616 research outputs found
Verification of the Thomson-Onsager reciprocity relation for spin caloritronics
We investigate the Thomson-Onsager relation between the spin-dependent
Seebeck and spin-dependent Peltier effect. To maintain identical device and
measurement conditions we measure both effects in a single
NiFe/Cu/NiFe nanopillar spin valve device subjected
to either an electrical or a thermal bias. In the low bias regime, we observe
similar spin signals as well as background responses, as required by the
Onsager reciprocity relation. However, at large biases, deviation from
reciprocity occurs due to dominant nonlinear contribution of the temperature
dependent transport coefficients. By systematic modeling of these nonlinear
thermoelectric effects and measuring higher order thermoelectric responses for
different applied biases, we identify the transition between the two regimes as
the point at which Joule heating start to dominate over Peltier heating. Our
results signify the importance of local equilibrium for the validity of this
phenomenological reciprocity relation.Comment: 5 pages, 5 figure
Direct electronic measurement of Peltier cooling and heating in graphene
Thermoelectric effects allow the generation of electrical power from waste
heat and the electrical control of cooling and heating. Remarkably, these
effects are also highly sensitive to the asymmetry in the density of states
around the Fermi energy and can therefore be exploited as probes of distortions
in the electronic structure at the nanoscale. Here we consider two-dimensional
graphene as an excellent nanoscale carbon material for exploring the
interaction between electronic and thermal transport phenomena, by presenting a
direct and quantitative measurement of the Peltier component to electronic
cooling and heating in graphene. Thanks to an architecture including nanoscale
thermometers, we detected Peltier component modulation of up to 15 mK for
currents of 20 A at room temperature and observed a full reversal between
Peltier cooling and heating for electron and hole regimes. This fundamental
thermodynamic property is a complementary tool for the study of nanoscale
thermoelectric transport in two-dimensional materials.Comment: Final version published in Nature Communications under a Creative
Commons Attribution 4.0 International Licens
Spin Injection and Detection via the Anomalous Spin Hall Effect in a Ferromagnetic Metal
We report a novel spin injection and detection mechanism via the anomalous
Hall effect in a ferromagnetic metal. The anomalous spin Hall effect (ASHE)
refers to the transverse spin current generated within the ferromagnet. We
utilize the ASHE and its reciprocal effect to electrically inject and detect
magnons in a magnetic insulator in a non-local geometry. Our experiments reveal
that permalloy can have a higher spin injection and detection efficiency to
that of platinum, owing to the ASHE. We also demonstrate the tunability of the
ASHE via the orientation of the permalloy magnetization, thus creating new
possibilities for spintronic applications
Comparison of hot-electron transmission in ferromagnetic Ni on epitaxial and polycrystalline Schottky interfaces
The hot-electron attenuation length in Ni is measured as a function of energy
across two different Schottky interfaces viz. a polycrystalline Si(111)/Au and
an epitaxial Si(111)/NiSi_2 interface using ballistic electron emission
microscopy (BEEM). For similarly prepared Si(111) substrates and identical Ni
thickness, the BEEM transmission is found to be lower for the polycrystalline
interface than for the epitaxial interface. However, in both cases, the
hot-electron attenuation length in Ni is found to be the same. This is
elucidated by the temperature-independent inelastic scattering, transmission
probabilities across the Schottky interface, and scattering at dissimilar
interfaces.Comment: 5 pages, 5 figure
On-chip detection of ferromagnetic resonance of a single submicron permalloy strip
We measured ferromagnetic resonance of a single submicron ferromagnetic
strip, embedded in an on-chip microwave transmission line device. The method
used is based on detection of the oscillating magnetic flux due to the
magnetization dynamics, with an inductive pick-up loop. The dependence of the
resonance frequency on applied static magnetic field agrees very well with the
Kittel formula, demonstrating that the uniform magnetization precession mode is
being driven
Comparison of the magneto-Peltier and magneto-Seebeck effects in magnetic tunnel junctions
Understanding heat generation and transport processes in a magnetic tunnel
junction (MTJ) is a significant step towards improving its application in
current memory devices. Recent work has experimentally demonstrated the
magneto-Seebeck effect in MTJs, where the Seebeck coefficient of the junction
varies as the magnetic configuration changes from a parallel (P) to an
anti-parallel (AP) configuration. Here we report the study on its
as-yet-unexplored reciprocal effect, the magneto-Peltier effect, where the heat
flow carried by the tunneling electrons is altered by changing the magnetic
configuration of the MTJ. The magneto-Peltier signal that reflects the change
in the temperature difference across the junction between the P and AP
configurations scales linearly with the applied current in the small bias but
is greatly enhanced in the large bias regime, due to higher-order Joule heating
mechanisms. By carefully extracting the linear response which reflects the
magneto-Peltier effect, and comparing it with the magneto-Seebeck measurements
performed on the same device, we observe results consistent with Onsager
reciprocity. We estimate a magneto-Peltier coefficient of 13.4 mV in the linear
regime using a three-dimensional thermoelectric model. Our result opens up the
possibility of programmable thermoelectric devices based on the Peltier effect
in MTJs
Spin-dependent Seebeck coefficients of Ni_{80}Fe_{20} and Co in nanopillar spin valves
We have experimentally determined the spin-dependent Seebeck coefficient of
permalloy (Ni_{80}Fe_{20}) and cobalt (Co) using nanopillar spin valve devices.
The devices were specifically designed to completely separate heat related
effects from charge related effects. A pure heat current through the nanopillar
spin valve, a stack of two ferromagnetic layers (F) separated by a non-magnetic
layer (N), leads to a thermovoltage proportional to the spin-dependent Seebeck
coefficient S_{S}=S_{\uparrow}-S_{\downarrow} of the ferromagnet, where
S_{\uparrow} and S_{\downarrow} are the Seebeck coefficient for spin-up and
spin-down electrons. By using a three-dimensional finite-element model (3D-FEM)
based on spin-dependent thermoelectric theory, whose input material parameters
were measured in separate devices, we were able to accurately determine a
spin-dependent Seebeck coefficient of -1.8 microvolt/Kelvin and -4.5
microvolt/Kelvin for cobalt and permalloy, respectively corresponding to a
Seebeck coefficient polarization P_{S}=S_{S}/S_{F} of 0.08 and 0.25, where
S_{F} is the Seebeck coefficient of the ferromagnet. The results are in
agreement with earlier theoretical work in Co/Cu multilayers and spin-dependent
Seebeck and spin-dependent Peltier measurements in Ni_{80}Fe_{20}/Cu spin valve
structures
Comment on "Conductance fluctuations in mesoscopic normal-metal/superconductor samples"
Recently, Hecker et al. [Phys. Rev. Lett. 79, 1547 (1997)] experimentally
studied magnetoconductance fluctuations in a mesoscopic Au wire connected to a
superconducting Nb contact. They claimed to have observed an enhancement of the
rms magnitude of these conductance fluctuations in the superconducting state
(rms(Gns)) relative to that in the normal state (rms(Gn)) by a factor of 2.8.
In this comment, we argue that the measured rms(Gns) is NOT significantly
enhanced compared to rms(Gn) when we correct for the presence of an incoherent
series resistance from the contacts, which is different when Nb is in the
superconducting or normal state.Comment: 1 pag
Electrical detection of spin pumping: dc voltage generated by ferromagnetic resonance at ferromagnet/nonmagnet contact
We describe electrical detection of spin pumping in metallic nanostructures.
In the spin pumping effect, a precessing ferromagnet attached to a normal-metal
acts as a pump of spin-polarized current, giving rise to a spin accumulation.
The resulting spin accumulation induces a backflow of spin current into the
ferromagnet and generates a dc voltage due to the spin dependent conductivities
of the ferromagnet. The magnitude of such voltage is proportional to the
spin-relaxation properties of the normal-metal. By using platinum as a contact
material we observe, in agreement with theory, that the voltage is
significantly reduced as compared to the case when aluminum was used.
Furtheremore, the effects of rectification between the circulating rf currents
and the magnetization precession of the ferromagnet are examined. Most
significantly, we show that using an improved layout device geometry these
effects can be minimized.Comment: 9 pages, 11 figure
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