Xingxing Li

Half-Metallicity in MnPSe3 Exfoliated Nanosheet with Carrier Doping

Searching two-dimensional (2D) half-metallic crystals that are feasible in experiment is essential to develop next-generation nanospintronic devices. Here, a 2D exfoliated MnPSe3 nanosheet with novel magnetism is first proposed based on first-principles calculations. In particular, the evaluated low cleavage energy and high in-plane stiffness indicate that the free-standing MnPSe3 nanosheet can be exfoliated from its bulk structure in experiment. The MnPSe3 nanosheet is an antiferromagnetic semiconductor at its ground state, whereas both electron and hole doping induce its transition from antiferromagnetic semiconductor to ferromagnetic half-metal. Moreover, the spin-polarization directions of 2D half-metallic MnPSe3 are opposite for electron and hole doping, which can be controlled by applying an external voltage gate. The Monte Carlo simulation based on the Ising model suggests the Curie temperature of the doped 2D MnPSe3 crystal is up to 206 K. These advantages render the 2D MnPSe3 crystal with great potentials for application in electric-field controlled spintronic devices.

Bipolar magnetic semiconductors: a new class of spintronics materials

Electrical control of spin polarization is very desirable in spintronics, since electric fields can be easily applied locally, in contrast to magnetic fields. Here, we propose a new concept of bipolar magnetic semiconductors (BMS) in which completely spin-polarized currents with reversible spin polarization can be created and controlled simply by applying a gate voltage. This is a result of the unique electronic structure of BMS, where the valence and conduction bands possess opposite spin polarization when approaching the Fermi level. BMS is thus expected to have potential for various applications. Our band structure and spin-polarized electronic transport calculations on semi-hydrogenated single-walled carbon nanotubes confirm the existence of BMS materials and demonstrate the electrical control of spin-polarization in them.

CrXTe3 (X = Si, Ge) nanosheets: two dimensional intrinsic ferromagnetic semiconductors

Two-dimensional (2D) ferromagnetic semiconductors hold a great potential for nano-electronic and spintronic devices. Nevertheless, their experimental realization remains a big challenge. Through first-principles calculations, we here demonstrate the possibility of realizing 2D ferromagnetic semiconductors simply by exfoliating layered crystals of CrXTe3 (X = Si, Ge). The exfoliation of CrXTe3 is feasible due to its small cleavage energy, and CrXTe3 nanosheets can form free-standing membranes. Interestingly, upon exfoliation, the ferromagnetism and semiconducting character are well preserved from bulk to the nanosheet form. Long-range ferromagnetic order with a magnetization of 3 μB per Cr atom is confirmed in 2D CrXTe3 from classical Heisenberg model Monte Carlo simulations. Both bulk and 2D CrXTe3 are indirect-gap semiconductors with their valence and conduction bands fully spin-polarized in the same direction, which is promising for spin-polarized carrier injection and detection. We further demonstrate the tunability and enrichment of the properties of CrXTe3 nanosheets via external operations. Under moderate tensile strain, the 2D ferromagnetism can be largely enhanced. By pure electron doping or adsorbing nucleophilic molecules, CrXTe3 nanosheets become 2D half metals.

Room-Temperature Half-Metallicity in La(Mn,Zn)AsO Alloy via Element Substitutions

Exploring half-metallic materials with high Curie temperature, wide half-metallic gap, and large magnetic anisotropy energy is one of the effective solutions to develop high-performance spintronic devices. Using first-principles calculations, we design a practicable half-metal based on a layered La(Mn0.5Zn0.5)AsO alloy via element substitutions. At its ground state, the pristine La(Mn0.5Zn0.5)AsO alloy is an antiferromagnetic semiconductor. Either hole doping via (Ca(2+)/Sr(2+),La(3+)) substitutions or electron doping via (H(-)/F(-),O(2-)) substitutions in the [LaO](+) layer induce half-metallicity in the La(Mn0.5Zn0.5)AsO alloy. The half-metallic gap is as large as 0.74 eV. Monte Carlo simulations based on the Ising model predict a Curie temperature of 475 K for 25% Ca doping and 600 K for 50% H doping, respectively. Moreover, the quasi two-dimensional structure endows the doped La(Mn,Zn)AsO alloy a sizable magnetic anisotropy energy with the magnitude of at least one order larger than those of Fe, Co, and Ni bulks.

SiN-SiC nanofilm: A nano-functional ceramic with bipolar magnetic semiconducting character

Nowadays, functional ceramics have been largely explored for application in various fields. However, magnetic functional ceramics for spintronics remain little studied. Here, we propose a nano-functional ceramic of sphalerite SiN-SiC nanofilm with intrinsic ferromagnetic order. Based on first principles calculations, the SiN-SiC nanofilm is found to be a ferromagnetic semiconductor with an indirect band gap of 1.71u2009eV. By mean field theory, the Curie temperature is estimated to be 304u2009K, close to room temperature. Furthermore, the valence band and conduction band states of the nanofilm exhibit inverse spin-polarization around the Fermi level. Thus, the SiN-SiC nanofilm is a typical bipolar magnetic semiconductor in which completely spin-polarized currents with reversible spin polarization can be created and controlled by applying a gate voltage. Such a nano-functional ceramic provides a possible route for electrical manipulation of carriers spin orientation.

Electrical control of carriers' spin orientation in the FeVTiSi Heusler alloy

The direct control of carrier spin by an electric field at room temperature is one of the most important challenges in the field of spintronics. For this purpose, we here propose a quaternary Heusler alloy FeVTiSi. Based on first principles calculations, the FeVTiSi alloy is found to be an intrinsic bipolar magnetic semiconductor in which the valence band and conduction band approach the Fermi level through opposite spin channels. Thus the FeVTiSi alloy can conduct completely spin-polarized currents with a tunable spin-polarization direction simply by applying a gate voltage. Furthermore, by Monte Carlo simulations based on the classical Heisenberg Hamiltonian, the Curie temperature of the FeVTiSi alloy is predicted to be well above the room temperature. The bipolar magnetic semiconducting character and the room temperature magnetic ordering endow the FeVTiSi alloy with great potential for developing electrically controllable spintronic devices working at room temperature.

Control of spin in a La(Mn,Zn)AsO alloy by carrier doping

The control of spin without magnetic field is one of the challenges in developing spintronic devices. In an attempt to solve this problem, we proposed a novel hypothetical La(Mn0.5Zn0.5)AsO alloy from two experimentally synthesized rare earth element transition metal arsenide oxides, i.e. LaMnAsO and LaZnAsO. On the basis of the first-principles calculations with strong-correlated correction, we found that the La(Mn0.5Zn0.5)AsO alloy is an antiferromagnetic semiconductor at the ground state, and a bipolar magnetic semiconductor at the ferromagnetic state. Both electron and hole doping in the La(Mn0.5Zn0.5)AsO alloy induce the transition from antiferromagnetic to ferromagnetic, as well as semiconductor to half metal. In particular, the spin-polarization direction is switchable depending on the doped carrier type. As carrier doping can be realized easily in experiment by applying a gate voltage, the La(Mn0.5Zn0.5)AsO alloy stands as a promising spintronic material to generate and control the spin-polarized carriers with an electric field.