Decay Of Spin Polarization Research Articles

Giant Spin-Orbit Torque in Antiferromagnetic-Coupled Pt/[Co/Gd]N Multilayers with Suppressed Spin Dephasing and Robust Thermal Stability.

Manipulating magnetization via power-efficient spin-orbit torque (SOT) has garnered significant attention in the field of spin-based memory and logic devices. However, the damping-like SOT efficiency (ξDL) in heavy metal (HM)/ferromagnetic metal (FM) bilayers is relatively small due to the strong spin dephasing accompanied by additional spin polarization decay. Furthermore, the perpendicular magnetic anisotropy (PMA) originating from the HM/FM interface is constrained by the thickness of FM, which is unfavorable for thermal stability in practical applications. Consequently, it is valuable to develop systems that not only exhibit large ξDL but also balance thermal stability. In this work, we designed antiferromagnetic-coupled [Co/Gd]N multilayers, where staggered Co and Gd magnetic moments effectively suppress the spin dephasing and additional spin polarization decay. The ordered Co-Gd arrangements along the out-of-plane direction provide bulk PMA, endowing Pt/[Co/Gd]N high thermal stability. The SOT of Pt/[Co/Gd]N was systematically studied with N, demonstrating a significantly large ξDL of up to 0.66. The ξDL of Pt/[Co/Gd]N is greater than those of Pt/Co and Pt/ferrimagnetic alloys. This significant enhancement relies on the effective suppression of spin dephasing in [Co/Gd]N. Our work highlights that the antiferromagnetic-coupled [Co/Gd]N multilayer is a promising candidate for low-consumption and high-density spintronic devices.

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe_{2}.

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe_{2} with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe_{2} layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K^{'} valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides.

Observation of Optically Induced Spin Dephasing and Dynamics Under Combined Electric and Magnetic Fields in Semiconductor Quantum Wells

Magnetic field dependence of the electronic spin polarization (P), polarization decay and spin oscillation in gallium arsenide quantum wells is investigated by spin-resolved photoluminescence measurements. In the presence of a magnetic field in the Voigt configuration, the spin-polarized electrons, generated by a circularly polarized light, are drifted by an electric field. The results show that P decays and oscillates with time as well as with the magnetic field. However, the electric field is found to play a vital role in spin dynamics and exchange coupling between the electron and hole spins in the wells.

Reversible out-of-plane spin texture in a two-dimensional ferroelectric material for persistent spin helix

Although spin-orbit coupling (SOC) in ferroelectrics contributes to the manipulation of the electron spin by an electric field, it also breaks the spin rotation symmetry resulting in the decay of spin polarization. Here, we showed that the [110] Dresselhaus model in the two-dimensional ferroelectrics with in-plane ferroelectricity can host a persistent spin helix and render the spin lifetime infinite. Taking a ${\mathrm{WO}}_{2}{\mathrm{Cl}}_{2}$ monolayer as an example, we demonstrated from first principles that SOC leads to a sizable [110] Dresselhaus-type band splitting near the conduction-band minimum accompanied by an out-of-plane Dresselhaus-type spin texture which is decisive for a long spin lifetime. More interestingly, such a spin texture can be reversed by switching the ferroelectric polarization, suggesting the fully electrically controllable persistent spin helix. Our findings uncover the possibility of an electrically controllable persistent spin helix in ferroelectrics, offering a promising platform for novel spintronics applications.

Interdot carrier and spin dynamics in a two-dimensional high-density quantum-dot array of InGaAs with quantum dots embedded as local potential minima

Interdot carrier and spin dynamics were studied in a two-dimensional array of high-density small quantum dots (SQDs) of InGaAs with an average diameter of 16 nm and a sheet density of 1.2 , in which 24 nm diametric large QDs (LQDs) were embedded as local potential minima. We observed a delayed photoluminescence (PL) rise from the lower-lying LQD states and a considerably faster PL decay from the higher-lying SQD states, indicating carrier transfer from the two-dimensionally coupled SQDs into the LQDs. In addition, inverse carrier tunneling from the LQDs into the SQDs was thermally induced, which is characterized by the thermal activation energy between the LQDs and SQDs. Moreover, circularly polarized transient PL behavior from the SQD states exhibits a suppression of the spin polarization decay in the initial time region, depending on the excited spin density. This tentatively suppressed spin relaxation can be quantitatively explained by selective interdot transfer of minority-spin electrons from the SQDs into LQDs, when the majority spin states in both QDs are sufficiently populated by excited spins. These findings indicate that the high-density SQDs behave as the main emitters with suppressed spin relaxation, while the scattered LQDs with lower potential behave as the receivers of minority-spin electrons.

Decay and revival of electron spin polarization in an ensemble of (In,Ga)As quantum dots

The periodic optical orientation of electron spins in (In,Ga)As/GaAs quantum dots leads to the formation of electron spin precession modes about an external magnetic field which are resonant with the pumping periodicity. As the electron spin is localized within a nuclear spin bath, its polarization imprints onto the spin polarization of the bath. The latter acts back on the electron spin polarization. We implement a pulse protocol where a train of laser pulses is followed by a long, dark gap. It allows us to obtain a high-resolution precession mode spectrum from the free evolution of the electron spin polarization. Additionally, we vary the number of pump pulses in a train to investigate the build-up of the precession modes. To separate out nuclear effects, we suppress the nuclear polarization by using a radio-frequency field. We find that a long-living nuclear spin polarization imprinted by the periodic excitation significantly speeds up the buildup of the electron spin polarization and induces the formation of additional electron spin precession modes. To interpret these findings, we extend an established dynamical nuclear polarization model to take into account optically detuned quantum dots for which nuclear spins activate additional electron spin precession modes.

Tailoring multilayer quantum wells for spin devices

The electron spin dynamics in multilayer GaAs/AlGaAs quantum wells, containing high-mobility dense two-dimensional electron gases, have been studied using time-resolved Kerr rotation and resonant spin amplification techniques. The electron spin dynamics was regulated through the wave function engineering and quantum confinement in multilayer quantum wells. We observed the spin coherence with a remarkably long dephasing time T2* > 13 ns for the structure doped beyond metal-insulator transition. Dyakonov-Perel spin relaxation mechanism, as well as the inhomogeneity of electron g-factor, was suggested as the major limiting factors for the spin coherence time. In the metallic regime, we found that the electron-electron collisions become dominant over microscopic scattering on the electron spin relaxation with the Dyakonov-Perel mechanism. Furthermore, the data analysis indicated that in our structure, due to the spin relaxation anisotropy, Dyakonov-Perel spin relaxation mechanism is efficient for the spins oriented in-plane and suppressed along the quantum well growth direction resulting in the enhancement of T2*. Our findings, namely, long-lived spin coherence persisting up to about room temperature, spin polarization decay time with and without a magnetic field, the spin-orbit field, single electron relaxation time, transport scattering time, and the electron-electron Coulomb scattering time highlight the attractiveness of n-doped multilayer systems for spin devices.

Long-lived nanosecond spin coherence in high-mobility 2DEGs confined in double and triple quantum wells

We investigated the spin coherence of high-mobility two-dimensional electron gases confined in multilayer GaAs quantum wells. The dynamics of the spin polarization was optically studied using pump-probe techniques: time-resolved Kerr rotation and resonant spin amplification. For double and triple quantum wells doped beyond the metal-to-insulator transition, the spin-orbit interaction was tailored by the sample parameters of structural symmetry (Rashba constant), width, and electron density (Dresselhaus linear and cubic constants) which allow us to attain long dephasing times in the nanoseconds range. The determination of the scales, namely, transport scattering time, single-electron scattering time, electron-electron scattering time, and spin polarization decay time further supports the possibility of using n-doped multilayer systems for developing spintronic devices.

Localization and spin transport in honeycomb structures with spin-orbit coupling

Transfer-matrix methods are used for a tight-binding description of electron transport in graphene-like geometries, in the presence of spin-orbit couplings. Application of finite-size scaling and phenomenological renormalization techniques shows that, for strong enough spin-orbit interactions and increasing on-site disorder, this system undergoes a metal-insulator transition characterized by the exponents $\nu=2.71(8)$, $\eta=0.174(2)$. We show how one can extract information regarding spin polarization decay with distance from an injection edge, from the evolution of wave-function amplitudes in the transfer-matrix approach. For (relatively weak) spin-orbit coupling intensity $\mu$, we obtain that the characteristic length $\Lambda_s$ for spin-polarization decay behaves as $\Lambda_s \propto \mu^{-2}$.

Persistent coherence and spin polarization of topological surface states on topological insulators

Gapless surface states on topological insulators are protected from elastic scattering on non-magnetic impurities which makes them promising candidates for low-power electronic applications. However, for wide-spread applications, these states should remain coherent and significantly spin polarized at ambient temperatures. Here, we studied the coherence and spin-structure of the topological states on the surface of a model topological insulator, Bi2Se3, at elevated temperatures in spin and angle-resolved photoemission spectroscopy. We found an extremely weak broadening and essentially no decay of spin polarization of the topological surface state up to room temperature. Our results demonstrate that the topological states on surfaces of topological insulators could serve as a basis for room temperature electronic devices.

Spin Kinetics in Low-Dimensional Semiconductor Systems

ABSTRACT Spin kinetics in low-dimensional semiconductor systems was investigated spectroscopically. In the structures, owing to the quantum confinement, the degeneracy of the heavy-hole (HH) and light-hole valence bands was removed. Semiconductor systems were pumped for governing transitions from the HH valence band to the conduction band for the generation of the conduction-band-electron spins, and a maximum ∼80% initial spin polarization was obtained in the systems at liquid helium temperature. Distinct spin oscillations and polarization decay were also observed. Spin kinetics of the drifting electrons was studied as a function of the external magnetic field as well as that of the system temperature in the exact Voigt configuration.

A Carbon Based Spintronic Material Fe<SUB><I>x</I></SUB>–C<SUB>1−<I>x</I></SUB>/Si Structure

The decay of spin polarization poses serious problems for spintronic devices. It will be greatly helped by the availability of spintronic materials with a long spin diffusion length. Carbon has small spin-orbital interaction and longer coherent length. This makes carbon suitable material for exploitation in the spintronic materials and devices. A great deal of magnetoresistance (MR) research has been carried out in carbon nanotubes, grapheme and small carbon molecules. However, the MRs of these materials are normally observed at low temperature, making these carbon materials difficult used in information industry. In this paper, we introduce a novel class of carbon based hybrid materials Fe(x)-C(1-x)/Si structure which show larger MR at room temperature. These materials have also some other novel physical properties, such as electromagnetoresistance, switch effect, pressure sensitivity, gas sensitivity and photoconductivity. This kind of carbon based materials has shown early sign of being excellent candidates for spintronic materials operating at room temperature.

Spin polarization decay in magnetic tunnel junctions with semimetal-inserted layers

Magnetic tunnel junctions (MTJs) were fabricated with a thin layer of semimetallic bismuth inserted between the tunnel barrier and the top ferromagnetic electrode. The tunneling magnetoresistance (TMR) was measured on a set of samples for which the thickness of the inserted layer varied from 0 to 20 nm. The TMR decreased with an exponential decay length that was found to be ΛBi=4.1 nm=0.48 λF,Bi, where λF,Bi is the Fermi wavelength measured in comparable Bi films. This result is in remarkably good agreement with the decay length previously measured in MTJs with inserted copper layers, λCu=0.58 λF,Cu, even though the values of λF differ by an order of magnitude. It thereby gives a confirmation that the characteristic length scale of the tunneling density of states is the Fermi wavelength. Measurements of TMR as a function of bias voltage show a large asymmetry and the peak TMR is shifted to a nonzero value.

Spin orientation of holes in quantum wells

This paper reviews the spin orientation of spin-3/2 holes in quantum wells. We discuss the Zeeman and Rashba spin splitting in hole systems that are qualitatively different from their counterparts in electron systems. We show how a systematic understanding of the unusual spin-dependent phenomena in hole systems can be gained using a multipole expansion of the spin density matrix. As an example we discuss spin precession in hole systems that can give rise to an alternating spin polarization. Finally, we discuss the qualitatively different regimes of hole spin polarization decay in clean and dirty samples.

Long nuclear spin polarization decay times controlled by optical pumping in individual quantum dots

Nuclear polarization dynamics are measured in the nuclear spin bistability regime in a single optically pumped $\mathrm{In}\mathrm{Ga}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dot. The controlling role of nuclear spin diffusion from the dot into the surrounding material is revealed in pump-probe measurements of the nonlinear nuclear spin dynamics. We measure nuclear spin polarization decay times in the range of $0.2--5\phantom{\rule{0.3em}{0ex}}\mathrm{s}$, strongly dependent on the optical pumping time. The long nuclear spin decay arises from polarization of the material surrounding the dot by spin diffusion for long $(>5\phantom{\rule{0.3em}{0ex}}\mathrm{s})$ pumping times. The time-resolved methods allow the detection of the unstable nuclear polarization state in the bistability regime otherwise undetectable in cw experiments.

Spin polarization decay in spin-12and spin-32systems

We present a general unifying theory for spin polarization decay due to the interplay of spin precession and momentum scattering that is applicable to both spin-1/2 electrons and spin-3/2 holes. Our theory allows us to identify and characterize a wide range of qualitatively different regimes. For strong momentum scattering or slow spin precession we recover the D'yakonov-Perel result, according to which the spin relaxation time is inversely proportional to the momentum relaxation time. On the other hand, we find that, in the ballistic regime the carrier spin polarization shows a very different qualitative behavior. In systems with isotropic spin splitting the spin polarization can oscillate indefinitely, while in systems with anisotropic spin splitting the spin polarization is reduced by spin dephasing, which is non-exponential and may result in an incomplete decay of the spin polarization. For weak momentum scattering or fast spin precession, the oscillations or non-exponential spin dephasing are modulated by an exponential envelope proportional to the momentum relaxation time. Nevertheless, even in this case in certain systems a fraction of the spin polarization may survive at long times. Finally it is shown that, despite the qualitatively different nature of spin precession in the valence band, spin polarization decay in spin-3/2 hole systems has many similarities to its counterpart in spin-1/2 electron systems.

Dynamics of Quantum Dot Nuclear Spin Polarization Controlled by a Single Electron

We present measurements of the buildup and decay of nuclear spin polarization in a single semiconductor quantum dot. Our experiment shows that we polarize the nuclei in a few milliseconds, while their decay dynamics depends drastically on external parameters. We show that a single electron can very efficiently depolarize nuclear spins in milliseconds whereas in the absence of the electron the nuclear spin lifetime is on the scale of seconds. This lifetime is further enhanced by 1-2 orders of magnitude by quenching the nonsecular nuclear dipole-dipole interactions with a magnetic field of 1 mT.

Magnetoresistance in Sr2FeMoO6:x glass composites

The effects of interfacial states on the temperature dependence of the magnetoresistance (MR) of Sr2FeMoO6-glass composites have been studied. X-ray diffraction analyses show that the glass is most likely located at the grain boundary without causing a change of the crystal structure of Sr2FeMoO6. The variation of the resistance with temperature and magnetic field indicates that the added glass layer has profound influence on the MR properties. At low temperature, the MR in low fields is enhanced notably because the insulating barrier for the intergranular tunneling is improved by adding the glass layer at the grain surface. However, at high temperature, the MR decreases rapidly with the increase of temperature due to, in addition to the enhancement of spin-independent hopping of electrons through the localized states, the fast decay of spin polarization at the surfaces of the grains. This decay is induced by the separation of the ferromagnetic grains with the nonmagnetic glass layer at the grain boundaries.

Carrier spin dynamics in modulation-doped InAs∕GaAs quantum dots

Photoexcited electron and hole spin relaxation was studied in modulation-doped and undoped InAs∕GaAs quantum dots by means of time-resolved photoluminescence. After excitation into the barriers or the wetting layer, the electron spin polarization is preserved during the capture and relaxation in the dots, especially in the p-doped structures, and decays with a characteristic time of about 100ps. Spin state admixture in combination with electron interaction with acoustic phonons is suggested as the spin relaxation mechanism. Rapid spin polarization decay during carrier relaxation in undoped quantum dots is attributed to electron-optical phonon interaction. For carrier excitation directly into the dots, no significant spin polarization was observed, which points to the mixed nature of hole levels in quantum dots. The hole spin polarization randomizes on a much shorter time scale and is not detected in the experiment.

A New Basic Motif in Cyanometallate Coordination Polymers: Structure and Magnetic Behavior of M(μ‐OH2)2[Au(CN)2]2(M=Cu, Ni)

The structures of two cyanoaurate-based coordination polymers, M(mu-OH(2))(2)[Au(CN)(2)](2) (M=Cu, Ni), were determined by using a combination of powder and single-crystal X-ray diffraction techniques. The basic structural motif for both polymers contains rarely observed M(mu-OH(2))(2)M double aqua-bridges, which generate an infinite chain; two trans [Au(CN)(2)](-) units also dangle from each metal center. The chains form ribbons that interact three dimensionally through CNH hydrogen bonding. The magnetic properties of both compounds and of the dehydrated analogue Cu[Au(CN)(2)](2) were investigated by direct current (dc) and alternating current (ac) magnetometry; muon spin-relaxation data was also obtained to probe their magnetic properties in zero-field. In M(mu-OH(2))(2)[Au(CN)(2)](2), ferromagnetic chains of M(mu-OH(2))(2)M are present below 20 K. Interchain magnetic interactions mediated through hydrogen bonding, involving water and cyanoaurate units, yield a long-range magnetically ordered system in Cu(mu-OH(2))(2)[Au(CN)(2)](2) below 0.20 K, as indicated by precession in the muon spin polarization decay. Ni(mu-OH(2))(2)[Au(CN)(2)](2) undergoes a transition to a spin-glass state in zero-field at 3.6 K, as indicated by a combination of muon spin-relaxation and ac-susceptibility data. This transition is probably due to competing interactions that lead to spin frustration. A phase transition to a paramagnetic state is possible for Ni(mu-OH(2))(2)[Au(CN)(2)](2) upon application of an external field; the critical field was determined to be 700 Oe at 1.8 K. The dehydrated compound Cu[Au(CN)(2)](2) shows weak antiferromagnetic interactions at low temperatures.