1.Laboratory of Phase Transtions and Microstructures in Condensed Matter Physics of Xinjiang, College of Physical Science and Technology, Yili Normal University, Yining 835000, China 2.National Laboratory of Solid State Microstructures, College of Physics, Nanjing University, Nanjing 210093, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11664042)
Received Date:01 May 2020
Accepted Date:29 May 2020
Available Online:16 June 2020
Published Online:05 October 2020
Abstract:It is still in controversy whether the transition metal doped TiO2 will generate room temperature ferromagnetism and where its magnetism originates from. In order to solve this problem, in this paper we use the GGA+U method based on density functional theory to conduct a first-principle study of the magnetic and optical properties for each of the systems of Ti0.875X0.125O2, X = Cr, Mn, Fe, Co. We calculate the ground state energy of each system, on the supposition that they are ferromagnetic or antiferromagnetic. After comparison, the ferromagnetic state is speculated to be its ground state. The binding energy and magnetism calculation results show that Ti0.875Cr0.125O2 has the best stability in all doped systems, that the transition metal element doped TiO2 system has a net magnetic moment, and that the doped systems are ferrimagnetic. In comparison, the net magnetic moment produced by Cr, Mn and Fe doped with TiO2 are substantial, showing that these three systems have good ferromagnetic properties. The Curie temperatures of all doped systems are above room temperature, which is of great significance for the electron spin to retain the information in dilute magnetic semiconductors (DMS), and also greatly helps with the practical application of magnetic materials. The analysis of the energy band structure reveals that intrinsic TiO2 is non ferromagnet, Ti0.875Cr0.125O2 and Ti0.875Mn0.125O2 maintain semiconductor properties, and Ti0.875Fe0.125O2 and Ti0.875Co0.125O2 exhibit metal characteristics. The doped systems produce room temperature ferromagnetism, the main magnetic source is the transition metal elements (Cr, Mn, Fe, Co) 3d electron orbit induced polarization of the surrounding O-2p state spin electrons, causing the systems to produce a net magnetic moment and be ferromagnetic. The absorption spectrum of the doped system is red-shifted, which shows that the doping causes the range of its absorption spectrum to extend to the visible range. At the same time, in all the doped systems in this article, Fe and Co doped TiO2 have the best light absorption intensity, and the magnetic property of the Fe doped system is the strongest, which indicates that when the system is ferromagnetic, the spin up and spin down splitting will occur in the local magnetic field, which will change the electronic structure of TiO2 and enhance its photocatalytic performance. The calculation results in this paper are of theoretical significance for preparing TiO2 with curie temperature above room temperature by being doped with transition metal elements of Cr, Mn, Fe, and Co. Keywords:TiO2/ electronic structure/ magnetism/ optical properties
表1掺杂前后体系结构参数计算结果 Table1.Calculation results of system parameters before and after doping.
图 2 (a) TiO2掺杂前后体系的结合能; (b)各掺杂TiO2体系自旋净磁矩与自旋密度模 Figure2. (a) Binding energy of TiO2 system before and after doping; (b) spin net magnetic moment and spin density mode of the doped TiO2 system.
图 1 结构模型 (a) 本征TiO2; (b) Ti0.875X0.125O2 (X = Cr, Mn, Fe, Co)
图 2 (a) TiO2掺杂前后体系的结合能; (b)各掺杂TiO2体系自旋净磁矩与自旋密度模
图 3 本征TiO2体系: (a)能带图; (b)态密度图
图 4 Cr掺杂TiO2体系 (a) 能带图; (b) 自旋态密度图; (c) Fermi能级附近放大图
图 5 Mn掺杂TiO2体系 (a) 能带图; (b) 自旋态密度图; (c) Fermi能级附近放大图
图 6 Fe掺杂TiO2体系 (a)能带图; (b)自旋态密度图
图 7 Co掺杂TiO2体系 (a) 能带图; (b) 自旋态密度图; (c) Fermi能级附近放大图
图 8 掺杂前后TiO2体系 (a) 吸收光谱图; (b) 可见光波长范围内的吸收光谱图