Spin Signal Research Articles

High spin polarization and spin signal enhancement in non-local spin valves with Co–Fe alloy injectors and detectors

For applications such as spin accumulation sensors for next-generation hard disk drive read heads, and for fundamental research, it is desirable to increase the spin signal in metallic non-local spin valves, which are central devices in spintronics. To this end, here, we report on the integration of high-spin-polarization Co–Fe binary alloy ferromagnetic injectors and detectors in Al-based non-local spin valves. Room-temperature deposition on amorphous substrates from an alloy target is shown to generate smooth, polycrystalline (110-textured), solid-solution body-centered-cubic Co75Fe25 films, which we characterize by energy dispersive x-ray spectroscopy, x-ray diffraction, x-ray reflectivity, atomic force microscopy, and electronic transport. Simple integration into transparent-interface Al non-local spin valves is then shown to realize up to a factor of ∼5 enhancement of the spin signal relative to Co, with full quantitative analysis yielding strikingly temperature-independent current spin polarizations exceeding 60%. We make a detailed quantitative comparison of these values with prior literature, concluding that Co–Fe alloys present a remarkably facile route to higher spin polarization and spin signals in non-local spin valves, with minimal barrier to adoption.

High-Resolution NMR Spectroscopy at Large Fields with Nitrogen Vacancy Centers.

Ensembles of nitrogen-vacancy (NV) centers are used as sensors to detect nuclear magnetic resonance signals from micron-sized samples at room temperature. In this scenario, the regime of large magnetic fields is especially interesting as it leads to a large nuclear thermal polarization-thus, to a strong sensor response even in low concentration samples-while chemical shifts and J couplings become more accessible. Nevertheless, this regime remains largely unexplored owing to the difficulties of coupling NV-based sensors with high-frequency nuclear signals. In this Letter, we circumvent this problem with a method that maps the relevant energy shifts in the amplitude of an induced nuclear spin signal that is subsequently transferred to the sensor. This stage is interspersed with free-precession periods of the sample nuclear spins where the sensor does not participate. Thus, our method leads to high spectral resolutions ultimately limited by the coherence of the nuclear spin signal.

Fiber-integrated silicon carbide silicon-vacancy-based magnetometer.

Silicon vacancies in silicon carbide have drawn much attention for various types of quantum sensing. However, most previous experiments are realized using confocal scanning systems, which limits their practical applications. In this work, we demonstrate a compact fiber-integrated silicon carbide silicon-vacancy-based magnetometer at room temperature. First, we effectively couple the silicon vacancy in a tiny silicon carbide slice with an optical fiber tip and realize the readout of the spin signal through the fiber at the same time. We then study the optically detected magnetic resonance spectra at different laser and microwave powers, obtaining an optimized magnetic field sensitivity of 12.3 μT/Hz 12. Based on this, the magnetometer is used to measure the strength and polar angle of an external magnetic field. Through these experiments, we have paved the way for fiber-integrated silicon-vacancy-based magnetometer applications in practical environments, such as geophysics and biomedical sensing.

Incomplete hetero-structure molecular layer enhancing room-temperature spin-photovoltaic performances

Molecular spin-photovoltaic device (MSP) has creatively integrated photovoltaic and spintronic functionalities, where output signal can be modulated even reaching fully spin polarized at room temperature by photo-generated voltage (Vph) as never before. Hence, to enhance Vph on a premise of effective spin transport is pivotal for improving MSP performance since it determines the capability for modulating spin signal. However, such capability for Vph to modulate spin signal is severely limited to the weak driving force of spin-photovoltaic interaction caused by single-component configuration of current MSP, and in this case, it can hardly be improved since MSP birth. Herein, an incomplete hetero-structure active layer is firstly employed to construct a novel-designed MSP, and a more efficient working mechanism has therefore been triggered and endows MSP with greater potential in the future. In such an MSP, the Vph has been enhanced by 15 times than in traditional single-component MSP at room temperature, which is far better than any previous reports. Moreover, desired properties of molecules for constructing ideal incomplete hetero-structure configuration and further optimizing above mechanism have been sketched theoritically, which will open a brand-new research chapter in both academic and practical aspects.

A Non-Perturbative, Low-Noise Surface Coating for Sensitive Force-Gradient Detection of Electron Spin Resonance in Thin Films.

The sensitivity of magnetic resonance force microscopy (MRFM) is limited by surface noise. Coating a thin-film polymer sample with metal has been shown to decrease, by orders of magnitude, sample-related force noise and frequency noise in MRFM experiments. Using both MRFM and inductively detected measurements of electron-spin resonance, we show that thermally evaporating a 12 nm gold layer on a 40 nm nitroxide-doped polystyrene film inactivates the nitroxide spin labels to a depth of 20 nm, making single-spin measurements difficult or impossible. We introduce a "laminated sample" protocol in which the gold layer is first evaporated on a sacrificial polymer. The sample is deposited on the room-temperature gold layer, removed using solvent lift-off, and placed manually on a coplanar waveguide. Electron spin resonance (ESR) of such a laminated sample was detected via MRFM at cryogenic temperatures using a high-compliance cantilever with an integrated 100-nm-scale cobalt tip. A 20-fold increase of spin signal was observed relative to a thin-film sample prepared instead with an evaporated metal coating. The observed signal is still somewhat smaller than expected, and we discuss possible remaining sources of signal loss.

Realization of high-dynamic-range broadband magnetic-field sensing with ensemble nitrogen-vacancy centers in diamond.

We present a new magnetometry method integrating an ensemble of nitrogen-vacancy (NV) centers in a single-crystal diamond with an extended dynamic range for monitoring a fast changing magnetic-field. The NV-center spin resonance frequency is tracked using a closed-loop frequency locked technique with fast frequency hopping to achieve a 10kHz measurement bandwidth, thus allowing for the detection of fast changing magnetic signals up to 0.723 T/s. This technique exhibits an extended dynamic range subjected to the working bandwidth of the microwave source. This extended dynamic range can reach up to 4.3 mT, which is 86 times broader than the intrinsic dynamic range. The essential components for NV spin control and signal processing, such as signal generation, microwave frequency control, data processing, and readout, are integrated in a board-level system. With this platform, we demonstrate a broadband magnetometry with an optimized sensitivity of 4.2 nT Hz-1/2. This magnetometry method has the potential to be implemented in a multichannel frequency locked vector magnetometer suitable for a wide range of practical applications, such as magnetocardiography and high-precision current sensors.

Artificial Graphene Spin Polarized Electrode for Magnetic Tunnel Junctions.

2D materials offer the ability to expose their electronic structure to manipulations by a proximity effect. This could be harnessed to craft properties of 2D interfaces and van der Waals heterostructures in devices and quantum materials. We explore the possibility to create an artificial spin polarized electrode from graphene through proximity interaction with a ferromagnetic insulator to be used in a magnetic tunnel junction (MTJ). Ferromagnetic insulator/graphene artificial electrodes were fabricated and integrated in MTJs based on spin analyzers. Evidence of the emergence of spin polarization in proximitized graphene layers was observed through the occurrence of tunnel magnetoresistance. We deduced a spin dependent splitting of graphene's Dirac band structure (∼15 meV) induced by the proximity effect, potentially leading to full spin polarization and opening the way to gating. The extracted spin signals illustrate the potential of 2D quantum materials based on proximity effects to craft spintronics functionalities, from vertical MTJs memory cells to logic circuits.

Synergistic modulation of spin and fluorescence signals in a nano-Saturn assembled by a metallofullerene and cycloparaphenylene nanohoop

Synergistic modulation of spin and fluorescence signals in a nano-Saturn assembled by a metallofullerene and cycloparaphenylene nanohoop

Magnetic control of heat generation in a magnetic tunnel contact with spin accumulation

It is described how the heat generation by an electrical current in a ferromagnetic tunnel contact to a nonmagnetic material depends on the spin accumulation in the nonmagnetic electrode for both Joule and Peltier heating. This enables the control of the heating in the tunnel contact by an external applied magnetic field that induces spin precession of the spin accumulation. Thereby, a Hanle type of spin signal is imprinted in the heating power. We derive expressions for the magnitude and sign of the magnetic-field-dependent heating power for Joule and Peltier heating, and discuss the important parameters. For contacts with small resistance-area product, the spin accumulation provides the dominant contribution to the Joule heating power, which can therefore be modulated by more than a factor of 2 with a magnetic field. The described phenomenon can also produce genuine spin signals in various devices, including nonlocal lateral spin valves.

Spin alignment around TNG300-1 voids

ABSTRACT Using a new statistical approach, we study the alignment signal of galactic spins with respect to the centre of voids identified in the TNG300-1 simulation. We explore this signal in different samples of galaxies, varying their distance from the void centre, mass, spin norm, local density, and velocity. We find a strong tendency (>9σ) of massive, high-spin, and low-radial velocity galaxies to be aligned perpendicularly to the void-centric direction in a wide range of distances corresponding to 0.9–1.4 void radii. Furthermore, we find that in these subdense environments, local density is irrelevant in the amplitude of spin alignment, while the largest impact is associated to the galaxy void-centric radial velocity in the sense that those at the lowest expansion rate are more strongly aligned perpendicularly to the centre of the void. Our results suggest that further analysis focused at understanding intrinsic alignments and their relation to large-scale structures may probe key for weak lensing studies in upcoming large surveys, such as Euclid and LSST.

Electrical detection of nuclear spins via silicon vacancies in silicon carbide at room temperature

Color centers in wide-bandgap semiconductors, including diamond and silicon carbide (SiC), are attractive systems for quantum information and quantum sensor devices with excellent spin properties at room temperature. In addition, nuclear spins in crystals are expected to serve as the quantum memory and to enhance the sensitivity of quantum sensors with the combination with color centers as a result of an extremely long spin coherence time. Although the spin state of both color centers and nuclear spins coupled through hyperfine interactions is usually optically read out, an electrical readout technique is important for miniaturizing and integrating devices. In the present study, we report the electrical detection of silicon vacancy (V2) centers in 4H-SiC by photocurrent-detected magnetic resonance (PDMR) using a frequency-sweep technique. We electrically observe the spin coherence of the V2 centers and clearly resolve the hyperfine splitting of the electron spin signal for the V2 centers coupled with next-nearest-neighbor 29Si atoms. In addition, we apply PDMR to electron–nuclear double resonance (PD-ENDOR) to detect nuclear magnetic resonance of 29Si at room temperature and find that this method can resolve nuclear spins coupled with neighboring electron spins in the V2 centers. The realization of PD-ENDOR is expected to be a critical step toward the development of electrically driven integrated quantum devices.

Chirality-Induced Spin Selectivity in Heterochiral Short-Peptide-Carbon-Nanotube Hybrid Networks: Role of Supramolecular Chirality.

Supramolecular short-peptide assemblies have been widely used for the development of biomaterials with potential biomedical applications. These peptides can self-assemble in a multitude of chiral hierarchical structures triggered by the application of different stimuli, such as changes in temperature, pH, solvent, etc. The self-assembly process is sensitive to the chemical composition of the peptides, being affected by specific amino acid sequence, type, and chirality. The resulting supramolecular chirality of these materials has been explored to modulate protein and cell interactions. Recently, significant attention has been focused on the development of chiral materials with potential spintronic applications, as it has been shown that transport of charge carriers through a chiral environment polarizes the carrier spins. This effect, named chirality-induced spin selectivity or CISS, has been studied in different chiral organic molecules and materials, as well as carbon nanotubes functionalized with chiral molecules. Nevertheless, this effect has been primarily explored in homochiral systems in which the chirality of the medium, and hence the resulting spin polarization, is defined by the chirality of the molecule, with limited options for tunability. Herein, we have developed heterochiral carbon-nanotube-short-peptide materials made by the combination of two different chiral sources: that is, homochiral peptides (l/d) + glucono-δ-lactone. We show that the presence of a small amount of glucono-δ-lactone with fixed chirality can alter the supramolecular chirality of the medium, thereby modulating the sign of the spin signal from "up" to "down" and vice versa. In addition, small amounts of glucono-δ-lactone can even induce nonzero spin polarization in an otherwise achiral and spin-inactive peptide-nanotube composite. Such "chiral doping" strategies could allow the development of complementary CISS-based spintronic devices and circuits on a single material platform.

Power-law scaling of correlations in statistically polarised nano-NMR

Diffusion noise is a major source of spectral line broadening in liquid state nano-scale nuclear magnetic resonance with shallow nitrogen-vacancy centres, whose main consequence is a limited spectral resolution. This limitation arises by virtue of the widely accepted assumption that nuclear spin signal correlations decay exponentially in nano-NMR. However, a more accurate analysis of diffusion shows that correlations survive for a longer time due to a power-law scaling, yielding the possibility for improved resolution and altering our understanding of diffusion at the nano-scale. Nevertheless, such behaviour remains to be demonstrated in experiments. Using three different experimental setups and disparate measurement techniques, we present overwhelming evidence of power-law decay of correlations. These result in sharp-peaked spectral lines, for which diffusion broadening need not be a limitation to resolution.

Development of an optically gated Fe/n-GaAs spin-polarized transistor

Efficient modulation of electrically injected spin signals that is suitable for modern-day transistor functionality is yet to be established. In this work, we demonstrate in detail the fabrication of a $\mathrm{Fe}/n\text{-GaAs}$ spin injection device and the experimental setup for an optical gating of the nonlocal spin transport signal. In situ scanning electron microscopy interface imaging reveals more uniform current distribution at the $\mathrm{Fe}/n\text{-GaAs}$ injector interface at bias voltages higher than the Schottky barrier height. Three- and four-terminal Hanle measurements confirm successful spin injection into $n$-GaAs, with strong interfacial spin dephasing at high magnetic fields. A time-resolved pump-probe Kerr rotation setup was used to illuminate circularly polarized light in the region of the pure spin current in $\mathrm{Fe}/n\text{-GaAs}$ lateral spin injection devices, where ($0.4\ifmmode\pm\else\textpm\fi{}0.3)%$ modulation of the nonlocal signal depending on the light helicity was observed at 30 K.

Gate modulation of the spin current in graphene/WSe2 van der Waals heterostructure at room temperature

Two-dimensional transition metal dichalcogenide semiconductors owning a large intrinsic spin-orbit coupling (SOC) are considered the best candidates to generate, detect and manipulate the spin currents. The SOC defines the interconversion of spin and charge currents via Rashba Edelstein effect (spin Hall effect) and its reciprocal as inverse Rashba Edelstein effect (inverse spin Hall effect). However, the spin signal originated in low dimensional materials because of Rashba Edelstein effect or spin Hall effect yet needed to be addressed with distinguishable measurement technique. Here, we demonstrate experimentally the room temperature interconversion of spin and charge currents in Graphene/WSe 2 van der Waals heterostructure which is induced by proximity effect. Remarkably, the spin currents induced by Rashba Edelstein effect and spin Hall effect are discriminated and extracted individually via external magnetic field, respectively. The magnitude of spin transport and their corresponding spin efficiencies ( α RE E = 1.47 ± 0.03 % ) and (θ SHE = = 4.5 ± 0.07 % ) are modulated via applied electric field and temperature. Such electric, and magnetic field tunability of the spin transport through the non-magnetic materials may provide a new approach to fabricate the fast and low-power spintronic devices for quantum scale applications. • Experimental demonstration of interconversion of spin and charge currents in Graphene/WSe 2 van der Waals heterostructure, induced by proximity of WSe 2 , at room temperature. • Remarkably, the spin currents induced by Rashba Edelstein effect and spin Hall effect are discriminated and their corresponding spin efficiencies are extracted individually via external magnetic field, respectively.

Temperature Dependence of Two-Terminal Local Magnetoresistance in Co-Based Heusler Alloy/Ge Lateral Spin-Valve Devices

We investigate temperature dependence of two-terminal local magnetoresistance (MR) effect for germanium (Ge)-based lateral spin-valve (LSV) devices with three different ferromagnet (FM)/Ge Schottky tunnel contacts. When we insert a 0.7-nm-thick Fe layer between Co-based Heusler alloy and Ge interfaces in LSV devices, the magnitude of the local MR signals is significantly improved from low temperatures to room temperature. However, the temperature dependence of the MR ratio is still large even for LSV devices with Fe insertion. From the analysis based on the standard theory, the temperature-dependent MR ratio can be interpreted in terms of thermally excited spin waves ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T^{3/{2}}$ </tex-math></inline-formula> law) in FM contacts. The physical origin of the decay of the two-terminal MR ratio in Ge-based LSV devices is the same as that of the four-terminal nonlocal spin signals because small temperature dependence of contact resistance is demonstrated in FM/Ge Schottky tunnel contacts which show almost linear <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|J|$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> characteristics and low interface resistance.

Spin injection by spin–charge coupling in proximity induced magnetic graphene

Within the field of spintronics major efforts are directed towards developing applications for spin-based transport devices made fully out of two-dimensional materials. In this work we present an experimental realization of a spin-valve device where the generation of the spin signal is exclusively attributed to the spin-dependent conductivity of the magnetic graphene resulting from the proximity of an interlayer antiferromagnet, chromium sulfide bromide (CrSBr). We clearly demonstrate that the usage of the conventional air-sensitive 3D magnetic contacts can be fully avoided when graphene/CrSBr heterostructures are employed. Moreover, apart from providing exceptionally long spin relaxation length, the usage of graphene for both generation and transport of the spin allows to automatically avoid the conductivity mismatch between the source and the channel circuits that has to be considered when using conventional low-resistive contacts. Our results address a necessary step in the engineering of spintronic circuitry out of layered materials and precede further developments in the area of complex spin-logic devices. Moreover, we introduce a fabrication procedure where we designed and implemented a recipe for the preparation of electrodes via a damage-free technique that offers an immediate advantage in the fields of air-sensitive and delicate organic materials.

Zero- and Low-Field Sensing with Nitrogen-Vacancy Centers

Over the years, an enormous effort has been made to establish nitrogen vacancy (N-$V$) centers in diamond as easily accessible and precise magnetic field sensors. However, most of their sensing protocols rely on the application of bias magnetic fields, preventing their usage in zero- or low-field experiments. We overcome this limitation by exploiting the full spin $S=1$ nature of the N-$V$ center, allowing us to detect nuclear spin signals at zero and low field with a linearly polarized microwave field. As conventional dynamical decoupling protocols fail in this regime, we develop robust pulse sequences and optimize pulse pairs, which allow us to sense temperature and weak ac magnetic fields and achieve an efficient decoupling from environmental noise. Our work allows for much broader and simpler applications of N-$V$ centers as magnetic field sensors in the zero- and low-field regime and can be further extended to three-level systems in ions and atoms.

Influence of low-energy magnons on magnon Hanle experiments in easy-plane antiferromagnets

Antiferromagnetic materials host pairs of spin-up and spin-down magnons which can be described in terms of a magnonic pseudospin. The close analogy between this magnonic pseudospin systems and that of electronic charge carriers led to the prediction of fascinating phenomena in antiferromagnets. Recently, the associated dynamics of antiferromagnetic pseudospin has been experimentally demonstrated and, in particular, the first observation of the magnon Hanle effect has been reported. We here expand the magnonic spin transport description by explicitly taking into account contributions of finite-spin low-energy magnons. In our experiments we realize the spin injection and detection process by two Platinum strips and investigate the influence of the Pt-strips on the generation and diffusive transport of magnons in films of the antiferromagnetic insulator hematite. For both a 15 nm and a 100 nm thick film, we find a distinct signal caused by the magnon Hanle effect. However, the magnonic spin signal exhibits clear differences in both films. In contrast to the thin film, for the thicker one, we observe an oscillating behavior in the high magnetic field range as well as an additional offset signal in the low magnetic field regime. We attribute this offset signal to the presence of finite-spin low-energy magnons.

Nonequilibrium topological spin textures in momentum space

SignificanceOptically excited systems can host unprecedented phenomena and reveal key information. The spin-channel physics in the photoexcited dynamics of quantum matter remains largely unexplored. This study finds the topological surface state under contemporary time-resolved pump-probe spectroscopy an exceptionally capable platform in this regard. Spin signals exhibit interesting tornado-like spiral patterns, and the unusual topological optical activity can be indicative of spintronic applications. This exemplifies a purely nonequilibrium topological winding phenomenon, where all the hidden helicity factors in the light-matter-coupled system are robustly encoded. These results open a direction of nonequilibrium topological spin states in quantum materials.