Spin Signal Research Articles

Non-Local and Local Spin Signals in a Lateral Spin Transport Device With ${\rm Co}_{2}{\rm FeAl}_{0.5}{\rm Si}_{0.5}/n$-GaAs Schottky Tunnel Junctions

We investigated magnetoresistance (MR) effects in four-terminal (4T) nonlocal and local measurements of a lateral spin transport device with Co2FeAl0.5Si0.5 (CFAS)/ -GaAs Schottky tunnel junctions. Clear voltage changes as a function of magnetic field were observed in both measurements, and these signals were confirmed to originate from spin injection, transport, and detection in the device. The values of voltage change in both measurements, ΔVlocal and ΔVlocal, increased linearly as a function of bias current. The MR ratio in local measurement could be explained by the conductivity mismatch theory. The nonlocal and local measurements in various circuit configurations indicated that the magnitude of was ΔVlocal enhanced not only by an increase in spin transport length in the n-GaAs channel but also by an enhancement of spin detection efficiency at the CFAS/n-GaAs interface.

Nonlocal spin correlations mediated by a superconductor

Nonlocal charge correlations induced in two normal metals contacted separately to a superconductor have been studied intensively in the past few years. Here we investigate nonlocal correlations induced by the transfer of pure spin currents through a superconductor on a scale comparable to the superconducting coherence length. As with charge currents, two processes contribute to the nonlocal spin signal: crossed Andreev reflection (CAR), where an electron with spin-up injected from one normal metal into the superconductor results in a hole with spin-down being injected into the second normal metal, and elastic cotunneling (EC), where the electron with spin-up injected from the first normal metal results in an electron with spin-up being injected into the second normal metal. Unlike charge currents, however, the spin currents associated with CAR and EC add due to the fact that the bulk superconductor cannot sustain a net spin current.

Effect of Cu interfacial layer thickness on spin-injection efficiency in NiFe/Cu/graphene spin valves

The spin injection efficiency in NiFe/Cu/graphene spin-valves has been investigated systematically with emphasis on the Cu thickness effect. An optimum Cu thickness is found to be around 2.5 nm for which spin injection efficiency up to 8.4% has been measured. With a further increase of Cu thickness, the efficiency decreases to 3.6%−3.8% at 3.5 nm and is hardly measureable at 5 nm and beyond. The decrease is discussed in the context of enhanced spin-flip scattering at the NiFe/Cu interface and grain boundaries in the polycrystalline Cu as well as current shunting induced lengthening of travelling distance of electrons inside Cu. On the other hand, a further reduction of Cu thickness below 2.5 nm results in pinholes which also lowers the spin-injection efficiency. In addition to the Cu thickness, another factor which affects the spin signal is the DC bias voltage. The non-local magnetoresistance is highest when measured under zero-bias condition and becomes smaller when a DC bias is applied. The reduction of efficiency correlates well with the decrease in Cu-graphene contact resistance, which is consistent with the non-ohmic nature of Cu-graphene interface. Hanle spin precession curves were measured at different back gate biases from which the spin relaxation length was determined to be 1.6 μm−2.7 μm.

Qualitative study of temperature-dependent spin signals in n-Ge-based lateral devices with Fe3Si/n+-Ge Schottky-tunnel contacts

We study electrical spin injection and detection in n-Ge across Fe3Si/n+-Ge Schottky tunnel barriers. Spin-accumulation signals detected electrically by the three-terminal Hanle-effect measurements have large temperature dependence, and the spin signals disappear at around 200 K. We find that the temperature variation in the spin signals is strongly related to that in the interface resistance of the Fe3Si/n+-Ge contacts. We also observe marked reduction in nonlocal spin-valve signals with increasing temperature in the four-terminal device with almost the same Fe3Si/n+-Ge/n-Ge heterostructure. Note that the nonlocal spin signals depend on not only the measurement temperature but also on the electrical characteristics of the spin injector. Considering the results of three-terminal and four-terminal measurements, we conclude that temperature dependence of the spin signals in our Fe3Si/n+-Ge/n-Ge devices is governed by the electrical characteristics of the spin injector. This study means that it is necessary to consider the effective Schottky-tunnel contacts with the stability on temperature variations for next-generation Ge-based spintronic applications.

Magnetic field dependence of non-local lateral spin-valve signals beyond the Hanle effect

We present a theoretical model of spin transport in metallic lateral valves that takes into account spin scattering on magnetic impurities. We show that the model agrees with recent experimental findings of increasing non-local spin signals by in-plane magnetic field, which is parallel to the injected spins. The increase arises due to reduction of conduction electron spin flips on magnetic impurities present at the metal–ferromagnet interfaces as they freeze out under application of the magnetic field.

Preparation of lateral spin-valve structure using doped conducting polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)

To perform four-terminal nonlocal spin-valve measurements on organic spin-valves, we fabricated lateral spin-valve devices consisting of doped conducting polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) [PEDOT:PSS] and ferromagnetic Ni80Fe20 narrow line (width: 530nm) electrodes. Although the formula of the nonlocal magnetoresistance with the parameters of doped conducting polymers predicts sufficient nonlocal magnetoresistance, we could not observe any spin signal. The spin diffusion length in the doped PEDOT:PSS device does not appear to be as long as those predicted by both the Elliott–Yafet mechanism and the theory of spin relaxation in organic disordered solids.

Transition from spin accumulation into interface states to spin injection in silicon and germanium conduction bands

Electrical spin injection into semiconductors paves the way for exploring new phenomena in the area of spin physics and new generations of spintronic devices. However the exact role of interface states in the electrical spin injection mechanism from a magnetic tunnel junction into a semiconductor is still under debate. Here we demonstrate a clear transition from spin accumulation into interface states to spin injection in the conduction band of n-Si and n-Ge using a CoFeB/MgO tunnel contact. We observe spin signal amplification at low temperature due to spin accumulation into interface states followed by a clear transition towards spin injection in the conduction band from approximately 150 K up to room temperature. In this regime, the spin signal is reduced down to a value compatible with the standard spin diffusion model. More interestingly, in the case of germanium, we demonstrate a significant modulation of the spin signal by applying a back-gate voltage to the conduction channel. We also observe the inverse spin Hall effect in Ge by spin pumping from the CoFeB electrode. Both observations are consistent with spin accumulation in the Ge conduction band.

In-plane and out-of-plane spin precession in lateral spin-valves

The spin signal variations of Al/NiFe lateral spin-valves with AlOx interfaces are studied under different applied field orientations. For applied fields perpendicular to the sample plane, the spin signal is governed by the spin precession and by the angular dephasing of spins in the channel, i.e., the Hanle effect. We show that using narrow permalloy wires with large shape anisotropies, it is also possible to observe spin precession with in-plane magnetic fields. In this case, the precessing spin current possesses an out-of plane component, which means that by tuning properly the external field it is possible to control along the three dimensions the orientation of the spins arriving on the ferromagnetic detector. Finally, fits of our data allow for both in and out-of plane fields extracting the relevant characteristics of our lateral spin-valves.

Colossal enhancement of spin–orbit coupling in weakly hydrogenated graphene

Graphene’s extremely small intrinsic spin–orbit (SO) interaction1 makes the realization of many interesting phenomena such as topological/quantum spin Hall states2,3 and the spin Hall effect4 (SHE) practically impossible. Recently, it was predicted1,5,6,7 that the introduction of adatoms in graphene would enhance the SO interaction by the conversion of sp2 to sp3 bonds. However, introducing adatoms and yet keeping graphene metallic, that is, without creating electronic (Anderson) localization8, is experimentally challenging. Here, we show that the controlled addition of small amounts of covalently bonded hydrogen atoms is sufficient to induce a colossal enhancement of the SO interaction by three orders of magnitude. This results in a SHE at zero external magnetic fields at room temperature, with non-local spin signals up to 100 Ω; orders of magnitude larger than in metals9. The non-local SHE is, further, directly confirmed by Larmor spin-precession measurements. From this and the length dependence of the non-local signal we extract a spin relaxation length of ∼1 μm, a spin relaxation time of ∼ 90 ps and a SO strength of 2.5 meV.

(Invited) SiGe Spintronics with Single-Crystalline Ferromagnetic Schottky-Tunnel Contacts

We present two important technologies for silicon-germanium (SiGe) spintronics. First, we show high-quality epitaxial growth of bcc-type ferromagnetic alloys on Si or Ge with atomically controlled interfaces. Second, we establish electrical spin injection and detection across the high-quality interfaces in Si- or Ge-based devices. Combining these technologies with a metal-oxide-semiconductor field-effect transistor structure, we can demonstrate room-temperature detection of spin accumulation and control the magnitude of the spin signals. These novel and interesting results are expected to build the basis of a key technology toward developing next-generation SiGe spintronic devices.

Room-temperature detection of spin accumulation in silicon across Schottky tunnel barriers using a metal–oxide–semiconductor field effect transistor structure (invited)

Using a metal-oxide-semiconductor field effect transistor structure with a high-quality CoFe/n+-Si contact, we systematically study spin injection and spin accumulation in a nondegenerated Si channel with a doping density of ∼4.5 × 1015 cm−3 at room temperature. By applying the gate voltage (VG) to the channel, we obtain sufficient bias currents (IBias) for creating spin accumulation in the channel and observe clear spin-accumulation signals even at room temperature. Whereas the magnitude of the spin signals is enhanced by increasing IBias, it is reduced by increasing VG interestingly. These features can be understood within the framework of the conventional spin diffusion model. As a result, a room-temperature spin injection technique for the nondegenerated Si channel without using insulating tunnel barriers is established, which indicates a technological progress for Si-based spintronic applications with gate electrodes.

Transport of perpendicular spin in a semiconductor channel via a fully electrical method

The transport of perpendicular spins in a GaAs channel is investigated via a fully electrical method. A Tb20Fe62Co18/Co40Fe40B20/MgO contact is used to inject perpendicular spin into the GaAs channel, where the Tb20Fe62Co18 layer produces perpendicular magnetization and the Co40Fe40B20 layer enhances the spin polarization of the injection current. By measuring the three-terminal Hanle effect with an in-plane field, we obtained a spin signal of 0.65 Ω (0.04 Ω) and a spin lifetime of 0.30 ns (0.17 ns) at 1.8 K (300 K). The observed spin signal with a MgO barrier is more than double of that without a MgO barrier.

Anomalous Nernst-Ettingshausen effect in nonlocal spin valve measurement under high-bias current injection

We have observed unconventional asymmetric field dependence of the nonlocal spin signal under a high dc bias current injection. From the systematic studies on the bias current and the field-direction dependencies of the nonlocal signals, the origin of the asymmetry is understood as the anomalous Nernst-Ettingshausen effect driven by the bias current. The bias current dependencies of the spin accumulation signal and the background resistance are also well explained by the thermal effects induced by the bias current.

Room-temperature sign reversed spin accumulation signals in silicon-based devices using an atomically smooth Fe3Si/Si(111) contact

We demonstrate a reliable sign-reversed spin signal detected by three-terminal Hanle effect measurements at room temperature in the Si-based lateral devices with one Fe3Si/Si(111) Schottky-tunnel contact. Theoretical calculations of the spin polarized density of states suggest the sign difference in the spin polarization (P) between the two types of Fe3Si/Si(111) interfaces. Actually, we directly observe the atomic steps at the Fe3Si/Si(111) interface, implying that there is a possible origin of the sign difference in the spin polarization (P) between spin-injection region and spin-detection one in one contact. The reliable sign-reversed spin signals support that the injected spins are transported laterally in the Si channel even for the three-terminal Hanle-effect measurements.

Radio-Frequency Magnetometry Using a Single Electron Spin

We experimentally demonstrate a simple and robust protocol for the detection of weak radio-frequency magnetic fields using a single electron spin in diamond. Our method relies on spin locking, where the Rabi frequency of the spin is adjusted to match the MHz signal frequency. In a proof-of-principle experiment we detect a 7.5 MHz magnetic probe field of ~40 nT amplitude with <10 kHz spectral resolution. Rotating-frame magnetometry may provide a direct and sensitive route to high-resolution spectroscopy of nanoscale nuclear spin signals.

Computation of DNP coupling factors of a nitroxide radical in toluene: seamless combination of MD simulations and analytical calculations

Dynamic nuclear polarization (DNP) employs paramagnetic species to increase the NMR signal of nuclear spins. In liquids, the efficiency of the effect depends on the strength of the interaction between the electron and nuclear spins and the time scales on which this interaction is modulated by the physical motion of the spin-bearing molecules. An approach to quantitatively predict the contribution of molecular motions to the DNP enhancement using molecular dynamics (MD) simulations is developed and illustrated for the nitroxide radical TEMPOL in liquid toluene. A multi-resolution strategy that combines explicit treatment of the solvent at short distances from the free radical with implicit description at large intermolecular distances is adopted. Novel analytical expressions are obtained to correct for the finite spatial extent of the MD simulations. The atomistic and analytical descriptions are sewn seamlessly together by ensuring that for molecular trajectories that start in the near (explicit) region and end in the distant (implicit) region the analytical dipolar spectral densities reproduce the MD estimates. The spectral densities obtained from the developed approach are used to calculate DNP coupling factors separately for the ring and methyl protons of toluene. The agreement with previously reported experimental DNP data at a magnetic field of 3.4 T is noteworthy and encouraging. Maximum obtainable DNP enhancements at other magnetic fields are predicted.

Proposal of an Experimental Scheme for Determination of Penetration Depth of Transverse Spin Current by a Nonlocal Spin Valve

We theoretically propose an experiment to determine the penetration depth of a transverse spin current using a nonlocal spin valve with three ferromagnetic (F) layers, where the F_1, F_2, and F_3 layers act as the spin injector, detector, and absorber, respectively. We show that the penetration depth can be evaluated by measuring the dependence of the spin signal (magnetoresistance) on the thickness of the F_3 layer.

Large local Hall effect in pin-hole dominated multigraphene spin-valves

We report local and non-local measurements in pin-hole dominated mesoscopic multigraphene spin-valves. Local spin-valve measurements show spurious switching behavior in resistance during magnetic field sweeping similar to the signal observed due to spin injection into multigraphene. The switching behavior has been explained in terms of a local Hall effect due to a thickness irregularity of the tunnel barrier. The local Hall effect appears due to a large local magnetostatic field produced near the roughness in the AlOx tunnel barrier. In our samples the resistance change due to the local Hall effect remains negligibly small above 75 K. A strong local Hall effect might hinder spin injection into multigraphene, resulting in no spin signal in non-local measurements.

Thermal spin injection and accumulation in CoFe/MgO/n-type Ge contacts.

Understanding the interplay between spin and heat is a fundamental and intriguing subject. Here we report thermal spin injection and accumulation in CoFe/MgO/n-type Ge contacts with an asymmetry of tunnel spin polarization. Using local heating of electrodes by laser beam or electrical current, the thermally-induced spin accumulation is observed for both polarities of the temperature gradient across the tunnel contact. We observe that the magnitude of thermally injected spin signal scales linearly with the power of local heating of electrodes, and its sign is reversed as we invert the temperature gradient. A large Hanle magnetothermopower (HMTP) of about 7.0% and the Seebeck spin tunneling coefficient of larger than 0.74 meV K−1 are obtained at room temperature.

Detecting external electron spins using nitrogen-vacancy centers

Near-surface nitrogen-vacancy (NV) centers have been created in diamond through low energy implantation of 15N to sense electron spins that are external to the diamond. By performing double resonance experiments, we have verified the presence of g=2 spins on a diamond crystal that was subjected to various surface treatments, including coating with a polymer film containing the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). Subsequent acid cleaning eliminated the spin signal without otherwise disrupting the NV center, providing strong evidence that the spins were at the surface. A clear correlation was observed between the size of the detected spin signal and the relaxation time T2 for the six NV centers studied. We have developed a model that takes into account the finite correlation time of the fluctuating magnetic fields generated by the external spins, and used it to infer the signal strength and correlation time of the magnetic fields from these spins. This model also highlights the sensitivity advantage of active manipulation of the longitudinal spin component via double resonance over passive detection schemes that measure the transverse component of spin.