Yongjia Zhang

Transformation of electrical transport from variable range hopping to hard gap resistance in Zn1−xFexO1−v magnetic semiconductor films

Transformation of the electrical transport from the Efros and Shklovskii [J. Phys. C 8, L49 (1975)] variable range hopping to the “hard gap” resistance was experimentally observed in a low temperature range as the Fe compositions in Zn1−xFexO1−v ferromagnetic semiconductor films increase. A universal form of the resistance versus temperature, i.e., ρ∝exp[TH∕T+(TES∕T)1∕2], was theoretically established to describe the experimental transport phenomena by taking into account the electron-electron Coulomb interaction, spin-spin exchange interaction, and hard gap energy. The spin polarization ratio, hard gap energy, and ratio of exchange interaction to Coulomb interaction were obtained by fitting the theoretical model to the experimental results. Moreover, the experimental magnetoresistance was also explained by the electrical transport model.

Magnetic properties of transition metal doped AlN nanosheet: First-principle studies

We carry out our first-principles calculations within density functional theory to study the 3d transition metal (TM) doped AlN nanosheets. The calculated results indicate that a stoichiometric AlN nanosheet is graphene-like structure and nonmagnetic. The TM impurities can induce magnetic moments, localized mainly on the 3d TM atoms and neighboring N atoms. Our calculated results of TM-doped nanosheet systems indicate a strong interaction between 3d orbit of TM atom and the 2p orbit of N atoms. In addition, the Mn- and Ni-doped AlN nanosheet with half-metal characters seems to be good candidates for spintronic applications. When substituting two Al atoms, the relative energies of the states between ferromagnetic and antiferromagnetic coupling are investigated sufficiently. The exchange coupling of Co- and Ni-doped AlN nanosheets exhibits a transformation with different distances of two TM atoms and that of Cr-, Mn-, and Fe-doped AlN nanosheets is not changed.

Magneto-optical Kerr rotation enhancement in Co–ZnO inhomogeneous magnetic semiconductor

The polar Kerr rotation and ellipticity spectra of the as-deposited and annealed Zn1−xCoxO inhomogeneous magnetic semiconductors were measured. The Kerr rotation spectra versus the photon energy can be greatly modulated by adjusting the Co concentration or annealing the samples. Moreover, the observed maximal Kerr rotation, 0.72°, in an annealed sample is higher than that of pure Co films, Pt∕Co multilayers, and PtxCo1−x alloys. The enhanced Kerr rotation in the annealed samples can be explained by the fact that the annealed samples became a nanocomposite system consisting of Co clusters and Zn1−xCoxO magnetic semiconductor.

Room-temperature magnetoelectric coupling in nanocrystalline Na0.5Bi0.5TiO3

Nanocrystalline Na0.5Bi0.5TiO3 was prepared by sol-gel method. The nanocrystalline Na0.5Bi0.5TiO3 plates present room temperature ferromagnetism (FM). The reduction of FM for the plate with subsequent long time and high temperature air-annealing indicating that the observed ferromagnetism is connected with the vacancies at/near the surface of nanograins. For Na0.5Bi0.5TiO3 powders after annealing at 900 °C in air, a subsequent annealing in vacuum at 900 °C for 20 min weakens the room-temperature FM, but a subsequent annealing in oxygen atmosphere at 900 °C for 20 min enhances the room-temperature FM, which means that the observed FM in nanocrystalline Na0.5Bi0.5TiO3 may originate from cation vacancy at/near the surface of nanograins. The results of density functional theory calculation with the local density approximation plus on-site effect (LDA + U) method on the magnetism of Na0.5Bi0.5TiO3 (100) surface show that Na vacancies can introduce a nonzero magnetic moment. The Na0.5Bi0.5TiO3 plates annealed a...

Magnetoelectric coupling in nanocrystalline Pb0.82La0.18TiO3

The combined effect of Pb vacancies and La dopant can induce ferromagnetism below 75 K for Pb0.82La0.18TiO3 plates annealed at 700,1000, and 1200 °C for 1 h. Pb0.82La0.18TiO3 plates annealed at 1000 °C for 1 h show the coexistence of ferromagnetism resulting from oxygen vacancies at/near surfaces of nanograins and ferroelectricity near room temperature. Under application of magnetic field, the dielectric constant decreases and the ferroelectric transition temperature shifts to high temperature for Pb0.82La0.18TiO3 multiferroic plates. In addition, the electric field treatment leads to an enormous enhancement of saturation magnetization for Pb0.82La0.18TiO3 multiferroic plates, showing a strong magnetoelectric coupling.

Controllable spin-polarized electrical transport in wide-band-gap oxide ferromagnetic semiconductors

A family of wide-band-gap ternary oxide ferromagnetic semiconductor films with high transition metal concentration was prepared. The resistivity of these films can be changed up to four orders of magnitude by varying the composition or the concentration of the oxygen vacancies. Moreover, all these films show common features in electrical transport, i.e., Mott variable range hopping (VRH) in the lower resistivity range, Efros VRH in the middle resistivity range, and “hard gap” resistance in the higher resistivity range. The above phenomena are well understood by considering the relative magnitude of three characterization lengths, i.e., Coulomb screening length, localization length of the carriers, and optimal hopping distance. Furthermore, spin polarization ratio of these magnetic semiconductors was obtained by fitting the experimental results of electrical transport. Therefore, the wide gap oxide ferromagnetic semiconductors with controllable spin-polarized electrical transport are expected to have appli...

Ferromagnetism induced by intrinsic defects and boron substitution in single-wall SiC nanotubes.

On the basis of density functional theory (DFT) methods, we study the magnetic properties and electronic structures of the armchair (4, 4) and zigzag (8, 0) single-wall SiC nanotubes with various vacancies and boron substitution. The calculation results indicate that a Si vacancy could induce the magnetic moments in both armchair (4, 4) and zigzag (8, 0) single-wall SiC nanotubes, which mainly arise from the p orbital of C atoms surrounding Si vacancy, leading to the ferromagnetic coupling. However, a C vacancy could only bring about the magnetic moment in armchair (4, 4) single-wall SiC nanotube, which mainly originates from the polarization of Si p electrons, leading to the antiferromagnetic coupling. In addition, for both kinds of single-wall SiC nanotubes, magnetic moments can be induced by a boron atom substituting for C atom. When two boron atoms locate nearest neighbored, both kinds of single-wall Si(C, B) nanotubes exhibit antiferromagnetic coupling.

Room-temperature ferromagnetism induced and controlled by electric field in BaNbO3 films

The application of electric field can induce the room-temperature ferromagnetism (FM) in paramagnetic BaNbO3 films on the conductive Nb:SrTiO3 substrate. The electric field-induced ferromagnetism in BaNbO3 is connected with the hybridization between electrons of Nb-4d1 and the injected electrons from the conductive substrate, possibly leading to large density of states at the Fermi level and satisfying the Stoner criterion for ferromagnetism. By application of electric field in forward and reversed directions, the magnetization of electric field-induced FM can be adjusted in some extent. The application of electric field can also enhance the oxygen vacancies-induced room-temperature ferromagnetism in BaNbO3 films.

Electrical transport properties of (CoxAl1−x)2O3−v oxide magnetic semiconductor and corresponding Co–Al2O3 granular films

(CoxAl1−x)2O3−v oxide magnetic semiconductor films were synthesized by introducing an impurity band in the insulating Al2O3 band gap, and the corresponding granular films were obtained by annealing. For both kinds of films, their electrical transport properties are well described by spin dependent variable range hopping mechanism instead of the usually expected intergrain tunnelling. The magnetoresistance was also discussed.

Magnetic Properties in

Based on first-principles study, the magnetic properties of the substitution for Li by A ( A=Ca, Sr, and Ba) and for O by M ( M=F, Cl, and Br) in LiNbO3 have been investigated. These substitutions in LiNbO3 could bring about the magnetic moments, which mainly arise from the s orbital of Nb atoms surrounding the doping element. The magnetism induced by the substitutions in ferroelectric LiNbO3 provides one alternative route to fabricate the multiferroic materials.