Potential Applications In Spintronics Research Articles

Multiple dimensions of spin-gapless semiconducting states in tetragonal Sr2CuF6

Spin-gapless semiconductors (SGSs), with intrinsic magnetism, $100%$ spin polarization, and zero-gap band crossing points, have attracted great scientific interest owing to their potential applications in spintronics. In this Letter, using first-principles calculations and symmetry analysis, we demonstrate that the realistic material ${\mathrm{Sr}}_{2}{\mathrm{CuF}}_{6}$ is a spintronic material with multiple dimensions of spin-gapless semiconducting states. Tetragonal ${\mathrm{Sr}}_{2}{\mathrm{CuF}}_{6}$ has a zero-dimensional zero-gap nodal point, a one-dimensional zero-gap nodal line, and a two-dimensional nearly zero-gap nodal surface in one spin direction. Moreover, it hosts a wide band gap in the other spin direction. Our results extend the SGS members from nodal point SGSs and nodal line SGSs to nodal surface SGSs. Furthermore, we report a SGS candidate in an experimentally realized material exhibiting different dimensions of zero-gap points in momentum space. It is hoped that spintronic materials with multiple dimensions of spin-gapless semiconducting states may have significant applications in new-generation spintronics.

Manipulated magnetic coercivity and spin reorientation transition in NiCo2O4 films

Half-metallic NiCo2O4 with high spin polarizability has great potential applications in spintronics. The manipulation of magnetic anisotropy is crucial for spintronics based on spin-transfer or spin–orbit torques, as it is directly related to the critical switching current density. Here, we report epitaxial growth of metallic NiCo2O4 film with perpendicular magnetic anisotropy on MgAl2O4 single crystal substrates. The modulation of the magnetic anisotropy was achieved by changing the growth conditions (deposition temperature and thickness) of NiCo2O4 films and by means of protonation. Strong dependence of magnetic coercivity on deposition temperature (350–500 °C) has been observed due to variable phase configuration. Furthermore, the magnetic coercive field can also be effectively controlled by the film thickness (3–78 nm) through strain relaxation. More importantly, spin reorientation transition has been achieved by proton and electron doping in the NiCo2O4 films, resulting in reconfigured valence states of Ni and Co cations and a magnetic easy axis rotation from out-of-plane to in-plane. The effective modulation of the magnetic anisotropy provides important insights into the functional design of NiCo2O4-based spintronics with ultralow energy dissipation.

Interfacial phase frustration stabilizes unconventional skyrmion crystals

Chiral magnetic phases with an unconventional topological twist in the magnetization are of huge interest due to their potential in spintronics applications. Here, we present a general method to induce such exotic magnetic phases using interfacial phase frustration within artificially grown superlattices. To demonstrate our method, we consider a multilayer with two different chiral magnetic phases as the competing orders at the top and bottom and show, using Monte Carlo calculations, that the interfacial phase frustration is realized at the central layer. In particular, we obtain three unconventional phases: a checkerboard skyrmion crystal, an incommensurate skyrmion stripe, and a ferrimagnetic skyrmion crystal. In these frustration-induced phases, the spin chirality-driven topological Hall conductivity can be largely enhanced. This method provides a playground to realize unconventional magnetic phases in any family of materials that can be grown in superlattices.

Geometric origin of intrinsic spin hall effect in an inhomogeneous electric field

In recent years, the spin Hall effect has received great attention because of its potential application in spintronics and quantum information processing and storage. However, this effect is usually studied under the external homogeneous electric field. Understanding how the inhomogeneous electric field affects the spin Hall effect is still lacking. Here, we investigate a two-dimensional two-band time-reversal symmetric system and give an expression for the intrinsic spin Hall conductivity in the presence of the inhomogeneous electric field, which is shown to be expressed through the geometric quantities: quantum metric and interband Berry connection. We show that for Rashba and Dresselhaus systems, the inhomogeneous intrinsic spin Hall conductivity can be tuned with the Fermi energy. On the other hand, when people get physical intuition on transport phenomena from the wave packet, one issue appears. It is shown that the conductivity obtained from the conventional wave packet approach cannot be fully consistent with the one predicted by the Kubo-Greenwood formula. Here, we attempt to solve this problem.

Magnetic band representations, Fu-Kane-like symmetry indicators, and magnetic topological materials

To realize novel topological phases and to pursue potential applications in low-energy consumption spintronics, the study of magnetic topological materials is of great interest. Starting from the theory of nonmagnetic topological quantum chemistry [Bradlyn et al., Nature 547, 298 (2017)], we have obtained irreducible (co)representations and compatibility relations (CRs) in momentum space, and we constructed a complete list of magnetic band (co)representations (MBRs) in real space for other magnetic space groups (MSGs) with antiunitary symmetries (i.e.,type-III and type-IV MSGs). The results are consistent with the magnetic topological quantum chemistry [Elcoro et al., Nat. Commun. 12, 5965 (2021)]. Using the CRs and MBRs, we reproduce the symmetry-based classifications for MSGs, and we obtain a set of Fu-Kane-like formulas of symmetry indicators (SIs) in both spinless (bosonic) and spinful (fermionic) systems, which are implemented in an automatic code---TopMat---to diagnose topological magnetic materials. The magnetic topological materials, whose occupied states cannot be decomposed into a sum of MBRs, are consistent with nonzero SIs. Lastly, using our online code, we have performed spin-polarized calculations for magnetic compounds in the materials database, and we find many magnetic topological candidates.

Electron mediation enhanced magnetocrystalline anisotropy and Curie temperature of FeCl2 monolayer by an electride substrate

Two-dimensional magnetic materials offer an ideal platform to study the fundamental research and potential applications in spintronics. However, it is still a challenging issue to raise their magnetic anisotropy energy and Curie temperature. Based on the density functional theory and the Monte Carlo method, we investigate the electron-mediated electronic structures and magnetic properties in a FeCl2/Ca2N heterostructure. It is found that a Ca2N monolayer devotes about 0.46 electrons/f.u. to a FeCl2 monolayer due to the low work function. This provides a high electron doping density of 4.28 × 1014 cm−2 and avoids the disorder effect of the traditional doping methods simultaneously such as the intercalation, adsorption, and substitution. Moreover, the magnetocrystalline anisotropy energy and Curie temperature of the FeCl2/Ca2N heterostructure enhance 66% and 100% compared with the FeCl2 monolayer, respectively. Our findings provide a practical and effective avenue to modulate the magnetic properties of two-dimensional magnetic materials.

First-Principles Study of Irn (n = 3-5) Clusters Adsorbed on Graphene and Hexagonal Boron Nitride: Structural and Magnetic Properties.

Magnetic clusters have attracted great attention and interest due to their novel electronic properties, and they have potential applications in nanoscale information storage devices and spintronics. The interaction between magnetic clusters and substrates is still one of the challenging research focuses. Here, by using the density functional theory (DFT), we study the structural stability and magnetic properties of iridium clusters (Irn, n = 3–5) adsorbed on two-dimensional (2D) substrates, such as graphene and hexagonal boron nitride (hBN). We find that the most favorable configurations of free Irn clusters change when adsorbed on 2D substrates. In the meantime, the magnetic moments of the most stable Irn reduce to 53% (graphene) and 23.6% (hBN) compared with those of the free−standing ones. Interestingly, about 12-times enlargement on the magnetic anisotropy energy can be found on hBN substrates. These theoretical results indicate that the cluster–substrate interaction has vital effects on the properties of Irn clusters.

Anomalous valley Hall effect and Nernst effect in strain engineered graphene

Anomalous valley Hall effect (AVHE), which forwards a strategy for combining valleytronics and spintronics, has recently attracted much interest. Usually, this effect is associated with the anomalous velocity acquired by the carriers due to the Berry curvature of the Bloch bands. Here we propose a new strategy to generate AVHE in a graphene-based normal/strained/normal junction, where AVHE originates from the spin-valley tunneling asymmetry for the transmission through the junction. When the system is driven by a temperature bias, an anomalous valley Nernst effect is demonstrated, in which the transverse current is completely spin- and valley-polarized simultaneously. In particular, the thermally induced longitudinal charge current can become zero with the finite transverse one, causing the ratio between them to be infinite, which is usually small for the Hall effect. It is expected that our findings could provide potential applications in valleytronics and spintronics.

Weyl-type nodal chains in X[formula omitted]MnO[formula omitted] (X = Li, Na)

Recently, magnetic topological semimetals have received a lot of attention due to their potential applications in spintronics. By using first-principles calculations, we propose that two ferromagnetic spinel materials of X2MnO4 (X = Li, Na) have Weyl-type nodal chains around the Fermi level. Their stability is validated by the cohesive energies, phonon dispersions, and elastic constants. The nodal chains are composed of two kinds of nodal loops that are protected by the glide operation M̃z, the mirror operation M1¯01 and their equivalents. The drumhead surface states are observed on the (001) surface, which exhibits nontrivial topological features. In addition, the compounds’ semimetal states are well preserved under electron correlation and lattice strains. Our work provides two promising candidates for exploring the combination of magnetic materials and topological semimetal states.

Electronic, Magnetic, and Optical Properties of Metal Adsorbed g-ZnO Systems

2D ZnO is one of the most attractive materials for potential applications in photocatalysis, gas and light detection, ultraviolet light-emitting diodes, resistive memory, and pressure-sensitive devices. The electronic structures, magnetic properties, and optical properties of M (Li, Na, Mg, Ca, or Ga) and TM (Cr, Co, Cu, Ag, or Au) adsorbed g-ZnO were investigated with density functional theory (DFT). It is found that the band structure, charge density difference, electron spin density, work function, and absorption spectrum of g-ZnO can be tuned by adsorbing M or TM atoms. More specifically, the specific charge transfer occurs between g-ZnO and adsorbed atom, indicating the formation of a covalent bond. The work functions of M adsorbed g-ZnO systems are obviously smaller than that of intrinsic g-ZnO, implying great potential in high-efficiency field emission devices. The Li, Na, Mg, Ca, Ga, Ag, or Au adsorbed g-ZnO systems, the Cr adsorbed g-ZnO system, and the Co or Cu adsorbed g-ZnO systems exhibit non-magnetic semiconductor proprieties, magnetic semiconductor proprieties, and magnetic metal proprieties, respectively. In addition, the magnetic moments of Cr, Co, or Cu adsorbed g-ZnO systems are 4 μ B, 3 μ B, or 1 μ B, respectively, which are mainly derived from adsorbed atoms, suggesting potential applications in nano-scale spintronics devices. Compared with the TM absorbed g-ZnO systems, the M adsorbed g-ZnO systems have more obvious absorption peaks for visible light, particularly for Mg or Ca adsorbed g-ZnO systems. Their absorption peaks appear in the near-infrared region, suggesting great potential in solar photocatalysis. Our work contributes to the design and fabrication of high-efficiency field emission devices, nano-scale spintronics devices, and visible-light responsive photocatalytic materials.

Transition metal substituted MoS2/WS2 van der Waals heterostructure for realization of dilute magnetic semiconductors

Atomically thin doped two dimensional (2D) layered materials manifest excellent magnetic features beneficial to the potential applications in spintronics. With the implementation of extensive first principles calculations, we demonstrate the MS2 (M = Mo, W) monolayers, as well as their van der Waals (vdW) hetero-bilayers as promising candidates for the successful realization of 2D dilute magnetic semiconductors with the incorporation of Mn and Co dopants. Under various pairwise doping configurations at different host atom sites, we report the electronic properties modifications induced change in magnetic exchange interactions. The magnetic coupling among the dopant pairs can be tuned between FM and AFM orderings via suitable doping adjustments. The developed interlayer exchange coupling between the vdW layers leads to strong and long-ranged ferromagnetic interactions which unleash robust magnetic moments with stable doping configurations. Our findings address the novel magnetic behavior of the layered vdW heterostructures and may further guide the future experimental efforts for the possible applications in modern electronics and nanoscale magnetic storage devices.

Air-stable ultrathin Cr3Te4 nanosheets with thickness-dependent magnetic biskyrmions

Two-dimensional (2D) materials with intrinsic magnetism have drawn intense interest for fundamental research and potential application in spintronics and valleytronics. Here we synthesized 2D Cr3Te4 nanosheets with controllable thickness by chemical vapor deposition approach. Reflection magnetic circular dichroism and magneto-transport measurements demonstrated that the Cr3Te4 nanosheets feature excellent environmental stability and have a thickness-dependent ferromagnetic behavior with a high Curie temperature of 165–235 K. Lorentz transmission electron microscopy studies revealed topologically nontrivial magnetic-spin states in the ultrathin Cr3Te4 nanosheets. The magnetic stripe domains in Cr3Te4 can transform into biskyrmion bubbles when an external magnetic field is applied perpendicularly to the nanosheet. The size and density of the biskyrmions can be tuned by the external applied magnetic field and thickness of the nanosheets, respectively. Our findings of topologically nontrivial magnetic-spin states in air-stable 2D magnets could promise new designs of magnetic memory storage, sensors, and spintronics.

Half-metallic magnetism in triazines g-C4N3and g-C3N4tailored with H, Li, and 2p elements

The graphitic carbon nitride g-C4N3, since its experimental realisation, and particularly being proven a free - standing monolayer with half - metallic ferromagnetism, has drawn growing research interest in spintronics. Here, the author presents a study using density functional calculations, on the electronic structure and magnetic ordering of g-C4N3 and its sister monolayer triazine g-C3N4, tailored with H, Li, and 2p elements. Half-metallic ferromagnetism of pristine g-C4N3 is first elucidated with an emphasis on its localised spin magnetic moments due to pyridinic N-sp2 electrons. The author then proposes various scenarios tailoring the structure of these two materials, in order to induce itinerant magnetism therein, towards greater potential applications in spintronics. The key to that problem lies in the chemistry of H, Li, and other 2p atoms at the graphitic N or C and vacant sites of our crystal lattices. Interestingly, in certain elaborate scenarios involving pairs of 2p adatoms at the latter, the author have obtained new half-metallic material structures where both localised and itinerant magnetism mechanisms coexisted. This result is explained in a clear physicochemical picture where the effect of spin - charge transfer shapes the electronic structures of these new monolayers.

Vanadium-Doped Molybdenum Diselenide Atomic Layers with Room-Temperature Ferromagnetism.

Diluted two-dimensional magnetic semiconductors with high Curie temperature are highly sought after because of their potential applications in spintronics. Development of new techniques for preparation of high quality diluted magnetic semiconductors is critical for their applications. In this study, vanadium-doped molybdenum selenide, a new diluted magnetic semiconductor, was synthesized by a single-step chemical vapor deposition method. The merit of this method is that the molybdenum and vanadium precursors can be supplied to the growth substrate uniformly. Photoluminescence measurements reveal that the band gap of MoSe2 decreases after doping, which can be attributed to the formation of impurity energy band caused by p-type doping at the valence band maximum. Thus, the V-doped MoSe2 still maintains the semiconducting characteristics. Vibrating sample magnetometer studies clearly show the ferromagnetism of V-doped MoSe2 at room temperature. DFT calculations illustrates the joint contribution of V dopants and nearby atoms to the magnetic moments. This study provides future prospects for the multifunctional application of two-dimensional materials.

Two-dimensional transition metal triborides: Monolayers with robust intrinsic magnetism and high spin stability

Two-dimensional materials with intrinsic magnetism have provoked comprehensive attentions due to their potential application in spintronics. In this work, a class of stable transition metal triborides, $\mathrm{TM}{\mathrm{B}}_{3}$ ($\mathrm{TM}=\mathrm{Ti}$, V, Cr, Mn, Fe) with various electronic and magnetic features were predicted by density functional theory. Based on static energy, phonon spectrum, and $ab\phantom{\rule{0.16em}{0ex}}initio$ molecular dynamics simulations, all the $\mathrm{TM}{\mathrm{B}}_{3}$ monolayers have thermal and kinetic stability. Meanwhile, $\mathrm{TM}{\mathrm{B}}_{3}\mathrm{s}$ exhibit mechanical anisotropy reflected by Young's modulus and Poisson's ratio. Moreover, $\mathrm{Ti}{\mathrm{B}}_{3}$ and $\mathrm{Fe}{\mathrm{B}}_{3}$ are robust ferromagnetic metals with the respective Curie temperatures of 248 and 367 K. $\mathrm{Cr}{\mathrm{B}}_{3}$ is antiferromagnetic semiconductor with direct bandgap of 513.6 meV and magnetic transform temperatures of as high as 378 K. Finally, it is predicted that the magnetic properties can be further modulated by exerting external strains. Our results may provide new clues for searching and designing two-dimensional magnetic materials.

Controllable spin diode based on a semiconductor quantum dot

We theoretically propose an all-electrically controlled spin-current diode consisting of a quantum dot sandwiched between one normal electrode and one ferromagnetic electrode. By applying a spin bias V S across one electrode, the spin current distribution shows a rectification effect; that is, in the forward spin bias regime, a spin current can tunnel through the junction, while in the reverse bias regime, the current is tiny. Such asymmetry in the spin-current profile suggests diode-like behaviour with respect to the spin bias. Moreover, the polarity direction of this spin-current diode can be manipulated and reversed by adjusting the gate voltage, which is much more feasible than the approach with traditional charge-current diodes. The present device can be realized by current technologies and has potential applications in spintronics or quantum information processing.

Magnetism and spin transport at permalloy/Cu1−xTbx interfaces

The significance of spin transport over an interface in energy-efficient spintronic devices has stimulated interest in the spintronic society during the last few decades. Here, interfaces of $\text{permalloy}/{\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ (Py/Cu-Tb) were investigated in depth. As the Cu-Tb thickness increases, we found that the saturation magnetization of the bilayers falls and then plateaus. Element-specific x-ray magnetic circular dichroism studies suggest that the Tb moment aligns opposite to the Fe and Ni moments, forming a self-assembled antiferromagnetic interface. As a result, the Cu-Tb adjacent layer to Py and the interface have a significant impact on spin transport. Relevant parameters, such as spin mixing conductance, spin diffusion length, and damping, can be tuned by inserting a thin Cu layer between Py and Tb or varying the compositions of Cu-Tb alloys. Using rare-earth Tb, we provide an effective method for controlling the spin transport and magnetism of ferromagnet/normal-metal interfaces. This approach is expected to have a great deal of potential in spintronic applications.

Electronic and vibrational properties of bulk Cr2Ge2Te6 from first-principles calculations

We use density functional theory based ab initio calculations to investigate the structural, vibrational, magnetic, and electronic properties of the layered ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT) that has attracted attention for potential spintronic applications. We optimize the structure for a fixed $c/a$ ratio. Our results are in excellent agreement with experimental data on structure, phonons, and electronic properties. The use of van der Waals interactions and relativistic spin-orbit coupling yields accurate lattice constants and interlayer distances. Simultaneously, the adopted theoretical methods lead to a rigorous description of the vibrational normal modes, as well as the valence bands, that are in excellent agreement with Raman spectroscopy and angle-resolved photoemission spectroscopy spectra, respectively. The magnetic moment is slightly overestimated, and the magnetic anisotropy has the correct sign but is greater in magnitude than that in experiment. We find that inclusion of the on-site Coulomb repulsion on $\mathrm{Cr}\text{\ensuremath{-}}3d$ orbitals worsens agreement with experiment, in contrast to previous studies. Our results provide a path toward ab initio analysis of magnetic heterostructures based on CGT.

Helicity dependent Wigner phase shift for photoionization in a circularly polarized laser field

The sensitivity of strong-field ionization to atomic orbital helicity has attracted much attention from physicists, due to its potential application in attosecond spectroscopy and spintronics. In order to intuitively observe the physical mechanisms of helicity-dependent ionization rates during photoionization, the concept of the Wigner phase can be used to characterize the different interactions between the rotating electrons and the Coulomb potential. Here, we find that in both one- and three-photon ionization schemes, the electrons liberated more easily by the circularly polarized laser field suffer less influence of the Coulomb potential during the propagation and then accumulate less Winger phase. This result indicates that the strength of the interaction between the rotating electrons and the Coulomb potential can explain the helicity-dependent ionization for different ionization mechanisms universally, which is also supported by our classical ensemble analysis. Our work provides an intuitive perspective towards the physics picture of ionization propensity rules.

Real-space observation of non-collinear spin structure in centrosymmetric TbGa rare-earth magnet

The exploration of non-collinear spin texture in the f-electron-based rare-earth magnets has attracted fascinating interest for its fundamental physics and potential spintronic applications. Here, the real-space observation of nanometric helical spin order with the period down to 20 nm in centrosymmetric TbGa magnets was presented by Lorentz transmission electron microscopy. It was found that the helical period and the initial temperature for the appearance of the periodic helix can be tuned by the thickness of single-crystal TbGa plates (71–169 nm). Furthermore, the domain evolution under the perpendicular magnetic fields and with the shape constriction reveals the high stability of the stripe domains.