RAPID COMMUNICATIONSPHYSICAL REVIEW B 95, 100403(R) (2017)

Nonreciprocity of electrically excited thermal spin signals in CoFeAl-Cu-Py lateral spin valves
Shaojie Hu,1,2,3,* Xiaomin Cui,4 Tatsuya Nomura,5 Tai Min,1,2 and Takashi Kimura3,5,†
1

Center of Spintronics and Quantum System, Xi’an Jiaotong University, No. 28 Xianning West Road Xi’an,
Shaanxi 710049, People’s Republic of China
2
School of Materials Science and Engineering, Xi’an Jiaotong University, No. 28 Xianning West Road Xi’an,
Shaanxi 710049, People’s Republic of China
3
Research Center for Quantum Nano-Spin Sciences, Kyushu University, 744 Motooka, Fukuoka 812-8581, Japan
4
Shaanxi Key Laboratory of Condensed Matter Structures and Properties, School of Science, Northwestern Polytechnical University,
Xi’an 710072, People’s Republic of China
5
Department of Physics, Kyushu University, 744 Motooka, Fukuoka 812-8581, Japan(Received 26 September 2016; revised manuscript received 18 February 2017; published 20 March 2017)
Electrical and thermal spin currents excited by an electric current have been systematically investigated inlateral spin valves consisting of CoFeAl and Ni80 Fe20 (Py) wires bridged by a Cu strip. In the electrical spinsignal, the reciprocity between the current and voltage probes was clearly confirmed. However, a significant
nonreciprocity was observed in the thermal spin signal. This provides clear evidence that a large spin-dependent
Seebeck coefficient is more important than the spin polarization for efficient thermal spin injection and detection.
We demonstrate that the spin-dependent Seebeck coefficient can be simply evaluated from the thermal spinsignals for two configurations. Our experimental description paves a way for evaluating a small spin-dependent
Seebeck coefficient for conventional ferromagnets without using complicated parameters.
DOI: 10.1103/PhysRevB.95.100403

The efficient generation and detection of the spin current isa crucial issue for the application of spintronic devices [1–3].
In addition, the integration of spintronic circuits with conventional semiconductor devices is another important milestone
for the practical application of spin devices. Recently, theinterplay between the heat and spin has attracted great attention [4–8], while the temperature gradient is found to excite
spin current injection from the ferromagnet to nonferromagnet
in various bilayer systems based on different mechanisms,
such as the so-called spin Seebeck effect in ferromagnetic
metals and insulators [9–12], the spin-dependent Seebeck
effect in metallic junctions [13–18], the Seebeck spin tunnel
effect [19–22], the magnon-driven effect [23,24], and spinheat accumulation [25–27]. Since the heat can be generated
by microwave absorption in an open circuit without an electric
current [28–31], this approach using heat may open an avenue
for simplifying the integration of spin devices.
In metallic ferromagnetic/nonmagnetic (F/N) hybrid structures, the spin-dependent Seebeck effect is one paramountmechanism for thermal spin injection [13,32]. The generation
efficiency of thermally excited spin current is dominated by
the spin-dependent Seebeck coefficient SS = S↑ − S↓ , whichis the difference in a Seebeck coefficient for the up-spin anddown-spin electrons in the ferromagnet. Therefore, the determination of SS is indispensable to quantify thermally excitedspin current as well as the optimization of the thermospin
properties of ferromagnets [33]. However, the quantitative
estimation of SS is still a difficult problem to solve for aconventional ferromagnets because the tiny spin asymmetry of
the Seebeck coefficient results in a poor generation efficiencyof the spin current. Moreover, the spin-related signals are
smeared out by spurious signals induced by the intrinsic ther-*†

shaojiehu@mail.xjtu.edu.cn
t-kimu@phys.kyushu-u.ac.jp2469-9950/2017/95(10)/100403(5)moelectric effects in the ferromagnet, such as the anomalous
Nernst effect and the anisotropic magneto-Seebeck effect,
especially at room temperature [34–38]. The exploration of
appropriate device structures and excellent materials could be
an approach to assist the estimation of SS of conventional
ferromagnets.
Recently, demonstrations of the spin-dependent Seebeck
effect have been reported by extending the nonlocal spin
valve measurement in a lateral spin valve structure combinedwith the second harmonic detection technique. This approach
not only evaluates the spin transport properties but also
extracts the thermospin effect from the spurious thermoelectriceffect [13,17]. Additionally, we demonstrate that CoFeAl
has excellent thermal spin injection efficiency because of
its favorable band structures [16,39]. However, the large
thermal spin signal is not only contributed by the excellent
spin-dependent Seebeck coefficient but also by the large
spin polarization of CoFeAl. It is more effective to show
the large spin-dependent Seebeck coefficient of CoFeAl bya persuasive experiment. In this Rapid Communication, by
utilizing the excellent electrical and thermal spin injection
properties of CoFeAl, we show that the thermospin property inconventional ferromagnetic metals with small spin-dependent
Seebeck coefficients can be precisely evaluated.
A lateral spin valve (LSV) consisting of CoFeAl (CFA)
and NiFe (Py) wires bridged by a Cu strip was fabricated on a
SiO2 /Si substrate by multistep electron-beam lithography andlift-off techniques. Figure 1(a) shows the scanning electron
microscopy (SEM) image and the schematic illustration of
the present lateral spin valve, where the center-center distance
between the two ferromagnetic wires L is 350 nm. Py and
CoFeAl were deposited by an electron-beam evaporator underan ultrahigh vacuum of 1.5×10−7 Pa. The thicknesses of thetwo ferromagnetic wires are 30 nm. Here, the CoFeAl wire wasconnected to the large contact pads at the wire end, although
the NiFe wire has flat ends. This leads to the difference in the100403-1©2017 American Physical Society

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HU, CUI, NOMURA, MIN, AND KIMURAPHYSICAL REVIEW B 95, 100403(R) (2017)

FIG. 1. (a) SEM image and the schematic illustration of thepresent lateral spin valve. (b) Measurement configuration for theelectrically and thermally driven spin injections. In electrically driven
spin injection, when the bias current flows through the F/N junction,
the generated spin current is detected by measuring the first harmonic
responding voltage. In thermally driven spin injection, when the bias
current flows through the ferromagnetic wire without passing throughthe junction, the spin current generated by the temperature gradient
due to Joule heating is detected by measuring the second harmonic
responding voltage.

switching field, which enables us to control the magnetization
configuration [parallel (P) and antiparallel (AP) alignments]
by sweeping the magnetic field. The 160-nm-thick Cu channel
was deposited by thermal evaporation under a base pressureof 10−6 Pa. Before the deposition of Cu, both the surfaces of
the CoFeAl and Py wires were well cleaned by an Ar+ ion
beam with a low acceleration voltage. The lateral dimensions
for the CoFeAl/Cu and Py/Cu junctions are 120×100 nm2 .
The interface transparency was confirmed by a four-terminal
contact-resistance measurement.
As shown in Fig. 1(b), for the detection of the electricallydriven spin current, a small ac bias current [I0 sin(ωt)] wasapplied across the F/N junction. Since the influence of theJoule heating effect is negligibly small because of the low
bias current density, the electrical spin current shows the firstharmonic frequency response. On the other hand, when thebias current in the F wire is very large, local heating will
be produced in the two segments of the F wire, as shownin Fig. 1(b). In this situation, owing to the high thermal
conductivity of the Cu strip, which is much larger than thatof the substrate, most of the produced heat current flows into
the Cu strip across the F/N junction, resulting in thermal
spin injection. Since the thermally excited spin current due
to the electric current is proportional to the current square

RSele =FIG. 2. Electrically driven nonlocal spin valve curves for thedevice with 350 nm interval distance together with the measurement
probe configuration. (a) Bias current flows through the Py/Cu
junction, and the electrically driven spin current will be detectedby the first harmonic voltage with sweeping magnetic field at the
CoFeAl/Cu(CFA/Cu) junction. The spin signal RSele is defined as
the normalized value of the voltage difference between the parallel
and antiparallel by the injection current. (b) The CoFeAl/Cu junction
works as the spin injector and the Py/Cu junction works as thespin current detector. The dashed and solid arrows correspond to
the directions of the sweeping field.

[I0 sin(ωt)2 ], we can extract the thermal contribution in theelectrical signal by detecting the second harmonic voltage.
First, the electrically excited spin injection and detection
properties are evaluated by conventional nonlocal spin valve
measurements with the probe configuration shown in the inset
of Fig. 2(a). We measured the nonlocal spin signal (RS )which is the voltage change between the P and AP statesnormalized by the bias current. Here, we evaluate RS for
two configurations, as schematically shown in the insets ofFigs. 2(a) and 2(b). One is the CoFeAl injector and Py detector(configuration A). The other one is the Py injector and the
CoFeAl detector (configuration B). Figures 2(a) and 2(b) show
the field dependences of the nonlocal spin valve curves forconfigurations A and B, respectively. The spin signals (RSele )for configuration A are 3.53 ± 0.02 m, which is almost thesame as that for configuration B. We additionally studied theother three LSVs consisting of CoFeAl and Py wires withdifferent intervals L and confirmed a similar relationship.
These probe configuration dependences can be understoodas follows. According to the one-dimensional spin diffusion
model [40,41], the electrical spin signal in the lateral spin valve
consisting of two different kinds of ferromagnetic wires could
be expressed by the following equation [42–44],

PI PD RFI RFD RN,[2RFI RFD + RN (RFI + RFD )][cosh(L/λN ) + sinh(L/λN )] + RN2 sinh(L/λN )where PI and PD are the spin polarizations for the injector anddetector. λN is the spin diffusion length for the nonferromagnet.
RFI , RFD , and RN are the spin resistances for the injector,
detector and nonferromagnet, respectively. The spin resistanceis defined as 2ρλ/[A(1 − P 2 )], where P , ρ, and λ are the spinpolarization, resistivity, and spin diffusion length, respectively.
For a nonmagnet, P is zero and A is given by the area of the(1)

effective cross section for the spin current. For a ferromagnet,
since the spin diffusion length is known to be quite short, theeffective cross section should be given by the area of the F/Njunction.
Figure 3 shows the spin signals for the CFA-Cu-Py LSVs
as a function of interval distance L. For comparisons, the spinsignals for the Py/Cu/Py and CFA/Cu/CFA were also plotted

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PHYSICAL REVIEW B 95, 100403(R) (2017)

FIG. 3. Interval distance dependence of the electrical spin signals
in different lateral spin valves. CFA-Cu-Py and Py-Cu-CFA correspond to the reversed injector and detector for the same lateral spin
valve containing two kinds of ferromagnets CFA and Py.

in the same graph. Here, it should be noted that all results
are well fitted by Eq. (1) with the parameters of PCFA =0.62, PPy = 0.36, λCFA = λPy = 2 nm, and λCu = 450 nm
at room temperature. This indicates the validity of the onedimensional spin diffusion model and high reproducibilityof our lateral spin valves. Using Eq. (1) and the data on
the CFA/Cu/Py sample in both configurations A and B,
we obtained almost the same value of the spin signal,
which indicates a reciprocal relationship between the spin
injector and detector in the nonlocal spin valve signal. Slightdifferences between the two configurations observed in theexperiments may be caused by extrinsic contributions, suchas an inhomogeneous current distribution and higher-ordereffects [45–48].
We then evaluated the thermally excited spin current in the
CFA/Cu/Py LSVs. Figures 4(a) and 4(b) show the nonlocal
spin valve signals under thermal spin injection with the probe
configurations shown in the insets of each figure. Here, thecurrent flows only in the ferromagnetic wire and did not
flow across the interface between F and N, meaning anelectrically excited spin current does not exist in the Cu wire. In
Fig. 4(a), we can see a quite unconventional feature in the fielddependence of the nonlocal signal. The field dependence ofthe voltage change is asymmetric with respect to H = 0. Thisasymmetric feature can be explained by the anomalous Nernst
effect in the ferromagnetic voltage probe [38,49,50]. Owing

VSth =

FIG. 4. Nonlocal spin curve for thermally driven spin injection
together with the schematic illustration of the measurement probe
configurations. The two solid arrows represent the direction of themagnetization for the two ferromagnets. (a) Thermally induced
nonlocal signals with the Py wire as an injector and the CoFeAl
wire as a detector. We refer to this configuration as “configuration A.”
Here, the bias current in the Py wire is 0.71 mA. (a) Thermally induced
nonlocal signals with the CoFeAl wire as an injector and the Py wire
as a detector. We refer to this configuration as “configuration B.”
Here, the bias current is 0.60 mA.

to the relatively large thermal conductivity in the Cu wire, theinjected heat current diffuses in the Cu wire isotropically andreaches the detecting junction. A finite heat current diffuses
into the CFA detector across the CFA/Cu junction. This
induces a measurable anomalous Nernst voltage in between
the CFA and Cu voltage probes. The field dependence ofthe voltage change due to the Nernst effect should show odd
functions, although the spin signal shows an even function.
Therefore, we can clearly separate the thermally excitedspin signal and extrinsic Nernst effect. The obtained thermal
spin signal RSth is approximately 0.04 μV/mA2 using the
Py injector and CoFeAl detector shown in Fig. 4(a). On
the other hand, as can be seen in Fig. 4(b), we observea clearer spin signal with a magnitude of 0.34 μV/mA2
with a relatively small asymmetric field dependence. Thisis because the CoFeAl wire has a large spin-dependent
Seebeck coefficient and the flat-end shape of the Py wire
suppressed the heat propagation from the Cu wire [38]. Thus,in thermal spin signals, nonreciprocal relations were clearlyobserved.
To understand these unique characteristics observed in thethermal spin signals, we analyze the experimental results morequantitatively. By adapting the one-dimensional spin diffusion
model in a conventional lateral spin valve structure, we canderive the thermally driven spin voltage as follows [13,16],

PD RFD RN λFI SS ∇TI,[2RFI RFD + RN (RFI + RFD )][cosh(L/λN ) + sinh(L/λN )] + RN2 sinh(L/λN )where ∇TI is the temperature gradient around the interfaceof the ferromagnetic injector. Most of the parameters except
for SS have been obtained from the nonlocal electrical spin
injection experiments describe above. Regarding ∇TI , the heat
power due to Joule heating is proportional to the current square,
and we can assume that ∇TI = γρI 2 . Here, γ is a constant(2)

parameter which mainly depends on the thermal conductivityof the ferromagnet and can be estimated from the numericalcalculation based on COMSOL [15,16,33].
We performed similar measurements for the other three
LSVs with different L and obtained a similar nonreciprocal
relationship. Figure 5 shows the thermally excited spin signal

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HU, CUI, NOMURA, MIN, AND KIMURAPHYSICAL REVIEW B 95, 100403(R) (2017)

FIG. 5. Interval distance dependence of the thermal spin signals
for various types of lateral spin valves.

as a function of the interval distance L for various types oflateral spin valves. It should be noted that all results are well
reproduced by Eq. (2) with the same material parameters.
Especially, the spin diffusion lengths for the Cu in all LSVs are
the same value of 450 nm as that in Fig. 3 even under thermal
spin injection. This indicates that the temperature change of
the Cu channel is negligibly small because of its large heat
capacity. Since the physical parameters in Eq. (2) do not change
when interchanging the probe configuration, we can simplify
Eq. (2) by introducing the ratio η of the thermally excited spinsignals between two configurations as follows,


Py
th
2
RSPy
SS γPy PCFA 1 − PCFA

 .
η=
= CFA
(3)
th
2
RSCFA
SS γCFA PPy 1 − PPyfor Py, 29.8 W m−1 K−1 for CFA, 400 W m−1 K−1 for Cu,
1.3 W m−1 K−1 for SiO2 , and 1.4 W m−1 K−1 for Si, we
obtained γCFA /γPy = 1.1. Moreover, the final part in Eq. (3)corresponding to the ratio of P 2 /(1 − P 2 ) is 2.4. Therefore,
we obtain SPy /SCFA as 0.043. From these values, we canestimate the spin-dependent Seebeck coefficient for Py as−2.86 μV/K, and this value is in the same range with previous
reports [33]. However, it is 25 times smaller than the value
for CFA, providing clear evidence that the large thermal
spin signals in the CFA/Cu/CFA LSVs originate from thelarge thermal spin-dependent Seebeck coefficient for the CFA.
Moreover, it also indicates that we can estimate SS , which is
the key experimental factor for thermal spin injection, without
using complicated experimental parameters.
In summary, we have investigated the transport propertiesof the electrically and thermally excited spin currents in
CFA-Cu-Py lateral spin valves. Although the electrical spinsignal showed a clear reciprocal relationship between thecurrent and voltage probes, a significant nonreciprocity wasobserved in the thermal spin signals. We showed that bothresults are quantitatively understood by the one-dimensional
spin diffusion model. These results provide clear evidence thata large thermal spin signal was mainly contributed by the large
spin-dependent Seebeck coefficient of the CFA. In addition,
we demonstrated that the spin-dependent Seebeck coefficient
can be simply evaluated from the ratio of thermal spinsignals between two configurations. This approach provides
a quantitative estimation of a small spin-dependent Seebeckcoefficient without using complex experimental parameters.

Here, most of the experimental parameters were eliminatedand the remaining parameters are only P , SS , and γ .
By assuming a thermal conductivity of 46.4 W m−1 K−1This work is partially supported by a Grant-in-Aid for
Scientific Research on Innovative Area, “Nano Spin Conversion Science” (26103002) and that for Scientific Research(S)(25220605). This work is also partially supported
by National Natural Science Foundation for Distinguished
Young Scholar of China (Grant No.51601139) and National
Key Research Program of China (2016YFA0300702).

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