Spin-flip Scatterings Research Articles

Skyrmion-deriven topological spin and charge Hall effects in diffusive antiferromagnetic thin films

We investigate topological Hall effects in a metallic antiferromagnetic (AFM) thin film and/or at the interface of an AFM insulator–normal metal bilayer with a single skyrmion in the diffusive regime. To determine the spin- and charge Hall currents, we employed a Boltzmann kinetic equation with both spin-dependent and spin-flip scatterings. The interaction between conduction electrons and static skyrmions is included in the Boltzmann equation via the corresponding emergent magnetic field arising from the skyrmion texture. We compute intrinsic and extrinsic contributions to the topological spin Hall effect and spin accumulation, induced by an AFM skyrmion. We show that although the spin Hall current vanishes rapidly outside the skyrmion, the spin accumulation can be finite at the edges far from the skyrmion, provided the spin diffusion length is longer than the skyrmion radius. In addition, We show that in the presence of a spin-dependent relaxation time, the topological charge Hall effect is finite and we determine the corresponding Hall voltage. Our results may help to explore antiferromagnetic skyrmions by electrical means in real materials.

Magnetic Configuration Driven Femtosecond Spin Dynamics in Synthetic Antiferromagnets.

Ultrafast demagnetization in diverse materials has sparked immense research activities due to its captivating richness and contested underlying mechanisms. Among these, the two most celebrated mechanisms have been the spin-flip scattering (SFS) and spin transport (ST) of optically excited carriers. In this work, we have investigated femtosecond laser-induced ultrafast demagnetization in perpendicular magnetic anisotropy-based synthetic antiferromagnets (p-SAFs) where [Co/Pt]n-1/Co multilayer blocks are separated by Ru or Ir spacers. Our investigation conclusively shows that the ST of optically excited carriers can have a significant contribution to the ultrafast demagnetization in addition to SFS processes. Moreover, we have also achieved an active control over the individual mechanisms by specially designing the SAF samples and altering the external magnetic field and excitation fluence. Our study provides a vital understanding of the underlying mechanism of ultrafast demagnetization in synthetic antiferromagnets, which will be crucial in future research and applications of antiferromagnetic spintronics.

Superconducting triplet pairings and anisotropic magnetoresistance effects in ferromagnet/superconductor/ferromagnet double-barrier junctions

Ferromagnetic spin valves offer the key building blocks to integrate giant- and tunneling-magnetoresistance effects into spintronics devices. Starting from a generalized Blonder-Tinkham-Klapwijk approach, we theoretically investigate the impact of interfacial Rashba and Dresselhaus spin-orbit couplings on the tunneling conductance, and thereby the magnetoresistance characteristics, of ferromagnet/superconductor/ferromagnet spin-valve junctions embedding thin superconducting spacers between the either parallel or antiparallel magnetized ferromagnets. We focus on the unique interplay between usual electron tunnelings-that fully determine the magnetoresistance in the normal-conducting state-and the peculiar Andreev reflections in the superconducting state. In the presence of interfacial spin-orbit couplings, special attention needs to be paid to the spin-flip ("unconventional") Andreev-reflection process that is expected to induce superconducting triplet correlations in proximitized regions. As a transport signature of these triplet pairings, we detect conductance double peaks around the singlet-gap energy, reflecting the competition between the singlet and an additionally emerging triplet gap; the latter is an effective superconducting gap that can be ascribed to the formation of triplet Cooper pairs through interfacial spin-flip scatterings (i.e., to the generation of an effective triplet-pairing term in the order parameter). We thoroughly analyze the Andreev reflections' role in connection with superconducting magnetoresistance phenomena, and eventually unravel huge conductance and magnetoresistance magnetoanisotropies-easily exceeding their normal-state counterparts by several orders of magnitude-as another experimentally accessible fingerprint of unconventional Andreev reflections.

Spin Relaxation in s-Wave Superconductors in the Presence of Resonant Spin-Flip Scatterers.

Employing analytical methods and quantum transport simulations we investigate the relaxation of quasiparticle spins in graphene proximitized by an s-wave superconductor in the presence of resonant magnetic and spin-orbit active impurities. Off resonance, the relaxation increases with decreasing temperature when electrons scatter off magnetic impurities-the Hebel-Slichter effect-and decreases when impurities have spin-orbit coupling. This distinct temperature dependence (not present in the normal state) uniquely discriminates between the two scattering mechanisms. However, we show that the Hebel-Slichter picture breaks down at resonances. The emergence of Yu-Shiba-Rusinov bound states within the superconducting gap redistributes the spectral weight away from magnetic resonances. The result is opposite to the Hebel-Slichter expectation: the spin relaxation decreases with decreasing temperature. Our findings hold for generic s-wave superconductors with resonant magnetic impurities, but also, as we show, for resonant magnetic Josephson junctions.

Spin relaxation in fluorinated single and bilayer graphene

We present a joint experiment-theory study on the role of fluorine adatoms in spin and momentum scattering of charge carriers in dilute fluorinated graphene and bilayer graphene. The experimental spin-flip and momentum scattering rates and their dependence on the density of fluorine and carrier doping are obtained through weak localization and conductivity measurements, respectively, and suggest the role of fluorine as resonant magnetic impurities. For the estimated fluorine concentration of a few 100 ppm, the observed spin lifetimes are in the range of 1-10\,ps. Theoretically, we established tight-binding electronic structures of fluorinated graphene and bilayer graphene by fitting to density functional supercell calculations and performed a comprehensive analysis of the spin-flip and momentum scattering rates within the same devices, aiming to develop a consistent description of both scattering channels. We find that resonant scattering in graphene is very sensitive to the precise position of the resonance level, as well as to the magnitude of the exchange coupling between itinerant carriers and localized spins. The experimental data point to the presence of weak spin-flip scatterers that, at the same time, relax the electron momentum strongly, nearly preserving the electron-hole symmetry. Such scatterers would exhibit resonance energies much closer to the neutrality point than what density functional theory predicts in the dilute limit. The inclusion of a magnetic moment on fluorine adatoms allowed us to qualitatively capture the carrier density dependence of the experimental rates but predicts a greater (weaker) spin (momentum) relaxation rate than the measurements. We discuss possible scenarios that may be responsible for the discrepancies. Our systematic study exposes the complexities involved in accurately capturing the behavior of adatoms on graphene.

Modified exchange interaction between magnetic impurities in spin–orbit coupled quantum wires

Indirect exchange interaction between magnetic impurities in one dimensional systems is a matter of long discussions since Kittel has established that in the asymptotic limit it decays as the inverse of distance x between the impurities. In this work we investigate this problem in a quantum wire with Rashba spin–orbit coupling (SOC). By employing a second order perturbation theory we find that one additional oscillatory term appears in each of the Ruderman–Kittel–Kasuya–Yosida (RKKY), the Dzaloshinkii–Moryia and the Ising couplings. Remarkably, these extra terms resulting from the spin precession of the conduction electrons induced by the SOC do not decay as in the usual RKKY interaction. We show that these extra oscillations arise from the finite momenta band splitting induced by the spin–orbit coupling that modifies the spin-flip scatterings occurring at the Fermi energy. Our findings open up a new perspective in the long-distance magnetic interactions in solid state systems.

Ultrafast Orbital-Oriented Control of Magnetization in Half-Metallic La0.7 Sr0.3 MnO3 Films.

Manipulating spins by ultrafast pulse laser provides a new avenue to switch the magnetization for spintronic applications. While the spin-orbit coupling is known to play a pivotal role in the ultrafast laser-induced demagnetization, the effect of the anisotropic spin-orbit coupling on the transient magnetization remains an open issue. This study uncovers the role of anisotropic spin-orbit coupling in the spin dynamics in a half-metallic La0.7 Sr0.3 MnO3 film by ultrafast pump-probe technique. The magnetic order is found to be transiently enhanced or attenuated within the initial sub-picosecond when the probe light is tuned to be s- or p-polarized, respectively. The subsequent slow demagnetization amplitude follows the fourfold symmetry of the orbitals as a function of the polarization angles of the probe light. A model based on the Elliott-Yafet spin-flip scatterings is proposed to reveal that the transient magnetization enhancement is related to the spin-mixed states arising from the anisotropic spin-orbit coupling. The findings provide new insights into the spin dynamics in magnetic systems with anisotropic spin-orbit coupling as well as perspectives for the ultrafast control of information process in spintronic devices.

Review of Ultrafast Demagnetization After Femtosecond Laser Pulses: A Complex Interaction of Light with Quantum Matter

When a magnetic film is excited by a femtosecond laser pulse, either with THz or with optical frequencies, then there is at least a partial demagnetization within a few hundred femtoseconds, followed by a remagnetization to the original state on a bit longer time scale. This phenomenon is caused by a complex interaction of light with quantum matter. This paper gives a review of the present knowledge of the underlying physics. It discusses first the situation of a direct change of the magnetization by its interaction with the electromagnetic wave of the laser pulse, which appears during THz laser pulses with small field amplitudes. Then it considers the situation of an indirect change which appears after THz laser pulses with large field amplitudes and after optical laser pulses. In these cases the laser photons primarily excite electrons, with subsequent modifications of their spin-angular momenta by spin-flip scatterings of these electrons at quasiparticles, either at other electrons or at phonons or at magnons. The contributions of these various spin-flip scatterings to demagnetization are investigated. Then the transfer of angular momentum from the electronic spin system to the lattice during ultrafast demagnetization is discussed by describing the lattice vibrations in terms of magnetoelastic spin-phonon modes. Finally, the effect of electronic correlations in the sense of the density-matrix theory is investigated.

Unexpectedly marginal effect of electronic correlations on ultrafast demagnetization after femtosecond laser-pulse excitation

The treatment of ultrafast demagnetization after femtosecond laser-pulse excitation of a ferromagnetic film is usually done by a theory based on Fermi's golden rule which neglects the effects of electronic correlations. In the present paper the contribution of spin-flip electron-phonon scatterings to the ultrafast demagnetization of Ni is calculated by this theory and by the density-matrix theory in which the correlations are taken into account. The unexpected result is that the correlations which are essential for the ultrafast dynamics of nonmagnetic phenomena have only a marginal effect for the considered magnetic problem. From this point of view the use of Fermi's golden rule in all former papers on ultrafast demagnetization is justified.

Impurity-Driven Two-Dimensional Spin Relaxation Induced by Intervalley Spin-Flip Scattering in Silicon

Through the theoretical study of electron spin lifetime in the 2DEG of doped Si, we highlight a dominant spin relaxation mechanism induced by the impurity central-cell potential near an interface via intervalley electron scattering. At low temperatures and with modest doping, this Yafet spin flip mechanism can become much more important than the Dyakonov-Perel spin relaxation indcued by the structural Rashba spin-orbit coupling field. As the leading-order impurity-induced spin flip happens only between two non-opposite valleys in Si, two-dimensional electron gas (2DEG) systems in Si MOSFETs or SiGe heterostructures are a natural platform to test and utilize this spin relaxation mechanism due to the valley splitting near the interface and its tunability by electrical gating or applied stress. Our proposed new spin relaxation mechanism may explain a part of the spin relaxation contribution to Si-based 2DEG systems.

Role of electron-magnon scatterings in ultrafast demagnetization

Possible contributions of electron-magnon scatterings for the ultrafast demagnetization after femtosecond laser irradiation of films of Ni and Fe are investigated by the ab initio spin-density-functional electron theory. The calculations are based on Fermi's golden rule for transition rates, and the nonequilibrium after the action of the laser pulse is modeled by different chemical potentials for electrons which are in a ``dominant spin-up'' or in a ``dominant spin-down'' state. First, it is shown that the experimentally observed demagnetization cannot be described by electron-magnon spin-flip scatterings with concomitant changes of the electronic orbital moments which are immediately quenched by the crystal field, a mechanism which has been suggested by Carpene et al. Second, it is argued that the experimentally observed demagnetization possibly can be explained by a combination of individual spin-flip electron-phonon and spin-flip electron-magnon processes. A precondition for this is (among others) that the magnon emission rate is much larger than the magnon absorption rate. It is shown that this precondition is indeed fulfilled for Ni and especially for Fe.

Anomalous spin-triplet current in Ho/Co/Ho-based Josephson junctions

The supercurrent through a holmium/cobalt/holmium-based Josephson junction is investigated by using the extended the Blonder-Tinkham-Klapwijk (BTK) approach. It is found that the spiral magnetism of the Ho layer can induce both spin-split and spin-flip scatterings, which control the conversion between the spin-singlet and equal spin-triplet pairing correlations, generating a tunable long-range spin-triplet pair ordering. Spin-triplet current sharp peaks variations with the Ho layer thickness as well as a slow decay of the current amplitude with the Co layer thickness are revealed. The theoretical results are in good agreement with the relevant experimental observations (Robinson J. W. A. et al., Science, 329 (2010) 59).

Ultrafast demagnetization after laser irradiation in transition metals:Ab initiocalculations of the spin-flip electron-phonon scattering with reduced exchange splitting

Despite intensive research, the underlying mechanisms for ultrafast demagnetization after laser irradiation in transition metals are still not understood. We discuss the possible processes which have been suggested in order to explain the ultrafast demagnetization within several hundreds of femtoseconds and argue that the spin angular momentum has to go to the lattice in the end. Based on this argument, we consider spin-flip electron-phonon scatterings. The demagnetization time ${\ensuremath{\tau}}_{M}$ and the demagnetization rate $dM/dt$ due to spin-flip electron-phonon scattering is calculated for fcc Ni and bcc Fe. Thereby, the electronic states and phononic states are calculated ab initio. We find that the demagnetization rates for fcc Ni and bcc Fe are too small to explain experimental demagnetization rates, which is in agreement with earlier publications. In addition, the demagnetization rates for band structures with reduced exchange splitting are calculated, however, also these demagnetization rates are too small. Finally, the phase space for scattering processes which is related to the maximum possible demagnetization is estimated for band structures with ground-state exchange splitting and with reduced exchange splitting. The maximum possible demagnetization is too small for bcc Fe and fcc Co but not necessarily for fcc Ni. We suggest to include magnons and to consider independent combinations of spin-flip electron-phonon and spin-flip electron-magnon scattering processes as a possible explanation for the ultrafast demagnetization.

Spin-Flip Scattering of Ion on a Metal Surface by the Auger Process

Spin-Flip Scattering of Ion on a Metal Surface by the Auger Process

Controlling the Competition between Optically Induced Ultrafast Spin-Flip Scattering and Spin Transport in Magnetic Multilayers

The study of ultrafast dynamics in magnetic materials provides rich opportunities for greater fundamental understanding of correlated phenomena in solid-state matter, because many of the basic microscopic mechanisms involved are as-yet unclear and are still being uncovered. Recently, two different possible mechanisms have been proposed to explain ultrafast laser induced magnetization dynamics: spin currents and spin-flip scattering. In this work, we use multilayers of Fe and Ni with different metals and insulators as the spacer material to conclusively show that spin currents can have a significant contribution to optically induced magnetization dynamics, in addition to spin-flip scattering processes. Moreover, we can control the competition between these two processes, and in some cases completely suppress interlayer spin currents as a sample undergoes rapid demagnetization. Finally, by reversing the order of the Fe/Ni layers, we experimentally show that spin currents are directional in our samples, predominantly flowing from the top to the bottom layer.

Imaging single electron spin in a molecule trapped within a nanocavity of tunable dimension

Control of magnetism at the nanoscale is shown by a reversible transfer of an electron to and from a single molecule within the tunable gap of a scanning tunneling microscope. The addition of an electron to magnesium porphine changes the molecule from the diamagnetic state to the paramagnetic state. The existence of the single unpaired electron in the molecule is confirmed by spectroscopy and spatial imaging of the many body Kondo state and inelastic spin excitation between the Zeeman levels at 600 mK and up to 9 Tesla magnetic field. Here, we show that the spin is delocalized in an extended molecular orbital, in contrast to the spatially confined d and f states in atoms and magnetic centers in molecules. Furthermore, by tuning the dimension of the tunneling gap and visualizing the spectroscopic images, the inelastic spin-flip scatterings are shown to underlie the formation of the Kondo state.

Comment on “Evidence for Spin-Flip Scattering and Local Moments in Dilute Fluorinated Graphene”

Comment on “Evidence for Spin-Flip Scattering and Local Moments in Dilute Fluorinated Graphene”

Spin–triplet current in half metal/conical helimagnet/superconductor heterojunctions

The BTK theory is extended to investigate spin–triplet current and differential conductance spectrum in the half metal/conical helimagnet (Holmium)/s-wave superconductor heterojunctions. We show that the effective spin–split and spin–flip scatterings of the Holmium layer control the conversion efficiency between the spin–singlet and equal-spin triplet pair correlations, leading to a tunneling current oscillation with the thickness of the Holmium layer. This can provide qualitative explanations on the current oscillation in Ho/Co/Ho-based Josephson junction experiment. The differential conductance spectrum confirms spin–flip Andreev reflection induced long-ranged equal-spin triplet pair correlations.

Evidence for Spin-Flip Scattering and Local Moments in Dilute Fluorinated Graphene

The issue of whether local magnetic moments can be formed by introducing adatoms into graphene is of intense research interest because it opens the window to fundamental studies of magnetism in graphene, as well as of its potential spintronics applications. To investigate this question, we measure, by exploiting the well-established weak localization physics, the phase coherence length L(Φ) in dilute fluorinated graphene. L(Φ) reveals an unusual saturation below ~10 K, which cannot be explained by nonmagnetic origins. The corresponding phase-breaking rate increases with decreasing carrier density and increases with increasing fluorine density. These results provide strong evidence for spin-flip scattering and point to the existence of an adatom-induced local magnetic moment in fluorinated graphene. Our results will stimulate further investigations of magnetism and spintronics applications in adatom-engineered graphene.

Tunable Spin Current Through a Vibrating Molecular Quantum Dot with Spin Bias and Spin Flip Scattering

By applying the Lang-Firsov canonical transformation and the Keldysh nonequilibrium Green’s function approach, the effect of the spin-flip scattering on the spin current through a vibrating molecular quantum dot with spin bias is theoretically investigated. We can obtain the spin current from the output terminal, and find that the sign of the spin current can be changed by adjusting the spin-flip strength, and a pure spin current can be generated via the charge bias and the spin bias. In the presence of the electron-phonon interaction, the positions of the current peaks are shifted and the spin current is remarkably suppressed, which leads to the Franck-Condon blockade. Furthermore, it is found that the competition between the EPI and the spin-flip scattering jointly determines the character of the spin current. These results offer us a way to manipulate the spin current in the spin current setup. The proposed device should be realizable with use of the present technology at low temperature.