180 research outputs found
Direct electronic measurement of the spin Hall effect
The generation, manipulation and detection of spin-polarized electrons in
nanostructures define the main challenges of spin-based electronics[1]. Amongst
the different approaches for spin generation and manipulation, spin-orbit
coupling, which couples the spin of an electron to its momentum, is attracting
considerable interest. In a spin-orbit-coupled system, a nonzero spin-current
is predicted in a direction perpendicular to the applied electric field, giving
rise to a "spin Hall effect"[2-4]. Consistent with this effect,
electrically-induced spin polarization was recently detected by optical
techniques at the edges of a semiconductor channel[5] and in two-dimensional
electron gases in semiconductor heterostructures[6,7]. Here we report
electrical measurements of the spin-Hall effect in a diffusive metallic
conductor, using a ferromagnetic electrode in combination with a tunnel barrier
to inject a spin-polarized current. In our devices, we observe an induced
voltage that results exclusively from the conversion of the injected spin
current into charge imbalance through the spin Hall effect. Such a voltage is
proportional to the component of the injected spins that is perpendicular to
the plane defined by the spin current direction and the voltage probes. These
experiments reveal opportunities for efficient spin detection without the need
for magnetic materials, which could lead to useful spintronics devices that
integrate information processing and data storage.Comment: 5 pages, 4 figures. Accepted for publication in Nature (pending
format approval
Thermoelectric spin voltage in graphene
In recent years, new spin-dependent thermal effects have been discovered in
ferromagnets, stimulating a growing interest in spin caloritronics, a field
that exploits the interaction between spin and heat currents. Amongst the most
intriguing phenomena is the spin Seebeck effect, in which a thermal gradient
gives rise to spin currents that are detected through the inverse spin Hall
effect. Non-magnetic materials such as graphene are also relevant for spin
caloritronics, thanks to efficient spin transport, energy-dependent carrier
mobility and unique density of states. Here, we propose and demonstrate that a
carrier thermal gradient in a graphene lateral spin valve can lead to a large
increase of the spin voltage near to the graphene charge neutrality point. Such
an increase results from a thermoelectric spin voltage, which is analogous to
the voltage in a thermocouple and that can be enhanced by the presence of hot
carriers generated by an applied current. These results could prove crucial to
drive graphene spintronic devices and, in particular, to sustain pure spin
signals with thermal gradients and to tune the remote spin accumulation by
varying the spin-injection bias
Strongly anisotropic spin relaxation in graphene/transition metal dichalcogenide heterostructures at room temperature
Graphene has emerged as the foremost material for future two-dimensional
spintronics due to its tuneable electronic properties. In graphene, spin
information can be transported over long distances and, in principle, be
manipulated by using magnetic correlations or large spin-orbit coupling (SOC)
induced by proximity effects. In particular, a dramatic SOC enhancement has
been predicted when interfacing graphene with a semiconducting transition metal
dechalcogenide, such as tungsten disulphide (WS). Signatures of such an
enhancement have recently been reported but the nature of the spin relaxation
in these systems remains unknown. Here, we unambiguously demonstrate
anisotropic spin dynamics in bilayer heterostructures comprising graphene and
WS. By using out-of-plane spin precession, we show that the spin lifetime
is largest when the spins point out of the graphene plane. Moreover, we observe
that the spin lifetime varies over one order of magnitude depending on the spin
orientation, indicating that the strong spin-valley coupling in WS is
imprinted in the bilayer and felt by the propagating spins. These findings
provide a rich platform to explore coupled spin-valley phenomena and offer
novel spin manipulation strategies based on spin relaxation anisotropy in
two-dimensional materials
Ageing memory and glassiness of a driven vortex system
Many systems in nature, glasses, interfaces and fractures being some
examples, cannot equilibrate with their environment, which gives rise to novel
and surprising behaviour such as memory effects, ageing and nonlinear dynamics.
Unlike their equilibrated counterparts, the dynamics of out-of- equilibrium
systems is generally too complex to be captured by simple macroscopic laws.
Here we investigate a system that straddles the boundary between glass and
crystal: a Bragg glass formed by vortices in a superconductor. We find that the
response to an applied force evolves according to a stretched exponential, with
the exponent reflecting the deviation from equilibrium. After the force is
removed, the system ages with time and its subsequent response time scales
linearly with its age (simple ageing), meaning that older systems are slower
than younger ones. We show that simple ageing can occur naturally in the
presence of sufficient quenched disorder. Moreover, the hierarchical
distribution of timescales, arising when chunks of loose vortices cannot move
before trapped ones become dislodged, leads to a stretched-exponential
response.Comment: 16 pages, 5 figure
Electronic measurement and control of spin transport in Silicon
The electron spin lifetime and diffusion length are transport parameters that
define the scale of coherence in spintronic devices and circuits. Since these
parameters are many orders of magnitude larger in semiconductors than in
metals, semiconductors could be the most suitable for spintronics. Thus far,
spin transport has only been measured in direct-bandgap semiconductors or in
combination with magnetic semiconductors, excluding a wide range of
non-magnetic semiconductors with indirect bandgaps. Most notable in this group
is silicon (Si), which (in addition to its market entrenchment in electronics)
has long been predicted a superior semiconductor for spintronics with enhanced
lifetime and diffusion length due to low spin-orbit scattering and lattice
inversion symmetry. Despite its exciting promise, a demonstration of coherent
spin transport in Si has remained elusive, because most experiments focused on
magnetoresistive devices; these methods fail because of universal impedance
mismatch obstacles, and are obscured by Lorentz magnetoresistance and Hall
effects. Here we demonstrate conduction band spin transport across 10 microns
undoped Si, by using spin-dependent ballistic hot-electron filtering through
ferromagnetic thin films for both spin-injection and detection. Not based on
magnetoresistance, the hot electron spin-injection and detection avoids
impedance mismatch issues and prevents interference from parasitic effects. The
clean collector current thus shows independent magnetic and electrical control
of spin precession and confirms spin coherent drift in the conduction band of
silicon.Comment: Single PDF file with 4 Figure
Spin Seebeck insulator
Thermoelectric generation is an essential function of future energy-saving
technologies. However, this generation has been an exclusive feature of
electric conductors, a situation which inflicts a heavy toll on its
application; a conduction electron often becomes a nuisance in thermal design
of devices. Here we report electric-voltage generation from heat flowing in an
insulator. We reveal that, despite the absence of conduction electrons, a
magnetic insulator LaY2Fe5O12 converts a heat flow into spin voltage. Attached
Pt films transform this spin voltage into electric voltage by the inverse spin
Hall effect. The experimental results require us to introduce thermally
activated interface spin exchange between LaY2Fe5O12 and Pt. Our findings
extend the range of potential materials for thermoelectric applications and
provide a crucial piece of information for understanding the physics of the
spin Seebeck effect.Comment: 19 pages, 5 figures (including supplementary information
The 2017 Magnetism Roadmap
Building upon the success and relevance of the 2014 Magnetism Roadmap, this 2017 Magnetism Roadmap edition follows a similar general layout, even if its focus is naturally shifted, and a different group of experts and, thus, viewpoints are being collected and presented. More importantly, key developments have changed the research landscape in very relevant ways, so that a novel view onto some of the most crucial developments is warranted, and thus, this 2017 Magnetism Roadmap article is a timely endeavour. The change in landscape is hereby not exclusively scientific, but also reflects the magnetism related industrial application portfolio. Specifically, Hard Disk Drive technology, which still dominates digital storage and will continue to do so for many years, if not decades, has now limited its footprint in the scientific and research community, whereas significantly growing interest in magnetism and magnetic materials in relation to energy applications is noticeable, and other technological fields are emerging as well. Also, more and more work is occurring in which complex topologies of magnetically ordered states are being explored, hereby aiming at a technological utilization of the very theoretical concepts that were recognised by the 2016 Nobel Prize in Physics. Given this somewhat shifted scenario, it seemed appropriate to select topics for this Roadmap article that represent the three core pillars of magnetism, namely magnetic materials, magnetic phenomena and associated characterization techniques, as well as applications of magnetism. While many of the contributions in this Roadmap have clearly overlapping relevance in all three fields, their relative focus is mostly associated to one of the three pillars. In this way, the interconnecting roles of having suitable magnetic materials, understanding (and being able to characterize) the underlying physics of their behaviour and utilizing them for applications and devices is well illustrated, thus giving an accurate snapshot of the world of magnetism in 2017. The article consists of 14 sections, each written by an expert in the field and addressing a specific subject on two pages. Evidently, the depth at which each contribution can describe the subject matter is limited and a full review of their statuses, advances, challenges and perspectives cannot be fully accomplished. Also, magnetism, as a vibrant research field, is too diverse, so that a number of areas will not be adequately represented here, leaving space for further Roadmap editions in the future. However, this 2017 Magnetism Roadmap article can provide a frame that will enable the reader to judge where each subject and magnetism research field stands overall today and which directions it might take in the foreseeable future. The first material focused pillar of the 2017 Magnetism Roadmap contains five articles, which address the questions of atomic scale confinement, 2D, curved and topological magnetic materials, as well as materials exhibiting unconventional magnetic phase transitions. The second pillar also has five contributions, which are devoted to advances in magnetic characterization, magneto-optics and magneto-plasmonics, ultrafast magnetization dynamics and magnonic transport. The final and application focused pillar has four contributions, which present non-volatile memory technology, antiferromagnetic spintronics, as well as magnet technology for energy and bio-related applications. As a whole, the 2017 Magnetism Roadmap article, just as with its 2014 predecessor, is intended to act as a reference point and guideline for emerging research directions in modern magnetism
Thermally driven spin injection from a ferromagnet into a non-magnetic metal
Creating, manipulating and detecting spin polarized carriers are the key
elements of spin based electronics. Most practical devices use a perpendicular
geometry in which the spin currents, describing the transport of spin angular
momentum, are accompanied by charge currents. In recent years, new sources of
pure spin currents, i.e., without charge currents, have been demonstrated and
applied. In this paper, we demonstrate a conceptually new source of pure spin
current driven by the flow of heat across a ferromagnetic/non-magnetic metal
(FM/NM) interface. This spin current is generated because the Seebeck
coefficient, which describes the generation of a voltage as a result of a
temperature gradient, is spin dependent in a ferromagnet. For a detailed study
of this new source of spins, it is measured in a non-local lateral geometry. We
developed a 3D model that describes the heat, charge and spin transport in this
geometry which allows us to quantify this process. We obtain a spin Seebeck
coefficient for Permalloy of -3.8 microvolt/Kelvin demonstrating that thermally
driven spin injection is a feasible alternative for electrical spin injection
in, for example, spin transfer torque experiments
Nanoscale phase-engineering of thermal transport with a Josephson heat modulator
Macroscopic quantum phase coherence has one of its pivotal expressions in the
Josephson effect [1], which manifests itself both in charge [2] and energy
transport [3-5]. The ability to master the amount of heat transferred through
two tunnel-coupled superconductors by tuning their phase difference is the core
of coherent caloritronics [4-6], and is expected to be a key tool in a number
of nanoscience fields, including solid state cooling [7], thermal isolation [8,
9], radiation detection [7], quantum information [10, 11] and thermal logic
[12]. Here we show the realization of the first balanced Josephson heat
modulator [13] designed to offer full control at the nanoscale over the
phase-coherent component of thermal currents. Our device provides
magnetic-flux-dependent temperature modulations up to 40 mK in amplitude with a
maximum of the flux-to-temperature transfer coefficient reaching 200 mK per
flux quantum at a bath temperature of 25 mK. Foremost, it demonstrates the
exact correspondence in the phase-engineering of charge and heat currents,
breaking ground for advanced caloritronic nanodevices such as thermal splitters
[14], heat pumps [15] and time-dependent electronic engines [16-19].Comment: 6+ pages, 4 color figure
Spin-injection Hall effect in a planar photovoltaic cell
Successful incorporation of the spin degree of freedom in semiconductor
technology requires the development of a new paradigm allowing for a scalable,
non-destructive electrical detection of the spin-polarization of injected
charge carriers as they propagate along the semiconducting channel. In this
paper we report the observation of a spin-injection Hall effect (SIHE) which
exploits the quantum-relativistic nature of spin-charge transport and which
meets all these key requirements on the spin detection. The two-dimensional
electron-hole gas photo-voltaic cell we designed to observe the SIHE allows us
to develop a quantitative microscopic theory of the phenomenon and to
demonstrate its direct application in optoelectronics. We report an
experimental realization of a non-magnetic spin-photovoltaic effect via the
SIHE, rendering our device an electrical polarimeter which directly converts
the degree of circular polarization of light to a voltage signal.Comment: 14 pages, 4 figure
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