Promising Applications In Spintronic Devices Research Articles

High-topological-number skyrmions with tunable diameters in two-dimensional frustrated J1−J2 magnets

Skyrmions are intriguing quasiparticles in the field of condensed matter due to their unique physics and promising applications in spintronic devices. However, despite abundant studies on skyrmions with a topological charge of Q = 1, there have been only few on those with higher Q (≥2) due to their intrinsic instability in Dzyaloshinskii–Moriya interaction (DMI) systems. In this work, applying the frustrated J1−J2 Heisenberg spin model, we investigate the stability of high-Q skyrmions and the manipulation of their diameters in a hexagonal close-packed lattice through atomistic simulations and first-principles calculations. First, three spin textures, called spiral, skyrmion, and ferromagnetic, are identified by varying (J1, J2), and it is shown that skyrmions with higher Q can occupy a wider range of (J1, J2) values. The diameter of the skyrmions can then be finely tuned using the frustration strength (|J2/J1|), the single-ion anisotropy (K), and an external magnetic field (B). As B increases, the high-Q skyrmions split into skyrmions with smaller Q and can be annihilated by a larger B. Furthermore, we find that the CoCl2 monolayer satisfies the criteria for a frustrated J1−J2 magnet, and its magnetic behaviors align with the aforementioned conclusions. In addition, high-Q skyrmions are identified in the CoCl2 monolayer, and the corresponding energy barriers for skyrmion collapse are investigated. Our findings pave the way for prospective spintronic applications based on high-Q and nanoscale skyrmionic textures.

Chirality induced spin selectivity in chiral hybrid organic-inorganic perovskites.

Chiral materials exhibit many interesting physical properties including circular dichroism, circularly polarized photoluminescence, and spin selectivity. Since its discovery, chirality-induced spin selectivity (CISS) has been demonstrated in many chiral material systems, which indicates promising applications in spintronic devices. Thus, searching for compounds that possess both sizable chirality and excellent spin transport properties is in order. Hybrid organic-inorganic perovskites have attracted intensive research interest due to their long carrier lifetime, high carrier mobility, chemically tunable electronic properties, and long spin lifetime, which make this emerging class of semiconductors promising candidate for spintronics. Moreover, hybrid perovskites integrate inorganic octahedral framework and organic ligands, which may introduce chirality into the materials, especially in quasi-two-dimensional structures. Recently, CISS has been observed in 2D chiral hybrid perovskites, showing the spin filtering effect. Studies of CISS in chiral hybrid perovskites not only help deepen our understanding of CISS mechanism but also shed new light on designing novel spintronic devices. In this review, we summarize the state-of-the-art studies of CISS effect in 2D chiral hybrid organic-inorganic perovskites system. We also discuss the remaining challenges and research opportunities of employing CISS in next-generation spintronic devices.

Manipulating two-dimensional magnetic states via electric field and pressure.

Topological spin configurations have been an intriguing topic due to the exotic transport properties and promising applications in spintronic devices. The discovery of two-dimensional (2D) magnetic materials such as CrI3 provides new platforms for manipulating magnetic structures. Here, by first-principles calculations and Monte Carlo methods, we investigated the exchange interaction and magnetic states of 2D van der Waals ferromagnetic/ferroelectric heterostructure CrI3/In2Se3. By switching the polarization in the ferroelectric In2Se3 layer under an electric field and changing the interlayer distance between CrI3 and In2Se3 under pressure, four spin configurations, ferromagnetic states, topological domain wall skyrmions, topological bimerons, and stripe domains can be realized. These striking tunable magnetic states can be understood from the Dzyaloshinskii-Moriya interaction and single-ion anisotropy parameters being modified by switching the polarization and changing the interlayer distance. Our results of controllable topological/non-topological spin states broaden the spin phenomena and potential of spintronic applications in van der Waals heterostructures.

Superior spin transport properties based on VS2 and VCl2 ferromagnetic monolayers

Two-dimensional ferromagnetic monolayers have attracted growing interest due to their promising applications in spintronic devices. To explore the potential application of monolayer VS2 and VCl2 in spintronic devices, previously reported ferromagnetic semiconductor and half-metal, respectively, we investigate the spin transport properties of VS2 homo-junction, VCl2 homo-junction, and lateral VS2–VCl2 heterostructure using first-principles combined with non-equilibrium Green's function. We show that monolayer VS2 exhibit superior spin Seebeck effect along an armchair direction, monolayer VCl2 is an excellent platform to realize a spin valve, and the magnetoresistance ratio is up to 1.3 × 104. Moreover, the VS2–VCl2 heterostructure exhibits an excellent spin diode effect. We explain these effects from the calculated spin-dependent band structure and transmission spectrum. The superior spin transport properties make monolayer VS2 and VCl2 promising candidates for spintronic applications.

Quantum anomalous Hall effect of Dirac half-metal monolayer TiCl3 with high Chern number

Based on first-principles calculations, monolayer TiCl3 is predicted to be a 100% spin-polarized Dirac half-metal with ferromagnetic Curie temperature T c of 63 K as predicted from Monte Carlo simulations. When considering the spin-orbit coupling, the Dirac point in the spin-up opens a ∼3 meV band gap. The calculated result of the anomalous Hall conductivity shows the Chern number , indicating that three corresponding gapless chiral edge states have emerged inside the bulk gap. Our findings suggest a feasible new member of the quantum anomalous Hall insulator family with promising applications in spintronic devices without dissipation edge states.

Theoretical study of enhanced ferromagnetism and tunable magnetic anisotropy of monolayer CrI3 by surface adsorption

Magnetic anisotropy energy (MAE) plays a key role for 2D magnetic materials, which have attracted significant attention for their promising applications in spintronic devices. Based on first-principles calculations, we have investigated the influence of surface adsorption on the ferromagnetism and MAE of monolayer CrI3. We find that Li adsorption can dramatically enhance its ferromagnetism, and tune its easy magnetization axis to the in-plane direction from original out-of-plane at certain coverage of Li. The monotonic enhancement of in-plane magnetism in CrI3 as the coverage of Li increases are attributed to electrostatic doping induced by charge transfer between Li atoms and I atoms, as supported by the charge doping simulation. The tunable robust magnetic anisotropy may open new promising applications of CrI3–based materials in spintronic devices.

First-principles investigation of a new 2D magnetic crystal: Ferromagnetic ordering and intrinsic half-metallicity.

The development of two-dimensional (2D) magnetic materials with half-metallic characteristics is of great interest because of their promising applications in spintronic devices with high circuit integration density and low energy consumption. Here, by using density functional theory calculations, ab initio molecular dynamics, and Monte Carlo simulation, we study the stability, electronic structure, and magnetic properties of a OsI3 monolayer, of which crystalline bulk is predicted to be a van der Waals layered ferromagnetic (FM) semiconductor. Our results reveal that the OsI3 monolayer can be easily exfoliated from the bulk phase with small cleavage energy and is energetically and thermodynamically stable at room temperature. Intrinsic half-metallicity with a wide bandgap and FM ordering with an estimated TC = 35 K are found for the OsI3 monolayer. Specifically, the FM ordering can be maintained under external biaxial strain from -2% to 5%. The in-plane magnetocrystalline anisotropy energy of the 2D OsI3 monolayer reaches up to 3.89 meV/OsI3, which is an order larger than that of most magnetic 2D materials such as the representative monolayer CrI3. The excellent magnetic features of the OsI3 monolayer therefore render it a promising 2D candidate for spintronic applications.

Rashba-like spin splitting along three momentum directions in trigonal layered PtBi2

Spin-orbit coupling (SOC) has gained much attention for its rich physical phenomena and highly promising applications in spintronic devices. The Rashba-type SOC in systems with inversion symmetry breaking is particularly attractive for spintronics applications since it allows for flexible manipulation of spin current by external electric fields. Here, we report the discovery of a giant anisotropic Rashba-like spin splitting along three momentum directions (3D Rashba-like spin splitting) with a helical spin polarization around the M points in the Brillouin zone of trigonal layered PtBi2. Due to its inversion asymmetry and reduced symmetry at the M point, Rashba-type as well as Dresselhaus-type SOC cooperatively yield a 3D spin splitting with αR ≈ 4.36 eV Å in PtBi2. The experimental realization of 3D Rashba-like spin splitting not only has fundamental interests but also paves the way to the future exploration of a new class of material with unprecedented functionalities for spintronics applications.

Fully spin-polarized open and closed nodal lines in β -borophene by magnetic proximity effect

Recently, the experimental realization of Dirac fermions in ${\ensuremath{\beta}}_{12}$- and ${\ensuremath{\chi}}_{3}$-boron sheets has attracted tremendous attention and simulated a broad exploration for the novel topologically nontrivial states in two-dimensional materials. Herein, by combining first-principles and tight-binding calculations, we discover a coexistence of open nodal arc and closed nodal loop produced by three-band touching in the recently synthesized $\ensuremath{\beta}$-boron sheet [Q. Zhong et al., Phys. Rev. Mater. 1, 021001 (2017)]. The symmetry analysis reveals that the nodal lines are protected by both time-reversal symmetry (TRS) and mirror-reflection symmetry (MRS). Intriguingly, when TRS is broken, e.g., via introducing magnetic proximity effect, a topological phase transition occurs, namely, the original spin-degenerate nodal line (loop) decays into fully spin-polarized nodal line (loop). Furthermore, when Rashba spin-orbit coupling (SOC) is turned on to break the MRS, spin-up and -down states are shifted toward opposite momentum directions, leading to the emergence of two new nodal loops around the $\mathrm{\ensuremath{\Gamma}}$ point. In addition, inclusion of both Rashba SOC and exchange field leads to band-gap opening of the Dirac points and nodal lines. Our findings not only reveal a form of nodal line in the $\ensuremath{\beta}$-boron sheet, but also offer an alternative approach to realize spin-polarized nodal line semimetals with promising applications in spintronic devices.

Surface-vacancy-induced metallicity and layer-dependent magnetic anisotropy energy in Cr2Ge2Te6

Two-dimensional van der Waals materials have attracted considerable attention because of their promising applications in spintronic devices. This paper reports on first-principles calculations of the electronic structure and ferromagnetism of Cr2Ge2Te6 with surface Ge vacancies. These vacancies are found to remove the bandgap and induce metallicity in Cr2Ge2Te6 that persists with decreasing Ge-vacancy concentration. Meanwhile, the Ge vacancies cause an unexpected sharp increase in the magnetic anisotropy energy compared to that of perfect Cr2Ge2Te6. More importantly, how thickness affects the magnetic anisotropy energy is studied to show that the latter oscillates upon increasing the number of layers, thereby switching between the out-of-plane and in-plane magnetization directions. The present findings regarding a surface-vacancy-tuned bandgap and magnetism controlled by the layer thickness in a two-dimensional van der Waals magnet could lead to potential applications in next-generation magnetic memory storage, sensors, and spintronics.

Tunable band gap and enhanced ferromagnetism by surface adsorption in monolayer Cr2Ge2Te6

Two-dimensional (2D) van der Waals (vdW) materials have attracted significant attention for their promising applications in spintronic devices. Here, using first-principles calculations and renormalized spin-wave theory, we investigate the influence of surface adsorption (H and alkali metals) on the bandgap and ferromagnetism of monolayer ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$. We find that H-atom adsorption maintains the bandgap of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ around 0.95 eV but leads to a nearly indirect-to-direct band gap transition, while alkali-metal adsorption removes the bandgap and induces metallicity. More importantly, both H and alkali-metal adsorption surprisingly make the magnetic anisotropy energy four times larger than that of pristine ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$, leading to an increase of ${T}_{\mathrm{c}}$ by about 33%. Our findings of adsorption-controlled bandgap and magnetism in a 2D vdW magnet may open up opportunities for potential applications for new-generation magnetic memory storage, sensors, and spintronics.

Enhanced multiferroic, magnetodielectric and electrical properties of Sm doped Lanthanum ferrite nanoparticles

In this work, we observe an enhancement of multiferroic property correlated with magnetic, magnetodielectric and ferroelectric properties in lanthanum ferrite (LaFeO3) nanoparticles by virtue of Samarium (Sm) doping in La site. The pure and doped lanthanum ferrite nanoparticle was synthesized by a simple solgel auto combustion route. We observe the positive magnetodielectric coupling in the samples and maximum 0.9% enhancement at the 1 T field in 10% Sm doped LaFeO3 nanoparticles. Highest value of saturation magnetization (1.84 emu/gm) is achieved in Sm doped LaFeO3 system. Particle size is decreased from 58 nm to 44 nm because of doping which is confirmed by transmission electron microscopy (TEM). The purity and nanocrystallinity nature of the prepared samples are confirmed by X-ray diffraction spectroscopy (XRD) pattern. DC and AC conduction mechanism and charge transport mechanism are illustrated in association with Mott’s variable range hopping (VRH) model and correlated barrier hopping (CBH) model respectively. The resistivity (DC & AC) is increased with a markable amount by virtue of Sm substitution. An improvement of dielectric properties is observed where dielectric loss is reduced due to the reduction of oxygen vacancies. The enhancement of magnetic properties is attributed to smaller crystallite size, inhomogeneous and disorder magnetic spin. The Sm doped LaFeO3 has been presented here which has promising applications in spintronic devices.

Structural, magnetic, and electron-transport properties of epitaxial Mn2PtSn films

The growth of new magnetic materials on suitable insulating substrates is an important part of the development of spin-electronics devices for memory or information processing. Epitaxial thin films of Mn2PtSn were grown on a MgO [001] substrate by magnetron co-sputtering of the constituents. Structural, magnetic, and electron-transport properties were investigated. The epitaxial Mn2PtSn film has an inverse tetragonal structure with the c-axis aligned in the plane of the MgO substrate. The lattice constants determined using XRD and TEM analysis are c = 6.124 Å and a = b = 4.505 Å. The orientation of Mn2PtSn c-axis which is 45° away from the a-axis of MgO has resulted in a small lattice mismatch of about 2.8%. The measured saturation magnetization is 5.3 μB/f.u., which is smaller than the first-principles calculated value of 6.4 μB/f.u. for ferromagnetic spin arrangement. Magnetization measurements determined the bulk magnetocrystalline anisotropy constant Kv of about 11.3 Merg/cm3 (1.13 MJ/m3). The electron-transport behavior is similar to that of normal magnetic metals. These results indicate that Mn2PtSn may have promising applications in spintronic devices.

Room-Temperature Spin-Orbit Torque Switching Induced by a Topological Insulator.

The strongly spin-momentum coupled electronic states in topological insulators (TI) have been extensively pursued to realize efficient magnetic switching. However, previous studies show a large discrepancy of the charge-spin conversion efficiency. Moreover, current-induced magnetic switching with TI can only be observed at cryogenic temperatures. We report spin-orbit torque switching in a TI-ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature. The obtained effective spin Hall angle of TI is substantially larger than the previously studied heavy metals. Our results demonstrate robust charge-spin conversion in TI and provide a direct avenue towards applicable TI-based spintronic devices.

Strain tunable magnetism in SnX2 (X = S, Se) monolayers by hole doping.

By first-principles calculations, the magnetism of hole doped tin dichalcogenides SnX2 (X = S, Se) monolayers is systematically studied. It is found that a phase transition from nonmagnetic to ferromagnetic ground state appears once above the critical hole density (~1014 cm−2). The spin magnetic moment can maintain a magnitude of 1.0 μB/hole with excellent stability of ferromagnetic state. Furthermore, we demonstrate that strain is very useful to modulate the DOS near the valence band, resulting in the reduction of the critical hole density to ~1013 cm−2 when the strain reaches 4% (6%) in SnS2 (SnSe2), which can be realized in common field effect transistors. Moreover, the phonon dispersion calculations for the strained SnX2 monolayers indicate that they can keep the dynamical stability under the hole doping. Therefore, the strain tunable magnetic transition in hole doped tin dichalcogenides indicates their potential promising applications in spintronic devices.

Tunable Electronic and Magnetic Properties of Graphene Flake-Doped Boron Nitride Nanotubes

Carbon-doped boron nitride nanostructures including nanosheets, nanoribbons, and nanotubes have drawn enormous research attention because of their tunable electronic properties and widespread applications. In this work, we explore the electronic and magnetic properties of graphene flake-doped single-walled boron nitride nanotubes (BNNTs) on the basis of first-principles calculations. Theoretical results reveal that the band structures of these doped BNNTs can be effectively engineered by embedding graphene flakes with different sizes and shapes. Moreover, the Lieb theorem works for the triangle graphene flake-doped BNNTs, and the corresponding doped systems are ferromagnetic, originating from the spin-polarized interface states. All BNNTs embedded with the triangular graphene flakes with relatively small sizes are typical bipolar magnetic semiconductors, which can be easily tuned into half-metals by carrier doping, opening the door to their promising applications in spintronic devices.