arXiv subjects

Bart J. van Wees

Publications and source records attributed to Bart J. van Wees.

At least 55 records · Page 3Linked to original sources

arXiv2017

Electrical spin injection, transport, and detection in graphene-hexagonal boron nitride van der Waals heterostructures: progress and perspectives

The current research in graphene spintronics strives for achieving a long spin lifetime, and efficient spin injection and detection in graphene. In this article, we review how hexagonal boron nitride (hBN) has evolved as a crucial substrate, as an encapsulation layer, and as a tunnel barrier for manipulation and control of spin lifetimes and spin injection/detection polarizations in graphene spin valve devices. First, we give an overview of the challenges due to conventional SiO$_2$ substrate for spin transport in graphene followed by the progress made in hBN based graphene heterostructures. Then we discuss in detail the shortcomings and developments in using conventional oxide tunnel barriers for spin injection into graphene followed by introducing the recent advancements in using the crystalline single/bi/tri-layer hBN tunnel barriers for an improved spin injection and detection which also can facilitate two-terminal spin valve and Hanle measurements, at room temperature, and are of technological importance. A special case of bias induced spin polarization of contacts with exfoliated and chemical vapour deposition (CVD) grown hBN tunnel barriers is also discussed. Further, we give our perspectives on utilizing graphene-hBN heterostructures for future developments in graphene spintronics.

cond-mat.mes-hall↗

arXiv2017

A two-channel model for Spin-relaxation noise

We develop a two-channel resistor model for simulating spin transport with general applicability. Using this model, for the case of graphene as a prototypical material, we calculate the spin signal consistent with experimental values. Using the same model we also simulate the charge and spin- dependent 1/f noise, both in the local and nonlocal four-probe measurement schemes, and identify the noise from the spin-relaxation resistances as the major source of spin-dependent 1/f noise.

cond-mat.mes-hall↗

arXiv2017

Spin transport in two-layer-CVD-hBN/graphene/hBN heterostructures

We study room temperature spin transport in graphene devices encapsulated between a layer-by-layer-stacked two-layer-thick chemical vapour deposition (CVD) grown hexagonal boron nitride (hBN) tunnel barrier, and a few-layer-thick exfoliated-hBN substrate. We find mobilities and spin-relaxation times comparable to that of SiO$_2$ substrate based graphene devices, and obtain a similar order of magnitude of spin relaxation rates for both the Elliott-Yafet and D'Yakonov-Perel' mechanisms. The behaviour of ferromagnet/two-layer-CVD-hBN/graphene/hBN contacts ranges from transparent to tunneling due to inhomogeneities in the CVD-hBN barriers. Surprisingly, we find both positive and negative spin polarizations for high-resistance two-layer-CVD-hBN barrier contacts with respect to the low-resistance contacts. Furthermore, we find that the differential spin injection polarization of the high-resistance contacts can be modulated by DC bias from -0.3 V to +0.3 V with no change in its sign, while its magnitude increases at higher negative bias. These features mark a distinctive spin injection nature of the two-layer-CVD-hBN compared to the bilayer-exfoliated-hBN tunnel barriers.

cond-mat.mes-hall↗

arXiv2017

Spin transport in high-mobility graphene on WS$_2$ substrate with electric-field tunable proximity spin-orbit interaction

Graphene supported on a transition metal dichalcogenide substrate offers a novel platform to study the spin transport in graphene in presence of a substrate induced spin-orbit coupling, while preserving its intrinsic charge transport properties. We report the first non-local spin transport measurements in graphene completely supported on a 3.5 nm thick tungsten disulfide (WS$_2$) substrate, and encapsulated from the top with a 8 nm thick hexagonal boron nitride layer. For graphene, having mobility up to 16,000 cm$^2$V$^{-1}$s$^{-1}$, we measure almost constant spin-signals both in electron and hole-doped regimes, independent of the conducting state of the underlying WS$_2$ substrate, which rules out the role of spin-absorption by WS$_2$. The spin-relaxation time $τ_{\text{s}}$ for the electrons in graphene-on-WS$_2$ is drastically reduced down to~10 ps than $τ_{\text{s}}$ ~ 800 ps in graphene-on-SiO$_2$ on the same chip. The strong suppression of $τ_{\text{s}}$ along with a detectable weak anti-localization signature in the quantum magneto-resistance measurements is a clear effect of the WS$_2$ induced spin-orbit coupling (SOC) in graphene. Via the top-gate voltage application in the encapsulated region, we modulate the electric field by 1 V/nm, changing $τ_{\text{s}}$ almost by a factor of four which suggests the electric-field control of the in-plane Rashba SOC. Further, via carrier-density dependence of $τ_{\text{s}}$ we also identify the fingerprints of the D'yakonov-Perel' type mechanism in the hole-doped regime at the graphene-WS$_2$ interface.

cond-mat.mes-hall↗

arXiv2017

Observation of bright and dark exciton transitions in monolayer MoSe2 by photocurrent spectroscopy

We investigate the excitonic transitions in single- and few-layer MoSe2 phototransistors by photocurrent spectroscopy. The measured spectral profiles show a well-defined peak at the optically active (bright) A0 exciton resonance. More interestingly, when a gate voltage is applied to the MoSe2 to bring its Fermi level near the bottom of the conduction band, another prominent peak emerges at an energy 30 meV above the A0 exciton. We attribute this second peak to a gate-induced activation of the spin-forbidden dark exciton transition, AD0. Additionally, we evaluate the thickness-dependent optical bandgap of the fabricated MoSe2 crystals by characterizing their absorption edge.

cond-mat.mtrl-sci↗

arXiv2017

Large Proximity-Induced Spin Lifetime Anisotropy in Transition Metal Dichalcogenide/Graphene Heterostructures

Van-der-Waals heterostructures have become a paradigm for designing new materials and devices, in which specific functionalities can be tailored by combining the properties of the individual 2D layers. A single layer of transition metal dichalcogenide (TMD) is an excellent complement to graphene (Gr), since the high quality of charge and spin transport in Gr is enriched with the large spin-orbit coupling of the TMD via proximity effect. The controllable spin-valley coupling makes these heterostructures particularly attractive for spintronic and opto-valleytronic applications. In this work, we study spin precession in a monolayer MoSe2/Gr heterostructure and observe an unconventional, dramatic modulation of the spin signal, showing one order of magnitude longer lifetime of out-of-plane spins (40 ps) compared with that of in-plane spins (3.5 ps). This demonstration of a large spin lifetime anisotropy in TMD/Gr heterostructures, is a direct evidence of induced spin-valley coupling in Gr and provides an accessible route for manipulation of spin dynamics in Gr, interfaced with TMDs.

cond-mat.mes-hall↗

arXiv2017

Negative spin Hall magnetoresistance of Pt on the bulk easy-plane antiferromagnet NiO

We report on spin Hall magnetoresistance (SMR) measurements of Pt Hall bars on the antiferromagnetic NiO(111) single crystal. An SMR with a sign opposite of conventional SMR is observed over a wide range of temperatures as well as magnetic fields stronger than 0.25T. The negative sign of the SMR can be explained by the alignment of magnetic moments being almost perpendicular to the external magnetic field within the easy plane (111) of the antiferromagnet. This correlation of magnetic moment alignment and external magnetic field direction is realized just by the easy-plane nature of the material without the need of any exchange coupling to an additional ferromagnet. The SMR signal strength decreases with increasing temperature, primarily due to the decrease in Néel order by including fluctuations. An increasing magnetic field increases the SMR signal strength as there are less domains and the magnetic moments are more strongly manipulated at high magnetic fields. The SMR is saturated at an applied magnetic field of $6$~T resulting in a spin-mixing conductance of $\sim10^{18}~ Ω^{-1}$m$^{-2}$, which is comparable to that of Pt on insulating ferrimagnets such as yttrium iron garnet. An argon plasma treatment doubles the spin-mixing conductance.

cond-mat.mes-hall↗

arXiv2017

Bias induced up to 100% spin-injection and detection polarizations in ferromagnet/bilayer-hBN/graphene/hBN heterostructures

We study spin transport in a fully hBN encapsulated monolayer-graphene van der Waals (vdW) heterostructure, at room temperature. A top-layer of bilayer-hBN is used as a tunnel barrier for spin-injection and detection in graphene with ferromagnetic cobalt electrodes. We report surprisingly large and bias induced (differential) spin-injection (detection) polarizations up to 50% (135%) at a positive voltage bias of +0.6 V, as well as sign inverted polarizations up to -70% (-60%) at a reverse bias of -0.4 V. This demonstrates the potential of bilayer-hBN tunnel barriers for practical graphene spintronics applications. With such enhanced spin-injection and detection polarizations, we report a record two-terminal (inverted) spin-valve signals up to 800 $Ω$ with a magnetoresistance ratio of 2.7%, and we achieve spin accumulations up to 4.1 meV. We propose how these numbers can be increased further, for future technologically relevant graphene based spintronic devices.

cond-mat.mes-hall↗

arXiv2017

Graphene-WS$_2$ heterostructures for tunable spin injection and spin transport

We report the first measurements of spin injection in to graphene through a 20 nm thick tungsten disulphide (WS$_2$) layer, along with a modified spin relaxation time (τs) in graphene in the WS$_2$ environment, via spin-valve and Hanle spin-precession measurements, respectively. First, during the spin-injection into graphene through a WS$_2$-graphene interface, we can tune the interface resistance at different current bias and modify the spin injection efficiency, in a correlation with the conductivity-mismatch theory. Temperature assisted tunneling is identified as a dominant mechanism for the charge transport across the interface. Second, we measure the spin transport in graphene, underneath the WS$_2$ crystal and observe a significant reduction in the τs down to 17 ps in graphene in the WS$_2$ covered region, compared to that in its pristine state. The reduced τs indicates the WS$_2$-proximity induced additional dephasing of the spins in graphene.

cond-mat.mes-hall↗

arXiv2016

Influence of yttrium iron garnet thickness and heater opacity on the nonlocal transport of electrically and thermally excited magnons

We studied the nonlocal transport behavior of both electrically and thermally excited magnons in yttrium iron garnet (YIG) as a function of its thickness. For electrically injected magnons, the nonlocal signals decrease monotonically as the YIG thickness increases. For the nonlocal behavior of the thermally generated magnons, or the nonlocal spin Seebeck effect (SSE), we observed a sign reversal which occurs at a certain heater-detector distance, and it is influenced by both the opacity of the YIG/heater interface and the YIG thickness. Our nonlocal SSE results can be qualitatively explained by the bulk-driven SSE mechanism together with the magnon diffusion model. Using a two-dimensional finite element model (2D-FEM), we estimated the bulk spin Seebeck coefficient of YIG at room temperature. The quantitative disagreement between the experimental and modeled results indicates more complex processes going on in addition to magnon diffusion and relaxation, especially close to the contacts.

cond-mat.mes-hall↗

arXiv2016

Temperature dependence of the magnon spin diffusion length and magnon spin conductivity in the magnetic insulator yttrium iron garnet

We present a systematic study of the temperature dependence of diffusive magnon spin transport, using a non-local device geometry. In our measurements, we detect spin signals arising from electrical and thermal magnon generation, and we directly extract the magnon spin diffusion length $λ_m$ for temperatures from 2 to 293 K. Values of $λ_m$ obtained from electrical and thermal generation agree within the experimental error, with $λ_m=9.6\pm0.9$ $μ$m at room temperature to a minimum of $λ_m=5.5\pm0.7$ $μ$m at 30 K. Using a 2D finite element model to fit the data obtained for electrical magnon generation we extract the magnon spin conductivity $σ_m$ as a function of temperature, which is reduced from $σ_m=5.1\pm0.2\times10^5$ S/m at room temperature to $σ_m=0.7\pm0.4\times10^5$ S/m at 5 K. Finally, we observe an enhancement of the signal originating from thermally generated magnons for low temperatures, where a maximum is observed around $T=7$ K. An explanation for this low temperature enhancement is however still missing and requires additional investigations.

cond-mat.mes-hall↗

arXiv2016

Magnon spin transport driven by the magnon chemical potential in a magnetic insulator

We develop a linear-response transport theory of diffusive spin and heat transport by magnons in magnetic insulators with metallic contacts. The magnons are described by a position dependent temperature and chemical potential that are governed by diffusion equations with characteristic relaxation lengths. Proceeding from a linearized Boltzmann equation, we derive expressions for length scales and transport coefficients. For yttrium iron garnet (YIG) at room temperature we find that long-range transport is dominated by the magnon chemical potential. We compare the model's results with recent experiments on YIG with Pt contacts [L.J. Cornelissen, et al., Nat. Phys. 11, 1022 (2015)] and extract a magnon spin conductivity of $σ_{m}=5\times10^{5}$ S/m. Our results for the spin Seebeck coefficient in YIG agree with published experiments. We conclude that the magnon chemical potential is an essential ingredient for energy and spin transport in magnetic insulators.

cond-mat.mes-hall↗

arXiv2016

98% directional guiding of spin currents with 90 micrometer relaxation length in bilayer graphene using carrier drift

Electrical control of spin signals and long distance spin transport are major requirements in the field of spin electronics. Here we report the efficient guiding of spin currents at room temperature in high mobility hexagonal boron nitride encapsulated bilayer graphene using carrier drift. Our experiments, together with modelling, show that the spin relaxation length can be tuned from 2 to 88 micrometers when applying a DC current of $\mp$40 uA respectively. Our model predicts that, extending the range up to $\mathrm{I_{dc}}=\mp$150 uA, the spin relaxation length can be tuned from 0.6 to 320 um respectively, indicating that spin relaxation lengths in the millimeter range are within scope in near future with moderate current densities. Our results also show that we are able to direct spin currents on either side of a spin injection contact. 98% of the injected spins flow to the left when $\mathrm{I_{dc}}$= -40 uA and 65% flow to the right when the drift current is reversed. Our model shows that, for $\mathrm{I_{dc}}=\mp$150 uA the numbers reach 99.8% and 95% respectively showing the potential of carrier drift for spin-based logic operations and devices.

cond-mat.mes-hall↗

arXiv2016

Detection of spin pumping from YIG by spin-charge conversion in a Au/Ni$_{80}$Fe$_{20}$ spin-valve structure

Many experiments have shown the detection of spin-currents driven by radio-frequency spin pumping from yttrium iron garnet (YIG), by making use of the inverse spin-Hall effect, which is present in materials with strong spin-orbit coupling, such as Pt. Here we show that it is also possible to directly detect the resonance-driven spin-current using Au/permalloy (Py, Ni$_{80}$Fe$_{20}$) devices, where Py is used as a detector for the spins pumped across the YIG/Au interface. This detection mechanism is equivalent to the spin-current detection in metallic non-local spin-valve devices. By finite element modeling we compare the pumped spin-current from a reference Pt strip with the detected signals from the Au/Py devices. We find that for one series of Au/Py devices the calculated spin pumping signals mostly match the measurements, within 20%, whereas for a second series of devices additional signals are present which are up to a factor 10 higher than the calculated signals from spin pumping. We also identify contributions from thermoelectric effects caused by the resonant (spin-related) and non-resonant heating of the YIG. Thermocouples are used to investigate the presence of these thermal effects and to quantify the magnitude of the Spin-(dependent-)Seebeck effect. Several additional features are observed, which are also discussed.

cond-mat.mes-hall↗

arXiv2015

Spin-Hall magnetoresistance and spin Seebeck effect in spin-spiral and paramagnetic phases of multiferroic CoCr2O4 films

We report on the spin-Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in multiferroic CoCr2O4 (CCO) spinel thin films with Pt contacts. We observe a large enhancement of both signals below the spin-spiral (Ts = 28 K) and the spin lock-in transitions (T_{lock_in} = 14 K). The SMR and SSE response in the spin lock-in phase are one order of magnitude larger than those observed at the ferrimagnetic transition temperature (Tc = 94 K), which indicates that the interaction between spins at the Pt|CCO interface is more efficient in the non-collinear magnetic state below Ts and T_{lock-in}. At T > Tc, magnetic field-induced SMR and SSE signals are observed, which can be explained by a high interface susceptibility. Our results show that the spin transport at the Pt|CCO interface is sensitive to the magnetic phases but cannot be explained solely by the bulk magnetization.

cond-mat.mtrl-sci↗

arXiv2015

24 \textmu m length spin relaxation length in boron nitride encapsulated bilayer graphene

We have performed spin and charge transport measurements in dual gated high mobility bilayer graphene encapsulated in hexagonal boron nitride. Our results show spin relaxation lengths $λ_s$ up to 13~\textmu m at room temperature with relaxation times $τ_s$ of 2.5~ns. At 4~K, the diffusion coefficient rises up to 0.52~m$^2$/s, a value 5 times higher than the best achieved for graphene spin valves up to date. As a consequence, $λ_s$ rises up to 24~\textmu m with $τ_s$ as high as 2.9~ns. We characterized 3 different samples and observed that the spin relaxation times increase with the device length. We explain our results using a model that accounts for the spin relaxation induced by the non-encapsulated outer regions.

cond-mat.mes-hall↗

arXiv2015

Sign of inverse spin Hall voltages generated by ferromagnetic resonance and temperature gradients in yttrium iron garnet|platinum bilayers

We carried out a concerted effort to determine the absolute sign of the inverse spin Hall effect voltage generated by spin currents injected into a normal metal. We focus on yttrium iron garnet (YIG)|platinum bilayers at room temperature, generating spin currents by microwaves and temperature gradients. We find consistent results for different samples and measurement setups that agree with theory. We suggest a right-hand-rule to define a positive spin Hall angle corresponding to with the voltage expected for the simple case of scattering of free electrons from repulsive Coulomb charges.

cond-mat.mtrl-sci↗

arXiv2014

Surface sensitivity of the spin Seebeck effect

We have investigated the influence of the interface quality on the spin Seebeck effect (SSE) of the bilayer system yttrium iron garnet (YIG) - platinum (Pt). The magnitude and shape of the SSE is strongly influenced by mechanical treatment of the YIG single crystal surface. We observe that the saturation magnetic field H_{sat} for the SSE signal increases from 55.3 mT to 72.8 mT with mechanical treatment. The change in the magnitude of H_{sat} can be attributed to the presence of a perpendicular magnetic anisotropy due to the treatment induced surface strain or shape anisotropy in the Pt/YIG system. Our results show that the SSE is a powerful tool to investigate magnetic anisotropy at the interface.

cond-mat.mtrl-sci↗