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Effective suppression of thermoelectric voltage innonlocal spin-valve measurement
To cite this article: Taisei Ariki et al 2017 Appl. Phys. Express 10 063004

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Applied Physics Express 10, 063004 (2017)https://doi.org/10.7567/APEX.10.063004

Effective suppression of thermoelectric voltage in nonlocal spin-valve measurement
Taisei Ariki1, Tatsuya Nomura1, Kohei Ohnishi1,2, and Takashi Kimura1,2*
Department of Physics, Kyushu University, Fukuoka 819-0395, Japan
Research Center for Quantum Nano-Spin Sciences, Kyushu University, Fukuoka 819-0395, Japan12*E-mail: t-kimu@phys.kyushu-u.ac.jp
Received April 13, 2017; accepted April 27, 2017; published online May 18, 2017
We demonstrate that the background signal in the nonlocal spin-valve measurement can be sufﬁciently suppressed by optimizing the electrodedesign of the lateral spin valve. A relatively long length scale of heat propagation produces spin-independent thermoelectric signals under thecombination of the Peltier and Seebeck effects. These unfavorable signals can be reduced by mixing the Peltier effects in two transparent
ferromagnetic/nonmagnetic junctions. Proper understanding of the contribution from the heat current in no spin-current area is a key for effective
reduction of the spin-independent background signal. © 2017 The Japan Society of Applied Physics

pintronics is a research ﬁeld for understanding andutilizing quantum properties of electron spin, which
is one of the exciting subjects in condensed matterphysics.1–5) Recent developments in nanofabrication techniques enable us to realize laterally conﬁgured ferromagnetic=nonmagnetic metal hybrid nanostructures. In such
systems, various intriguing phenomena can be induced inassociation with spin-dependent transport. However, owing
to a small magnitude of spin-related signal, it is very hard toevaluate spin transport precisely. The nonlocal spin-injection
technique is a powerful mean for detecting small spin-related
signals because current-induced spurious signals can besuppressed signiﬁcantly.6–10) However, even in the nonlocal
conﬁguration, the detecting voltage includes large spinindependent background signals, which are serious obstacles
for the reliable operation of spin-based memory and logiccircuits.
There are several models for explaining the origin of spinindependent background signal in the nonlocal conﬁguration.
One is the current-spreading eﬀect in the vicinity of theinjecting junction, as discussed by Johnson and Silsbee.11)
This eﬀect becomes important when the linewidth of themetallic wire is larger than the interval and=or the ﬁlm
thickness. Spin-dependent scattering at the detecting junction
has been considered as one of the possible reasons for the
background voltage with the diﬀerent mechanism.12) However, this mechanism is diﬃcult to explain the experimental
results quantitatively and systematically. The most important
contribution for the nonlocal background voltage is believed
to be the interplay between the Peltier and Seebeck eﬀects.13)
The spin injection across the ferromagnetic=nonmagnetic
junction produces the Peltier heating=cooling eﬀect, resulting
in heat propagation through the nonmagnetic strip. This
heat current produces the measurable electrical voltage
because of the Seebeck eﬀect in the detecting junction witha relatively large Seebeck coeﬃcient for a ferromagneticmetal. Although this mechanism provides the quantitativeexplanation, nobody focuses on the suppression of the background voltage. In particular, in nanosized electronic devices,suppression and=or reduction of the background signals areimportant for the reliable operation of spin-based devices.14)
Here, we demonstrated that the background voltage can beminimized by preparing an additional injecting junction. In
addition, we showed that the outside terminal of the lateralspin valve contributes to the nonlocal background signal
signiﬁcantly.

SFig. 1. SEM image for the fabricated lateral spin valve consisting of triple
ferromagnetic Py wires, Py1, Py2, and Py3 with a nonmagnetic Cu strip
together with a schematic illustration of the fabricated lateral spin valve.
Here, the location of the Cu strip is shifted to the upper side from the centerof the Py wires.

A lateral spin valve consisting of three ferromagneticwires bridged by a Cu strip has been fabricated on a SiO2=Si
substrate by the two-step lift-oﬀ process. First, the ferromagnetic permalloy (Py) wires were fabricated by using
electron-beam lithography and an electron-beam evaporator.
The base pressure during Py evaporation was 10−8 Pa. The
thickness of the Py ﬁlm was 20 nm. Then, we deposited a
200-nm-thick Cu ﬁlm by using a Joule evaporator in order tomake a strip bridging the ferromagnetic Py wires. Here, thewidths of the Py wires and the Cu strip are ﬁxed to be 80 nm.
The electrical resistivities of the Py and Cu wires are 35 and2.8 µΩ cm at room temperature, respectively. The interfaces
between Py and Cu were well cleaned by low-voltage ion
milling, resulting in highly transparent interfaces. Figure 1
shows the schematic illustration of the fabricated lateral spin
valve together with the scanning electron microscopy (SEM)
image. Here, the three ferromagnetic wires have diﬀerent end
shapes, resulting in diﬀerent coercive ﬁelds.9) We emphasizethat the Cu strip is slightly shifted to the upper side from thecenter of the Py wires. The spin and heat transports wereevaluated by the standard current-bias lock-in detection
technique with the ﬁrst-harmonic voltage response under an
excitation current of 100 µA. In this measurement, we canneglect the inﬂuence of the thermal spin injection because the063004-1© 2017 The Japan Society of Applied Physics

Appl. Phys. Express 10, 063004 (2017)

T. Ariki et al.

Fig. 2. (a) Schematic illustration of the probe conﬁguration of theconventional nonlocal spin-valve measurement using Py2 injector and Py1
detector. (b) Field dependence of the nonlocal spin valve signal measured atroom temperature.

thermal spin injection is mainly induced by the Joule heatingeﬀect, which shows the second-harmonic response. Moreover, we emphasize that the thermal spin injection eﬃciency
under an excitation current of 100 µA should be very low
because of a small spin-dependent coeﬃcient of Py.15)First, we performed a conventional nonlocal spin-valve
measurement with the probe conﬁguration shown in Fig. 2(a).
We observed a clear spin-valve signal with a magnitude of0.95 mΩ, as shown in Fig. 2(b). The obtained signal is slightly
larger than typical values reported in Py=Cu lateral spin valvesat room temperature.7,9,10) This is because the junction size ofour Py=Cu is smaller than those in previous devices. We also
see a large background signal with a magnitude of 16.1 mΩ.
By using the Seebeck coeﬃcients for Py, SPy = −20 µV=K,and Cu, SCu = 1.6 µV=K, the background signal can be understood by a small temperature change of 70 mK due to thePeltier eﬀect.13,14) Thus, a relatively large Seebeck coeﬃcientfor Py produces non-negligible nonlocal voltage, which ismuch larger than the nonlocal spin-valve signal at room
temperature.
We then changed the current-probe conﬁguration as shownin Fig. 3(a) and again measured the nonlocal spin-valve
signal. The current ﬂows across the two interfaces comprising
the Py2=Cu (J2) and Cu=Py3 (J3) junctions. These twojunctions produce the Peltier heating and cooling eﬀects
depending on the current direction. In this case, owing to alarge heat conductivity for the Cu strip, the heat current
generated from J2 is mostly absorbed into J3. Therefore, theheat current propagating to the voltage junction, Py1=Cu (J1)junction, should decrease suﬃciently, resulting in a signiﬁcant reduction in the Seebeck voltage. As can be seen in
Fig. 3(b), a small background signal occurs if the Peltiereﬀects in the two junctions are canceled.
Here, we carefully consider the ﬁeld dependence of thenonlocal spin-valve signal shown in Fig. 3(b). We observed
three abrupt changes consisting of −0.75, +0.98, and −0.23mΩ both in the forward and backward ﬁeld sweeps of thenonlocal signal. These three sudden changes correspond tomagnetization switching of the three ferromagnetic wires.
From the shapes of the wire ends, we know that the ﬁrst,
second, and third sudden changes correspond to the switching of the spin voltage detector (Py1), middle spin injector(Py2), and right-hand-side spin injector (Py3), respectively.
As shown in Fig. 3(c), when Py2 and Py3 are aligned antiparallel to each other, spin-splitting of the chemical potential

Fig. 3. (a) Schematic illustration of the probe conﬁguration for thenonlocal spin-valve measurement under the spin injection with the twointerfaces comprising of Py2=Cu and Cu=Py3. (b) Field dependence of thenonlocal signal measured at room temperature. (c) Schematic illustrations of
the induced spin-dependent chemical potential when the magnetizations for
Py2 and Py3 are aligned anti-parallel and (d) that for the parallel alignment.
J1, J2, and J3 represent the Py1=Cu, Py2=Cu, and Py3=Cu junctions,respectively.

is relatively large because of the eﬀective spin accumulation.
On the other hand, as shown in Fig. 3(d), when Py2 andPy3 are aligned parallel to each other, spin-splitting of
the chemical potential becomes small. The detected voltage
is given by the voltage diﬀerence between the chemical
potential for Py1 and that for Cu. Thus, the change of thespin-valve signal can be simply understood by spin splittingof the nonequilibrium chemical potential in the Cu strip basedon the one-dimensional spin-diﬀusion model with a transparent interface.9,16)
From the above consideration, we understand that thebackground resistance is given by the mean value between
the resistance at state B and that at state D. The estimatedbackground resistance is 0.07 mΩ, which is much smaller
than the spin-valve signal. Thus, two Peltier eﬀects with different polarities signiﬁcantly reduce the heat current propagating to the voltage terminals (J1 junction). An observed
small ﬁnite background voltage indicates that the heat current
due to the Peltier eﬀect generated in the J3 junction partiallydiﬀuses into the substrate during the propagation in the Cu
strip between Py2 and Py3.17)
Then, we measure the nonlocal spin-valve signal forthe half conﬁguration, where the current and voltage probes
are located at the same side, as schematically shown in
Fig. 4(a).9) Figure 4(b) shows the nonlocal spin signal forthe half conﬁguration. The obtained spin-valve signal is also
0.97 mΩ, the same as that in Fig. 2(b). Thus, we can conﬁrm
that the diﬀerence of the spin signal between the two
conﬁgurations (the half and cross conﬁgurations) is negligibly small. This is because the diﬀerence in the eﬀective
distance between the two conﬁgurations is much smaller than
the spin diﬀusion length for the Cu wire, which is 500 nm atroom temperature.18) On the other hand, we obtain a small
diﬀerence of the background signal between the two conﬁgurations. This diﬀerence can be explained by considering
the asymmetric geometry of the lateral spin valve withrespect to the Cu strip as follows. As shown in Fig. 1, the063004-2© 2017 The Japan Society of Applied Physics

Appl. Phys. Express 10, 063004 (2017)

T. Ariki et al.

Fig. 4. (a) Schematic illustration of the probe conﬁguration for thenonlocal spin-valve measurement with the half conﬁguration, where thecurrent and voltage probes are located at the same side and (b) the nonlocal
spin-valve signal observed at room temperature. (c) Schematic illustration of
the probe conﬁguration for the Peltier canceling eﬀect using the Py2=Cu and
Cu=Py1 junctions and (d) the nonlocal spin-valve signal observed at room
temperature.

length of the distance between the Cu strip and the Cu contact
for the upper-side Py wire is 200 nm, which is shorter thanthat for the lower side (500 nm). Therefore, the temperature
diﬀerence between the Cu strip and the Cu contact for theupper side is smaller than that for the lower side, resulting
in a smaller Seebeck voltage. Thus, the geometry of thevoltage terminal including the outside of the junction is also
important for the quantitative understanding of the background signal.
Finally, we examine a similar Peltier canceling eﬀect byinterchanging the current and voltage probes. Figure 4(d)shows the nonlocal spin-valve signal with the probe conﬁguration shown in Fig. 4(c). The aspect of the ﬁeld dependence is diﬀerent from that in Fig. 3(b). This is because theroles of Py1 and Py3 are diﬀerent from those in Fig. 3.
We emphasize that the observed three resistance changes
consisting of 0.23, 0.75, and 0.98 mΩ are completely thesame as those in Fig. 3(b). Therefore, each resistance change
due to the switching of the Py wires can be understood by thesimilar one-dimensional spin diﬀusion model.15)
According to a simple one-dimensional spin-heat diﬀusion
model,13,14,19) we should observe the same ﬁeld dependenceof the nonlocal voltage as that in Fig. 3(b) because eachinterface produces the reversible contribution both for thespin and heat currents. Indeed, the obtained spin signal iscompletely the same as that in Fig. 3(b), as we expected.
However, the magnitude of the background signal is muchlarger than that in the previous conﬁguration, indicating thenonreversible relationship.
To understand this discrepancy, we reconsider the wholedevice structure of the lateral spin valve including the outsideof the spin-transport area. If we focus on the current path in
this conﬁguration, we ﬁnd another Py=Cu interface on thelarge Py pad. This extra Py=Cu interface is located in thevicinity of the Py1=Cu injecting junction (J1) and induces
an additional Peltier eﬀect.13) Therefore, a similar Peltier
canceling eﬀect occurs via the Py wire. As a result, the main
Peltier heat current generated in J2 does not reduce
considerably. It also should be noted that the backgroundsignal in this conﬁguration is larger than that in Fig. 2(b)although the probe conﬁguration in Fig. 2(a) does not include
any negative Peltier eﬀect. This can be explained byconsidering the temperature proﬁle in the ferromagnetic
voltage probe. As discussed above, the Seebeck voltage
depends on the length of the ferromagnetic voltage probe
because the heat current propagates over one micron even in
the Cu and Py wires. This means that the background signal
in the previous nonlocal spin-valve measurement with theprobe conﬁguration shown in Fig. 3(a) is reduced not only bycanceling the Peltier eﬀects, but also by the suppression ofthe Seebeck voltage. Thus, the present probe conﬁgurationinduces the actual Seebeck voltage. These extra eﬀects fromthe outside of the spin-transport area only aﬀect the heattransport. The spin-current distribution is not modiﬁed bythese outside structures because of its extremely short spindiﬀusion length for a ferromagnetic metal.
In summary, we conﬁrmed that the background signal in thenonlocal spin valve measurement was induced by the thermoelectric voltage under the combination between the Peltier andSeebeck eﬀects. We found that the background signal waseﬀectively suppressed by combining the Pelteir eﬀects of two
Py=Cu interfaces. The thermoelectric eﬀect from the outsideof the spin transport regime is found to produce a nonnegligible eﬀect in the nonlocal background signal.
Acknowledgment This work is partially supported by a 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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