University of Groningen
Spin caloritronics in magnetic/non-magnetic nanostructures and graphene field effect
devices
Dejene, Fasil
DOI:
10.1038/nphys2743
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Publication date:
2015
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Dejene, F. (2015). Spin caloritronics in magnetic/non-magnetic nanostructures and graphene field effect
devices [Groningen]: University of Groningen DOI: 10.1038/nphys2743
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Summary
Electrons in a (magnetic) conductor transport charge, heat and angular momentum
(spin) when a voltage or temperature gradient is applied. The past century has
seen great progress in the understanding of the coupled flow of charge and heat
(thermoelectricity) as well as charge and spin (spintronics) both in bulk and thin
metallic films. While most electronic devices we use today are based on the charge
property of the electron, it is not uncommon to find applications that make use of
the interaction of the charge with other transport properties. For instance, accurate
measurement of temperatures and solid-state refrigeration applications are based on
two of the most common thermoelectric effects—the Seebeck effect, the conversion
of heat into an electrical current, and its inverse the Peltier effect, respectively. The
significant improvement in the data-storage and information processing technologies
we witnessed over the past decade hinges on the giant (tunneling) magnetoresistance
effect used in the read-heads of the magnetic hard disk drive.
Very recently, the combination of spintronics and thermoelectricity has led to the
birth of spin caloritronics—a field envisioned to provide multifunctional spintronic
concepts that may, in the future, provide alternative ways for managing heat flow at
the nanoscale and controlling spin information by using heat. The research described
in this thesis was aimed at gaining a deeper understanding of the origin and working
principles of various spin-dependent thermoelectric effects in ferromagnetic/normal
metal structures.
Spin caloritronics in metals (chapters 3−5)
In a ferromagnetic metal, due to the strong exchange interaction, the Fermi energy
density of states for spin up and spin down electrons is shifted with respect to
each other. The transport of charge and heat can thus be described by a two-spin
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Summary
channel model, one for spin up (majority spins) and another for spin down (minority
spins) with each spin-channel having its own electrical and thermal conductivities
as well as Seebeck and Peltier coefficients. A charge current flowing through a
ferromagnet is thus accompanied by a spin-polarized current that, when injected into
another nonmangetic metal (N), results in a non-equilibrium magnetization (spin
accumulation). A spin valve device, comprising of two F layers separated by an
N layer, is prototypically utilized to study the process of electrical spin injection,
transport and detection in various systems.
In addition to charge current, heat current driven spin injection into non magnetic
materials has recently been demonstrated by Slachter et al., in nonlocal spin valve
devices, where a Joule-heated ferromagnet is used to inject spins into an adjacent
normal metal. The size of the spin accumulation is proportional to the spin-dependent
Seebeck coefficient, the difference in the Seebeck coefficients of spin up and spin
down electrons, of the ferromagnetic metal used. In chapter 3, we verified this earlier
work in specifically designed nanopillar spin valve devices and determined the
spin-dependent Seebeck coefficient for permalloy and cobalt. The Thomson-Onsager
relation, that relates the Seebeck coefficient with the Peltier coefficient, also predicts a
spin current driven heating/cooling effect (spin-dependent Peltier effect). Flipse et al.
demonstrated this process earlier from which a spin-dependent Peltier coefficient of
−1 mV was obtained that was in agreement with the Thomson Onsager reciprocity
relation. This symmetry relation was rigorously tested in chapter 5 by measuring
both spin-dependent quantities in a single device. The reciprocity relation holds
both in the linear as well as in the nonlinear regime. In the latter, contributions from
nonlinear thermoelectric effects cause deviation in the current-voltage relationships.
In chapter 4 the first experimental observation of the magnetic heat valve was
presented. Due to the spin-dependence of the thermal conductivity, a heat current
through a ferromagnetic metal is also spin polarized, that when injected into a
non-magnetic metal, causes a spin heat accumulation (SHA) or a difference in the
effective temperature of spin up and spin down electrons. In a pillar spin valve, it is
possible to modulate the heat conductance of the spin valve by changing the relative
magnetization direction of the ferromagnets. When the two ferromagnetic layers
are aligned parallel (antiparallel) to each other, the total heat conductance of the
nanopillar is larger (smaller) corresponding to the absence (presence) of SHA in the
normal metal spacer. This non-equilibrium SHA thermalizes by inelastic scattering
mediated by either electron-electron or electron-phonon interactions and/or spin-flip
scattering processes. The length scale over which the spin temperature thermalizes is
directly linked to the inelastic scattering length in the metal. This technique therefore
offers a unique possibility to estimate the inelastic scattering length at low energies
and elevated temperatures, not accessible by other spectroscopic methods.
Summary
119
Manipulation of spin currents by a magnetic insulator (chapter 6)
The long-term goal of spintornics is to achieve efficient manipulation of a spin current
using an external gate fabricated atop a spin-transport channel as in the Datta-Das
spin field effect transistor. In chapter 7 we demonstrated an alternative spin current
manipulation technique using a nonlocal spin valve fabricated on a magnetic Yttrium
Iron Garnett (YIG) substrate. Although exchange of electrons across the metal/YIG
is not allowed, spins can be exchanged due to the spin-mixing interface conductance.
When the spin magnetic moment in the metal is aligned (antialigned) with the
magnetization of the YIG, most of the spins are back-reflected. However, when the
spin magnetic moment is perpendicular to the YIG magnetization, majority of the
spins are absorbed by the YIG thereby resulting in a reduction in the nonlocal spin
valve signal. We quantified the results by using a three dimensional spin transport
model as well as comparing the results with devices fabricated on standard SiO2
substrate, for which the parameters governing spin transport are well known. We also
quantified the size of the spin-mixing conductance and highlighted the role of thermal
magnons and other interfacial spin-orbit induced spin-relaxation mechanisms for the
observed small modulation in the signal.
Thermoelectric effects in graphene (chapter 7)
In recent years, extensive research has been devoted to two dimensional systems
such as metal chalcogenides, topological insulators and graphene. Graphene is a two
dimensional one atom thick honeycomb lattice of carbon atoms that has prominent
electronic, spintronic and thermoelectric properties. It was discovered in 2004 by
Andre Geim and Konstantin Noveselov for which they shared the 2010 Nobel prize
in physics. It has well documented properties, among others, long spin relaxation
times, very large thermal conductivity and Seebeck coefficient. Graphene’s unique
electronic band structure presents the possibility of tunning the Seebeck and Peltier
coefficient from large negative values (in the electron regime) to large positive values
(in the hole regime) opening up possibilities for tunable thermoelectric conversion or
refrigeration applications.
In chapter 7, we studied electronic and thermoelectric properties of single and
bilayer graphene in a device architecture that allowed us to detect both the Seebeck
and Peltier effect in a single device. The devices studied for this purpose have,
in addition to conventional electrical contacts, a micropatterned electrical heater
as well as a thermocouple that is used to measure local temperature changes at a
graphene/metal interface. Two separate measurements aimed at the understanding
of the Peltier heating/cooling and Seebeck effects were performed. In the first
experiment, by sending an electrical current through a metal/graphene interface
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Summary
and tunning the charge carriers in the graphene, it was possible to reversibly heat
or cool the interface. In another measurement, on the same device, we performed
thermopower measurements where the graphene was subjected to an in-plane heat
current and the thermovoltage that develops over the graphene was obtained as a
function of the carrier density. Using a three dimensional thermoelectric model, we
further verified the validity of the Thomson-Onsager reciprocity relation between
the Peltier and Seebeck coefficients.
Conclusion
Spin caloritronics is a broad research field that encompasses the study of the
coupling between heat and spin transport in metallic nanostructures, magnetic
insulators, magnetic tunnel junctions and other technologically important devices.
Although it is yet at its infancy, it has already provided us with additional insight
on the thermoelectrical transport properties of nanoscale spintronic devices. While
heating effects remain detrimental to the performance of current microprocessors,
future spin caloritronic devices might be useful for nanoscale heat scavenging and
waste heat management applications as well as adding more functionality to current
spintronic devices.