【作者】 尤瑟夫；

【导师】 闫羽；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2010， 博士

【摘要】 采用基于密度泛函理论(DFT)的第一原理计算研究了Fe, Co, Ni金属在高压下铁磁态的稳定性。计算结果表明高压下Fe, Co, Ni金属的铁磁态是不同的,与晶体结构密切相关。对于Fe,在常压下呈现铁磁性的体心立方结构(bcc), GGA和GGA+U计算表明其分别在12和115Gpa由铁磁性的体心立方结构(bcc)转变为非磁性的六角(hcp)结构。对于金属Co, GGA计算发现在压力为107GPa附近,Co从铁磁性的hcp转变到非磁性的fcc结构。与Fe和Co不同,Ni在200Gpa的高压下也能保持面心立方(fcc)的铁磁态。计算的DOS表明,Fe, Co, Ni磁性的压力抑制是由于压力引起的费米能级附近的态密度下降造成的。采用第一原理计算研究了B掺杂ZnO的电子结构与磁性,GGA与GGA+U的计算表明替代Zn的B不产生磁性,替代O的B产生约3.0μB的磁矩,磁矩主要由替代的B和次近邻的O原子的2p轨道及其近邻Zn的2p与3d轨道产生,并且B的原子磁矩最大。总能量的计算表明B掺杂ZnO的基态是反铁磁的(AFM)。更多还原

【Abstract】 The appearance of ferromagnetism in bcc Fe, hcp Co, and fcc Ni metals is one of the most fundamental questions in solid-state physics. Although a large number of experimental and theoretical studies have been carried out for a deeper understanding of the nature of magnetism in these metals, it is still not a trivial problem why ferromagnetism is stable in these 3d metals under ambient conditions. Therefore, we investigate the stability of the ferromagnetic (FM) state in Fe, Co, and Ni metals under high pressure using generalized gradient approximation (GGA) and GGA+U within the density functional theory (DFT). It is found that the ferromagnetic state under pressure is very different for Fe, Co, and Ni metals, and is closely associated with the crystal structure. In the case of Fe, a ferromagnetic bcc ground states obtained at ambient pressure and the GGA calculations show that the Fe metal undergoes structural phase transitions from the most stable phase FM bcc to NM hcp phase at around 12 GPa, while calculated phase transitions by GGA+U occurred around 115 GPa. Moreover, the calculations show that the bcc to hcp structure transition in Fe is accompanied by the disappearance of magnetism in the hcp Fe. For Co, the phase transition from a ferromagnetic hcp to a nonmagnetic fcc is found around 107 GPa for GGA. In contrast to Fe and Co, a ferromagnetic fcc state in Ni is maintained even at 200 GPa. The calculated results suggest that the suppression of ferromagnetism in Fe, Co, and Ni is due to pressure-induced decrease of the density of state at the Fermi level.Compared to conventional magnetic semiconductors, one obvious advantage of d0 ferromagnetism is that clusters or secondary phases formed by the dopant do not contribute to magnetism. However, the mechanism of d0 ferromagnetism is not well understood. A complete understanding of the physics of d0 ferromagnetism is essential for identifying robust DMS for practical applications. The structural, electrical and optical properties of B-doped ZnO films (ZnO:B), are widely studied. In order to explore possible magnetic properties of B-doped ZnO, the first-principles calculations are performed to study the electronic structures and magnetic properties of nonmagnetic B doped ZnO. Both generalized gradient approximation (GGA) and GGA+U calculations show that substitutional B atom at Zn site can not introduce magnetic moment in ZnO, while a substitutional B atom at O site in ZnO induces magnetic moment of about 3.0μB. The moment mainly comes from delocalized p orbitals of B atom and its second neighboring 0 atoms, and the p and d orbitals of its nearest neighboring Zn atoms. The addition of UZn lead to downward shift of the Zn 3d states, which weakens the hybridization of the Zn 3d and O 2p states, while the UO+UB correction enlarge splitting between the occupied and unoccupied 2p states of B and its second neighboring 0 atom. For three approximations, the impurity states near the Fermi level remains almost the same. Accordingly, the magnetic moment and its distribution calculated by the GGA, GGA+UZn and GGA+UO+UB are almost the same. Moreover, the AFM state is the ground state for B-doped ZnO. The calculated DOS and the spin density distribution show that long-range magnetic coupling between the magnetic moment of two B atoms are mediated by the overlap of the spin density.更多还原