In-plane Spin Research Articles

Rashba-induced spin texture and spin-layer-locking effects in the antiferromagnetic CrI3 bilayer

The antiferromagnetic (AFM) CrI3 bilayer is a particularly interesting representative of van der Waals 2D semiconductors, which are currently being studied for their magnetism and for their potential in spintronics. Using ab initio density-functional theory calculations, we investigate the spin texture in momentum space of the states of the (doubly degenerate) highest valence band of the AFM CrI3 bilayer with Cr-spin moments perpendicular to the layers. We find the existence, in the main central part of the Brillouin zone, of a Rashba in-plane spin texture of opposite signs on the two layers, resulting from the intrinsic local electric fields acting on each layer. To study the layer segregation of the wavefunctions, we apply a small electric field that splits the degenerate states according to their layer occupancy. We find that the wavefunctions of the highest valence band are layer-segregated, belonging to only one of the two layers with opposite in-plane spin textures, and the segregation occurs over nearly the whole Brillouin zone. The corresponding layer locking of the in-plane-canted spin is related to the separation in energy of the highest AFM band from the rest of the valence bands. We explain how the band interactions destroy the layer locking at the K, K′, and Γ points. Furthermore, we compare the layer locking of the in-plane-canted spin in our AFM bilayer system with the hidden spin polarization in centrosymmetric nonmagnetic materials, pointing out the differences in segregation mechanisms and their consequences for the layer locking. We note that a similar Rashba effect with layer locking of in-plane-canted spin could occur in other van der Waals AFM bilayers with strong spin–orbit coupling and an isolated energy band.

Spin-polarization of topological crystalline and normal insulator Pb[formula omitted]Sn[formula omitted]Se (111) epilayers probed by photoelectron spectroscopy

The helical spin texture on the surface of topological crystalline insulators (TCI) makes these materials attractive for application in spintronics. In this work, spin-polarization and electronic structure of surface states of (111)-oriented Pb1−xSnxSe TCI epitaxial films are examined by angle- as well as spin-resolved photoemission spectroscopy (SR-ARPES). High-quality epilayers with various Sn content are grown by the molecular beam epitaxy (MBE) method. Topological-normal insulator transition manifesting itself as band gap opening is observed. It is shown that the gap opening can be induced not only by changing the Sn content of the epilayer but also depositing a transition metal (TM) on its surface. In the latter case, the observed gaping of the surface states is caused by change in surface composition and not by magnetism. We also show that helical spin polarization is present not only for samples of topological composition but also for trivial ones (with an open band gap). The observed spin polarization reaches a value of 30% for the in-plane spin component and is almost absent for the out-of-plane one. We believe that our work will pave the way for the application of surface states not only of topological but also normal insulators based on lead–tin chalcogenides in spin-charge conversion devices.

Enhanced spin current by anti-reflective magnetic tunnel junctions

Charge current and in-plane spin current are calculated in magnetic tunnel junctions (MTJs). The transmission function is investigated using nonequilibrium Green’s function formalism in a single-barrier MTJ, a resonant-tunneling MTJ and an anti-reflective MTJ (A-MTJ). The results show an enhancement in magnitude of the charge and in-plane spin currents for an A-MTJ compared to the other MTJs due to the presence of anti-reflective regions.

Spin density wave, charge density wave, and bond disproportionation wave instabilities in hole-doped infinite-layer RNiO2

Using \emph{ab initio} band structure methods and DFT+dynamical mean-field theory approach we explore the possible formation of spin and charge stripes in the Ni-O plane of hole-doped infinite-layer nickelates, $R$NiO$_2$. Our results reveal a remarkable instability of the $C$-type $(110)$ spin state with undistorted lattice towards the formation of the spin-density, charge- and bond-disproportionation stripe phases accompanied by in-plane``breathing''-like distortions of the crystal structure. Our work gives a comprehensive picture of competing charge and spin stripe states, with possible frustration of different stripe patterns upon doping. It suggests that the spin and charge stripe state likely arises from strong magnetic correlations (with concomitant lattice distortions), which play a key role for understanding the anomalous properties of hole-doped layered nickelates.

Modeling of Spin Transport in Hybrid Magnetic Tunnel Junctions for Magnetic Recording Applications

We have demonstrated modeling of phonon and defect-induced spin relaxation length (LS) in Fe3O4 and organic semiconductor (OSC) Alq3. LS of Alq3 decreases with enhanced disorder and film thickness at a low film width regime. Exponential change of LS at low width regime is found for Alq3 which is, however, absent for Fe3O4 indicating comparable spin-dependent scattering and LS in Fe3O4. LS also decreases with spin-flip probability both for Alq3 and Fe3O4. Voltage-dependent tunnel magnetoresistance (TMR) response in Fe3O4/Alq3/Co and La0.7Sr0.3MnO3 (LSMO)/Alq3/Co hybrid magnetic tunnel junction (MTJ) devices has been attributed to modified spin filter effect across magnetic/OSC junction at high bias regime. TMR reduction with Alq3 thickness for Fe3O4 device has been attributed to spin relaxation at the organic spacer layer. A low bias peak from differential TMR indicates spin-polarized injection for both MTJ devices. Enhanced in-plane spin transfer torque for both MTJ is associated with modified spin filtering at magnetic/OSC junctions. Lower TMR signal for LSMO device indicates reduced tunneling and enhanced carrier injection across the OSC, which is also supported by the band structure profile. The TMR response observed from simulation results matches well with previously reported experimental results. Higher TMR response for Fe3O4 device indicates the possibility of device employment in room temperature magnetic recording applications.

Spin- and angle-resolved inverse photoemission setup with spin orientation independent from electron incidence angle.

A new spin- and angle-resolved inverse photoemission setup with a low-energy electron source is presented. The spin-polarized electron source, with a compact design, can decouple the spin polarization vector from the electron beam propagation vector, allowing one to explore any spin orientation at any wavevector in angle-resolved inverse photoemission. The beam polarization can be tuned to any preferred direction with a shielded electron optical system, preserving the parallel beam condition. We demonstrate the performances of the setup by measurements on Cu(001) and Au(111). We estimate the energy resolution of the overall system at room temperature to be ∼170meV from kBTeff of a Cu(001) Fermi level, allowing a direct comparison to photoemission. The spin-resolved operation of the setup has been demonstrated by measuring the Rashba splitting of the Au(111) Shockley surface state. The effective polarization of the electron beam is P = 30% ± 3%, and the wavevector resolution is ΔkF ≲ 0.06 Å-1. Measurements on the Au(111) surface state demonstrate how the electron beam polarization direction can be tuned in the three spatial dimensions. The maximum of the spin asymmetry is reached when the electron beam polarization is aligned with the in-plane spin polarization of the Au(111) surface state.

Topological spin memory of antiferromagnetically coupled skyrmion pairs in Co/Gd/Pt multilayers

Magnetic skyrmions are topologically protected spin textures that have attracted considerable interest recently. This paper reports the discovery of a topological spin memory effect, which provides a promising new avenue for information encryption and recovery. The authors demonstrate experimentally that antiferromagnetically (AFM)-coupled bubble skyrmion pairs stabilized in a Co/Gd/Pt multilayered film at room temperature evolve into complex in-plane spin textures as the temperature is lowered and then reform completely when warmed back up. Simulations show that Dzyaloshinskii-Moriya interactions play a key role in this spin memory effect and reveal that the topological charge is preserved throughout the spin reorientation transition and recovery.

Quantum critical spin-liquid-like behavior in the S=12 quasikagome-lattice compound CeRh1−xPdxSn investigated using muon spin relaxation and neutron scattering

We present the results of muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) and neutron scattering on the Ce-based quasikagome lattice ${\mathrm{CeRh}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}\mathrm{Sn}$ ($x=0.1$ to 0.75). Our zero-field (ZF) $\ensuremath{\mu}\mathrm{SR}$ results reveal the absence of both static long-range magnetic order and spin freezing down to 0.05 K in the single-crystal sample of $x=0.1$. The weak temperature-dependent plateaus of the dynamic spin fluctuations below 0.2 K in ZF-$\ensuremath{\mu}\mathrm{SR}$ together with its longitudinal-field (LF) dependence between 0 and 3 kG indicate the presence of dynamic spin fluctuations persisting even at $T=0.05\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ without static magnetic order. On the other hand, the magnetic specific heat divided by temperature ${C}_{\text{4f}}/T$ increases as $\ensuremath{-}\text{log}T$ on cooling below 0.9 K, passes through a broad maximum at 0.13 K, and slightly decreases on further cooling. The ac susceptibility also exhibits a frequency-independent broad peak at 0.16 K, which is prominent with an applied field $H$ along the $c$ direction. We, therefore, argue that such a behavior for $x=0.1$ [namely, a plateau in spin relaxation rate ($\ensuremath{\lambda}$) below 0.2 K and a linear $T$ dependence in ${C}_{\text{4f}}$ below 0.13 K] can be attributed to a metallic spin-liquid-like ground state near the quantum critical point in the frustrated Kondo lattice. The LF-$\ensuremath{\mu}\mathrm{SR}$ study suggests that the out of kagome plane spin fluctuations are responsible for the spin-liquid (SL) behavior. Low-energy inelastic neutron scattering (INS) of $x=0.1$ reveals gapless magnetic excitations, which are also supported by the behavior of ${C}_{\text{4f}}$ proportional to ${T}^{1.1}$ down to 0.06 K. Our high-energy INS study shows very weak and broad scattering in $x=0$ and 0.1, which transforms into well-localized crystal-field excitations with increasing $x$. The ZF-$\ensuremath{\mu}\mathrm{SR}$ results for the $x=0.2$ polycrystalline sample exhibit similar behavior to that of $x=0.1$. A saturation of $\ensuremath{\lambda}$ below 0.2 K suggests a spin-fluctuating SL ground state down to 0.05 K. The ZF-$\ensuremath{\mu}\mathrm{SR}$ results for the $x=0.5$ sample are interpreted as a long-range antiferromagnetic (AFM) ground state below ${T}_{\text{N}}=0.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, in which the AFM interaction of the enlarged moments probably overcomes the frustration effect. The long-range AFM ordering is also supported by the evolution of magnetic Bragg peaks in $x=0.75$ sample observed below 5 K in the neutron diffraction data.

Orbital ordering and quasi-two-dimensional magnetism in AMnF4 (A=K,Rb) : A first-principles study

Strongly correlated systems with the interplay of electronic, charge, spin, and orbital degrees of freedom have, in recent times, received a surge of interest because of their rich physics, novel physical properties, and potential applications. In the present work, we study the structural, magnetic, and electronic properties and orbital ordering in layered perovskite-type $A\mathrm{Mn}{\mathrm{F}}_{4} (A=\mathrm{K},\mathrm{Rb})$ from first principles. A detailed analysis of the electronic properties in both compounds reveals an interesting nodal-line-like dispersion at $\ensuremath{\sim}0.4$ eV below the Fermi level at the hinges of the Brillouin zone. Magnetic properties reflect the quasi-two-dimensional magnetism in these compounds, with very weak exchange interaction between the layers. Our results report the robust in-plane ferromagnetic spin order in $A\mathrm{Mn}{\mathrm{F}}_{4}$ ($A$=K, Rb) with the critical temperatures estimated to be around 30--60 K. We also find an anti-ferro-orbital ordering within the $ab$ plane and a ferro-orbital ordering out of plane favoring the $C$-type orbital order in these compounds.

Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe3

Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and breakdown of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons.

Incommensurate magnetic orders and topological Hall effect in the square-net centrosymmetric EuGa2Al2 system

Neutron diffraction on the centrosymmetric square-net magnet EuGa$_2$Al$_2$ reveals multiple incommensurate magnetic states (AFM1,2,3) in zero field. In applied field, a new magnetic phase (A) is identified from magnetization and transport measurements, bounded by two of the $\mu_0H$~=~0 incommensurate magnetic phases (AFM1,helical and AFM3, cycloidal) with different moment orientations. Moreover, magneto-transport measurements indicate the presence of a topological Hall effect, with maximum values centered in the A phase. Together, these results render EuGa$_2$Al$_2$ a material with non-coplanar or topological spin texture in applied field. X-ray diffraction reveals an out-of-plane (OOP) charge density wave (CDW) below $T_{CDW} \sim$ 50 K while the magnetic propagation vector lies in plane below $T_N$ = 19.5 K. Together these data point to a new route to realizing in-plane non-collinear spin textures through an OOP CDW. In turn, these non-collinear spin textures may be unstable against the formation of topological spin textures in an applied field.

Spin-wave modes of magnetic bimerons in nanodots

We report the resonance excitations and the spin-wave modes of a single bimeron in a confined nanodot by using micromagnetic simulations. Magnetic bimerons can be considered as in-plane topological spin textures of magnetic skyrmions, which means that the spin-wave modes of bimerons also rotate in-plane compared to skyrmions, for example, through the application of out-of-plane microwave magnetic fields, the spin-wave mode of bimerons is no longer a breathing mode but contains a counterclockwise mode at low frequencies and a clockwise mode at high frequencies. When in-plane microwave magnetic fields rotated at different angles are applied, the spin-wave mode of bimerons has an anisotropic property, i.e., the spin-wave mode presents as a breathing mode for the microwave magnetic field applied along the x-direction, and a couple of azimuthal modes for the microwave magnetic field applied along the y-direction. Moreover, we demonstrate that the breathing mode, the counterclockwise rotation mode, and the clockwise rotation mode can simultaneously appear together when the microwave magnetic field is applied at a specific angle in the plane. In addition to the three typical spin-wave modes, two high-phase counterclockwise rotation modes lead to the periodic deformation of bimerons due to the broken rotational symmetry of the spin texture. Our results reveal the rich spin-wave modes of bimerons, which may contribute to the applications in spintronics and magnonics.

Gapless Dirac magnons in CrCl3

Bosonic Dirac materials are testbeds for dissipationless spin-based electronics. In the quasi two-dimensional honeycomb lattice of CrX3 (X = Cl, Br, I), Dirac magnons have been predicted at the crossing of acoustical and optical spin waves, analogous to Dirac fermions in graphene. Here we show that, distinct from CrBr3 and CrI3, gapless Dirac magnons are present in bulk CrCl3, with inelastic neutron scattering intensity at low temperatures approaching zero at the Dirac K point. Upon warming, magnon-magnon interactions induce strong renormalization and decreased lifetimes, with a ~25% softening of the upper magnon branch intensity from 5 to 50 K, though magnon features persist well above TN. Moreover, on cooling below ~50 K, an anomalous increase in the a-axis lattice constant and a hardening of a ~26 meV phonon feature are observed, indicating magnetoelastic and spin-phonon coupling arising from an increase in the in-plane spin correlations that begins tens of Kelvin above TN.

Ising and XY paramagnons in two-dimensional 2H−NbSe2

Paramagnons are the collective modes that govern the spin response of nearly magnetic conductors. In some cases they mediate electron pairing leading to superconductivity. This scenario may occur in 2H-NbSe$_2$ monolayers, that feature spin-valley coupling on account of spin-orbit interactions and their lack of inversion symmetry. Here we explore spin anisotropy of paramagnons both for non-centrosymmetric Kane-Mele-Hubbard models for 2H-NbSe$_2$ monolayers described with a DFT-derived tight-binding model. In the infinite wavelength limit we find spatially uniform paramagnons with energies around $1$~meV that feature a colossal off-plane uniaxial magnetic anisotropy, with quenched transversal spin response. At finite wave vectors, longitudinal and transverse channels reverse roles: XY fluctuations dominate within a significant portion of the Brillouin zone. Our results show that 2H-NbSe$_2$ is close to a Coulomb-driven in-plane (XY) spin density wave instability.

Fermi surface chirality induced in a TaSe2 monosheet formed by a Ta/Bi2Se3 interface reaction.

Spin-momentum locking in topological insulators and materials with Rashba-type interactions is an extremely attractive feature for novel spintronic devices and is therefore under intense investigation. Significant efforts are underway to identify new material systems with spin-momentum locking, but also to create heterostructures with new spintronic functionalities. In the present study we address both subjects and investigate a van der Waals-type heterostructure consisting of the topological insulator Bi2Se3 and a single Se-Ta-Se triple-layer (TL) of H-type TaSe2 grown by a method which exploits an interface reaction between the adsorbed metal and selenium. We then show, using surface x-ray diffraction, that the symmetry of the TaSe2-like TL is reduced from D3h to C3v resulting from a vertical atomic shift of the tantalum atom. Spin- and momentum-resolved photoemission indicates that, owing to the symmetry lowering, the states at the Fermi surface acquire an in-plane spin component forming a surface contour with a helical Rashba-like spin texture, which is coupled to the Dirac cone of the substrate. Our approach provides a route to realize chiral two-dimensional electron systems via interface engineering in van der Waals epitaxy that do not exist in the corresponding bulk materials.

Direct Spectroscopic Evidence of Magnetic Proximity Effect in MoS2 Monolayer on Graphene/Co.

A magnetic field modifies optical properties and provides valley splitting in a molybdenum disulfide (MoS2) monolayer. Here we demonstrate a scalable approach to the epitaxial synthesis of MoS2 monolayer on a magnetic graphene/Co system. Using spin- and angle-resolved photoemission spectroscopy we observe a magnetic proximity effect that causes a 20 meV spin-splitting at the Γ̅ point and canting of spins at the K̅ point in the valence band toward the in-plane direction of cobalt magnetization. Our density functional theory calculations reveal that the in-plane spin component at K̅ is localized on Co atoms in the valence band, while in the conduction band it is localized on the MoS2 layer. The calculations also predict a 16 meV spin-splitting at the Γ̅ point and 8 meV K̅- valley asymmetry for an out-of-plane magnetization. These findings suggest control over optical transitions in MoS2 via Co magnetization. Our estimations show that the magnetic proximity effect is equivalent to the action of the magnetic field as large as 100 T.

Photonic spin Hall effect in twisted few-layer anisotropic two-dimensional atomic crystals

Motivated by the emergence of stacked two-dimensional materials with a twist angle, we examine the photonic spin Hall effect in twisted few-layer anisotropic two-dimensional atomic crystals. By developing a generalized model for arbitrary layers of anisotropic two-dimensional atomic crystals, the photonic spin Hall effect of twisted few-layer black phosphorus without interlayer interaction is demonstrated and discussed. We find that the twist between different layers results in the arise of the in-plane spin Hall shift, and the shift is very sensitive to the twist angle. While the in-plane spin Hall shifts are symmetric about the origin as a function versus the twist angle, the transverse spin Hall shifts are symmetric about the axis. And the photonic spin Hall shifts exhibit periodic changes versus the thickness of the interlayer due to the optical Fabry-P\'erot resonance. The results may provide different aspects for analysis and control of the optical properties of twisted multilayer two-dimensional materials.

Surface states-dependent giant quantized photonic spin Hall effect in a magnetic topological insulator thin film

We theoretically investigate the in-plane and transverse photonic spin Hall effect (PSHE) in a magnetic topological ultra-thin film (MTF) in the terahertz regime in the presence of an external magnetic field. We take into account the hybridization between the top and bottom surface states (SSs) and demonstrate that both the in-plane and transverse PSHE are quantized due to the Landau levels (LLs) quantization of the magneto-optical (MO) conductivities. The interplay between the effective energy induced by the hybridization interaction and the Zeeman energy plays an important role as it results in topological and normal insulating phases. By manipulating the interplay between these two interaction energies we drive the system through several topological quantum phase transitions (TQPTs). We show that the PSHE can be greatly enhanced and the magnitude of the giant PSHE in MTF is several hundreds times of the incident gaussian beam wavelength. Furthermore, we show that the PSHE is sensitive to the SSs of the Dirac fermions which is due to its topological character. This work sheds important light both on the PSHE and on the MO transport properties in MTFs in the presence of an external magnetic field which harbor great potentials in the development of new-generation photonics spin-Hall and optoelectronic devices.

Optimal Spin Polarization for Spin–Orbit-Torque Memory and Oscillator

Spin–orbit-torque arising from in-plane polarized spin current has attracted considerable attention for its potential in high-performance magnetic random access memories and spin torque oscillators (STOs). Recent studies have shown that an out-of-plane spin polarization can be even more efficient to drive the magnetization dynamics. In this work, we thoroughly examine the effect of the out-of-plane polarized spin current through macro-spin simulations. We propose separately the desirable conditions of spin polarization for spintronic memory and oscillator devices. It is found that for memory applications, a comparable strength of out-of-plane and in-plane spin polarization is necessary to simultaneously achieve a reduced switching current density and a faster switching speed. Besides, in STOs, the presence of a sizable out-of-plane polarization can provide an increase of oscillation amplitude and/or an improvement on frequency tunability. Our results address the importance of deliberate control on the spin polarization in designing efficient spintronic devices.

Impact of Polarization Phase of Incident Light on In-Plane Spin Splitting of Reflected Beam

In this paper, a physical model is established to reveal the relationship between polarization phase of incident light and in-plane spin splitting (IPSS) of reflected beam, and the impact of polarization phase on the IPSS is systematically investigated. Three interesting features of the effect of polarization phase on IPSS were discovered. Furthermore, it is found that the IPSS can be effectively enhanced and suppressed by adjusting the polarization phase near ±π/2. The enhanced IPSS shift can almost reach its upper limit (that is, half of the beam waist) and the suppressed IPSS shift can be reduced to zero. In addition, it is also found that the spin cumulative direction of spin components can be reversed adjusting the polarization phase at ±π/2. These findings not only provide a deeper insight into photonic spin Hall effect, but also open a path for the photon manipulation, precision metrology and the manufacture of photonic spin switching devices.