Weishi Tan

The geometric, optical, and magnetic properties of the endohedral stannaspherenes M@Sn12 (M=Ti, V, Cr, Mn, Fe, Co, Ni)

The geometric, optical, and magnetic properties of the M@Sn(12) clusters (M=Ti, V, Cr, Mn, Fe, Co, Ni) are studied using the relativistic density-functional method. The geometric optimization shows that the ground states of these clusters are probably very close to the I(h) structure. Our calculations demonstrate that the optical gaps of the M@Sn(12) can be tuned from infrared to green, and the magnetic moments of them vary from 2 mu(B) to 5 mu(B) by doping d transition metal atoms into Sn(12) cage, suggesting that M@Sn(12) could be a new class of potential nanomaterials with tunable magnetic and optical properties.

Multilevel Resistance Switching Memory in La2/3Ba1/3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (011) Heterostructure by Combined Straintronics-Spintronics

We demonstrate a memory device with multifield switchable multilevel states at room temperature based on the integration of straintronics and spintronics in a La2/3Ba1/3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) (011) heterostructure. By precisely controlling the electric field applied on the PMN-PT substrate, multiple nonvolatile resistance states can be generated in La2/3Ba1/3MnO3 films, which can be ascribed to the strain-modulated metal-insulator transition and phase separation of Manganite. Furthermore, because of the strong coupling between spin and charge degrees of freedom, the resistance of the La2/3Ba1/3MnO3 film can be readily modulated by magnetic field over a broad temperature range. Therefore, by combining electroresistance and magnetoresistance effects, multilevel resistance states with excellent retention and endurance properties can be achieved at room temperature with the coactions of electric and magnetic fields. The incorporation of ferroelastic strain and magnetic and resistive properties in memory cells suggests a promising approach for multistate, high-density, and low-power consumption electronic memory devices.

The theoretical search for half-metallic material: The non-stoichiometric peroskite oxide Sr2FeCoO6−δ

The non-stoichiometric peroskite oxide Sr2FeCoO6−δ is investigated and predicted to be half-metallic material using density-functional calculations. The results reveal that Sr2FeCoO5 shows antiferromagnetic half-metallic behavior and exhibits magnetic moment ordering with the magnetic moments of 2.97 and 3.72 μB on Fe(1) and Fe(2) sites, 2.96 and 3.20 μB on Co(1) and Co(2) sites antiparallel to those of Fe, and about 0.05 μB on O sites parallel to those of Co, respectively. Moreover, Sr2FeCoO4 and Sr2FeCoO6 both have half-metallic character. This hints that Sr2FeCoO6−δ possesses half-metallic nature in a large range of δ.

S doping effect on the properties of double perovskite La2FeMoO6

The effect of S-doping on the properties of double perovskite La2FeMoO6 (LFMO) is investigated by using density-functional calculations. The results reveal that the ground state of the series La2FeMoO6−xSx is ferrimagnetic, with I4/mmm symmetry for x = 0 and 6, and I4mm symmetry for the others. With increasing the amount of S ions, the volume increases nearly linearly. The estimated Neel temperature of the compounds decreases from 537 K for La2FeMoO6 to 454 K for La2FeMoS6 upon S doping. In addition, La2FeMoO6 and the end member La2FeMoS6 both exhibit half-metallic nature, while the others behave as full metal.

Three-dimensional strain state and spacer thickness-dependent properties of epitaxial Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3/Pr0.7Sr0.3MnO3 trilayer structure

Epitaxial colossal magnetoresistive trilayer structures consisting of ferromagnetic metallic Pr0.7Sr0.3MnO3 (PSMO) and antiferromagnetic insulator La0.5Ca0.5MnO3 (LCMO) were fabricated on (001)-oriented single crystal MgO substrates using pulsed laser deposition technique. The evolution of three-dimensional strain states and electrical and magnetic transport properties of PSMO/LCMO/PSMO trilayers have been studied as a function of LCMO spacer thickness and lattice strain. When the thickness of LCMO spacer is 6 nm, lattice strain in the trilayer begins to be relaxed. Furthermore, trilayers with thickness of LCMO spacer up to 36 nm are not fully strain relaxation. The unit cell volume of the films is not conserved and exhibits the variation with LCMO layer thickness. Strain relaxation states are determined by bulk strain (eB) and Jahn–Teller (eJT) strain together. The electrical and magnetic transport properties, including metal-insulator transition temperatures TMI and saturation magnetization MS, also sho...

Effect of high-pressure on the electronic and magnetic properties in double perovskite oxide Sr2FeMoO6

The high-pressure effect on the physical properties in double perovskite Sr2FeMoO6 is explored theoretically. The structure of Sr2FeMoO6 exhibits I4/mmm symmetry in the whole external hydrostatic pressure applied. A spin crossover of Fe ion from high-spin to low-spin state is found at the pressure of about 33–36 GPa, accompanied by a transition from ferrimagnetic half-metallic to nonmagnetic semiconductor state. When the pressure is above 45 GPa, the nonmagnetic compound behaves as full metal. By the mean-field theory, the estimated Curie temperature of the ferrimagnetic Sr2FeMoO6 increases from 472 to 639 K when the pressure increases from 0 to 30 GPa.The high-pressure effect on the physical properties in double perovskite Sr2FeMoO6 is explored theoretically. The structure of Sr2FeMoO6 exhibits I4/mmm symmetry in the whole external hydrostatic pressure applied. A spin crossover of Fe ion from high-spin to low-spin state is found at the pressure of about 33–36 GPa, accompanied by a transition from ferrimagnetic half-metallic to nonmagnetic semiconductor state. When the pressure is above 45 GPa, the nonmagnetic compound behaves as full metal. By the mean-field theory, the estimated Curie temperature of the ferrimagnetic Sr2FeMoO6 increases from 472 to 639 K when the pressure increases from 0 to 30 GPa.

Biaxial strain effect on the electronic and magnetic phase transitions in double perovskite La2FeMnO6: A first-principles study

The influence of biaxial strain on both the electronic and magnetic properties in double perovskite La2FeMnO6 is investigated by using density-functional calculations. The results show that La2FeMnO6 exhibits ferromagnetic semiconductor at ambient condition and turns into ferromagnetic half-metal under the whole tensile strain applied in this work, while the compound transfers into ferromagnetic metal under the compressive strain within −8% and into ferrimagnetic semiconductor with the compressive strain beyond −9%. For both ferromagnetic half-metallic and metallic La2FeMnO6, they exhibit very slight change in the electronic states and magnetic moments of the ions, comparing with those of the compound at ambient condition. The electronic configurations of Fe and Mn originating from the high-spin state Fe3+ and intermediate-spin state Mn3+ in ferromagnetic La2FeMnO6 emerge in the low-spin state and high-spin state in ferrimagnetic La2FeMnO6, respectively, when the compressive strain is beyond −9%.

Disorder effect on the electronic and magnetic properties of Sr2FeCoO6: A density-functional theoretical investigation

Using density-functional calculations within the generalized gradient approximation plus U framework, the electronic and magnetic properties of the ordered and the disordered Sr2FeCoO6 are investigated systematically. The results show that all disordered Sr2FeCoO6 have ferromagnetic ground state, as that of the ordered one. The magnetic coupling of both Fe and Co sublattices remains ferromagnetic, and the two sublattices are coupled ferromagnetically as well. Most interestingly, the disorder leads to the substantial increase of the magnetic moments on both Fe and Co sites comparing with those of the ordered Sr2FeCoO6. It is also significantly found that the disorder can control the transport property, decreasing the half-metallic band gap of the ordered Sr2FeCoO6 and even giving rise to a half-metal to metal transition at a critical level of disorder.

Manipulation of anisotropic magnetoresistance and domain configuration in Co/PMN-PT (011) multiferroic heterostructures by electric field

The electric field manipulation of magnetic anisotropy and domain configuration has been investigated in the artificial multiferroic Co/PMN-PT (011) heterostructure at room temperature. A uniaxial magnetic anisotropy is induced with the application of an electric field, which leads to an electrically switched anisotropic magnetoresistance with tunability as large as ∼29%. Furthermore, the magnetic domain structures of Co films are investigated by magnetic force microscopy under an in situ electric field, which exhibits direct evidence for electric field control of magnetism at the mesoscale. The converse magnetoelectric effect demonstrated in this multiferroic heterostructure has potential to be utilized in magnetoelectric devices with low power consumption.

Point defects and magnetic properties of neutron irradiated MgO single crystal

(100)-oriented MgO single crystals were irradiated to introduce point defects with different neutron doses ranging from 1.0×1016 to 1.0×1020 cm-2. The point defect configurations were studied with X-ray diffuse scattering and UV-Vis absorption spectra. The isointensity profiles of X-ray diffuse scattering caused by the cubic and double-force point defects in MgO were theoretically calculated based on the Huang scattering theory. The magnetic properties at different temperature were measured with superconducting quantum interference device (SQUID). The reciprocal space mappings (RSMs) of irradiated MgO revealed notable diffuse scattering. The UV-Vis spectra indicated the presence of O Frenkel defects in irradiated MgO. Neutron-irradiated MgO was diamagnetic at room temperature and became ferromagnetic at low temperature due to O Frenkel defects induced by neutron-irradiation.