Thermo-electric effect in a nano-sized crossed Permalloy/Cu junction under high bias
current
Congpu Mu, Shaojie Hu, Jianbo Wang, and Takashi Kimura
Citation: Applied Physics Letters 103, 132408 (2013); doi: 10.1063/1.4822330
View online: http://dx.doi.org/10.1063/1.4822330
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/13?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Characterization of spin pumping effect in Permalloy/Cu/Pt microfabricated lateral devices
J. Appl. Phys. 115, 17C505 (2014); 10.1063/1.4861681
Current-induced magnetization switching in permalloy-based nanopillars with Cu, Ag, and Au
J. Appl. Phys. 97, 10C706 (2005); 10.1063/1.1851882
Spin-polarized current-driven switching in permalloy nanostructures
J. Appl. Phys. 97, 10E302 (2005); 10.1063/1.1847292
Perpendicular giant magnetoresistance in Co/Cu and permalloy/Cu multilayered nanowires (abstract)
J. Appl. Phys. 81, 4569 (1997); 10.1063/1.365430
Magnetic viscosity effects in the giant magnetoresistance of NiO/Permalloy/Cu/Permalloy exchange-biased films
J. Appl. Phys. 81, 5218 (1997); 10.1063/1.364472

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Mon, 22 Aug 201610:08:39

APPLIED PHYSICS LETTERS 103, 132408 (2013)

Thermo-electric effect in a nano-sized crossed Permalloy/Cu junction underhigh bias current
Congpu Mu,1,2 Shaojie Hu,1,3 Jianbo Wang,2 and Takashi Kimura1,4,5,a)1

Advanced Electronics Research Division, INAMORI Frontier Research Center, Kyushu University,
744 Motooka, Fukuoka 819-0395, Japan
2
Institute of Applied Magnetics, Key Laboratory for Magnetism and Magnetic Materials of the Ministryof Education, Lanzhou University, Lanzhou 730000, People’s Republic of China
3
Graduate School of Information Science and Electrical Engineering, Kyushu University,
744 Motooka, Fukuoka 819-0395, Japan
4
Department of Physics, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan
5
CREST, Japan Science and Technology Agency, Sanbancho, Tokyo 102-0075, Japan(Received 15 June 2013; accepted 10 September 2013; published online 25 September 2013)
We show that the difference in the Seebeck coefficients between two voltage probes produces anadditional electric signal in the local resistance measurement of the submicron-sized junction. Thisis because the temperature increase at the junction induced by the Joule heating produces
unnegligible Seebeck voltage in addition to the Ohmic voltage. In nanostructured systems, since
the temperature variation becomes quite high under the high-bias current, the Seebeck voltage
dominates the detected electrical voltage. This provides a consistent description for unusual
bias-current dependences of the differential resistance in nano-sized metallic junction systems.
C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4822330]
V
Electrical transport measurement is a powerful means
for understanding the physical properties of metallic andsemiconductor materials. Recent developments of nanofabrications provide multi-terminal nano-sized electrical probes,
which enable to detect the transport properties in nanostructured materials and nano-sized elements selectively andsensitively.1–5 In such systems, since the sample dimension
is comparable or less than the characteristic lengths such asphase coherence length and spin-diffusion length, various intriguing phenomenon have been reported.6,7 Novel transport
properties such as spin-dependent and ballistic transports
have also reported recently.8–11 Indeed, the giant and tunnel
magneto resistance effects are widely utilized as high sensitive magnetic sensor in the practical applications.12,13
In nanostructured systems, thermally induced phenomenon becomes important in addition to the transport phenomenon because the temperature of the small element is easily
modified by extrinsic effects.14 Especially, the interaction
between the spin and heat currents, which is known as the
emerging field of spincaloritronics,15,16 is creating a renewedinterests because of possible application for spin manipulations. Moreover, novel spin-related thermal properties, such as
spin and spin-dependent Seebeck and Peltier effects, havebeen reported in unique spincaloritronic systems.17–20
Therefore, thermal transports in ferromagnetic/nonmagnetic
hybrid structures have been intensively investigated very
recently.21–23 However, there are still numerous controversial
issues and un-conventional phenomena in such nanostructured
thermoelectric effects.24,25 For proper understanding these
nontrivial thermal transport properties, first of all, the interplay between the charge and heat currents plays an importantrole. Especially, relatively large Seebeck coefficients in the
ferromagnetic transition metals as well as their anisotropies
may induce a large undesired electrical signal in addition toa)kimura@ifrc.kyushu-u.ac.jp0003-6951/2013/103(13)/132408/4/$30.00

the tiny spin-related signal.26 Indeed, the back ground signal
much larger than the spin related signal is found to be inducedin the nonlocal voltage probe under the electrical spin injection.27,28 Moreover, the structural changes due to the migration effects may have to be considered as the possible origin
for such unconventional phenomena.29,30 Here, we focus on
the influence of the heat current in a ferromagnetic/nonmagnetic junction. We show that the thermoelectric effect is dominant contribution of the electrical voltage even in a local
configuration.
We have fabricated a laterally configured ferromagnetic/
nonmagnetic metal junction. Figure 1 shows a scanning electron microscope (SEM) image of the fabricated device together with its schematic illustration. Here, Permalloy (Py)wires, 100 nm in width and 40 nm in thickness, have been
fabricated by electron-beam lithography with a conventional
lift-off technique on a thermally oxidized Si substrate. Then,a Cu strip, 300 nm in width and 180 nm in thickness, hasbeen fabricated by repeating the same lift-off process. Here,in order to obtain highly transparent interface, the surface of
the Py was cleaned by low-voltage Ar ion milling prior to
the Cu deposition. The transparency and cleanliness havebeen confirmed by the lateral spin transports, such as local
and nonlocal spin valve measurements.28 The representative
nonlocal spin signal of the present device measured at room
temperature is shown in Fig. 1(a). We have also fabricated asimple Cu cross with the same lateral dimension, for acomparison,
To study the thermo-electric effect induced by the current, we mainly study the bias current dependence of the resistance at the corner of the cross-shaped wire, whose probe
configuration is shown in Fig. 1(b). Here, the current is swept
from 1.5 mA to þ1:5 mA. The maximum current densities
for the Py, Cu, and the Py/Cu junctions are 3:75  1011 A/m2,
2:78  1010 A/m2, and 5:10  1011 A/m2, respectively. So,
the current density becomes the highest value in the Py wire.

103, 132408-1

C 2013 AIP Publishing LLC
V

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Mon, 22 Aug 201610:08:39

132408-2

Mu et al.

Appl. Phys. Lett. 103, 132408 (2013)

FIG. 1. (a) SEM image of the fabricated sample consisting of laterally
configured ferromagnetic/nonmagnetic
junctions together with the representative nonlocal spin signal observed atroom temperature. (b) Definition of thecorner resistance and the expected current distribution at the corner.

However, the value is lower the critical value for the migration in Py.31 When the cross consists of a polycrystalline homogeneous metal, the electrical voltage is mainly induced by
the Ohmic resistance because the mean free path of the polycrystalline metallic wire is a few ten nanometer, which ismuch shorter than the lateral dimension of the device.32 As
shown in Fig. 1(b), when the charge current flows from theleft to the bottom electrode, the positive voltage difference isobserved between the top and right electrodes because of the
Ohmic resistance. By increasing the bias current flowing in
the corner, the voltage linearly increases because of the
Ohm’s law. In addition, when the cross consists of a junction,
the interface resistance is additionally superimposed on the
Ohmic resistance. It should be noted that the sign of the voltage contribution from the interface resistance is opposite tothat from the Ohmic resistance, as shown in Fig. 1(b).
Moreover, the thermo-electric effects are also induced in thevicinity of the junction and modified the electric resistance via
Joule heating and/or Peltier effect.
First, we evaluate the bias-current dependence of thecorner resistance for the homogeneous Cu cross. As seen in
Fig. 2(a), the voltage increases almost linearly with the dc
bias current because of the Ohm’s law. However, if we
carefully check the dependence, a small deviation from thelinear dependence is observed under the high bias current.

To show this nonlinear behavior more clearly, the resistance defined by dV/dI is plotted in Fig. 2(b). A parabolic
dependence is clearly observed, meaning that the temperature of the measured region is increased by the Joule
heating.33
We then measure the bias-current dependence of thecorner resistance for the Cu/Py junction. As shown in Fig.
3(a), the bias-dependence of the voltage shows the quite
different features from that in the Cu cross. Especially, it issurprising that the voltage becomes negative under the positive bias current above þ1:4 mA. One of the possible origins is the increase of the interface resistance because thecontribution of the interface resistance is negative in thepresent measurement configuration. However, it is impossible to explain the parabolic enhancement of the voltage
under the negative bias current. The possibilities of the
migration effects can also be excluded by the experimental
fact that the observed characteristics are reversible
changes.29,30
In order to understand the parabolic increase of the voltage, we consider the Seebeck effect induced by the heating
at the junction. As mentioned in Fig. 1, under the high bias
current, the temperature at the junction increases because of
the Joule heating. When the temperature at the junction is
locally heated, the electrical voltage is induced by the
Seebeck voltage in the Py/Cu junction. The induced voltage
is given by the following equation:27
V ¼ ðSPy  SCu ÞDT:(1)

Here, SPy and SCu are the Seebeck coefficients for the Py andCu, respectively. By assuming that the temperature change
DT is proportional to I2, the parabolic dependence observedin Fig. 3(a) is well reproduced. As another important featureof the Seebeck voltage, we measure the probe configuration
dependence of the induced voltage. According to Eq. (1), thesign of the induced voltage should be reversed by interchange Vþ and V terminals. Since the voltage due to the
Ohmic resistance should satisfy the reciprocal relationship,
we should interchange Iþ and I in another probe configuration. Therefore, we measure the bias current dependence ofthe corner resistance with the reversed probe configuration.
As shown in Fig. 3(b), we observe the reversed parabolic dependence, supporting Eq. (1).
To confirm our expectation more clearly, we define
the average voltage Vavg and the voltage difference Vdif as
follows:
FIG. 2. (a) Dependence voltage as a function of the DC bias current in thehomogeneous Cu cross. (b) DC bias current dependences of the differential
resistance defined by dV=dI.

Vavg ¼

VA þ VB;
2(2)

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Mon, 22 Aug 201610:08:39

132408-3

Mu et al.

Appl. Phys. Lett. 103, 132408 (2013)

FIG. 3. (a) Induced voltage at the corner as a function of the DC bias current
in the submicron-scale Cu/Py junction
when the current flows from Cu to Py(configuration A). (b) Induced voltage
at the corner as a function of the DC
bias current when the current flows
from Py to Cu (configuration B). The
insets show the probe configuration foreach corner resistance measurement.

FIG. 4. DC bias current dependencesof (a) VAvg and (b) VDif . DC bias current dependences of (c) the differential
resistance defined by dVAvg =dI and (d)that defined by dVDif =dI.

Vdif ¼

VA  VB:
2(3)

Here, VA and VB are the observed voltages in the configuration of Figs. 3(a) and 3(b), respectively. As mentioned
before, the sign of the Seebeck voltage depends on the probe
configuration while that of the Ohmic voltage does notdepend on the probe configuration. Therefore, Vavg and Vdif
give the Ohmic and Seebeck voltages, respectively. Figures
4(a) and 4(b) show, respectively, Vavg and Vdif as a functionof the dc bias current. Vavg behaves similarly to Fig. 2,

indicating that the Vavg is produced by the Ohmic resistanceof the corner. In Fig. 4(b), we also confirm a perfect parabolic dependence without any background. This clearly supports that the voltage is induced by the Seebeck effectcombined with the Joule heating. The differential resistances
Ravg and Rdif defined by dVavg =dI and dVdif =dI, respectively,
are also plotted in Figs. 4(c) and 4(d). dVavg =dI shows thereversed parabolic dependence due to the negative contribution of the interface resistance. A small linear component,
which produces the horizontal shift of the parabolic curve, is
explained by the Peltier effect in the Py/Cu junction.

FIG. 5. Numerically simulated threedimensional spatial distributions for(a) the current density and (b) the temperature in the vicinity of the Py/Cu
junction under the bias current of 1.5
mA. In the right-hand sand, the crosssectional images focusing on the junction are plotted.

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Mon, 22 Aug 201610:08:39

132408-4

Mu et al.

dVdif =dI increases almost linearly, but includes a slight deviation from the linear dependence. This may be because theSeebeck coefficient under the high bias current is slightly
modified by the Joule heating.
Finally, in order to show the reliability of our scenario
more quantitatively, we have numerically calculated the spatial distributions of the current and the temperature in the vicinity of the Py/Cu junction. Figures 5(a) and 5(b) show thethree dimensional color plots for the calculated current density and the temperature, respectively. Here, 1.5 mA dc current flows from the Py wire into the Cu wire. As shown in
Fig. 5(a), the current distribution around the junction is quite
inhomogeneous because of the large conductivity of the Cu
wire with the bias electric field. On the other hand, the temperature distribution is homogeneous around the junction.
This means that one dimensional model can be applied forthe analysis of the thermoelectric effect. From Fig. 5(b), we
can evaluate the temperature increase DT at the junction isapproximately 2.5 K under the bias current of 1.5 mA. Since
the Seebeck coefficients for the Cu and Py are 1.6 lV/K and20 lV/K,17,18 the observed voltage 45 lV is quantitatively explained by the present scenario.
Thus, the Seebeck effect in the voltage probe produces asignificant measurable electrical voltage. Surprisingly, we
showed that, in the nano-sized Py/Cu junction, it is much larger
than the Ohmic voltage produced by the local current flow. A
similar situation occurs in a current-perpendicular-to-plane
giant magnetoresistive structure with a nano-sized metallic
junction.33 Therefore, unusual behaviors of the bias-current dependence of the electrical resistance34,35 may be explained bytaking into account the Seebeck voltage in the voltage probe.
This work was partially supported by CANONFoundation, NEDO, and CREST. The authors (C.M. andS.H.) acknowledge the China Scholarship Council for Young
Scientists.
1

J. Nygard, D. Cobden, and P. Lindelhof, Nature (London) 408, 342(2000).
2
A. K. Geim and K. S. Novoselov, Nature Mater. 6(3), 183–191 (2007).
3
M. Tsutsui, M. Taniguchi, and T. Kawai, Nat. Commun. 1, 138 (2010).
4
T. Ono, H. Miyajima, K. Shigeto, and T. Shinjo, Appl. Phys. Lett. 72,
1116 (1998).
5
M. M. Deshmukh and D. C. Ralph, Phys. Rev. Lett. 89, 266803 (2002).
6
K. Tsukagoshi, B. Alphenaar, and H. Ago, Nature 401, 572–574 (1999).

Appl. Phys. Lett. 103, 132408 (2013)
7

K. Yakushiji, F. Ernult, H. Imamura, K. Yamane, S. Mitani, K. Takanashi,
S. Takahashi, S. Maekawa, and H. Fujimori, Nature Mater. 4, 57 (2005).
H. D. Chopra and S. Z. Hua, Phys. Rev. B 66, 020403R (2002).
9
U. Ebels, A. Radulescu, Y. Henry, L. Piraux, and K. Ounadjela, Phys.
Rev. Lett. 84, 983 (2000).
10F. J. Jedema, A. T. Filip, and B. J. van Wees, Nature (London) 410, 345(2001).
11
T. Kimura, Y. Otani, T. Sato, S. Takahashi, and S. Maekawa, Phys. Rev.
Lett. 98, 156601 (2007).
12
S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M.
Samant, and S.-H. Yang, Nature Mater. 3, 862 (2004).
13
S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nature
Mater. 3, 868 (2004).
14
D. G. Cahill, W. K. Ford, K. E. Goodson, G. D. Mahan, A. Majumdar, H.
J. Maris, R. Merlin, and S. R. Phillpot, J. Appl. Phys. 93, 793 (2003).
15
G. E. W. Bauer, A. H. MacDonald, and S. Maekawa, Solid State
Commun. 150, 459 (2010).
16
G. E. W. Bauer, E. Saitoh, and B. J. van Wees, Nature Mater. 11, 391(2012).
17
K. Uchida, S. Takahashi, K. Harii, J. Ieda, W. Koshibae, K. Ando, S.
Maekawa, and E. Saitoh, Nature 455, 778 (2008).
18
A. Slachter, F. L. Bakker, J. P. Adam, and B. J. van Wees, Nat. Phys. 6,879 (2010).
19
J. Flipse, F. L. Bakker, A. Slachter, F. K. Dejene and B. J. van Wees, Nat.
Nanotechnol. 7, 166 (2012).
20
J. C. Le Breton, S. Sharma, H. Saito, S. Yuasa, and R. Jansen, Nature(London) 475, 82 (2011).
21
F. Giazotto, T. Heikkila, A. Luukanen, A. Savin, and J. Pekola, Rev. Mod.
Phys. 78, 217 (2006).
22
S. Petit-Watelot, R. Otxoa, M. Manfrini, W. Van Roy, L. Lagae, J. V.
Kim, and T. Devolder, Phys. Rev. Lett. 109, 267205 (2012).
23
S. Krompiewski, Phys. Rev. B 80, 075433 (2009).
24
J. Sinova, Nature Mater. 9, 880 (2010).
25
S. Y. Huang, X. Fan, D. Qu, Y. P. Chen, W. G. Wang, J. Wu, T. Y. Chen,
J. Q. Xiao, and C. L. Chien, Phys. Rev. Lett. 109, 107204 (2012).
26
A. Asamitsu, Y. Moritomo, and Y. Tokura, Phys. Rev. B 53, R2952(1996).
27
L. Bakker, A. Slachter, J.-P. Adam, and B. J. van Wees, Phys. Rev. Lett.
105, 136601 (2010).
28
S. J. Hu and T. Kimura, Phys. Rev. B 87, 014424 (2013).
29
P. Kumar, M. G. Krishna, and A. K. Bhattacharya, Int. J. Nanosci. 7,
255–261 (2008).
30
P. Kumar and K. Sangeeth, Nanosci. Nanotechnol. Lett. 1, 194–198(2009).
31
A. Yamaguchi, S. Nasu, H. Tanigawa, T. Ono, K. Miyake, K. Mibu, andT. Shinjo, Appl. Phys. Lett. 86, 012511 (2005).
32
G. Mihajlovic, J. E. Pearson, M. A. Garcia, S. D. Bader, and A. Hoffmann,
Phys. Rev. Lett. 103, 166601 (2009).
33
J. Katine, F. Albert, R. Buhrman, E. Myers, and D. Ralph, Phys. Rev. Lett.
84, 3149 (2000).
34
A. Fukushima, K. Yagami, A. A. Tulapukkar, Y. Suzuki, H. Kubota, A.
Yamamoto, and S. Yuasa, Jpn. J. Appl. Phys., Part 2 44, L12 (2005).
35
A. Sugihara, M. Kodzuka, K. Yakushiji, H. Kubota, S. Yuasa, A.
Yamamoto, K. Ando, K. Takanashi, T. Ohkubo, K. Hono, and A.
Fukushima, Appl. Phys. Express 3, 065204 (2010).
8

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 133.5.164.140 On: Mon, 22 Aug 201610:08:39