arXiv subjects

B. J. van Wees

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

At least 19 recordsLinked to original sources

arXiv2024

Non-Reciprocal Transport of Thermally-Generated Magnons

We demonstrate the non-reciprocity of electrically and thermally-generated incoherent magnon transport using the magnetization direction of a Py wire placed on top of an ultrathin YIG film. We show that the transport properties of thermally-generated magnons under a Py wire depends on the relative orientation between the temperature gradient and the Py-magnetization direction. The symmetries of this non-reciprocal magnon transport match with those predicted by the remote dipolar interaction between YIG and Py magnons, controlled by the chirality of the YIG magnon dipolar stray fields. We also show that the directional magnon generation by the spin Seebeck effect from the Py wire displays the symmetries expected from the chiral spin Seebeck effect.

cond-mat.mes-hall↗

arXiv2024

Efficient Magnon Injection and Detection via the Orbital Rashba Edelstein Effect

Orbital currents and accumulation provide a new avenue to boost spintronic effects in nanodevices. Here we use interconversion effects between charge current and orbital angular momentum to demonstrate a dramatic increase in the magnon spin injection and detection efficiencies in nanodevices consisting of a magnetic insulator contacted by Pt/CuOx electrodes. Moreover, we note distinct variations in efficiency for magnon spin injection and detection, indicating a disparity in the direct and inverse orbital Rashba Edelstein effect efficiencies.

cond-mat.mes-hall↗

arXiv2023

Soft Magnons in Anisotropic Ferromagnets

We discuss spin-wave transport in anisotropic ferromagnets with an emphasis on the zeroes of the band edges as a function of a magnetic field. An associated divergence of the magnon spin should be observable by enhanced magnon conductivities in non-local experiments, especially in two-dimensional ferromagnets.

cond-mat.mes-hall↗

arXiv2022

The spin-flop transition in the quasi two dimensional antiferromagnet MnPS3 detected via thermally generated magnon transport

We present the detection of the spin-flop transition in the antiferromagnetic van der Waals material MnPS3 via thermally generated nonlocal magnon transport using permalloy detector strips. The inverse anomalous spin Hall effect has the unique power to detect an out-of-plane spin accumulation which enables us to detect magnons with an out-of-plane spin polarization; in contrast to strips of high spin-orbit material such as Pt which only possess the spin Hall effect and are only sensitive to an in-plane spin polarization of the spin accumulation. We show that nonlocal magnon transport is able to measure the spin-flop transition in the absence of other spurious effects. Our measurements show the detection of magnons generated by the spin Seebeck effect before and after the spin-flop transition where the signal reversal of the magnon spin accumulation agrees with the OOP spin polarization carried by magnon modes before and after the SF transition.

cond-mat.mes-hall↗

arXiv2022

Observation of magnetization surface textures of the van der Waals antiferromagnet FePS3 by spin Hall magnetoresistance

Van der Waals materials are a new platform to study two-dimensional systems, including magnetic order. Since the number of spins is relatively small, measuring the magnetization is challenging. Here we report spin Hall magnetoresistance (SMR) up to room temperature caused by the magnetic surface texture of exfoliated flakes of magnetic van der Waals materials. For the antiferromagnet FePS3 the SMR amounts to 0.1 % for an applied magnetic field of 7 T at 5 K which implies a substantial canting of the magnetic moments relative to the colinear antiferromagnetic order. The canting is substantial even for a magnetic field along the Néel vector, which illustrates the unique power of the SMR to detect magnetic surface textures in van der Waals magnets.

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arXiv2022

Spin Hall magnetoresistance in paramagnetic NdGaO3

In recent years, spin Hall magnetoresistance (SMR) has emerged as an efficient way to probe the spontaneous magnetization state in ordered magnetic systems, by electrical current. Less known is its versatility as a probe of materials that do not possess spontaneous magnetization such as in paramagnets. In this work, SMR is used to probe paramagnetic NdGaO3 (NGO), a rare earth oxide, possessing a sizable spin orbit interaction (L=6). NGO has not been investigated earlier for its efficiency in propagating spins. We have performed extensive temperature and angle dependent-magnetoresistance (ADMR) studies along dissimilar crystallographic axes in NGO, using platinum (Pt) as spin injector and detector and utilizing (inverse) spin Hall effect. We find a close correlation between the temperature dependence of the ADMR response with magnetization in NGO and a linear current bias dependence of the ADMR amplitudes. These are chacteristics of SMR effect in Pt/NGO, arising from the torque acting on localized moments in NGO and considering crystal field induced intermultiplet transitions with temperature. Control experiments on Pt/SrTiO3 and Pt/SiO2 devices were also carried out in order to validate the observed SMR response in Pt/NGO bilayer and to rule out magnetoresistive contributions from Pt.

cond-mat.mtrl-sci↗

arXiv2022

Giant magnon spin conductivity approaching the two-dimensional transport regime in ultrathin yttrium iron garnet films

Conductivities are key material parameters that govern various types of transport (electronic charge, spin, heat etc.) driven by thermodynamic forces. Magnons, the elementary excitations of the magnetic order, flow under the gradient of a magnon chemical potential in proportion to a magnon (spin) conductivity $σ_{m}$. The magnetic insulator yttrium iron garnet (YIG) is the material of choice for efficient magnon spin transport. Here we report an unexpected giant $σ_{m}$ in record-thin YIG films with thicknesses down to 3.7 nm when the number of occupied two-dimensional (2D) subbands is reduced from a large number to a few, which corresponds to a transition from 3D to 2D magnon transport. We extract a 2D spin conductivity ($\approx1$ S) at room temperature, comparable to the (electronic) spin conductivity of the high-mobility two-dimensional electron gas in GaAs quantum wells at millikelvin temperatures. Such high conductivities offer unique opportunities to develop low-dissipation magnon-based spintronic devices.

cond-mat.mes-hall↗

arXiv2020

Magnetic order of Dy$^{3+}$ and Fe$^{3+}$ moments in antiferromagnetic DyFeO$_{3}$ probed by spin Hall magnetoresistance and spin Seebeck effect

We report on spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in single crystal of the rare-earth antiferromagnet DyFeO$_{3}$ with a thin Pt film contact. The angular shape and symmetry of the SMR at elevated temperatures reflect the antiferromagnetic order of the Fe$^{3+}$ moments as governed by the Zeeman energy, the magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya interaction. We interpret the observed linear dependence of the signal on the magnetic field strength as evidence for field-induced order of the Dy$^{3+}$ moments up to room temperature. At and below the Morin temperature of 50$\,$K, the SMR monitors the spin-reorientation phase transition of Fe$^{3+}$ spins. Below 23$\,$K, additional features emerge that persist below 4$\,$K, the ordering temperature of the Dy$^{3+}$ magnetic sublattice. We conclude that the combination of SMR and SSE is a simple and efficient tool to study spin reorientation phase transitions and sublattice magnetizations.

cond-mat.mtrl-sci↗

arXiv2020

Electrically induced strong modulation of magnons transport in ultrathin magnetic insulator films

Magnon transport through a magnetic insulator can be controlled by current-biased heavy-metal gates that modulate the magnon conductivity via the magnon density. Here, we report nonlinear modulation effects in 10$\,$nm thick yttrium iron garnet (YIG) films. The modulation efficiency is larger than 40\%/mA. The spin transport signal at high DC current density (2.2$\times 10^{11}\,$A/m$^{2}$) saturates for a 400$\,$nm wide Pt gate, which indicates that even at high current levels a magnetic instability cannot be reached in spite of the high magnetic quality of the films.

cond-mat.mes-hall↗

arXiv2020

Carrier Drift Control of Spin Currents in Graphene-Based Spin-Current Demultiplexers

Electrical control of spin transport is promising for achieving new device functionalities. Here we calculate the propagation of spin currents in a graphene-based spin-current demultiplexer under the effect of drift currents. We show that, using spin- and charge-transport parameters already obtained in experiments, the spin currents can be guided in a controlled way. In particular, spin-current selectivities up to 102 can be achieved for measurements over a distance of 10μm under a moderate drift current density of 20μA/μm, meaning that the spin current in the arm that is off is only 1% of the current in the arm that is on. To illustrate the versatility of this approach, we show similar efficiencies in a device with four outputs and the possibility of multiplexer operation using spin drift. Finally, we explain how the effect can be optimized in graphene and two-dimensional semiconductors.

cond-mat.mes-hall↗

arXiv2020

Nonlinear analog spintronics with van der Waals heterostructures

The current generation of spintronic devices, which use electron-spin relies on linear operations for spin-injection, transport and detection processes. The existence of nonlinearity in a spintronic device is indispensable for spin-based complex signal processing operations. Here we for the first time demonstrate the presence of electron-spin dependent nonlinearity in a spintronic device, and measure up to 4th harmonic spin-signals via nonlocal spin-valve and Hanle spin-precession measurements. We demonstrate its application for analog signal processing over pure spin-signals such as amplitude modulation and heterodyne detection operations which require nonlinearity as an essential element. Furthermore, we show that the presence of nonlinearity in the spin-signal has an amplifying effect on the energy-dependent conductivity induced nonlinear spin-to-charge conversion effect. The interaction of the two spin-dependent nonlinear effects in the spin transport channel leads to a highly efficient detection of the spin-signal without using ferromagnets. These effects are measured both at 4K and room temperature, and are suitable for their applications as nonlinear circuit elements in the fields of advanced-spintronics and spin-based neuromorphic computing.

cond-mat.mes-hall↗

arXiv2020

Colloquium: Spintronics in graphene and other two-dimensional materials

After the first unequivocal demonstration of spin transport in graphene (Tombros et al., 2007), surprisingly at room temperature, it was quickly realized that this novel material was relevant for both fundamental spintronics and future applications. Over the decade since, exciting results have made the field of graphene spintronics blossom, and a second generation of studies has extended to new two-dimensional (2D) compounds. This Colloquium reviews recent theoretical and experimental advances on electronic spin transport in graphene and related 2D materials, focusing on emergent phenomena in van der Waals heterostructures and the new perspectives provided by them. These phenomena include proximity-enabled spin-orbit effects, the coupling of electronic spin to light, electrical tunability, and 2D magnetism.

cond-mat.mes-hall↗

arXiv2019

Nonlocal Spin Transport as a Probe of Viscous Magnon Fluids

Magnons in ferromagnets behave as a viscous fluid over a length scale, the momentum-relaxation length, below which momentum-conserving scattering processes dominate. We show theoretically that in this hydrodynamic regime viscous effects lead to a sign change in the magnon chemical potential, which can be detected as a sign change in the nonlocal resistance measured in spin transport experiments. This sign change is observable when the injector-detector distance becomes comparable to the momentum-relaxation length. Taking into account momentum- and spin-relaxation processes, we consider the quasiconservation laws for momentum and spin in a magnon fluid. The resulting equations are solved for nonlocal spin transport devices in which spin is injected and detected via metallic leads. Because of the finite viscosity we also find a backflow of magnons close to the injector lead. Our work shows that nonlocal magnon spin transport devices are an attractive platform to develop and study magnon-fluid dynamics.

cond-mat.mes-hall↗

arXiv2019

Modulation of magnon spin transport in a magnetic gate transistor

We demonstrate a modulation of up to 18% in the magnon spin transport in a magnetic insulator (Y$_{3}$Fe$_{5}$O$_{12}$, YIG) using a common ferromagnetic metal (permalloy, Py) as a magnetic control gate. A Py electrode, placed between two Pt injector and detector electrodes, acts as a magnetic gate in our prototypical magnon transistor device. By manipulating the magnetization direction of Py with respect to that of YIG, the transmission of magnons through the Py|YIG interface can be controlled, resulting in a modulation of the non-equilibrium magnon density in the YIG channel between the Pt injector and detector electrodes. This study opens up the possibility of using the magnetic gating effect for magnon-based spin logic applications.

cond-mat.mes-hall↗

arXiv2018

Temperature dependence of the effective spin-mixing conductance probed with lateral non-local spin valves

We report the temperature dependence of the effective spin-mixing conductance between a normal metal (aluminium, Al) and a magnetic insulator ($\text{Y}_3\text{Fe}_5\text{O}_{12}$, YIG). Non-local spin valve devices, using Al as the spin transport channel, were fabricated on top of YIG and SiO$_2$ substrates. By comparing the spin relaxation lengths in the Al channel on the two different substrates, we calculate the effective spin-mixing conductance ($G_\text{s}$) to be $3.3\times10^{12}$~$Ω^{-1}\text{m}^{-2}$ at 293~K for the Al/YIG interface. A decrease of up to 84\% in $G_\text{s}$ is observed when the temperature ($T$) is decreased from 293~K to 4.2~K, with $G_\text{s}$ scaling with $(T/T_\text{c})^{3/2}$. The real part of the spin-mixing conductance ($G_\text{r}\approx 5.7\times10^{13}~ Ω^{-1}\text{m}^{-2}$), calculated from the experimentally obtained $G_\text{s}$, is found to be approximately independent of the temperature. We evidence a hitherto unrecognized underestimation of $G_\text{r}$ extracted from the modulation of the spin signal by rotating the magnetization direction of YIG with respect to the spin accumulation direction in the Al channel, which is found to be 50 times smaller than the calculated value.

cond-mat.mes-hall↗

arXiv2018

Gate-Controlled Magnetoresistance of a Paramagnetic Insulator|Platinum Interface

We report an electric field-induced in-plane magnetoresistance of an atomically flat paramagnetic insulator|platinum (Pt) interface at low temperatures with an ionic liquid gate. Transport experiments as a function of applied magnetic field strength and direction obey the spin Hall magnetoresistance phenomenology with perpendicular magnetic anisotropy. Our results establish the utility of ionic gating as an alternative method to control spintronic devices without using ferromagnets.

cond-mat.mes-hall↗

arXiv2018

Enhanced magnon spin transport in NiFe$_2$O$_4$ thin films on a lattice-matched substrate

We investigate magnon spin transport in epitaxial nickel ferrite (NiFe$_2$O$_4$, NFO) films grown on magnesium gallate spinel (MgGa$_2$O$_4$, 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 the nonlocal transport experiments, enhanced signals are observed for both electrically and thermally excited magnons, and the magnon relaxation length ($λ_m$) of NFO is found to be around 2.5 $μ$m 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, $λ_m$ and spin mixing conductance at the interface between Pt) of NFO films grown on a lattice-matched substrate that are comparable with those of yttrium iron garnet.

cond-mat.mes-hall↗

arXiv2018

Bias dependent spin injection into graphene on YIG through bilayer hBN tunnel barriers

We study the spin injection efficiency into single and bilayer graphene on the ferrimagnetic insulator Yttrium-Iron-Garnet (YIG) through an exfoliated tunnel barrier of bilayer hexagonal boron nitride (hBN). The contacts of two samples yield a resistance-area product between 5 and 30 k$Ωμ$m$^2$. Depending on an applied DC bias current, the magnitude of the non-local spin signal can be increased or suppressed below the noise level. The spin injection efficiency reaches values from -60% to +25%. The results are confirmed with both spin valve and spin precession measurements. The proximity induced exchange field is found in sample A to be (85 $\pm$ 30) mT and in sample B close to the detection limit. Our results show that the exceptional spin injection properties of bilayer hBN tunnel barriers reported by Gurram et al. are not limited to fully encapsulated graphene systems but are also valid in graphene/YIG devices. This further emphasizes the versatility of bilayer hBN as an efficient and reliable tunnel barrier for graphene spintronics.

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