The First-principles Calculations On Magnetism Of Doped PbTiO₃and NiO Nanotube

Materials that simultaneously exhibit ferroelectricity and Ferromagnetism(or antiferromagnetism) in the same phase are known as multiferroics. With potential applications in various fields, such as spin quantum devices, data accessors and detectors, etc., multiferroic materials have always been a hot spot in research. Recently, diluted magnetic semiconductors doping by non-magnetic elements, transition elements or rare-earth elements have been gradually attracted much attention. Using the first-principles calculations, we investigate the magnetism of PbTiO3doping by non-magnetic elements, transition elements or rare-earth elements in this thesis.As a prototype of strongly correlated materials, NiO has attracted a large number of attentions in the past decades. Meanwhile, it also has a wide variety of technological applications ranging from catalytic supports to spintronics. The magnetic structure of NiO consists of ferromagnetic sheets of Ni2+parallel to the (111) plane with opposite spin directions in neighboring sheets. Nanomaterial is a class of materials made up of basic units, at least one dimension of which are of size of1-100nm range. Their structure and properties are very different from those of bulk materials. In this paper, we use first-principles calculations to investigate the structure and magnetism of single-wall NiO nanotube.The main conclusions of our work are as follows:1.The stoichiometric bulk PbTiO3is non-magnetic, and there are two types of oxygen atoms in the ferroelectric PbTiO3unit cell. Spin polarization is induced by replacement of oxygen atoms by non-magnetic2p impurities (B, C and N), except replacing the oxygen atom inside the unit cell with nitrogen atom. The impurities concentration is4.2%. Large magnetic moments are found for the impurity centers. Smaller magnetic moments are induced on the oxygen atoms around impurities. Besides, some magnetic moments are also induced on the Ti atoms adjacent to boron atom. It is shown that the boron-doped PbTiO3may exhibit properties of a magnetic pseudo-half-metal, while the carbon-doped PbTiO3is expected to be a magnetic semiconductor, and the nitrogen-doped PbTiO3is a magnetic half-metal.2.Magnetism of transition-metal-doped PbTi1-XMXO3(M=V, Cr, Mn, Fe, or Co) is investigated. Magnetic moments are induced by V, Cr, Mn, and Fe, while Co-doped PbTi1-XMXO3is non-magnetic. Magnetic order alters with the distance between two doping atoms, when M is V, Mn, or Fe. The magnitude of magnetic moments increases at first and then decreases. It arrives the peak value when M is Mn. Most magnetic moments are found for3d electrons of transition metals. Much smaller magnetic moments are induced on the oxygen atoms around impurities. When replacing Ti with V, spin polarization is induced on one titanium ion adjacent to the vanadium ion.3.The magnetic property of ferroelectric (FE) and paraelectric (PE) PbTiO3doped with Bi or Gd atom is different. Magnetic moments are found on3d electrons of titanium ions, when PE PbTiO3doped with Bi or Gd atom. That Bi atom replaces with Pb atom in FE PbTiO3does not result in magnetism. Gd atom substituted for Pb atom in FE PbTiO3can cause small local magnetic moments, and spin polarization is induced on3d electrons. The difference in magnetic moments between the FE and PE phases indicates that the ferroelectricity strongly affects the ferromagnetism induced by the impurity. This fact implies potential magnetoelectric coupling.4.We fabricate (10,0) NiO single-wall nanotube, and perform ab initio calculations on its magnetism. Ni2+and O2-ions arrange in layers along the axial direction of the tube. Compared with bulk NiO, coordination number of ions in nanotube decreases, thus makes outer electrons more itinerant, that induces spin polarization of O2-ions. The magnetic moments of Ni2+and O2-ions are1.34μB and0.16μB. Ions belonging to the same plane experience a ferromagnetic coupling, while ions belonging to different planes experience an antiferromagnetic coupling. So the nanotube is antiferromagnetic.