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One-dimensional nanoribbons usually exhibit considerably different properties compared to their monolayer counterparts due to the formation of edge states and limited width. In this study, we systematically investigate the stability, electronic structures and magnetic properties of CrSiTe3 single-layer nanoribbons with different edge configurations and ribbon widths using first-principles calculations. The results show that the edge energies (less than 0.4 eV/A) for all studied CrSiTe3 nanoribbons are much lower than that of graphene and many transition-metal dichalcogenide nanoribbons, indicating their stability and easy formation. Compared to the CrSiTe3 monolayer with ferromagnetic semiconductor characteristics, some of CrSiTe3 nanoribbons (N-SiCr-ZNRs, N-Te-ZNRs, N-TeCr-ANRs and N-Te-ANRs) become half-metal due to the hybridization between the d orbitals of edge Cr atoms and the p orbitals of edge Te atoms. While N-SiTe-ANRs are bipolar magnetic semiconductors, in which the states near Fermi level are localized around the nanoribbons edge. Our results show that CrSiTe3 single-layer nanoribbons are promising candidates suitable for applications in spintronic devices.

We study the exchange between electron and magnon spins at the interface of an antiferromagnet and a heavy metal at finite temperatures. The underlying physical mechanism is based on spin torque associated with the creation/annihilation of thermal magnons with right-hand and left-hand polarization. The creation/annihilation process depends strongly on the relative orientation between the polarization of the electron and the magnon spins. For a sufficiently strong spin transfer torque (STT), the conversion process becomes nonlinear, generating a nonzero net spin pumping current in the AFM that can detected in the neighboring metal layer. Applying an external magnetic field renders possible the manipulation of the STT driving thermal spin pumping. Our theoretical results are experimentally feasible and are of a direct relevance to antiferromagnet-based spintronic devices., 10 pages, 6 figures

We report a two-dimensional (2D) Dirac semimetal in an orthorhombic $\mathit{Pmma}$ structure, an NP monolayer, based on first-principles calculations. The NP monolayer is found to be stable according to phonon dispersion and molecular dynamical simulations. It exhibits a pair of anisotropic Dirac cones exactly at the Fermi level, located along high symmetry lines in the Brillouin zone, and shows highly anisotropic in-plane stiffness. First-principles calculations show that Dirac cones are locally stable under strains along the $x$ or $y$ direction and electric fields perpendicular to the monolayer. The Dirac nodes are protected from gap opening by the mirror-reflection symmetry in the $x$ direction. We construct an effective two-band tight-binding model to characterize the Dirac states. Our result provides a promising 2D material for special electronics applications.

We study the nonlinear gyrotropic motion of a magnetic skyrmion in a nanodisk by means of micromagnetic simulations. The skyrmion is driven by a linearly polarized harmonic field with the frequency of counterclockwise gyrotropic mode. It is found that the motion of the skyrmion displays different patterns with increasing field amplitude. In the linear regime of weak driving field, the skyrmion performs a single counterclockwise gyrotropic motion. The guiding center of the skyrmion moves along a helical line from the centre of the nanodisk to a stable circular orbit. The stable orbital radius increases linearly with the field amplitude. When the driving field is larger than a critical value, the skyrmion exhibits complex nonlinear motion. With the advance of time, the motion trajectory of the skyrmion goes through a series of evolution process, from a single circular motion to a bird nest-like and a flower-like trajectory and finally, to a gear-like steady-state motion. The frequency spectra show that except the counterclockwise gyrotropic mode, the clockwise gyrotropic mode is also nonlinearly excited and its amplitude increases with time. The complex motion trajectory of the skyrmion is the result of superposition of the two gyrotropic motions with changing amplitude. Both the linear and nonlinear gyrotropic motions of the skyrmion can be well described by a generalized Thiele’s equation of motion.

We investigate the electronic structure and magnetic properties of layered compounds SrBO 2 (B = Co, Ni, Mn) based on first principles calculations in the framework of density functional theory with GGA+ U method. We compute the phonon dispersion of these compounds to probe the dynamical stability and find that all the compounds are dynamically stable. SrCoO 2 and SrNiO 2 are G-type antiferromagnetic insulators, and SrMnO 2 is an A-type antiferromagnetic conductor. The electronic configurations of 3d orbitals are (d xz ) 2 (d z 2 ) 2 ( d yz ) 1 (d xy ) 1 (d x 2 - y 2 ) 1 and (d xz ) 2 (dy z ) 2 (d z 2 ) 2 (d xy ) 1 (d x 2 - y 2 ) 1 in SrCoO 2 and SrNiO 2 , respectively. SrCoO 2 shows a Jahn-Teller distortion ( a > b ) because the down-spin Co 3d electron occupies the degenerate (d xz , d yz ) levels. Using Monte Carlo simulations based on the Heisenberg model with exchange parameters obtained from first principles calculations, we obtain the Neel temperatures ( T N ) of SrCoO 2 and SrNiO 2 , which are 249 K and 85 K, respectively.

Advance in the field of two-dimensional (2D) Van der Waals layered magnetic materials have made the atomic-thickness spintronic devices possible in the near future. The application of the 2D layered magnetic materials inevitably involves the magnetization reversal. Here we study the magnetization reversal of the 2D layered material Fe3GeTe2 by means of magnetic and anomalous Hall resistance measurements. We find the reversal process of Fe3GeTe2 flake is temperature and thickness dependent. For thin flakes and at low temperatures, the hysteresis loops have perfect rectangular shapes with large coercivity, indicating the single domain characteristic during the reversal process. With increasing temperature, a cross-over from single domain to multi-domain structure occurs. For bulk and very thick flakes Fe3GeTe2, the nearly zero magnetic remanence and small coercivity demonstrates that the reversal process is accomplished through the formation of multi-domain structure and its gradual evolution with the magnetic field. The change of magnetization reversal with temperature and flake thickness is explained based on the competition between the long-range magnetic dipolar interaction and the perpendicular magnetic anisotropy. The results obtained in this work provide important reference for designing Fe3GeTe2-based spintronic devices.

Different from the extensive progresses on two-dimensional (2D) intrinsic ferromagnets, modulation of magnetism in a 2D antiferromagnet is less explored. In this work, we investigate the impact of strain on magnetism of monolayer and bilayer FePS3 antiferromagnet using first-principles calculations. Our results show that zigzag antiferromagnetic (AFM) state is the ground state of monolayer FePS3 and the AFM order is very robust to strain. The in-plane and out-of-plane magnetic anisotropy energies (MAEs) of monolayer FePS3 are 0.31 and 1.41 meV/cell, and MAE is mainly provided by Fe atoms. Interestingly, compressive strain can enhance monotonically MAE of monolayer FePS3 while the MAE experiences a fall-rise-fall process under tensile strain. In addition, interlayer coupling of bilayer FePS3 is quite sensitive to strain and undergoes an AFM-FM-AFM transition under both tensile and compressive strain. The out-of-plane direction is also energetically favorable for unstrained bilayer FePS3 with in-plane and out-of-plane MAEs of 0.58 and 1.54 meV/cell, respectively. with the decrease of strain from 8% to −8%, the MAE of bilayer FePS3 can be enhanced except a small decrease at the strain of −8%. These findings are essential for applications of FePS3 based low-dimensional antiferromagnetic spintronic devices.

Controllable creation of magnetic skyrmions in nanostructures is a prerequisite for the application of skyrmions in spintronics. Here, we propose a new method for the creation of skyrmions. We show by using micromagnetic simulations that the skyrmions can be nucleated by resonantly exciting one of the skyrmion intrinsic oscillation modes. We first studied the dynamics of skyrmion in a ferromagnetic nanodisk with perpendicular anisotropy. One breathing mode and two non-degenerate gyrotropic modes are identified. Then we applied a circular-polarized microwave field to excite the uniformly magnetized nanodisk. When the frequency of the driving field is equal to the eigenfrequency of the skyrmion gyrotropic mode, stable skyrmions can be created from the initial uniform state. The number of skyrmions can be effectively controlled by appropriately choosing the duration of the driving field or tuning the field amplitude.

We present our first-principles calculations of a new two-dimensional material, arsenic nitrogen monolayer. The structural, electronic, and mechanical properties are investigated in detail by means of density functional theory computations. The calculated binding energy and the phonon spectra demonstrate that the AsN can form stable monolayer in puckered honeycomb structure. It is a semiconductor with indirect band gap of 0.73 eV, and displays highly anisotropic mechanical properties. Strain has obvious influence on the electronic properties of AsN monolayer. It is found that in the armchair direction, a moderate compression strain (−12%) can trigger an indirect to direct band gap transition and a tensile strain of 18% can make the AsN becoming a stable metal. In the zigzag direction, a rather smaller strain than armchair direction (12% for compression and 8% for stretch) can induce the indirect band gap to metal transition.

System Science Philosophy is a knowledge system constructed of universal principle and law sequences. Alloy gene is a characteristic atom existing in the center of coordination cluster and occupying the lattice point of a lattice cell, and carries holographic information about valence electron structure, physical and thermodynamic properties obtained by alloy gene theory. Alloy gene potential energy curve function has developed traditional atom pair interaction potential functions into many atoms’ interaction function associated with valence electron structure, bond length, bond energy, which makes alloy gene Gibbs energy function can be established. Alloy gene Gibbs energy transmissive function has developed traditional partition function. Its equilibrium and sub-equilibrium transmissive modes produce alloy holographic network positioning bank, which is operable platform to achieve transformation from “trial and error” method to the “whole obtained from a few part” law for getting advanced alloys. It has become possible to launch alloy gene sequence project.

Most of the two-dimensional layered materials discovered so far are intrinsically nonmagnetic. Introducing ferromagnetism into these materials will expand their application greatly in the field of spintronics. Here we study the introduction of ferromagnetism into the PtTe2 monolayer via doping the 3d transition metal atoms and the group VA and VIIB atoms by means of the first-principles calculations. Results show that the magnetic order can be introduced in the V-, Mn-, Fe-, Cu- and Zn-doped PtTe2 monolayers, among which the V-, Cu- and Zn-doped monolayers have ferromagnetic ground state, and the Mn- and Fe-doped monolayers have antiferromagnetic ordering in ground state. Among the group VA atom doped systems, N-, P- and As-doped monolayers exhibit ferromagnetic, and Sb doped monolayer is a nonmagnetic metal. All the VIIB atom doped PtTe2 monolayers are nonmagnetic, while the doping makes the PtTe2 monolayer transit from an indirect bandgap semiconductor to metal. The findings obtained in this work may initiate new experimental exploration and expand the application of the layered transition-metal ditellurides in spintronics.

Spin-wave propagation in periodic magnetization-modulated cylindrical nanowires is studied by micromagnetic simulation. Spin wave scattering at the interface of two magnetization segments causes a spin-wave band structure, which can be effectively tuned by changing either the magnetization modulation level or the period of the cylindrical nanowire magnonic crystal. The bandgap width is oscillating with either the period or magnetization modulation due to the oscillating variation of the spin wave transmission coefficient through the interface of the two magnetization segments. Analytical calculation based on band theory is used to account for the micromagnetic simulation results.

Skyrmion-based magnonic crystal (MC) provides the dynamic tunability of manipulating magnonic band structure, and this brings obvious advantages over geometry or material-modulated MCs with a static band. But the existence of stable skyrmion usually requires strong Dzyaloshinskii–Moriya interaction (DMI) in combination with an external magnetic field under specific strength, and all these features limit the experimental realization and practical designing of the skyrmion-based MC. Here, we introduce the concept of artificial magnetic skyrmion-based MC. The artificial skyrmion lattice is realized by patterning an array of magnetic nanodisks on a thin film. The coupling between nanodisks and thin film generates an array of skyrmions possessing the same period as the nanodisk array. Via applying the pulsed magnetic field, one can turn on and off the skyrmion lattice, which allows switching between two very different magnonic band structures. Furthermore, via a honeycomb lattice, we extend this design to the dynamic on and off for chiral magnon edge state. The on and off switching is fast and in the range of nanoseconds. Considering that the coupling from nanodisks can greatly enhance the stability of skyrmions, no matter whether the DMI or magnetic field exists or not, our design points to a simple realization of dynamic skyrmion MC and topological magnonic devices.

Two-dimensional (2D) material of few-layer black phosphorus (BP) has recently attracted extensive interest owing to its tunable band gap and high carrier mobility. We investigate the electronic transport properties of zigzag black phosphorene nanoribbons (ZBPNRs) with asymmetric H, Li, O and Co edge saturations by employing the density functional theory in combination with the non-equilibrium Green's function. The computational results forecast that different types of saturated atoms at both edge of ribbons mainly contribute to the electronic transport properties of molecular junctions. The metal edge saturation of Co atom is used to the one edge of ZBPNR which can induce an identical electronic transport property. Interestingly, the negative differential resistance (NDR) phenomena can be observed in our proposed ZBPNR junctions with an analysis of internal physical mechanism. Our theoretical results could support the possibility of potential applications to design 2D electronic devices based on the material of BP in future.

Polarization is a fundamental property of waves that refers to the orientation of the oscillations. It has been widely used to encode information in photonics and phononics. However, the polarization of spin waves is rarely used yet in magnonics. The reason for this is that only the right-handed polarized spin waves can be accommodated in ferromagnets. Here, we report that stable left-handed polarized spin waves can be introduced into ferromagnets if a spin-polarized electrical current is presented. The right-handed and left-handed polarized spin waves coexist when the current density is larger than a critical value while the system keeps stable. The results are confirmed by micromagnetic simulations. This work provides new playgrounds to study spin waves and points to new findings for future experimental studies., 17 papes, 5 figures

We investigate the electronic structures and electronic transport properties of zigzag phosphorene nanoribbons with oxygen-saturated edges (O-zPNRs) by using the spin-polarized density functional theory and the nonequilibrium Green's function method. The results show that the O-zPNR is an antiferromagnetic (AFM) or ferromagnetic (FM) semiconductor with spins localized at two ribbon edges anti-parallel or parallel with each other. The electronic transmission for the single AFM or FM O-zPNR is zero when a bias voltage is applied to the two electrodes made of the same type O-zPNR. Nonzero transmission arises for the AFM-AFM and FM-FM O-zPNR heterojunctions. The transmission spectrum and the electrical current are fully spin polarized for the FM-FM O-zPNR heterojunction. An in-plane transverse electrical field can effectively manipulate the electronic structure and spin-dependent electronic transport. It induces splitting of the spins of the two edges and makes the AFM O-zPNR become a half metal. Moreover, the transverse electrical field gives rise to the transmission spectrum and the spin polarized electrical current for the AFM-AFM O-zPNR heterojunction. The degree of spin polarization can be tuned by the strength of the transverse field.

Two-dimensional (2D) half-metallic materials are essential to develop next-generation spintronic devices. Moreover, electrical controllability and room-temperature magnetism are two important ingredients for applications in spintronics. Here, we report findings of a combination of these properties in 2D honeycomb structures of group-V $\mathrm{N}X$ ($X$=N, P, As, Sb, Bi) binary compounds from first-principles calculations. These novel 2D materials are stable semiconductors with indirect band gaps. Hole doping can induce magnetism due to their Mexican-hat band edges and make them half-metals. Some of these half-metals exhibit inverse spin-polarization direction when changing the doping level, which can be achieved by changing the external voltage gate. Monte Carlo simulations based on the Ising model suggest the Curie temperatures of these half-metals are much higher than room temperature. These outstanding properties make monolayer $\mathrm{N}X$ promising candidates for spintronic applications.

Electronic spin current is convertible to magnonic spin current via the creation or annihilation of thermal magnons at the interface of a magnetic insulator and a metal with a strong spin-orbital coupling. So far this phenomenon was evidenced in the linear regime. Based on analytical and fulledged numerical results for the non-linear regime we demonstrate that the generated thermal magnons or magnonic spin current in the insulator is asymmetric with respect to the charge current direction in the metal and exhibits a nonlinear dependence on the charge current density, which is explained by the tuning effect of the spin Hall torque and the magnetization damping. The results are also discussed in light of and are in line with recent experiments pointing to a new way of non-linear manipulation of spin with electrical means., to appear in Phys. Rev. B., Rapid Comm

Ferromagnetism of two-dimensional layered materials is of considerable interest in spintronics. In the present work, we report the ferromagnetism of the layered material SnSe2 induced by Bi doping. The pure SnSe2 and Bi-doped SnSe2 film samples are prepared by chemical vapor transport method. X-ray diffractions show that both the SnSe2 and Bi-doped SnSe2 have CdI2-type layered structure grown perfectly along the c-axis. Magnetic measurements indicate that the pure SnSe2 is a diamagnet. The doping of nonmagnetic Bi introduces ferromagnetism in SnSe2 with Curie temperature of 65 K. A coercivity as high as 4.4 kOe is observed at T = 10 K. Such high coercivity has never been reported in d0 two-dimensional layered materials. First-principles calculations indicate that the Bi-doping leads to the spin splitting of degenerate px and py orbital states and gives rise to a spin order aligned in ab plane. Calculations also show that the Bi-doped SnSe2 is a half metal. All these features make Bi-doped SnSe2 a potential candidate for spintronic material.

We present first-principles calculations of electronic structures of a class of two-dimensional (2D) honeycomb structures of group-V binary compounds. Our results show these new 2D materials are stable semiconductors with direct or indirect band gaps. The band gap can be tuned by applying lattice strain. During their stretchable regime, they all exhibit metal-indirect gap semiconductor-direct gap semiconductor-topological insulator (TI) transitions with increasing strain from negative (compressive) to positive (tensile) values. The topological phase transition results from the band inversion at $\Gamma$ point due to lattice strain and is irrelevant to spin-orbit coupling (SOC)., Comment: 16 pages, 6 figures

The magnetic dynamical properties of the soft layer in exchange-coupled soft/hard bilayers have been investigated numerically using a one-dimensional atomic chain model. The frequencies and spatial profiles of spin wave eigenmodes are calculated during the magnetization reversal process of the soft layer. The spin wave modes exhibit a spatially modulated amplitude, which is especially evident for high-order modes. A dynamic pinning effect of surface magnetic moment is observed. The spin wave eigenfrequency decreases linearly with the increase of the magnetic field in the uniformly magnetized state and increases nonlinearly with field when spiral magnetization configuration is formed in the soft layer.

The electronic properties of two-dimensional puckered arsenene have been investigated using first-principles calculations. The effective mass of electrons exhibits highly anisotropic dispersion in intrinsic puckered arsenene. Futhermore, we find that out-of-plane strain is effective in tuning the band gap, as the material undergoes the transition into a metal from an indirect gap semiconductor. Remarkably, we observe the emergence of Dirac-like cone with in-plane strain. Strain modulates not only the band gap of monolayer arsenene, but also the effective mass. Our results present possibilities for engineering the electronic properties of two-dimensional puckered arsenene and pave a way for tuning carrier mobility of future electronic devices.

Magnonic crystals are prototype magnetic metamaterials designed for the control of spin wave propagation. Conventional magnonic crystals are composed of single domain elements. If magnetization textures, such as domain walls, vortices and skyrmions, are included in the building blocks of magnonic crystals, additional degrees of freedom over the control of the magnonic band structure can be achieved. We theoretically investigate the influence of domain walls on the spin wave propagation and the corresponding magnonic band structure. It is found that the rotation of magnetization inside a domain wall introduces a geometric vector potential for the spin wave excitation. The corresponding Berry phase has quantized value $4 n_w \pi$, where $n_w$ is the winding number of the domain wall. Due to the topological vector potential, the magnonic band structure of magnonic crystals with domain walls as comprising elements differs significantly from an identical magnonic crystal composed of only magnetic domains. This difference can be utilized to realize dynamic reconfiguration of magnonic band structure by a sole nucleation or annihilation of domain walls in magnonic crystals., Comment: 21 pages, 9 figures