42 research outputs found
Interplay of Peltier and Seebeck effects in nanoscale nonlocal spin valves
We have experimentally studied the role of thermoelectric effects in
nanoscale nonlocal spin valve devices. A finite element thermoelectric model is
developed to calculate the generated Seebeck voltages due to Peltier and Joule
heating in the devices. By measuring the first, second and third harmonic
voltage response non locally, the model is experimentally examined. The results
indicate that the combination of Peltier and Seebeck effects contributes
significantly to the nonlocal baseline resistance. Moreover, we found that the
second and third harmonic response signals can be attributed to Joule heating
and temperature dependencies of both Seebeck coefficient and resistivity.Comment: 4 pages, 4 figure
Cooling and heating with electron spins: Observation of the spin Peltier effect
The Peltier coefficient describes the amount of heat that is carried by an
electrical current when it passes through a material. Connecting two materials
with different Peltier coefficients causes a net heat flow towards or away from
the interface, resulting in cooling or heating at the interface - the Peltier
effect. Spintronics describes the transport of charge and angular momentum by
making use of separate spin-up and spin-down channels. Recently, the merger of
thermoelectricity with spintronics has given rise to a novel and rich research
field named spin caloritronics. Here, we report the first direct experimental
observation of refrigeration/heating driven by a spin current, a new spin
thermoelectric effect which we call the spin Peltier effect. The heat flow is
generated by the spin dependency of the Peltier coefficient inside the
ferromagnetic material. We explored the effect in a specifically designed spin
valve pillar structure by measuring the temperature using an electrically
isolated thermocouple. The difference in heat flow between the two magnetic
configurations leads to a change in temperature. With the help of 3-D finite
element modeling, we extracted permalloy spin Peltier coefficients in the range
of -0.9 to -1.3 mV. These results enable magnetic control of heat flow and
provide new functionality for future spintronic devices
Thermoelectric Detection of Ferromagnetic Resonance of a Nanoscale Ferromagnet
We present thermoelectric measurements of the heat dissipated due to ferromagnetic resonance of a Permalloy strip. A microwave magnetic field, produced by an on-chip coplanar strip waveguide, is used to drive the magnetization precession. The generated heat is detected via Seebeck measurements on a thermocouple connected to the ferromagnet. The observed resonance peak shape is in agreement with the Landau-Lifshitz-Gilbert equation and is compared with thermoelectric finite-element modeling. Unlike other methods, this technique is not restricted to electrically conductive media and is therefore also applicable to for instance ferromagnetic insulators
Suppressed spin dephasing for 2D and bulk electrons in GaAs wires due to engineered cancellation of spin-orbit interaction terms
We report a study of suppressed spin dephasing for quasi-one-dimensional
electron ensembles in wires etched into a GaAs/AlGaAs heterojunction system.
Time-resolved Kerr-rotation measurements show a suppression that is most
pronounced for wires along the [110] crystal direction. This is the fingerprint
of a suppression that is enhanced due to a strong anisotropy in spin-orbit
fields that can occur when the Rashba and Dresselhaus contributions are
engineered to cancel each other. A surprising observation is that this
mechanisms for suppressing spin dephasing is not only effective for electrons
in the heterojunction quantum well, but also for electrons in a deeper bulk
layer.Comment: 5 pages, 3 figure
Optical probing of spin dynamics of two-dimensional and bulk electrons in a GaAs/AlGaAs heterojunction system
We present time-resolved Kerr rotation measurements of electron spin dynamics
in a GaAs/AlGaAs heterojunction system that contains a high-mobility
two-dimensional electron gas (2DEG). Due to the complex layer structure of this
material the Kerr rotation signals contain information from electron spins in
three different layers: the 2DEG layer, a GaAs epilayer in the heterostructure,
and the underlying GaAs substrate. The 2DEG electrons can be observed at low
pump intensities, using that they have a less negative g-factor than electrons
in bulk GaAs regions. At high pump intensities, the Kerr signals from the GaAs
epilayer and the substrate can be distinguished when using a barrier between
the two layers that blocks intermixing of the two electron populations. This
allows for stronger pumping of the epilayer, which results in a shift of the
effective g-factor. Thus, three populations can be distinguished using
differences in g-factor. We support this interpretation by studying how the
spin dynamics of each population has its unique dependence on temperature, and
how they correlate with time-resolved reflectance signals.Comment: 14 pages, 7 figure
Seebeck Effect in Magnetic Tunnel Junctions
Creating temperature gradients in magnetic nanostructures has resulted in a
new research direction, i.e., the combination of magneto- and thermoelectric
effects. Here, we demonstrate the observation of one important effect of this
class: the magneto-Seebeck effect. It is observed when a magnetic configuration
changes the charge based Seebeck coefficient. In particular, the Seebeck
coefficient changes during the transition from a parallel to an antiparallel
magnetic configuration in a tunnel junction. In that respect, it is the analog
to the tunneling magnetoresistance. The Seebeck coefficients in parallel and
antiparallel configuration are in the order of the voltages known from the
charge-Seebeck effect. The size and sign of the effect can be controlled by the
composition of the electrodes' atomic layers adjacent to the barrier and the
temperature. Experimentally, we realized 8.8 % magneto-Seebeck effect, which
results from a voltage change of about -8.7 {\mu}V/K from the antiparallel to
the parallel direction close to the predicted value of -12.1 {\mu}V/K.Comment: 16 pages, 7 figures, 2 table
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
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
Spin Caloritronics
This is a brief overview of the state of the art of spin caloritronics, the
science and technology of controlling heat currents by the electron spin degree
of freedom (and vice versa).Comment: To be published in "Spin Current", edited by S. Maekawa, E. Saitoh,
S. Valenzuela and Y. Kimura, Oxford University Pres
Acoustic spin pumping as the origin of the long-range spin Seebeck effect
The spin Seebeck effect (SSE) is known as the generation of 'spin voltage' in
a magnet as a result of a temperature gradient. Spin voltage stands for the
potential for spins, which drives a spin current. The SSE is of crucial
importance in spintronics and energy-conversion technology, since it enables
simple and versatile generation of spin currents from heat. The SSE has been
observed in a variety of materials ranging from magnetic metals and
semiconductors to magnetic insulators. However, the mechanism, the long-range
nature, of the SSE in metals is still to be clarified. Here we found that,
using a Ni81Fe19/Pt bilayer wire on an insulating sapphire plate, the
long-range spin voltage induced by the SSE in magnetic metals is due to
phonons. Under a temperature gradient in the sapphire, surprisingly, the
voltage generated in the Pt layer is shown to reflect the wire position,
although the wire is isolated both electrically and magnetically. This
non-local voltage is direct evidence that the SSE is attributed to the coupling
of spins and phonons. We demonstrate this coupling by directly injecting sound
waves, which realizes the acoustic spin pumping. Our finding opens the door to
"acoustic spintronics" in which phonons are exploited for constructing
spin-based devices.Comment: 18 pages, 6 figure