not consider lattice distortion. From this, we can assume that the atomic vacancy defect causes lattice distortion and lattice distortion bring magnetism in TiO2 and ZnO.

To study the effect of surface, we have calculated MgO(001) surface in section 5.3. We observed that the Mg vacancy defect can induced a spin polarization in oxygen atoms around the Mg vacancy site whereas the O vacancy defect has no influence on the magnetic state. We have obtained that the total magnetic moment per unit cell increases as the Mg vacancy defect moves to the surface layer.

Previous studies show that vacancy defect induced magnetic state around O atom, and the p-orbital is essential role. Thus, other 2p materials, such C and N, can show magnetic signals. To proof this, we have calculated carbon doped ZnO in section 5.4. Also, Ultrathin N and C films on MgO(001) surface have been investigated in section 5.5. MgN and MgC films on MgO(001) surface show ferromagnetic and half metallic features. One can see the details in section 5.6.

In section 5.7 and 5.8, magnetic properties of 5d and 4d transition metal is calculated. In nature, 4d and 5d transition metal is nonmagnetic materials, but these materials can be induced magnetic state by forming nano-structure or surface (ultrathin films). We have find that ultrathin 4d and 5d metal on NiAl(001) surface have giant perpendicular magnetic anisotropy energy.