Spin Detection Research Articles

High-Contrast Quantum Imaging with Time-Gated Fluorescence Detection

Optical detection of spin state has been widely used for the solid state spin qubit in the application of quantum information processing. The signal contrast determines the accuracy of quantum state manipulation, sensitivity of quantum sensing and resolution of quantum imaging. Here, we demonstrated a time-gated fluorescence detection method for enhancing the spin state signal contrast of nitrogen vacancy (NV) center in diamond. By adjusting the delay between time gate and the excitation laser pulse, we improved both the signal contrast and signal-to-noise ratio for NV spin detection. An enhancement ratio of 1.86 times was reached for the signal contrast. Utilizing the time-gated fluorescence detection, we further demonstrated a high contrast quantum imaging of nanoparticle's stray magnetic field. Without any additional manipulation of the quantum state, we expect that this method can be used to improve the performance of various applications with NV center.

Nanoscale detection of valley-dependent spin splitting around atomic defects of graphene

Knowing exact nature of broken-symmetry states around individual atomic defects of graphene is very important in understanding the electronic properties. Here, we realize measurement of valley-dependent spin splitting around atomic defects of graphene at nanoscale and single-electron level by using edge-free graphene quantum dots. Our experiments detect large valley-dependent spin splitting around atomic defects of graphene due to the coexistence of sublattice symmetry breaking, enhanced spin–orbit coupling and time reversal symmetry breaking. The spin splitting in the two distinct valleys has opposite direction and almost has the same amplitude. The strategy reported in this work is quite robust and could be extended to measure broken-symmetry states in other 2D systems.

Enhancement of spin transparency by interfacial alloying

We report that atomic-layer alloying (intermixing) at a Pt/Co interface can increase, by approximately 30%, rather than degrade the interfacial spin transparency, and thereby strengthen the efficiency of the dampinglike spin-orbit torque arising from the spin Hall effect in the Pt. At the same time, this interfacial alloying substantially reduces fieldlike spin-orbit torque. Insertion of an ultrathin magnetic alloy layer at heavy-metal/ferromagnet interfaces represents an effective approach for improving interfacial spin transparency that may enhance not only spin-orbit torques but also the spin detection efficiency in inverse spin Hall experiments.

Room temperature electrical spin injection from a new spin gapless ferromagnetic semiconducting inverse Heusler alloy Mn2CoSi into p-Si via SiO2 tunnel barrier

Inverse Heusler alloys possessing spin gapless semiconducting behavior have drawn great curiosity among researchers in the past few months on account of their unique transport characteristics that can be put into use in spin based electronic device implementations. Thin films of a possible ternary spin gapless semiconductor Mn2CoSi (MCS) inverse Heusler alloy have been deposited on a p-Si (100) substrate using the electron beam physical vapor deposition technique. The as-grown films exhibit a polycrystalline nature having a uniform and smooth surface with full coverage. A magnetic study reveals that the film is ferromagnetically soft along the direction parallel to its plane and its Curie temperature (TC) is much higher than room temperature (300 K). The formation of the MCS/SiO2/p-Si heterostructure is confirmed from cross-sectional transmission electron microscopy and cross-sectional scanning electron microscopy studies. The electronic- and magneto-transport properties of the heterostructure have been studied at various isothermal conditions. From current–voltage characteristics, a conventional magnetic diode like behavior has been observed throughout the working temperature regime of 78–300 K. The temperature coefficient of resistance (TCR) value of the film is estimated to be –2.09 × 10−9 Ω m/K, which is similar to the TCR values of reported spin gapless semiconductors. Room temperature spin injection and detection in a nonmagnetic semiconductor (p-Si) has been carried out using the three-terminal Hanle device in our MCS/SiO2/p-Si heterostructure. The estimated values of spin lifetime (78 ps) and spin diffusion length (167 nm) of the injected carriers at room temperature provide an indication of their industrial importance in future spin based electronic device applications.

Spin detection with a micromechanical trampoline: towards magnetic resonance microscopy harnessing cavity optomechanics

We explore the prospects and benefits of combining the techniques of cavity optomechanics with efforts to image spins using magnetic resonance force microscopy (MRFM). In particular, we focus on a common mechanical resonator used in cavity optomechanics—high-stress stoichiometric silicon nitride (Si3N4) membranes. We present experimental work with a ‘trampoline’ membrane resonator that has a quality factor above 106 and an order of magnitude lower mass than a comparable standard membrane resonators. Such high-stress resonators are on a trajectory to reach 0.1 force sensitivities at MHz frequencies by using techniques such as soft clamping and phononic-crystal control of acoustic radiation in combination with cryogenic cooling. We present a demonstration of force-detected electron spin resonance of an ensemble at room temperature using the trampoline resonators functionalized with a magnetic grain. We discuss prospects for combining such a resonator with an integrated Fabry–Perot cavity readout at cryogenic temperatures, and provide ideas for future impacts of membrane cavity optomechanical devices on MRFM of nuclear spins.

Superfluid spin transport in ferro- and antiferromagnets

This paper focuses on spin superfluid transport, observation of which was recently reported in antiferromagnet Cr$_2$O$_3$ [Yuan et al., Sci. Adv. 4, eaat1098 (2018)]. This paper analyzes the role of dissipation in transformation of spin current injected with incoherent magnons to a superfluid spin current near the interface where spin is injected. The Gilbert damping parameter in the Landau-Lifshitz-Gilbert theory does not describe dissipation properly, and the dissipation parameters are calculated from the Boltzmann equation for magnons scattered by defects. The two-fluid theory is developed similar to the two-fluid theory for superfluids. This theory shows that the influence of temperature variation in bulk on the superfluid spin transport (bulk Seebeck effect) is weak at low temperatures. The scenario that the results of Yuan et al. are connected with the Seebeck effect at the interface between the spin detector and the sample is also discussed. The Landau criterion for an antiferromagnet put in a magnetic field is derived from the spectrum of collective spin modes. The Landau instability starts in the gapped mode earlier than in the Goldstone gapless mode, in contrast to easy-plane ferromagnets where the Goldstone mode becomes unstable. The structure of the magnetic vortex in the geometry of the experiment is determined. The vortex core has the skyrmion structure with finite magnetization component normal to the magnetic field. This magnetization creates stray magnetic fields around the exit point of the vortex line from the sample, which can be used for experimental detection of vortices.

Large g factor in bilayer WS2 flakes

The valley of transition metal dichalcogenides provides an additional platform to manipulate spin due to its unique selection rule. Normally, intralayer optical transitions in the magnetic field show a Zeeman splitting with a g factor of about −4. Here, we report a remarkable valley Zeeman effect exhibited by the splitting of excitonic emission in a bilayer WS2, with a value of g factor as large as −16.5. The observed large g factor results from the interlayer recombination, as the conduction band and the valence band are modified in opposite directions by the magnetic field in different layers. The interlayer recombination is due to the defect induced inversion symmetry breaking, which is theoretically not accessible in ideal bilayer WS2 with inversion symmetry. The large g factor of interlayer emission offers potential benefits for future optical spin control and detection.

Comment on “Optical detection of transverse spin-Seebeck effect in permalloy film using Sagnac interferometer microscopy”

Measuring spin currents and spin accumulations is challenging. This is mainly because of their small size and the lack of a standard spin detector comparable to conventional ampere meters or voltage detectors. It is all the more pleasing to hear about new methods for detecting spin currents and spin accumulations. A recent example is the paper of McLaughlin et al. [Phys. Rev. B 95, 180401 (2017)], who report novel measurements of the transverse spin-Seebeck effect (TSSE) in metals by optical means using a Sagnac interferometer microscope. However, because no standard procedures or reference measurements are available for verification, critical discussions are crucial for evaluation of such novel experimental approaches. In our comment, we estimate the size of the effect observed by McLaughlin et al. To illustrate that the estimated size is exceptionally large, we use a comparison with another spin-caloritronic effect: the spin-dependent Seebeck effect. Such a large TSSE is in contradiction with previous experiments that could not resolve a TSSE signal. Therefore, we suggest that signal measured by McLaughlin et al. is not or not only caused by the TSSE.

Spin-momentum locked spin manipulation in a two-dimensional Rashba system

Spin-momentum locking, which constrains spin orientation perpendicular to electron momentum, is attracting considerable interest for exploring various spin functionalities in semiconductors and topological materials. Efficient spin generation and spin detection have been demonstrated using the induced helical spin texture. Nevertheless, spin manipulation by spin-momentum locking remains a missing piece because, once bias voltage is applied to induce the current flow, the spin orientation must be locked by the electron momentum direction, thereby rendering spin phase control difficult. Herein, we demonstrate the spin-momentum locking-induced spin manipulation for ballistic electrons in a strong Rashba two-dimensional system. Electron spin rotates in a circular orbital motion for ballistically moving electrons, although spin orientation is locked towards the spin-orbit field because of the helical spin texture. This fact demonstrates spin manipulation by control of the electron orbital motion and reveals potential effects of the orbital degree of freedom on the spin phase for future spintronic and topological devices and for the processing of quantum information.

Tunable Valley Polarized Plasmon-Exciton Polaritons in Two-Dimensional Semiconductors.

Monolayers of transition-metal dicalcogenides have emerged as two-dimensional semiconductors with direct bandgaps at degenerate but inequivalent electronic "valleys", supporting distinct excitons that can be selectively excited by polarized light. These valley-addressable excitons, when strongly coupled with optical resonances, lead to the formation of half-light half-matter quasiparticles, known as polaritons. Here we report self-assembled plasmonic crystals that support tungsten disulfide monolayers, in which the strong coupling of semiconductor excitons and plasmon lattice modes results in a Rabi splitting of ∼160 meV in transmission spectra as well as valley-polarized photoluminescence at room temperature. More importantly we find that one can flexibly tune the degree of valley polarization by changing either the emission angle or the excitation angle of the pump beam. Our results provide a platform that allows the detection, control, and processing of optical spin and valley information at the nanoscale under ambient conditions.

The linear relationship of spin pumping energy in a La:YIG/Pt heterostructure used in a microwave rectifier

ABSTRACTLa3+ doped yttrium iron garnet films have been grown on (111) oriented gadolinium gallium garnet substrates via Liquid phase epitaxy technique as a basic material for ISHE device fabrication. Pt as a material with a large spin hall angle was used as a spin detection layer. We investigated the dependence of the spin pumping effect on the power and frequency of the excitation microwaves in La:YIG/Pt bilayers by measuring the ISHE voltage. We demonstrated that the area under the ISHE curve(SISHE) across a wide power range had a nearly linear correlation with the input microwave power (Pin). The parameter SISHE can be used to describe the spin current energy in a Pt layer which can be a useful parameter for a microwave rectifier.

Vector-Resolved Magnetooptic Kerr Effect Measurements of Spin–Orbit Torque

We demonstrate simultaneous detection of current-driven dampinglike and fieldlike spin–orbit torques in heavy metal/ferromagnetic metal bilayers by measuring all three magnetization components $m_{x},m_{y}$ , and $m_{z} $ using a vector-resolved magnetooptic Kerr effect (MOKE) technique based on quadrant detection. We investigate the magnitude and direction of spin–orbit torques in a series of platinum/permalloy samples, finding good agreement with results obtained via polar and quadratic MOKE measurements without quadrant detection.

微纳螺旋凹槽结构检测光子自旋角动量

微纳螺旋凹槽结构检测光子自旋角动量

Theory of time-resolved Raman scattering in correlated systems: Ultrafast engineering of spin dynamics and detection of thermalization

Ultrafast characterization and control of many-body interactions and elementary excitations are critical to understanding and manipulating emergent phenomena in strongly correlated systems. In particular, spin interaction plays an important role in unconventional superconductivity, but efficient tools for probing spin dynamics especially out of equilibrium, are still lacking. To address this question, we develop a theory for nonresonant time-resolved Raman scattering, which can be a generic and powerful tool for nonequilibrium studies. We also use exact diagonalization to simulate the pump-probe dynamics of correlated electrons in the square-lattice single-band Hubbard model. Different ultrafast processes are shown to exist in the time-resolved Raman spectra and dominate under different pump conditions. For high-frequency and off-resonance pumps, we show that the Floquet theory works well in capturing the softening of bimagnon excitation. By comparing the Stokes/anti-Stokes spectra, we also show that effective heating dominates at small pump fluences, while a coherent many-body effect starts to take over at larger pump amplitudes and frequencies on resonance to the Mott gap. Time-resolved Raman scattering thereby provides the platform to explore different ultrafast processes and design material properties out of equilibrium.

Ferromagnet-Free All-Electric Spin Hall Transistors.

The spin field-effect transistor, an essential building block for spin information processing, shows promise for energy-efficient computing. Despite steady progress, it suffers from a low-output signal because of low spin injection and detection efficiencies. We demonstrate that this low-output obstacle can be overcome by utilizing direct and inverse spin Hall effects for spin injection and detection, respectively, without a ferromagnetic component. The output voltage of our all-electric spin Hall transistor is about two orders of magnitude larger than that of previously reported spin transistors based on ferromagnets or quantum point contacts. Moreover, the symmetry of the spin Hall effect allows all-electric spin Hall transistors to effectively mimic n-type and p-type devices, opening a way of realizing the complementary functionality.

Importance of spin current generation and detection by spin injection and the spin Hall effect for lateral spin valve performance

Lateral spin valves are attractive device geometries where functional spin currents can be generated and detected by various mechanisms, such as spin injection and the direct and the inverse spin Hall effect. To understand the mechanisms behind these effects better, as well as their potential for application in devices, we combine multiple mechanisms in multi-terminal Pt–Py–Cu lateral spin valves: we generate pure spin currents in the copper spin conduit both via the spin Hall effect in platinum and electric spin injection from permalloy and detect signals both via conventional non-local detection and via the inverse spin Hall effect in the same device at variable temperatures. Differences are observed, which we explain by the different spin injection and detection mechanisms, revealing their importance for the temperature dependence, which is not just governed by the spin transport in the conduit as previously claimed. By determining a different sensitivity of the observed effects on the interfaces, we highlight the importance of the exact current path for the device operation.

Co - Fe - B/MgO/Ge Spin Photodiode Operating at Telecommunication Wavelength with Zero Applied Magnetic Field

We report on the growth and study of Co-Fe-B/MgO/Ge(001) spin photodiode by using a combination of both molecular beam epitaxy (MBE) and sputtering methods. An epitaxial growth of MgO on Ge in MBE is achieved by the deposition of MgO at room temperature (RT) followed by a post-growth anneal-ing at 300 °C. The spin detector, which consists of 1.1-nm ultrathin Co-Fe-B layer capped with 5-nm Ta is subsequently grown by sputtering at RT. After a post-growth annealing of the whole structure at 250 °C, we obtain clear evidence of a strong perpendicular magnetic anisotropy in the temperature range 10-300 K. Co-Fe-B/MgO/Ge(001) heterojunctions are then processed into spin photodiodes demonstrating at zero magnetic field a photocurrent helicity asymmetry of about 0.9% at 9 K and 0.1% at RT at the telecommunication wavelength of 1310 nm. The demonstration of a spin photodiode working at a telecommunication wavelength with zero applied magnetic field is of great interest for future applications of the optical transport of spin information.

Spin Absorption by In Situ Deposited Nanoscale Magnets on Graphene Spin Valves

An in situ measurement of spin transport in a graphene nonlocal spin valve is used to quantify the spin current absorbed by a small (250 nm $\times$ 750 nm) metallic island. The experiment allows for successive depositions of either Fe or Cu without breaking vacuum, so that the thickness of the island is the only parameter that is varied. Furthermore, by measuring the effect of the island using separate contacts for injection and detection, we isolate the effect of spin absorption from any change in the spin injection and detection mechanisms. As inferred from the thickness dependence, the effective spin current $j_e = \frac{2e}{\hbar} j_s$ absorbed by Fe is as large as $10^8$ A/m$^2$. The maximum value of $j_e$ is limited by the resistance-area product of the graphene/Fe interface, which is as small as 3 $\Omega\mu$m$^2$. The spin current absorbed by the same thickness of Cu is smaller than for Fe, as expected given the longer spin diffusion length and larger spin resistance of Cu compared to Fe. These results allow for a quantitative assessment of the prospects for achieving spin transfer torque switching of a nanomagnet using a graphene-based nonlocal spin valve.

Observation of local magnetoresistance signals in a SiGe-based lateral spin-valve device

Utilizing a Si0.1Ge0.9 layer grown on Ge/Si(111) as a spin-transport channel and a Co2FeAl0.5Si0.5 ferromagnetic epilayer as a spin injector and detector, we demonstrate two-terminal local magnetoresistance signals at low temperatures in SiGe based lateral spin-valve devices. The magnitude of the local magnetoresistance signals is twice as large as that of nonlocal signals below 50 K. The local magnetoresistance signal can be observed up to 225 K, at which the nonlocal magnetoresistance signals already disappear. To observe the local magnetoresistance signal at higher temperatures, we should consider the spin detection sensitivity of the spin detector contact in the used lateral spin-valve devices.

Enhanced magnon spin transport in NiFe2O4 thin films on a lattice-matched substrate

We investigate magnon spin transport in epitaxial nickel ferrite (NiFe2O4, NFO) films grown on magnesium gallate spinel (MgGa2O4, MGO) substrates, which have a lattice mismatch with NFO as small as 0.78%, resulting in the reduction of antiphase boundary defects and thus in improved magnetic properties in the NFO films. In nonlocal transport experiments where platinum (Pt) strips function as magnon spin injectors and detectors, enhanced signals are observed for both electrically and thermally excited magnons, and the magnon relaxation length (λm) of NFO is found to be around 2.5 μm at room temperature. Moreover, at both room and low temperatures, we present distinct features from the nonlocal spin Seebeck signals which arise from magnon-polaron formation. Our results demonstrate excellent magnon transport properties (magnon spin conductivity, λm, and spin mixing conductance at the Pt/NFO interface) of NFO films grown on a lattice-matched substrate which are comparable with those of yttrium iron garnet.