Chuan Tian

Prediction for room-temperature half-metallic ferromagnetism in the half-fluorinated single layers of BN and ZnO

On the basis of density functional calculations we explored the possibility that BN and ZnO single layers can become ferromagnetic at room temperature and half-metallic upon fluorine adsorption. Fluorine atoms form stable bonds only with the B and Zn atoms on the graphenelike BN and ZnO single layers, respectively, and the resulting fluorinated BN and ZnO single layers are expected to become ferromagnetic half metals with Curie temperatures around room temperature. The fluorination of semiconductor single layers can lead to interesting spintronics and functional applications.

Theoretical Analysis of the Spin Exchange and Magnetic Dipole-Dipole Interactions Leading to the Magnetic Structure of Ni3TeO6

The origin of the collinear antiferromagnetic magnetic structure of Ni(3)TeO(6) below 52 K was analyzed by calculating its spin exchanges on the basis of density functional calculations, and the cause for the parallelc-spin orientation found for this magnetic structure by calculating the spin-orbit coupling and magnetic dipole-dipole interaction energies. The calculated exchanges correctly predict the observed magnetic structure below 52 K, and lead practically to no spin frustration. The perpendicularc- and parallelc-spin orientations are predicted by the spin-orbit coupling and the magnetic dipole-dipole interactions, respectively. However, the magnetic dipole-dipole interactions are stronger than the spin-orbit coupling interactions, and hence are responsible for the spin orientation observed for Ni(3)TeO(6).

On the magnetic insulating states, spin frustration, and dominant spin exchange of the ordered double-perovskites Sr2CuOsO6 and Sr2NiOsO6: density functional analysis.

The ordered double-perovskites Sr(2)MOsO(6) (M = Cu, Ni) consisting of 3d and 5d transition-metal magnetic ions (M(2+) and Os(6+), respectively) are magnetic insulators; the magnetic susceptibilities of Sr(2)CuOsO(6) and Sr(2)NiOsO(6) obey the Curie-Weiss law with dominant antiferromagnetic and ferromagnetic interactions, respectively, and the zero-field-cooled and field-cooled susceptibility curves of both compounds diverge below ∼20 K. In contrast, the available density functional studies predicted both Sr(2)CuOsO(6) and Sr(2)NiOsO(6) to be metals. We resolved this discrepancy on the basis of systematic density functional calculations. The magnetic insulating states of Sr(2)MOsO(6) are found only when a substantially large on-site repulsion is employed for the Os atom, although it is a 5d element. The cause for the divergence between the zero-field-cooled and field-cooled susceptibility curves in both compounds and the reason for the difference in their dominant magnetic interactions were investigated by examining their spin exchange interactions.

Magnetic Structure and Ferroelectric Polarization of MnWO4 Investigated by Density Functional Calculations and Classical Spin Analysis

The ordered magnetic states of MnWO4 at low temperatures were examined by evaluating the spin exchange interactions between the Mn2+ ions of MnWO4 on the basis of first principles density functional calculations and by performing classical spin analysis with the resulting spin exchange parameters. Our work shows that the spin exchange interactions are frustrated within each zigzag chain of Mn2+ ions along the c direction and between such chains of Mn2+ ions along the a direction. This explains the occurrence of a spiral-spin order along the c and a directions in the incommensurate magnetic state AF2, and that of a up arrow up arrow down arrow down arrow spin order along the c and a directions in the commensurate magnetic state AF1. The ferroelectric polarization of MnWO4 in the spiral-spin state AF2 was examined by performing Berry phase calculations for a model superstructure to find that the ferroelectric polarization occurs along the b direction, in agreement with experiment.

π-Back-Donation Effect of the Cyanide Ligands on the Electron Correlation and Charge Transfer in Prussian Blue RbMn[Fe(CN)6]

The temperature-induced charge transfer between the Mn and Fe sites in RbMn[Fe(CN)(6)] was analyzed by density functional theory calculations. Our analysis indicates that the extent of electron correlation (equivalently, the pairing energy or the on-site repulsion) is much greater for the Mn(n+) ion than for the Fe(n+) ion (n = 2, 3). This surprising and counterintuitive finding is a consequence of the pi-back-donation effect of the CN ligands.

Color properties and structural phase transition in penta- and hexacoordinate isothiocyanato Ni(II) compounds.

We investigated the optical properties of (NBu(4))(3)[Ni(NCS)(5)], a pentacoordinate Ni compound, and compared the results with the more traditional hexacoordinate analogue (NEt(4))(4)[Ni(NCS)(6)]. On the basis of our complementary electronic structure calculations, the color properties of this high spin complex can be understood in terms of excitations between strongly hybridized orbitals with significant Ni d and ligand character. Variable temperature vibrational studies show mode softening with decreasing temperature and splitting near 200 K, trends that we attribute to improved low temperature intermolecular interactions and a weak structural phase transition, respectively.