PHYSICAL REVIEW B 94, 014416 (2016)

First- and second-harmonic detection of spin accumulation in a multiterminal lateralspin valve under high-bias ac current
Shaojie Hu,1 Tatsuya Nomura,2 Ginga Uematsu,2 Nagarjuna Asam,2 and Takashi Kimura1,2,*
1

Research Center for Quantum Nano-Spin Sciences, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan2
Department of Physics, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan(Received 30 March 2016; revised manuscript received 9 June 2016; published 13 July 2016)

We have investigated the transport properties of electrically and thermally excited spin currents in a lateral spin
valve consisting of a spin injector and detector with a middle ferromagnetic wire by detecting the first- and secondharmonic voltages. The first-harmonic spin signal was significantly suppressed by the middle ferromagnetic wire
because of the spin absorption effect. On the other hand, in the second-harmonic signal, a small signal related tothe middle ferromagnetic wire was observed in addition to a conventional spin signal with a reduced magnitude.
This indicates that the additional ferromagnetic wire acts not only as the spin absorber but also as another spin
injector under thermal spin injection, because the heat current caused by direct spin injection propagates tothe middle ferromagnetic wire and creates another temperature gradient. By using this effect, we show that themagnetization direction of a ferromagnetic nanodot embedded in a nonmagnetic Cu wire becomes measurable.
DOI: 10.1103/PhysRevB.94.014416
I. INTRODUCTION

The electronic device utilizing electron spins is one ofthe promising approaches for next-generation nanoelectronic
devices [1,2]. Especially, the establishment of a concept of spincurrent, the flow of the spin angular momentum, provides two
important progresses in the operation of spin-based electric
devices [3]. One is the manipulation of the electric current
originating from spin-dependent transports such as giant
magnetoresistance [4,5] and tunnel magnetoresistance [6–8].
The electric signal reflecting the spin information enables
us to detect the magnetization direction precisely and sensitively [9,10]. The other one is the manipulation of themagnetization direction due to the transfer of the spin angularmomentum [11–13]. The injection of spin current into a small
ferromagnet exerts the torque on the local magnetizationand provides a reliable magnetization switching techniquewith low power consumption [14,15]. Thus, these innovativetechniques based on electrical means offer a highly selectablearchitecture, which seems to be suitable for an integrated
circuit. However, the preparation of nanoscale magnetic multilayers with individual electrodes significantly complicates the
fabrication procedure of nanoelectronic devices. Therefore,
seamless device integration is an important milestone fordeveloping nanospin devices as well as energy efficient device
operation.
Apart from the electrical control of the spin current,
recently, the manipulation of spin currents using heat has
been paid considerable attention under the concept ofspin caloritronics combining spintronics and thermoelectrics[16–20]. Numerous intriguing phenomena such as spin
Seebeck [16], spin-dependent Seebeck [17], spin-dependent
Peltier, and spin Peltier effects [21,22] have been reported invarious structures. Moreover, the spin transfer torque inducedby thermally excited spin currents was also studied theoretically [23,24] and experimentally [25–28]. Although some ofthe devices consisting of simple ferromagnetic/nonmagnetic*

t-kimu@phys.kyushu-u.ac.jp2469-9950/2016/94(1)/014416(5)hybrid structures are suitable for mass production [29], thestudy on thermally excited spin currents reported so far mainly
focuses on fundamental viewpoints. The performance at the
moment is quite far from a practical application because of thelow efficiencies of the related effects. We have demonstrated
that CoFeAl shows an excellent performance for thermal
spin injection because of its large spin-dependent Seebeckcoefficient [30]. This may open up possibilities for utilizing
the heat-induced spin current in a practical device. Moreover,a high thermal spin-injection efficiency provides the ideal
platform for a detailed experimental study on the thermally
excited spin current. Especially, the transport properties of
the thermally excited spin current in multiterminal lateralhybrid structures have not been well studied. Here, weinvestigate the spin absorption property due to an additional
ferromagnetic wire under thermal spin injection. We foundthat the thermally excited spin signal was strongly modulatedby an additionally connected middle ferromagnetic wire. By
extending this property, we propose and demonstrate that themagnetization direction of the middle wire can be indirectly
detected by using thermal spin injection. The waste heat due
to the electric current, namely, Joule heating, enables one toinduce additional spin injection, leading us to distinguish themagnetization direction without an independent electrode.

II. EXPERIMENTAL PROCEDURE

Figure 1 shows a scanning electron microscope image of alateral spin valve device used for the present study, together
with a schematic illustration. We fabricated the lateral spin
valve consisting of three ferromagentic CoFeAl (CFA) wires
bridged by a Cu strip. The device was fabricated by two-steplift-off processes. First, three CFA wires, 40 nm in thickness
and 120 nm in width, were deposited by electron-beam
evaporation on a thermally oxidized Si substrate under a basepressure of 2.0 × 10−7 Pa. Here, the center-to-center distances
for two neighboring CFA wires are 300 nm. In order to control
the relative configuration of the magnetizations for three CFA
wires by adjusting the magnetic field, the differences in the014416-1©2016 American Physical Society

HU, NOMURA, UEMATSU, ASAM, AND KIMURAPHYSICAL REVIEW B 94, 014416 (2016)

FIG. 1. Scanning electron microscope (SEM) image of thefabricated lateral spin valve consisting of three CoFeAl wires bridged
by a Cu strip, together with a schematic illustration of the device
geometry.

edge shapes for the CFA wires were introduced. Then, a Cu
strip (120 nm wide and 150 nm thick) bridging the CFA wires
was thermally evaporated. The electrical resistivities for Cu
and CFA are, respectively, 2.2 and 45.0 μ cm. Prior to Cu
deposition, the CFA surfaces were carefully cleaned by low
power Ar ion milling to obtain a highly transparent interface
with a few m interface resistance. The electrically andthermally excited spin transports were detected using a lowfrequency lock-in technique with first- and second-harmonic
voltage responses, respectively. Here, all the measurements
were performed in air at room temperature.
III. RESULTS AND DISCUSSIONSFirst, we evaluated the electrical spin-injection and detection property of the fabricated lateral spin valve device froma conventional nonlocal spin valve measurement. Figure 2(a)shows a room-temperature spin signal using the CFA1 injector
and CFA2 detector. A clear large spin valve signal with amagnitude of 4.2 m assures a high electrical spin polarization

FIG. 2. (a) Room-temperature first-harmonic spin valve signal
using a CFA1 injector and CFA2 detector. (b) First-harmonic spin
valve signal detected by CFA1 under spin injection from CFA2 to
CFA3. (c) Second-harmonic spin valve signal detected by CFA2 underthermal spin injection using CFA1. The right-hand sides for eachfigure correspond to the probe configuration for the measurements.

FIG. 3. Room-temperature first-harmonic spin valve signal for astandard CFA/Cu lateral spin valve (a) without the middle CFA wireand (b) with the middle CFA wire. In both devices, the center-to-center
distance between the injector and the detector is 600 nm.

for the CFA wires with ideal interface conditions. We also
confirmed that the interface for the CFA3/Cu is also in an
ideal condition from nonlocal detection with multiterminal
spin injection, where the spin signal detected by CFA1 is
modified by the relative magnetization configuration of CFA2
and CFA3 [35]. As shown in Fig. 2(b), a spin signal witha magnitude comparable to Fig. 2(a) and its modulation
depending on the relative magnetization configuration between
CFA2 and CFA3 were clearly confirmed. We also evaluatedthe thermal spin-injection property from the second-harmonic
voltage measurements. As shown in Fig. 2(c), a clear spin
valve signal has been observed at room temperature, assuringthat the spin-dependent Seebeck coefficient for our CFA wire
was sufficiently large, but the value is slightly smaller than ourprevious reports [30]. This is because the relatively thicker Cu
and CFA wires reduce the temperature gradient at the same
current. A small asymmetry with respect to the magnetic field
is due to the anomalous Nernst-Ettingshausen effect inducedin the CFA2 detector [19].
We then evaluated the spin absorption property for the CFA
wire. We fabricated a conventional CFA/Cu lateral spin valve
with a center-to-center distance of 600 nm and performed anonlocal spin valve measurement for comparison. As shownin Fig. 3(a), we obtained a clear bipolar spin signal with themagnitude of 2.56 m, which is a reasonable value compared
to previous reports. We then measured the room-temperature
spin signal with the middle absorber by using the device shownin Fig. 1. Because of the high electrical spin polarization for
CFA, we still clearly observed bipolar spin signals with amagnitude of 0.75 m even in the device with the middle CFA
absorber, as indicated in Fig. 3(b). Here, we quantitatively
evaluate the absorption efficiency. From the spin transportmodel with a transparent interface, the ratio of the spin signal
η can be analytically calculated as follows [31–34],η=

2[(1 + Q) cosh a + sinh a],(2 + 3Q) cosh a + (2 + Q + Q2 ) sinh a

(1)

S
S
where Q is defined as RCu/RCFA, which is the ratio ofSS
the spin resistance for Cu RCu to that for CFA RCFA
. a isdefined as l/2λCu with a center-to-center distance between the014416-2

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PHYSICAL REVIEW B 94, 014416 (2016)

by thermal spin injection from CFA2. Moreover, this result
indicates that one can distinguish the magnetization direction
without using direct spin injection and detection.
We quantitatively evaluate the magnitude of the thermal
spin signal induced by indirect spin injection from CFA2.
According to the one-dimensional spin diffusion model, thethermally induced spin voltage VS under the temperature
gradient ∇T of the CFA in the vicinity of the CFA/Cu interface,in a conventional CFA/Cu lateral spin valve with a separation
distance l, is given by the following equation,
FIG. 4. (a) Schematic illustration of the thermal spin injection anddetection in a lateral spin valve with the middle CFA wire, together
with the probe configuration for the measurement. (b) Heat flow underthermal spin injection in the triple CFA/Cu junction and induced spin
accumulation in the Cu strip. (c) Room-temperature second-harmonic
spin valve signal observed in the CFA/Cu lateral spin valve with the
middle CFA wire.

injector and detector l and the spin diffusion length for Cu.
By using the values obtained from the separately performed
S
S
experiments (λCu = 500 nm and PCFA = 0.65), RCu
and RCFA
can be calculated as 1.22 and 0.20 , respectively, leading
to Q = 6.0. As a result, we obtain η as 0.35, which is inreasonable agreement with the experimentally obtained value(0.29). This indicates that the interface condition between
CFA/Cu is highly transparent.
We evaluate the absorption property of the thermally excitedspin current by using the second-harmonic voltage detection
with the probe configuration shown in Fig. 4(a). Interestingly, asmall negative voltage change has been observed in addition to
the main spin valve signal changes due to injector and detector
switching. By comparing the field dependence of the signal
shown in Fig. 2(b), we conclude that the additional resistancechange around 70 mT was caused by the magnetization
switching of CFA2. According to the nonlocal detection
scheme, the detecting voltage depends on the relative angle
between the spin injector and detector. Therefore, it is not
so simple to explain the reason for the appearance of theadditional signal change due to the magnetization switching of
CFA2. To understand the reason for the additional signal, we
consider the heat flow in the present sample. Since the Cu has
a high thermal conductivity, the heat generated in the CFA3
easily propagates into the CFA2. This produces a temperature
gradient across the Cu/CFA2 junction, resulting in thermal spin
injection from CFA2 into the Cu. Therefore, as schematicallyshown in Fig. 4(b), in the heat-related second-harmonic signal
for detecting the heat-related signal, the thermal spin injections
both from CFA3 and CFA2 should be considered. Here, it
should be noted that the direction of the temperature gradientin the CFA2/Cu junction is in the opposite direction to that in
the CFA3/Cu junction. Therefore, the sign of spin signal due
to CFA2 switching should be reversed to that of CFA3. As
shown in Fig. 4(c), the spin signal due to CFA2 switching was
negative, which was indeed opposite to that of CFA3 [17,30].
Moreover, we confirmed that the additional voltage changeshows parabolic dependences on the magnitude of the ac
current as well as the main two voltage changes. Thus, theadditional spin signal observed in Fig. 4(b) can be understood

VS =

S
∇T
QPCFA λCFA SCFA,
2(1 + Q)(cosh x + sinh x) + Q2 sinh x(2)

where x is the normalized separation distance defined by l/λCu .
S
PCFS and SCFA
are the electrical spin polarization and the spindependent Seebeck coefficient for CFA, respectively. Since
the diffusion process of the spin current does not depend on
the excitation method, the spin voltage with the middle CFAVSAbs under thermal spin injection can be calculated as ηVwith a similar consideration of the absorption efficiency η
discussed in the electrical scheme. Moreover, the spin voltage
indirectly induced by the CFA2 VSInd can be approximately
calculated by assuming the temperature gradient of the CFA2
in the vicinity of the CFA2/Cu interface ∇T  with a normalized
distance x/2. Therefore, the ratio of the spin voltage due tothe CFA3 to that of CFA2 ξ can be approximately given by thefollowing equation,
ξ=≈VSIndVSAbs2(1+Q)(cosh x + sinh x)+Q2 sinh x∇T .
η∇T 2(1 + Q)[cosh(x/2) + sinh(x/2)] + Q2 sinh(x/2)(3)

Since ξ can be estimated experimentally to be 0.13 fromthe results shown in Fig. 4(a), we obtain the relation∇T  = 0.06∇T .
To confirm the reliability of the above relationship, we
have numerically calculated the spatial distributions of thecurrent and the temperature of the present CFA/Cu lateral

FIG. 5. Numerically simulated temperature distribution of thelateral spin valve under a bias current of 0.63 mA at ambient
temperature. (a) Top view of the temperature profile in the x-y plane.
(b) Cross-sectional view of the temperature profile in the x-z plane.
Line profiles of the temperature distributions along the z axis (c) at
the center of the CFA1/Cu junction and (d) that at the center of the
CFA2/Cu junction.

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HU, NOMURA, UEMATSU, ASAM, AND KIMURAPHYSICAL REVIEW B 94, 014416 (2016)

FIG. 6. (a) SEM image of the fabricated lateral spin valve with aferromagnetic nanodot embedded in a Cu wire. (b) Second-harmonic
voltage as a function of the magnetic field, which is swept fromthe negative to the positive direction. The inset shows a schematic
illustration of the probe configuration for the measurement.

spin valve under a bias current Iac = 0.63 mA in CFA3
by using the finite element program COMSOL MULTIPHYSICS.
Here, we assume that the thermal conductivities for Cu,
CFA, Si, and SiO2 are 400, 29.8, 130, and 1.4 W m−1 K−1 ,respectively. Figures 5(a) and 5(b) show the three-dimensional
color plots of the simulated temperature for the lateral andvertical profiles, respectively. CFA and Cu wires are aligned
along x and y, respectively. z is the direction normal to thejunction. From the results, we confirmed that the heat flow
effectively propagated in the Cu wire and reached the CFA2.
As can be seen in Figs. 5(c) and 5(d), the temperature gradientalong the z direction at the CFA2/Cu junction is found tobe approximately 5% of that at the CFA3/Cu junction, whichshows good agreement with the experimental estimation. Thus,
the additional signal observed in Fig. 4(c) is quantitatively
explained by indirect spin injection from CFA2.
Finally, we demonstrate that the magnetization direction ofa small ferromagnetic nanodot embedded in a Cu wire canbe detected by using the present indirect detection method.
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study. Here, the CFA nanodot was placed underneath theCu wire between the CFA injector and CFA detector. It is
impossible to detect the magnetization direction by using theconventional electrical detection scheme. However, as can beseen in Fig. 6(b), by using the second-harmonic detection
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IV. CONCLUSION

In summary, we have investigated the electrically andthermally excited spin current transports in a CoFeAl/Cu
lateral spin valve with a middle CoFeAl wire. In conventional
nonlocal spin detection, the spin signal expectedly showsa significant reduction due to the spin current absorption
effect. On the other hand, in the second-harmonic detection,
an unexpected additional signal change depending on themagnetization direction of the middle wire has been observedin addition to the conventional thermal spin signal. We foundthat this additional signal is quantitatively explained by indirect
thermal spin injection from the middle wire due to the heat flow
through the Cu strip. By using this scheme, we have succeeded
in detecting the magnetization direction of the ferromagnetic
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ACKNOWLEDGMENT

This work is partially supported by Grant-in-Aid for
Scientific Research on Innovative Area, “Nano Spin Conversion Science” (26103002) and that for Scientific Research(S)(25220605) from Ministry of Education, Culture, Sports,
Science and Technology in Japan.

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