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Efficient thermal spin injection in metallic
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To cite this article: Tatsuya Nomura et al 2017 J. Phys. D: Appl. Phys. 50 465003

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Journal of Physics D: Applied Physics
J. Phys. D: Appl. Phys. 50 (2017) 465003 (8pp)

https://doi.org/10.1088/1361-6463/aa8b5bEfficient thermal spin injection in metallic
nanostructures
Tatsuya Nomura1, Taisei Ariki1, Shaojie Hu2,3 and Takashi Kimura1,2
1

Department of Physics, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan
Research Center for Quantum Nano-Spin Science, Kyushu University, 744 Motooka, Fukuoka
819-0395, Japan
3
Center for Spintronics and Quantum System, State Key Laboratory for Mechanical Behavior ofMaterials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi
710049, People’s Republic of China
2E-mail: t-kimu@phys.kyushu-u.ac.jp
Received 9 July 2016, revised 18 August 2017
Accepted for publication 8 September 2017
Published 25 October 2017
Abstract

Thermal spin injection is a unique and fascinating method for generating spin current.
If magnetization can be controlled by thermal spin injection, various advantages will be
provided in spintronic devices, through its wireless controllability. However, the generation
efficiency of thermal spin injection is believed to be lower than that of electrical spin
injection. Here, we explore a suitable ferromagnetic metal for an efficient thermal spin
injection, via systematic experiments based on diffusive spin transport under temperature
gradients. Since a ferromagnetic metal with strong spin splitting is expected to have a large
spin-dependent Seebeck coefficient, a lateral spin valve based on CoFe electrodes has been
fabricated. However, the superior thermal spin injection property has not been observed,
because the CoFe electrode retained its crystalline signature—where s-like electrons dominate
the transport property in the ferromagnet. To suppress the crystalline signature, we adopt a
CoFeAl electrode, in which the Al impurity significantly reduces the contribution from s-likeelectrons. Highly efficient thermal spin injection has been demonstrated using this CoFeAlelectrode. Further optimization for thermal spin injection has been demonstrated by adjusting
the Co and Fe composition.
Keywords: spin injection, spin current, heat current, spin-dependent transport(Some figures may appear in colour only in the online journal)

1. Introduction

with a nonlocal spin-injection geometry, a pure spin current,
which does not carry any charge current, can be created [9–12]. This pure spin current has several advantages such as the
suppression of Joule heating and maximization of spin cur­rent. Although pure spin current is an attractive quantity, thepreparation of independent electrodes for generating pure spincurrent is meanwhile indispensable. This leads to complications in device fabrication and integration.
In place of electrically-driven spin current generation, utilizing heat has recently attracted considerable attention as a
new approach for controlling spin current in ferromagnetic/
nonmagnetic hybrid nanostructures [13–24]. The control ofthe spin current using heat was also studied in a pioneering
work on spin-dependent thermoelectric effects by Johnson

Spin current, which is a flow of spin angular momentum,
plays a central role in the operation of spin-based nano-electronic devices. The efficient manipulation of spin current is animperative, primary issue for developing functional, energy
efficient nano-spintronic devices [1–6]. Spin current is mainly
generated by applying an electric field across a ferromagn­
etic/nonmagnetic metal interface—this process, known aselectrical spin injection, was first proposed by Johnson andSilsbee over 30 years ago [7, 8]. With the development of
nano-fabrication techniques, electrical spin injection has been
extended to laterally configured multi-terminal ferromagnetic/
nonmagnetic hybrid nanostructures [9]. In these structures,1361-6463/17/465003+8$33.00

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J. Phys. D: Appl. Phys. 50 (2017) 465003

and Silsbee 30 years ago [8, 13]. With the development of
spintronics, the unique interactions between magnetism andthermoelectricity in magnetic multi-layered or granular systems have been reported by a few groups from the fundamental view point for the heat and charge transports [15–18].
After the discovery of spin-transfer switching, spin-dependent
thermoelectric effects have been paid considerable attention
from the application point of view, in the emerging field of
spin caloritronics. A fascinating, representative phenomenon
here is the spin Seebeck effect, in which the temper­ature
gradient across the interface between ferromagnet and nonmagnet creates a spin current [19]. The spin Seebeck effectcan create spin cur­rent not only in the ferromagnetic metal
but also from the ferromagnetic insulator, and opens the door
to novel applications in spintronics [22]. However, recent
experimental studies have pointed out the importance of ferromagnetic proximity effects in ferromagnetic/heavy metal
bilayer systems [25]. In particular, since the spin Seebeck
effect is always detected via inverse spin Hall effects in nonmagnetic heavy metals, the possibility of the contribution
from the anomalous Nernst effect cannot be overlooked [26].
Therefore, the quanti­tative understanding of the spin Seebeck
effect is still under consideration.
Along with the spin Seebeck effect, thermal spin injection is known as another approach to generating spin current
using heat [14], [23–25]. The origin of thermal spin injectionis the spin dependence of the Seebeck coefficients, while thespin Seebeck effect originates from thermally excited spin
pumping [13, 14]. Therefore, this phenomenon is often called
the spin-dependent Seebeck effect. The first demonstration ofthermal spin injection was reported by Slachter et al [23]. Bymanipulating the heat flow due to a Joule heating in speciallydeveloped laterally configured ferromagnetic/nonmagnetic
hybrid nano-structures, they have succeeded in detecting thethermally excited spin-valve signal even at room temper­
ature. This report was recognized as the pioneering work forthermal spin injection. However, the magnitude of the spinsignal obtained was much smaller than the electrically-driven
spin signals.
The thermal spin injection in a magnetic tunnel junction
was also demonstrated by Walter et al [24]. Since the microscopic mechanism of this phenomenon is the spin dependenceof the Seebeck coefficient of a tunnel junction for the ferromagnetic electrode, this is known as magneto-Seebeck tunneling or Seebeck spin tunneling effect. In [24], a laser beam
was used in order to produce an efficient heat flow acrossthe tunnel barrier, obtaining a relatively large spin-valve-likethermoelectric voltage, compared with the metallic-junction
device. The Seebeck spin tunneling effect has been demonstrated in a semiconductor by using a three-terminal Hanle
effect, which is a powerful technique for detecting spin accumulation in semiconductors [25]. Owing to the large resistivity of the semiconductor, the Joule heating from the electric
current efficiently produced a temperature gradient, resulting
in thermal spin injection. Moreover, voltage tuning of the thermally excited spin current was also demonstrated [27].

The induced thermoelectric spin voltages in the tunnel
junctions were much larger than those for the metallic devices[23, 28, 29]. However, the magnitude was ten times smaller
than the electrically induced signal. Moreover, because of
the large interface resistance, the generated spin current wasmuch smaller than the metallic structure. These features wereconsidered as disadvantages from the application view point.
However, thermal spin injection can be performed without
the use of electricity; an innovative approach to switching
the magnetization, such as wireless and/or energy harvesting
operation, may be developed. Indeed, the spin transfer torqueinduced by thermally excited spin currents has been exper­
imentally demonstrated in magnetic multi-layered films.
Therefore, the improvement of the generation efficiency ofthermally-driven spin current is an imperative and primaryissue.
Recently, we have demonstrated that a CoFeAl alloy shows
excellent performance in thermal spin injection, because of its
favorable band structure [30]. This may open up possibilities
for utilizing the thermally-driven spin current in a practicaldevice. By performing further optimization of the materials,
we may be able to demonstrate more efficient thermal spin
injection, and more functional device operation. In this article,
we explain a strategy for increasing thermal spin injection
efficiency using a CoFe-based alloy, with reference to oursystematic experiment. In addition, further possibilities forthermal spin injection will be introduced.

2. Analytical solution of thermal spin injection
To simplify the thermoelectric transport in the ferromagn­etic
metal system, we adopt the Mott model in diffusive metallic
systems [31]. The Seebeck coefficient is strongly correlated tothe band structure around the Fermi level, and, under a simple
approximation in a metal, the coefficient is proportional to theenergy derivative of the logarithmic density of state D at the
Fermi level [31–33]. According to Mott’s model, the Seebeckcoefficient for a degenerate metal is simply given by the following equation:
π 2 kB2 T ∂ ln D
(1)
,
S=−
3 e ∂E
where kB and e are, respectively, the Boltzman constant andthe elementary charge of the electron. T and E are the temper­
ature and the energy respectively.
Here, we focus on the experimental evaluation of theSeebeck coefficient. Because of the Seebeck effect, thetemperature gradient produces the effective electric field in
addition to any applied electric field. Specifically, under thegradients of the temperature T and electro-chemical potential
μ, the flow of the electrons, which is the current density J, canbe given by the following equation:
J = −σ(∇µ/e − S∇T),(2)where σ is the electrical conductivity.

2

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Therefore, the generation efficiency of the spin current due
to temperature gradient is significantly enhanced by the large
spin-dependent Seebeck coefficient. From this point of view,
we have investigated the thermal spin injection property forferromagnetic metals with strong spin splitting.
We analytically investigate the diffusion property of thethermally-excited spin current. First, we estimate the generated spin current due to a temperature gradient in a homogeneous ferromagnet. Since there will be no spin accumulation(µ↑ − µ↓ = 0 ) in a homogeneous ferromagnet under a onedimensional temperature gradient, the conventional Seebeck
voltage with the open circuit condition ( J = 0) can be calculated as follows.

We here emphasize the remarkable difference between theelectrically driven and thermally driven cases. In the electrically driven case, since the electrical conductivity is always
positive, the electrons always flow in the same direction. In
the thermally driven case, since the sign of the Seebeck coefficient can be either positive or negative, the direction of electron motion due to the temperature gradient depends on thematerial. Specifically, when the energy derivative of the density of state is positive, the Seebeck coefficient becomes negative (electron-like Fermi surface). On the other hand, when theenergy derivative is negative, the Seebeck coefficient becomes
positive (hole-like Fermi surface). This is a unique feature of
the thermally-driven motion of the electrons.
We are now focusing on the ferromagnetic metals, in whichmost of the material parameters depend on the spin direction.
In this case, the Seebeck coefficient should also be consideredseparately for up and down spins. Specifically,∇µ/e = (S↑ σ↑ + S↓ σ↓ )/(σ↑ + σ↓ )∇T = S0 ∇T.
(4)Here, S0 is the effective Seebeck coefficient in the ferromagn­
etic systems. We then calculate the generated spin current
under one dimensional temperature gradient using equation (3). Since we know the relationship between ∇µ and ∇T ,the generated spin current can be described as follows:J↑ = −σ↑ (∇µ↑ /e − S↑ ∇T), J↓ = −σ↓ (∇µ↓ /e − S↓ ∇T).
(3)This indicates that when the Seebeck coefficient depends on
the spin direction, the temperature gradient produces a spincurrent determined by J↑ − J↓. This is known as thermal spin
injection; it has often been called the spin-dependent Seebeck
effect.
In the electrically driven case, since the signs of the electrical conductivity for up and down spins are always positive, the electrical spin polarization for the ferromagnet,
which is defined by (σ↑ − σ↓ )/(σ↑ + σ↓ ) is always smaller
than 1. This means that the generated spin current J↑ − J↓ isless than the bias charge current J↑ + J↓. On the other hand,for the thermal spin injection, the sign of the Seebeck coefficient can take either positive or negative value. If the signsof the Seebeck coefficient for up and down spin are opposite,a temperature gradient produces the spin current J↑ − J↓. In
this case, up-spin and down-spin currents are canceled out,
leading to the reduction of the charge current J↑ + J↓. In thissituation, (J↑ − J↓ )/(J↑ + J↓ ) can be larger than 1. Thus, thegeneration efficiency of the spin current due to the thermal
spin injection is significantly enhanced by the sign reversal of
the Seebeck coefficient. To realize such a situation, we seekmaterials suitable for thermal spin injection.
According to the Stoner criterion, a ferromagnetic metal
can be described by the systematic shift of the d-band centerof the density of states to lower energies by an amount ofenergy equal to the spin splitting [34]. For conventional ferromagnets, the magnitude of the spin splitting between the up
and down spins can be considered approximately as a small
shift of the density of state. In this case, although the electrons
for up and down spins diffuse toward the same direction withdifferent magnitude, as shown in figures 1(a) and (b), the difference between S↑ and S↓ is very small. Therefore, the magnitude of the generated spin current diminishes. On the other
hand, in a ferromagnet with strong spin splitting, the large difference in the density of state between spins produces a sign
reversal of the Seebeck coefficient between the up and down
spins. In such a situation, the up-spin and down-spin electrons
flow in opposite directions, as shown in figures 1(c) and (d).

IS = I↑ − I↓ = A∇T(−S0 (σ↑ − σ↓ ) + (σ↑ S↑ − σ↓ S↓ ))(5)= (1 − P2 )AσSS ∇T/2.

Here, SS is the spin-dependent Seebeck coefficient, definedby S↑ − S↓. P is the electrical spin polarization, defined by(σ↑ − σ↓ )/(σ↑ + σ↓ ). From this equation, the spin-dependent
Seebeck coefficient SS can be seen to be the key parameter for
efficient thermal spin injection.
Then, we consider the spin diffusion process in the lateralspin valve (LSV) structure under the thermal spin injection.
When we introduce the temperature gradient across the ferromagnetic/nonmagnetic metal interface, the spin current given
by equation (3) is generated, depending on the magnitude of
the temperature gradient in the ferromagnet. The generated
spin current is injected into the nonmagnet, and induces the
non-equilibrium spin accumulation. This non-equilibrium
spin accumulation diffuses not only into the nonmagnet but
also back to the ferromagnet, similarly to the electrically generated spin current in the lateral hybrid structure. In this case,the position dependence of the spin signal due to spin accumulation is simply obtained by replacing the value of PF I , where
PF is the spin polarization for the ferromagnet, in the electricalspin injection. Finally, we obtain the following equation forthe spin signal due to thermal spin injection [23, 30]:
∆VS2f =PF RN RF λF SS ∇TF.
2RF (RF + RN )(cosh(L/λN ) + sinh(L/λN ) + R2N sinh(L/λN ))(6)
Using this equation with the spin signal under the thermal
spin injection, the spin-dependent Seebeck coefficient can be
estimated.
3. Comparison of thermal spin injection properties
between NiFe and CoFe
We have prepared lateral spin valves consisting of two ferromagnetic wires bridged by a Cu strip, as shown in figures 2(a)3

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J. Phys. D: Appl. Phys. 50 (2017) 465003

Figure 1. Schematic illustrations for the electron diffusion mechanism due to the temperature gradient. (a) Electron states at high andlow temperatures and (b) electron flows due to temperature gradient for a conventional ferromagnet. (c) Electron states at high and low
temperatures and (d) electron flows due to temperature gradient for a ferromagnet with strong spin splitting.

Figure 2. (a) Representative scanning electron microscope image for a fabricated lateral spin valve. (b) Schematic illustrations for the
fabricated device, together with the probe configuration for the thermal spin injection. Temperature profiles under the thermal spin injection
obtained by COMSOL simulation, (c) top view for the whole device and (d) cross section around the FM/Cu junctions.

and (b). Here, we adopt two representative ferromagnetic
metal alloys, as ferromagnets. One is NiFe, with a relatively
small magnetization, meaning that the spin splitting is not sostrong. The other is CoFe, with a large saturation magnetization. According to the Stoner criterion, the large satur­
ation magnetization is induced by the strong spin splitting.
Therefore, CoFe should have stronger spin splitting than NiFe.
Here, both ferromagnetic films were deposited by conventional e-gun evaporations under the base pressure of 10−6 Pa.
The composition of each film was measured with energy-dispersive x-ray (EDX) microscopy. For the NiFe film, Ni andFe were 81 wt% and 19 wt% respectively. For the CoFe film,
Co and Fe were 49 wt% and 51 wt% respectively. The electrical resistivities for the NiFe and CoFe are 35 µΩ cm and45 µΩ cm respectively. The Cu strips was deposited by a Jouleevaporator. Here, prior to the Cu deposition, the surfaces forthe ferromagnetic wires were well cleaned by Ar ion millingwith low acceleration voltage to obtain the highly transparent
interface. The resistivity of Cu is 2.8 µΩ cm at room temper­
ature. Here, we fixed a center–center interval between the ferromagnetic wires as 200 nm.
The thermal spin injection is performed using a local Joule
heating method. By introducing a large ac current in one ferromagnetic wire, the temperature of the ferromagnetic wire

significantly increases because of the Joule heating. This
results in the creation of a temperature gradient across theferromagnetic/nonmagnetic metal interface. The spin accumulation induced by the thermal spin injection is detected
electrically, by another ferromagnetic wire. Here, since thetemperature gradient due to the Joule heating is proportional
to the current square, the induced voltage due to the thermal
spin injection should be proportional to the current square.
Therefore, we detect the second harmonic comp­onent of theinduced voltage by using a lock-in amplifier. In this method,no contribution from the electrical spin injection will be
expected, ideally, because the electric current does not flow
across the interface. However, a partial current injection
from the ferromagnet may be injected into the Cu around thejunction, because of the low resistivity of the Cu, resulting
in spin accumulation via the electrical spin injection. Indeed,
we observe a spin-valve signal in the voltage component of
the first harmonic frequency. However, the magnitude of thespin signal is much smaller than the conventional electricallyinduced nonlocal spin signal [30]. Therefore, we can exclude
such contributions by the second harmonic lock-in technique.
We can estimate the temperature distribution under the
local Joule heating using COMSOL simulation. As shown infigure 2(c), a temperature gradient around the interface, which
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J. Phys. D: Appl. Phys. 50 (2017) 465003

Figure 3. Experimental results for 2nd harmonic spin valve signals for NiFe/Cu LSV measured at room temperature (a) and 5.5 K (b), andfor CoFe/LSV measured at room temperature (c).

Figure 4. (a) Schematic illustration for the multi-band structure for a 3D transition metal. Density of states for s-like electron (b) and3D electron (c).

can drive the thermal spin injection, is effectively produced byintroducing a large current in the ferromagnetic injector [23,28, 29]. From the COMSOL simulation, we can estimate thetemperature gradient ∇T in the vicinity of the interface, as
well as the temperature at the detecting junction, as functionsof the bias ac current. We confirmed that ∇T is proportional
to I2 by COMSOL simulation.
Figures 3(a) and (c) show the 2nd harmonic spin signals
under the thermal spin injection at room temperature as a function of the magnetic field for NiFe/Cu and CoFe/Cu LSVs,
respectively. Here, in order to fairly compare the thermal spin
injection efficiency, we take the normalized second harmonic
voltage V 2f /I 2 as the vertical axis. This is because the temper­
ature gradient is proportional to the Joule heating I2, which
was confirmed by COMSOL simulation. Here, we confirm
that the vertical axis does not depend on the magnitude of thecurrent amplitude below I = 1.0 mA. In the CoFe/Cu LSV,a typical spin-valve-like signal is clearly observed with themagnitude of 0.15 μV mA−2 . In the NiFe/Cu LSV, the strong
asymmetric field dependence dominates the overall signal
change. From the systematic study of our previous experiments, we can understand this large asymmetric signal as theanomalous Nernst effect in the ferromagnetic voltage probe[35]. When the sample is cooled to low temperature (5.5 K),
the anomalous Nernst effect is strongly suppressed. On theother hand, the spin diffusion length increases with decreasing
temperature [11]. Therefore, we can pick up the signal of thethermal spin injection for the NiFe/Cu LSV at low temper­
ature. As seen in figure 3(b), we see a spin-valve-like signal,
ensuring that the thermal spin injection can be achieved in the
NiFe/Cu LSV. From this knowledge, by carefully observing
the signal at RT, we can confirm the signature of the spin valve

signal at H ≈ 25 mT, which is caused by the magnetization
reversal of the NiFe injector.
From the aforementioned results with the electricallydriven spin-diffusion property [35], we now estimate theeffective Seebeck coefficient S0 and spin-dependent Seebeckcoefficient SS for each ferromagnetic metal. From the background signal with the Seebeck coefficient for the Cu
SCu = 1.6 μV K−1, we obtain S0 NiFe as −20.5 μV K−1. Thisis quite consistent with the previously reported value [19, 23].
This assures the validity of COMSOL simulation. Therefore,
we similarly estimate S0 for the CoFe. The obtained value is−24.3 µV K−1, which is almost the same as that for the NiFe.
We then estimate the spin-dependent Seebeck coefficient fromthe 2nd harmonic spin signal with the calculated temperature
gradient. The obtained SS for the Permalloy and CoFe are−6.23 μV K−1 and −19.8 μV K−1 respectively. The obtained
SS for the Permalloy is reasonable value compared to the previously reported value [23]. SS for the CoFe is larger than the
NiFe, but is smaller than S0, contrary to our expectation. Thus,
the thermal spin injection efficiency is not so high, even using
the CoFe alloy with strong spin splitting.
To find out the reason that SS for our CoFe is smaller
than S0, we consider the electron state in the CoFe film. As
schematically illustrated in figure 4, in 3D transition metals,
the conduction electrons can be divided into two major categories [36]. One is the 3D electron, which is the basis offerro-magnetism. As schematically shown in figure 4(c), we
expect a large spin-dependent Seebeck coefficient based on
the 3D band structure. The other one is the s-like electron.
The spin-dependent Seebeck coefficient for s-like electrons
is relatively small, because of the small difference between
the up-spin and down-spin band structures, as schematically5T Nomura et al

J. Phys. D: Appl. Phys. 50 (2017) 465003

Figure 5. (a) 2nd harmonic spin valve signals for CoFeAl/Cu LSV measured at room temperature. (b) Position dependence of the 2nd
harmonic spin valve signals. The red line is the fitting curve based on equation (6).

the definitions for S0 and SS , S↑ and S↓ can be calculated as−35.7 μV K−1 and 36.4 μV K−1. The sign difference between
S↑ and S↓ is clear evidence that CoFeAl is an excellent mat­erial for thermal spin injection [30].
Here, we emphasize that the temperature of the Cu wire isnot so high under the thermal spin injection. From the background signal of the second harmonic signal, the temperaturechange at the detection junction ∆T can be estimated to beapproximately 3 K. This is because the Cu wire has a large
heat conductivity and heat capacity. We can also roughly evaluate the influence of the heating effect on the Cu wire fromthe interval dependence of the thermal spin signal. Since thespin relaxation increases with the temperature, the spin diffusion length for the Cu should decrease at higher temperature.
However, as can be seen in figure 5(b), we have confirmedthat the spin diffusion length for the Cu wire is 500 nm ±50
nm, which is the same as that at room temperature. These
facts strongly support the validity of our analysis based on the
COMSOL simulation.
For more clear evidence of the excellent performance of
the CoFeAl sample, we fabricated a lateral spin valve consisting of CoFeAl and NiFe wires bridged by a Cu strip as
shown in figure 6(a) [38]. In this device, we measured the 2nd
harmonic spin signals in two probe configurations. One—configuration A—used NiFe as thermal spin injector and the
CoFeAl as spin detector; the other—configuration B—used
CoFeAl as thermal spin injector and NiFe as spin detector, as
schematically shown in the insets for figure 6(b) and (c). As
seen in figures 6(b) and (c), the significant difference between
two signals has been observed in the thermal spin signal. In
configuration A, the signal is dominated by the anomalous
Nernst effect of the CoFeAl wire and a tiny spin-valve-likesignal can be confirmed. This is because the poor thermal
spin-dependent Seebeck coefficient for the NiFe strip. On theother hand, in configuration B, a clear spin-valve signal has
been observed, because of the excellent performance of the
CoFeAl. This is further evidence that the CoFeAl has a large
spin-dependent Seebeck coefficient.
Thus, the strategy based on the Stoner criterion seems towork very well for efficient thermal spin injection. For further optimization of the spin-dependent Seebeck coefficient,
we investigated the composition dependence of the thermal
spin injection. Here, we prepared several CoFeAl alloys withshown in figure 4(b). In the crystalline structure, owing totheir long mean free path and small effective mass, the s-likeelectrons dominate the conduction in the ferromagnetic metal.
Thus, the spin-dependent Seebeck coefficient in the CoFe
becomes small, although the 3D electrons have the ideal spindependent band structure.
4. Efficient thermal spin injection based on CoFeAl
From the above considerations, when the contribution fromthe s-like electrons can be suppressed, we may exploit theideal signature of the 3D electrons. It is well known that themean free path for the electrons can be reduced by adding
impurities, because of the resulting strong randomness ofthe crystalline structure. Particularly in amorphous or dirty
metallic structures, the contribution of the s-electrons is significantly reduced because of their extremely short mean freepath [37]. In this situation, since the number of 3D electrons
is much larger than the number of s-electrons, the signatureof the 3D electrons becomes visible. To realize such amorphous-like CoFe structures, we added a small amount of Al,
which is one of the representative light elements. In this case,
we expect a large spin-dependent Seebeck effect in CoFeAl
because the Al impurities reduce the mean free path for thes-like electrons significantly.
To demonstrate the enhancement of the thermal spin-injection efficiency, we have fabricated the source of CoFeAl using
an arc furnace, and deposited a CoFeAl film using an electronbeam evaporator [30]. The composition of each metal in thedeposited film, evaluated using EDX, were Co 48 wt%, Fe
48 wt% and Al 4 wt% . The CoFeAl/Cu LSV was fabricatedby a two-step lift-off method, similarly to the conventional
LSV. We used the same device geometries, such as width andinterval, as those in the previous LSV. Thermal spin signal
has been measured by the 2nd harmonic detection techniqueof the nonlocal spin signal similarly to the previous measurements. As seen in figure 5(a), a clear spin-valve-like signal
with the magnitude of 0.873 μV has been observed. The signal
obtained is ten times larger than that for the CoFe/Cu LSV,
implying highly efficient thermal spin injection. Using the
COMSOL simulation together with the parameters from theelectrical spin injection experiments, S0 and SS can be estimated as −22.0 μV K−1 and −72.1 μV K−1. Also, by using
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J. Phys. D: Appl. Phys. 50 (2017) 465003

Figure 6. (a) Scacnning electron microscope image of the fabricated CoFeAl/Cu/NiFe LSV together with its schematic illustration. (b)2nd harmonic spin valve signal for NiFe injector and CoFeAl detector and (c) that for CoFeAl injector and NiFe detector measured at room
temperature.

Figure 7. Room temperature 2nd harmonic spin valve signals for various CoFeAl alloy with different compositions. (a) Co 7 wt%, Fe 88
wt% and Al 5 wt%. (b) Co 26 wt%, Fe 71 wt% and Al 3 wt%. (c) Co 43 wt%, Fe 53 wt% and Al 4 wt%. (d) Co 48 wt%, Fe 48 wt% and Al
4 wt%. (e) Spin-dependent Seebeck coefficient as a function of Co composition in CoFeAl alloy.

different composition. We performed similar lateral spin-valve
experiments under thermal spin injections. Figure 7(a) shows
the second harmonic signal under the thermal spin injections
for various CoFeAl alloys. We can confirm that the magnitude of the spin-valve-like signal changes systematically withthe composition. By performing the aforementioned analysis,
we estimated the spin-dependent Seebeck coefficient SS foreach alloy. Figure 7(b) shows the summary for the the spindependent Seebeck coefficient SS as a function of the Co composition. SS takes its maximum value at the composition of
Co 43 wt%, Fe 53 wt% and Al 4 wt%. So, when the composition of Co is too rich, the thermal spin injection efficiency
decreases. These features resemble Slater–Pauling behavior[38], suggesting a link between the two effects. However, ourexperiments are still very rough, and we may have to consider
other mechanisms for the large spin-dependent Seebeck coefficient. For example, it was reported that the Heusler compound was partially formed in the evaporated CoFeAl film[39]. Since the energy derivative of the density of state in the
Heusler compound is known to be very large, a large Seebeckcoefficient will be produced. Thus, further systematic study isneeded in order to conclude this matter.
5. Conclusion
We have investigated the thermal spin injection properties
for various ferromagnetic metal alloys, using lateral spin
valve structures. CoFe-based alloy is found to have a large
spin-dependent Seebeck coefficient, which produces excellent thermal spin injection. This property can be qualitatively
understood through the combination of the Mott model andStoner criterion. We have pointed out that a small amountof Al impurity is a key to significantly increasing the spindependent Seebeck coefficient. We have also demonstrated
that further optimization is possible by adjusting the Co and
Fe compositions.

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This work is partially supported by Grant-in-Aid for Scientific Research on Innovative Areas, ‘Nano Spin Conversion
Science’ (26103002) and that for Scientific Research (S)(25220605).
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