Investigations On Magnetic Properties Of Ni-doped SnO₂ By First-principles Calculations

Being one of the most hopeful host compound with high temperature ferromagnetism, the transition metal doping oxide compound semiconductors are considered to be very important for future semiconductor spintronics application. Recently, although a series of diluted magnetic oxide compound semiconductor with room temperature ferromagnetism have been prepared successfully, the experimental result, such as the Curie temperature and the magnetic moment, sensitively depend on the experimental condition, the preparation method and the substrate. Theoretically, the origin of ferromagnetism about the diluted magnetic oxide compound has been extensively investigated. However, the consistent conclusion has not been obtained. Ni is one of important 3d transition metal magnetic element, so the magnetism of Ni doped oxide semiconductor has been widely studied. The first principles calculations on Ni-doped TiO2 showed that the Ni atom which substitutes Ti in TiO2 has not magnetic moment and the Ni atom will have magnetic moment only when oxygen vacancy exist in Ni-doped system. In this close connection, the experiments report that the oxygen vacancy significantly influence on the magnetic properties of Ni-doped SnO₂ thin films and nano-particles. SnO₂ with the rutile structure is an important n type wide-band-gap semiconductor and it's band gap is about 3.6eV. In addition,Its excellent optical transparency, metal-like conductivity and the high chemical stability make it a highly multifunctional material with widespread applications. The mechanism of the magnetism in Ni-doped SnO₂ has not been studied by the calculations. First-principles calculation is a main tool to study the magnetic properties of transition metal doped oxide compound. In this paper,the electronic structure and magnetic properties of Ni doped SnO₂ with and without oxygen vacancy are investigated by the first-principles calculation in full potential linearized augmented plane wave(FLAPW)method based on the density functional theory, as implemented in the WIEN2k code. The thesis is arranged as follows:First, in order to explain the effect of oxygen vacancy on the magnetism in Ni-doped SnO₂, we study the magnetism of Ni-doped SnO₂ without oxygen vacancy. Two cases for one Ni substitution and two Ni substitutions are considered. The calculated DOS show that the Ni-3d and O-2p orbitals do not split. So the system without oxygen vacancy is nonmagnetic and the magnetic moment is zero, which consistent with the experimental result. The d_xy and dyz orbital locate below Fermi level, results in being occupied fully. The d_z², d_x²-y² and d_xz orbitals locate in the band gap and hybridize with a few of O-2p orbital. The number of Ni-3d electron in the muffin tins of Ni is about 7.8, which indicates that the valent state of Ni is close to +2, in agreement with the experimental result.Second, the pure Ni doping does not induce magnetism in SnO₂. But many experiments report that Ni doped-SnO₂ thin films have the room temperature ferromagnetism and the oxygen vacancy significantly influence on the magnetic properties of Ni-doped SnO₂ thin films and nano-particles. So we calculate Ni-doped SnO₂ with an oxygen vacancy. The calculations demonstrate that the total energy of the system is the lowest when an oxygen vacancy close to the Ni and the Ni-3d and O-2p orbitals split, which indicate that Ni and its surrounding O atoms in the Ni-doped SnO₂ with oxygen vacancy appear magnetic moment. The magnetic moment of supercell is 2.0μB. The valent state of Ni in the Ni-doped SnO₂ with oxygen vacancy is also close to +2.Third, Ni-doped SnO₂ system with oxygen vacancy shows ferromagnetism, so the magnetic coupling of Ni atoms are investigated. The Ni₁-Ni₄ configuration (the distance of Ni atoms is 7.419 A) and the Ni₁-Ni₃ configuration (the distance of Ni atoms is 4.737 ?) are considered. An oxygen vacancy is introduced near each Ni atom to compensate Sn+4 and Ni+2 ion charge difference. The result indicates that the energy difference between ferromagnetic and anti-ferromagnetic coupling for the Ni₁-Ni₄ configuration is only 2.0meV. So the ground state for the Ni₁-Ni₄ configuration is weak ferromagnetic state. The energy difference for the Ni₁-Ni₃ configuration is only -15.49meV, which suggests that the ground state for the Ni₁-Ni₃ configuration is anti-ferromagnetic state. The stronger polarization of Sn atom located in Ni-Vo-Sn-Vo-Ni chain induces the stronger magnetic coupling for the Ni₁-Ni₃ configuration. The magnetic moment difference of Ni ions and supercell between Ni₁-Ni₄ configuration and Ni₁-Ni₃ configuration indicate that the magnetism of materials also depend on the locations and the relative distance of Ni atom besides the concentration of oxygen vacancy and the doping. Fourth, the Ni-3d electron in Ni-doped SnO₂ being strong correlation, the electronic structure and the magnetic properties of Ni-doped SnO₂ are investigated by GGA+U. The result indicates that Ni magnetic moment for both two configurations slightly increase and the magnetic moments of supercell are more located on Ni atoms. The calculated energy difference by GGA+U shows that anti-ferromagnetic coupling between two Ni ions for Ni₁-Ni₃ configuration is weakened, while ferromagnetic coupling between two Ni ions for Ni₁-Ni₄ configuration is strengthened. The calculated Ni-3d states by GGA+U are more located than that by GGA.In conclusion, Ni-doped SnO₂ system without oxygen vacancy is nonmagnetic. Ni-doped SnO₂ system with oxygen vacancy is magnetic and the magnetic moments mainly locate on Ni atoms. For given Ni-Ni configurations, the polarization of atoms located on atom chain connecting Ni-Ni can induce the long-range ferromagnetic coupling between two Ni moments. GGA+U calculations show that the calculated Ni-3d states are more located and ferromagnetic coupling between two Ni ions is strengthened.