Handbook of Spintronics
DOI 10.1007/978-94-007-7604-3_55-1# Springer Science+Business Media Dordrecht 2014

Lateral Spin Transport (Diffusive Spin Current)
Takashi Kimura*
Department of Physics, Inamori Frontier Research Center, Kyushu University, Fukuoka, Japan

Abstract
The transport properties of the diffusive pure spin current induced in the metallic nanostructures are
discussed. The author introduces the methods for the efﬁcient generation, manipulation, anddetection of the pure spin current in laterally conﬁgured ferromagnetic/nonmagnetic metal hybrid
structures. The experimental demonstration of the magnetization switching using the pure spincurrent is also introduced.

Introduction
Ferromagnetic metal (FM)/nonmagnetic metal (NM) hybrid nanostructures show intriguing electrical transports in association with the spin accumulation and the spin momentum transfer [1–3]. Insuch spin-dependent transports, spin current, a ﬂow of the spin angular momentum, is the key
ingredient [4]. Therefore, the study on the transport properties of the spin current is essential for
deeply understanding the fundamental physics of the spin-dependent transports as well as for further
developing the spintronic devices. In order to investigate the spin current property, till to date, mostexperiments by other groups have been carried out in a vertical structure called the currentperpendicular-to-plane (CPP) conﬁguration [5, 6]. It is, however, difﬁcult to fabricate multiterminal
devices with vertical structures, so one can obtain only limited information about the series
resistance of the magnetic multilayers. On the other hand, laterally conﬁgured FM/NM hybridnanostructures have great advantage for developing the multiterminal spintronic devices because of
their ﬂexible conﬁguration [7, 8]. Especially, a pure spin current created by a nonlocal spin injection
is a powerful tool for detecting the electric signals purely related to the spin transports [9–12] as inthis case, the charge-current-induced spurious signals such as anisotropic magnetoresistance and theanomalous Hall effect can be removed. In this chapter, the author describes the transport property ofthe diffusive spin current in lateral nanostructures and introduces unique techniques for the efﬁcient
manipulation of spin current.

Generation, Diffusion, and Detection of Pure Spin Current
A laterally conﬁgured ferromagnetic (F)/nonmagnetic (N) hybrid structure combined witha nonlocal spin injection allows to create a ﬂow of spins without a ﬂow of electrical charges, i.e.,
pure spin current [7–11, 13, 14]. Figure 1 shows a schematic illustration of the nonlocal spin
injection. A bias voltage for the spin injection is applied between the ferromagnet and left-handside nonmagnet. In this case, the spin-polarized electrons are injected from the ferromagnet and are
extracted from the left-hand side of the nonmagnet. This results in the accumulation of the*Email: t-kimu@phys.kyushu-u.ac.jp
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FM
Ie

NM

Is

Is

Flowing
direction

Flowing
direction

eV

eV

Fig. 1 Schematic illustration of the nonlocal spin injection together with the density of states for the up-spin and downspin electrons in nonmagnetic metal for left-hand, center, and right-hand sides

nonequilibrium spins in the vicinity of the F/N junctions. Since the electrochemical potential in theleft-hand side is lower than that underneath the F/N junction, the electron ﬂows by the electric ﬁeld.
In the right-hand side, although there is no electric ﬁeld, the diffusion process from thenonequilibrium into the equilibrium state induces the motion of the electrons. Since the excess
up-spin electrons exist underneath the F/N junction, the up-spin electrons diffuse into the right-handside. On the other hand, the deﬁciency of the down-spin electrons induces the incoming ﬂow of thedown-spin electrons opposite to the motion of the up-spin electron. Thus, a pure spin current, which
carries the spin angular momentum without electrical charges, can be induced by the nonlocal spin
injection.
The induced pure spin current can be detected by using another FM voltage probe. Here, a fourterminal structure, in which two ferromagnetic electrodes are separately connected to a NM strip, isconsidered, as shown in Fig. 2a. When the pure spin current is injected into the FM, a shift in theelectrostatic potential of the FM is induced because of the spin-dependent conductivity. The sign ofthe potential shift depends on the relative angle between the spin direction of the injecting spincurrent and the magnetization direction. When the direction of the injecting spin is parallel to themajority (minority) spin for the spin detector, the electrostatic potential of the spin detector shifts

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a

b

c

Fig. 2 (a) Schematic illustration of the nonlocal spin injection and detection together with a SEM image of a typicallateral spin valve consisting of Py and Cu wires. (b) Typical nonlocal spin-valve signal for the Py/Cu device witha distance of 800 nm. (c) Position dependence of the nonlocal spin-valve signals at RT and 77 K

positively (negatively). Therefore, when the voltage between the FM and the right-hand side of the
NM is measured with sweeping the magnetic ﬁeld, a clear voltage change is observed. The voltage
normalized by the injecting current is known as the spin signal [7–11, 13, 14].
Figure 2b shows a representative nonlocal signal as a function of the external magnetic ﬁeld
observed in the Py/Cu lateral nonlocal spin valve measured at 77 K. Here, the center-center distance
between the Py injector and detector is 800 nm. The ﬁeld dependence of the nonlocal spin signal
exhibits a clear spin-valve effect corresponding to parallel (high) and antiparallel (low) states. Bychanging the distance between the injector and detector, one can experimentally estimate the spin
diffusion length of the Cu wire. As can be seen in Fig. 2c, the spin signal monotonically decreases
with increasing the distance d both at RT and 77 K because of the spin relaxation phenomenon.
From the one-dimensional spin diffusion model with the assumption of the transparent interface[14, 15], the dependence of the spin signal on the distance d between the injector and the detector canbe calculated as follows:
DRs 

SN
P2F
s2F l2F,
S inj S det 1  P2 2 rN lN sinhðd=lN ÞF(1)

where rF and rN are the electrical resistivities for the Py and Cu, respectively. lF and lN are the spin
diffusion length for Py and Cu, respectively. Sinj, Sdet, and SN are the size of the injecting junction, thedetecting junction, and the cross section of the Cu wire, respectively. PF is the spin polarization.
From the ﬁtting, one ﬁnds that the spin diffusion length for the Cu wire is 500 nm at RT and 1.5 mm at
77 K.

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In the nonmagnetic metals with weak spin orbit interactions such as Cu, Al, and Ag, the spin
diffusion length is a few hundred nm at RT [8–10, 13, 16–18]. This means that one can manipulate
the spin current in the Cu even in lateral conﬁguration by using recent nano-fabrication techniques.
On the other hand, In some metals with a strong spin-orbit interaction and most of FMs, the spin
diffusion length is known to be a few or a few 10 nm [19]. This implies that the manipulation of thespin current is difﬁcult in such metals using the lateral conﬁgurations. However, as introduced later,by the combination between the lateral conﬁgurations and the unique transport properties of the spincurrent, one can effectively manipulate the spin current even in such metals.

Advantage of Lateral Configuration
The conventional electrical spin injection is performed by injecting the spin-polarized current froma single FM injector into NM [7, 8, 10]. In such cases, the direction of the spin accumulation in the
NM is ﬁxed by the magnetization of the FM injector. When two or more FM injectors are utilized,
one may have greater control of the spin current and spin accumulation. Taking the advantage of the
ﬂexible probe conﬁguration in lateral geometry, one can easily expand the simple nonlocal spinvalve concept to a multiterminal device. This provides high tunability and the attractive device
performance [10]. Here, two representative advantages using multiterminal spin injection are
introduced.
The ﬁrst demonstration is a direction control of the spin current using noncollinear dual spin
injectors [20]. A lateral spin valve consisting of two Permalloy injectors Py1 and Py2 and a detector
Py3, bridged by a Cu strip, has been prepared, as shown in a scanning electron microscope (SEM)
image of Fig. 2. Here, the Py injectors are patterned into needle shapes so as to prevent the inﬂuenceof the demagnetizing ﬁeld in the remanent state. The easy axes of the Py injectors are tilted from thehorizontal axis. The direction of the accumulated spin under the dual spin injection is detected by the
Py strip with a horizontal easy axis as a nonlocal spin-valve signal.
The nonlocal spin-valve signal curves for various injected current fraction are shown in Fig. 3c. In
the measurement, the magnetic ﬁeld was swept between 600 and 400 Oe, to ﬁx the magnetization
direction of the detector Py3 during the sweep. In the nonlocal spin-valve signal under the dual
injection shown in Fig. 3, the accumulated spin is not collinear with the detector, except whenf ¼ 0 for i2 ¼ i2, where f is the angle of the accumulated spin with respect to x axis. The spin signal
DRs for noncollinear structures is deﬁned as
DRS DV ðfÞ DV ðfÞ¼,
I 0 ðfÞ
I s ðfÞ=p(2)

where Is (f) is the magnitude of the spin current as a function of f and p is the spin polarization. As
shown in Fig. 3d, under the condition that the charge current is held ﬁxed as f is varied, one ﬁnds
that DRs varies as cos f. Thus, by injecting current through two magnetic wires with noncollinear
conﬁguration, it is possible to control electrically the polarization direction (axis) of the spin current.
In this manner, one can rotate the polarization axis of the spin accumulation in the normal Cu wire
while maintaining the magnetization direction of the spin detector ﬁxed.
Then, the author focuses on the generation of the giant spin current using multiterminal spin
injectors [21], which is the second advantage of the multiterminal injection. Since the conventional
direct spin injection produces an extra Joule heating induced by the charge current, the magnitude of
the injecting spin current is limited by the maximum tolerance of the charge current. On the other

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a

c

b

d

Fig. 3 (a) SEM image of the lateral spin valve with the noncollinear dual spin injectors, (b) schematic illustration forelectrical control of the direction of the spin accumulation using the noncollinear dual spin injectors, (c) nonlocal spinsignal DV/I0 for various current fraction, and (d) relationship between the direction of the accumulated spin and the spinsignalhand, the pure spin current does not include any charge current. Therefore, the maximum toleranceof the pure spin current is much larger than that of the spin-polarized current. This implies that one
can inject giant spin currents, which cannot be realized by the conventional direct spin injection.
Moreover, pure spin current does not produce current-induced Oersted ﬁeld. Thus, pure spin current
is advantageous compared with the spin current induced by the conventional direct spin injection.
Another example in this regard is that one may reverse the magnetization in the large dimension of
the NM by the pure spin current injection [22–25]. Since the switching speed of the magnetization is
proportional to the magnitude of the injecting spin current [26], the generation of the giant pure spincurrent may open the door for the ultrahigh-speed operation of the spintronic devices. The pure spincurrent is, in general, created by a nonlocal spin injection, and the magnitude is limited by the
maximum allowed current in a ferromagnetic spin injector. If the pure spin current from two or more
ferromagnetic injectors can be superimposed, one obtains a giant pure spin current, which isunachievable in the conventional spin injection. Therefore, here, a method for generating giant
pure spin currents using multiterminal spin injection is introduced.
To demonstrate the multiterminal spin injection, a lateral spin valve consisting of the quadruple
spin injectors (Py1, Py2, Py3, and Py4) has been fabricated. Figure 4a shows the scanning electron
microscope image of the fabricated device together with its schematic. In order to create the spincurrent in Cu, the current IC is injected from each injector. This means that totally 4IC current isprovided from the current source. The spin signal can be evaluated by measuring the voltage
between the middle Py and the Cu wire, as schematically shown in Fig. 4a. Figure 4b shows a typicalnonlocal spin signal under the quadruple-terminal spin injection. The vertical axis corresponds to the

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a
V

V
I+I

I+I

b

Fig. 4 (a) Concept for multiterminal spin injection and SEM image of the multiterminal spin injection device. (b)Typical nonlocal spin-valve signal under quadruple spin injection

value of the induced voltage divided by the injecting current IC from each electrode. The signal
shows large clear spin-valve effect consisting of four downward jumps and one upward jump. The
four downward resistance jumps correspond to the magnetization switching of the four spin
injectors. The spin current injecting into the middle Py wire is roughly estimated from the following
equation [14, 22]:1  P2Py S det
DRS I C :IS ¼
2PPy rPy lPy(3)This means that the injection spin current is proportional to the current in each terminal. Since themaximum tolerance current in the lateral spin-valve device is, in general, limited by theelectromigration of the FM injectors, the maximum magnitude of the pure spin current is further
increased by using thicker spin injectors.
One should also consider the inﬂuence of the heating under the high bias current injection because
the heating of the sample strongly affects the spin transport. Therefore, the bias current dependenceof the nonlocal spin-valve signal with varying number of the injectors has been investigated. In thisexperiment, the thickness of the Py injector was set to 25 nm, in order to heat the sample effectively.
As shown in Fig. 5, if a single injector is used, at 1.5 mA, the spin signal reduces to 93 % of that at
near-zero bias. On the other hand, for four-injector conﬁguration, spin signal remains almost
unattenuated at this injection level. This is due to the less heating of the Py injector for multiterminal
case, compared to the single terminal case. It can be clearly conﬁrmed that the critical current, where
the spin signal starts to reduce, is improved by increasing the number of the spin injector. Thus, byusing multiterminal spin injector, the Joule heating problem and electromigration issue in nonlocal
spin injection can be tackled.

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V

V

I+I

I

V

V

I+I

I

I+I

I+I

Fig. 5 Bias dependence of the spin signals under single, dual, triple, and quadruple spin injections together with theschematics for each injection conﬁgurationab

Fig. 6 Schematic illustrations of the ﬂow of the spin current in (a) single F/N junction and (b) an F/N junction with an
F contact

Spin Absorption EffectInfluence of an Additional Contact
The driving force of the pure spin current is a diffusion of the nonequilibrium electrons into theequilibrium state. If one considers the spin current in a single FM/NM junction shown in Fig. 6a, thespatial distribution of the spin accumulation in the NM symmetrically decays from the junction.
Therefore, the spin current ﬂows also symmetrically into both sides. On the other hand, when an
additional material is connected in the right-hand side of the NM through the low resistive ohmic
junction as in Fig. 6b, the spatial distribution of the spin accumulation is strongly modiﬁed. Whenthe spin relaxation rate for the connected material is much larger than that for the NM, thenonequilibrium spins are preferably absorbed into the connected material. Thus, one can selectively
extract the spin current. This is known as the spin absorption effect [15].
To demonstrate the above spin current absorption effect, two kinds of the lateral spin valves havebeen prepared [27]. One is a conventional lateral spin valve consisting of the Py injector and thedetector bridged by a Cu strip (device A). The other one is a lateral spin valve with a middle Py wire
(device B). Here, the center-center distance between the injector and the detector for device A is
600 nm, while that for device B is 460 nm. Although the geometrical disorder due to the additional
ferromagnetic contact may also violate the spin coherence and the spin accumulation, such an effectshould be negligible because of large difference in thickness between Cu and Py.
Figure 7a shows the spin signal observed in device A, where a spin-valve signal with themagnitude of 0.2 mO is clearly observed. Since the center-center distance between the injector
and detector for device B is shorter than that for device A, one may naively expect that the larger spinsignal is expected to be observed in device B. However, as in Fig. 7b, a quite small spin signal lessthan 0.05 mO is observed in device B. This is due to the inﬂuence of the spin current absorption into

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Fig. 7 (a) Nonlocal spin-valve signal for a conventional lateral spin valve and that for a lateral spin valve with a middle
Py wire. The insets show SEM images of the measured device and the probe conﬁgurations for the nonlocal spin-valve
measurements

Fig. 8 Nonlocal spin-valve signals for Cu, Au, and Pt middle wire measured at room temperature

the middle Py wire and clearly support that the spin accumulation in the Cu is strongly suppressed by
the middle Py2 wire connected to the Cu.
It was also demonstrated that the spin accumulation in the Cu is suppressed by connecting thenonmagnetic wire with a strong spin relaxation [15]. Figure 8 shows the spin signals with various
nonmagnetic middle wires. Here, the center-center distance between the injector and detector is
ﬁxed at 600 nm. For the middle Cu wire, the obtained spin signal is 0.18 mO, which is almost thesame as that without the middle wire. The large reductions of the spin signals are observed in the Au
and Pt middle wires. These indicate that the nonequilibrium spin currents are strongly relaxed by thePt and Au wires while the Cu has weak relaxation of the spin current. Thus, one can evaluate themagnitude of the spin relaxation of a material from the magnitude of the spin signal.

Transverse Spin Current Absorption into Ferromagnetic Metal
Another interesting phenomenon is expected in the spin absorption into an FM. When the magnetization of the spin absorber is parallel or antiparallel to the injected spins (longitudinal spin
absorption), the effective spin polarization is the same as that of the bulk spin polarization. Thissituation is known as a collinear conﬁguration, which corresponds to most of the experimental
situations in the lateral spin-valve systems [8–11, 13, 17, 18]. On the other hand, when themagnetization of the spin absorber is normal to the direction of the spin current (transverse spin
absorption), the effective spin polarization becomes zero [15]. Since the spin relaxation rate
decreases with increasing the spin polarization, the spin relaxation rate for the transverse spin

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a

b

c

Fig. 9 (a) Scanning electron microscope (SEM) image of the specially fabricated Py/Cu lateral spin valve consisting of
two V-shaped nanowires with a middle strip. Expected domain structures in the Py wires and spin accumulation in a Cu
channel for the longitudinal conﬁguration (b) and the transverse conﬁguration (c)current becomes larger than that for the longitudinal spin current. Moreover, in the FM, the spin
relaxation length for the transverse spin current is known to be shorter than that for the longitudinal
one [28–32]. Since the spin relaxation rate is inversely proportional to the spin relaxation length, theshorter transverse spin relaxation length induces the further enhancement of the spin absorption into
the middle Py wire. Therefore, one expects that the spin absorption can be tuned by the direction of
the injected spin.
In order to evaluate the longitudinal and transverse spin current absorptions precisely, a lateralspin valve (LSV) using V-shaped ferromagnetic injector and detector shown in Fig. 9a has been
fabricated [33]. Here, an FM strip, which plays a role of the spin absorber, is located in the middle of
the V-shaped wires. The three ferromagnetic Py wires are bridged by a nonmagnetic Cu strip. It
should be noted that the V-shaped Py wires are connecting with the Cu strip at the corners of the
V-shaped wires. Therefore, the directions of the injecting and detecting spins reﬂect the domain
structures at the corners. When the strong magnetic ﬁeld is applied to the sample along thex direction, all of the magnetizations in the V-shaped wires and the strip are aligned with thex direction as shown in Fig. 9b. In this situation, the direction of the generated spins in the Cu
strip is parallel to the magnetization of the spin absorber. This situation corresponds to thelongitudinal spin current absorption. On the other hand, when the magnetic ﬁeld decreases tozero, the domain structure of each FM wire reﬂects its own shape, as shown in Fig. 9c. The
magnetization of the middle strip is aligned with the wire direction (y direction) because of the
shape anisotropy. However, the magnetizations of the V-shaped wire around the corner maintain theﬁeld direction (x direction) even at the remanent state [34]. In this situation, the injecting spin isperpendicular to the magnetization of the spin absorber, meaning the transverse spin current
absorption. Note that the magnetic conﬁguration between the injector and the detector should beroughly still in parallel at the remanent state. Therefore, the difference in the spin signal between two
situations should be mainly caused by the change of the spin relaxation rate due to the middle Py
wire. By comparing the nonlocal spin voltages between two situations, one can fairly evaluate thelongitudinal and transverse spin current absorptions.
Figure 10a, b shows the nonlocal spin-valve signals for the V-shaped LSV with and without
a middle ferromagnetic injector, respectively. The overall resistance change was estimated to be
0.3 mO, which is strongly reduced from the V-shaped LSV without the middle Py insertion(1.2 mO). This is due to the spin absorption effect into the middle Py strip [27]. More interestingly,
in the LSV with the middle wire, the spin signal at the remanent state shows the signiﬁcant reduction
from the value at the high magnetic ﬁeld. As mentioned above, the relative angle between the

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Fig. 10 Nonlocal spin-valve signals for V-shaped LSV without a middle Py strip (a) and with a middle strip (b). Here,the magnetic ﬁeld is applied along the x direction

injecting spin and the magnetization direction of the middle wire becomes 90 at zero magnetic ﬁeld.
Therefore, the reduction of the spin signal in the low magnetic ﬁeld is caused by the stronger spin
absorption due to the transverse spin current. From the analysis based on one-dimensional spin
diffusion model, the transverse spin relaxation length is 1.7 nm, which is much shorter than thelongitudinal one (5 nm). Thus, it was experimentally demonstrated that the transverse spin current isabsorbed by the FM more strongly than the longitudinal one.

Reabsorption Effect of Spin Current
One should also take into account the spin absorption into the spin injector (reabsorption effect). By
using the electrical spin injection, the nonequilibrium spin accumulation is induced at the interface
and diffused into the equilibrium state. In that case, the spin current diffuses not only into the NM but
also into the FM injector. Notably, as mentioned above, the conventional ferromagnetic injector has
much faster spin relaxation rate than that for the NM. Therefore, the injected spin current in the NM
mainly returns back to the FM (Fig. 11a). This phenomenon gives rise to an extremely low injection
efﬁciency of the spin current in the N [15, 35, 36]. However, if one utilizes a highly spin-polarized
ferromagnet (HSF) as a spin injector, so-called half-metallic ferromagnet (HMF) [37], the spinpolarized electrons can be efﬁciently injected into the N, and the backﬂow of the spin currents can bestrongly suppressed, resulting in a dramatical improvement of the injection efﬁciency of the spincurrents in the N (Fig. 11b). This is because the spin relaxation rate for the FM is proportional to 1 
P2. In this scheme, the use of HSF spin injectors is critical for generating a giant pure spin current in
the NM (Fig. 11c). Recently, signiﬁcant improvement of the generation efﬁciency of the pure spincurrent has been achieved by using the spin injector consisting of the Heusler compound [38–40].
The above-explained reabsorption effect can be characterized by the generation efﬁciency  givenby the following equation [15]:
rF lF = 1  P 2
:
(4)
¼P
rN lN þ rF lF = 1  P2
Here, rNlN and rFlF/(1  P2) correspond to the difﬁculty of the spin relaxations for the NM andFM, respectively. Therefore, the reabsorption effect can be understood as the competition of the spin
relaxation strengths between FM and NM. Since the spin diffusion length for the ferromagnet lF is
extremely short,  becomes very small in a conventional FM/NM junction. However, as explained
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a

b

c

Fig. 11 Schematic illustrations of the electrical spin injection from a conventional FM (a) and a HMF (b) into an
NM. (c) Generation of a pure spin current by using nonlocal spin injection and spatial distributions of the spin-dependent
electrochemical potentials in the N

above, by using a HMF with P  1, the difﬁculty of the spin relaxation becomes very high. As
a result,  is dramatically improved.
The difﬁculty of the electrical spin injection in an FM/semiconductor interface known asa conductance mismatch can be similarly understood by the spin reabsorption effect [36]. Since
the electrical resistivity for the semiconductor is much larger than that for the FM,  becomes verysmall. A similar obstacle occurs in the case for the spin injection into the molecular materials
because of their high resistivity. As explained above, a spin injector with a perfect HMF property cansolve this issue. The spin injection efﬁciency is also known to be improved by inserting the tunnel
barrier at the interface instead of the use of the highly spin-polarized material [41]. Since theinsertion of the tunnel barrier strongly prevents the diffusion of the spin current into the FM injector,
the difﬁculty of the spin relaxation for the FM is effectively enhanced. The preparation of a highquality tunnel barrier without reducing the spin polarization at the interface is required for theeffective spin injection. Since a long spin diffusion length is expected in silicon- and carbon-basedorganic materials, the efﬁcient generation of the spin current in such materials may open a new
avenue for the spintronic devices based on pure spin currents.
A contact size in the FM/NM junction is also an important factor for the spin injection, absorption,and reabsorption [42]. So far, the distance between the spin injector and detector is known as themost important geometrical parameter for the spin injection and detection. However, by reducing thesize of the injecting junction, the generation efﬁciency of the pure spin current is drastically
improved. This is because the spin relaxation rate in the FM is inversely proportional to the junction
size S. To demonstrate this, the junction-size dependence of the spin signal has been investigated.

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a

c

b

Fig. 12 Nonlocal spin-valve signals of (a) the large-junction device and (b) the small-junction device with the probe
conﬁgurations. (c) Spin signal in the NLSV measurement as a function of the junction size lpw. The dotted curve is thebest ﬁtting to the data points using Eq. 4

Here, the junction size for the lateral spin valve is changed from 0.006 mm2 to 0.2 mm2 although theedge-to-edge distance between the injector and detector is ﬁxed to 800 nm. Since the spin relaxation
rate of the vertical Cu arms is small, the spin current diffusions into the horizontal Cu arms can beneglected. Then, the spin signal for the present LSV is roughly obtained by Eq. As mentioned above,
reducing the size of the ohmic junction between the Py pad and the Cu wire decreases the spin
relaxation rate in the Py pad. To change the junction size between the Py pad and the Cu wire, thelength of the Cu wire on the Py pad is adjusted, as seen in the inset of Fig. 12c. The junction-size
dependence of the spin signal has been investigated by changing the size of the detecting junction
with keeping the same electrode spacing of 600 nm. The obtained spin signal is plotted as a functionof the junction size in Fig. 12c. The spin signal increases with reducing the junction size and is well
reproduced by Eq. 1, where the spin signal is inversely proportional to the junction size.

Magnetization Switching Due to Injection of Pure Spin Current
The switching mechanism due to spin torque is explained with a model proposed by Slonczewski inwhich the torque exerted on the magnetization is proportional to the injected spin current. This
clearly indicates that the spin current is essential to realize the magnetization switching due to thespin injection. Most of the present spin-transfer devices consist of vertical multilayered nanopillars
in which typically two magnetic layers are separated by a nonmagnetic metal layer [43, 44]. In suchvertical structures, the charge current always ﬂows together with the spin current; thereby, undesirable Joule heat is generated. As mentioned above, by optimizing the junction, the pure spin current
can be effectively injected into the nanomagnet because of the spin absorption [42]. Therefore, themagnetization of nanomagnet can be switched nonlocally. To test this idea, a nanoscale ferromagnetic particle is conﬁgured for a lateral nonlocal spin injection device as in Fig. 13a, b [22–25].
The device for the present study consists of a large Permalloy (Py) pad 30 nm in thickness, a Cu
cross 100 nm in width and 80 nm in thickness, and a Py nanoscale particle, 50 nm in width, 180 nm
in length, and 6 nm in thickness. A gold wire 100 nm in width and 40 nm in thickness is connected to
the Py particle to increase the effective spin relaxation rate, resulting in high spin current absorption
into the Py particle. The magnetic ﬁeld is applied along the easy axis of the Py particle. Here, thedimensions of Py pad and Cu wires are chosen large so that the charge current up to 15 mA can ﬂow
through them.
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a

c

b

d

Fig. 13 (a) Scanning electron microscope image of the fabricated lateral spin valve. (b) Schematic illustration of
nonlocal spin injection using lateral spin-valve geometry. (c) Field dependence of the nonlocal spin signal. (d) Nonlocal
spin-valve signal after the pulsed current injection as a function of the current amplitude with corresponding magnetization conﬁgurations

To conﬁrm that the spin current from the Py injector is injected into the Py particle, the nonlocal
spin-valve measurements are performed. As in Fig. 13c, the ﬁeld dependence shows a clear spinsignal with a magnitude of 0.18 mO, assuring that the spin current reaches the Py particle. Then, theeffect of the nonlocal spin injection into the Py particle has been examined with using the same probe
conﬁguration. Before performing the nonlocal spin injection, the magnetization conﬁguration is set
in the antiparallel conﬁguration by controlling the external magnetic ﬁeld.
The nonlocal spin injection is performed by applying large pulsed currents up to 15 mA in theabsence of magnetic ﬁeld. As shown in Fig. 13d, when the magnitude of the pulsed current isincreased positively in the antiparallel state, no signal change is observed up to 15 mA. On the other
hand, for the negative scan, the abrupt signal change is observed at 14 mA. The change inresistance at 14 mA is 0.18 mO, corresponding to that of the transition from antiparallel to parallelstates. This means that the magnetization of the Py particle is switched only by the spin current
induced by the nonlocal spin injection. The responsible spin current for switching is estimated fromthe experiment to be about 200 mA, which is reasonable compared with the values obtained forconventional pillar structures. However, the switching from the parallel to antiparallel state has notbeen achieved in the present device. This is mainly due to the low spin-injection efﬁciency.
To improve the efﬁciency of the injecting spin current, a newly designed sample has been
fabricated, as shown in Fig. 14a [23]. The new sample consists of two Py/Au nanopillars on a Cu
wire. As shown in Fig. 14a, the junction size between the Py and Cu in the new sample is effectively
diminished, leading to the efﬁcient generation of the pure spin current. Figure 14b shows thenonlocal spin-valve signal as a function of the external ﬁeld. The obtained spin signal is around
4 mO, much larger than that of the previous device. Then, nonlocal spin injection with variable DC

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Handbook of Spintronics
DOI 10.1007/978-94-007-7604-3_55-1# Springer Science+Business Media Dordrecht 2014a

V/I (mΩ)

b

3

P

c 3

2

2

1

1

0

0

−1
−2

AP
50 100−100 −50 0
Magnetic field (mT)

−1

P

AP

−2
−8 −6 −4 −2 0 2
I dc (mA)

4

6

8

Fig. 14 (a) SEM image and schematic illustration of the improved nonlocal spin injection device. (b) Giant spin signal
and (c) the reversible magnetization switching by the pure spin current injection observed in the improved device

current is applied to perform the magnetization switching. The sample is preset to a parallel state at
which both magnetizations are aligned in the positive ﬁeld direction. As can be seen in Fig. 14c,
when the current is increased, the nonlocal spin-valve signal sharply decreases at about 4.5 mA,
indicating a clear magnetization reversal. According to the change in the nonlocal spin-valve signal,
the parallel state is transformed into an antiparallel state which is switched back to the parallel state
by a negative DC current of 5 mA. Thus, reversible magnetization switching between antiparalleland parallel states is realized by means of nonlocal spin injection with the specially developed device
consisting of perpendicular nanopillars and lateral magnetic nanostructures.
Very recently, Zou and Ji demonstrate the nonlocal switching of the Py nanodot by using
a specially developed LSV structure [24]. They prepared a lateral spin valve with a 5-nm-thickferromagnetic Py detector. This structure enables to inject the pure spin current entirely in the Py
detector. As a result, the magnetization of the Py detector is reversed by a sufﬁciently large spin
torque. The interesting thing is that the structure includes the interface barriers both at the injecting
and detecting junctions. According to the spin diffusion model, the interface resistance strongly
suppresses the spin current diffusion into the ferromagnet. To understand the result more quantitatively, other effects such as the magnetic interface anisotropy may have to be considered.

Conclusion and Outlook
The author described the transport properties of the diffusive spin currents by introducing theexperimental studies on the electrical spin injection in metallic nanostructures. The author showsthat a nonlocal spin injection in a lateral spin-valve structure enables to create a pure spin current.
Electrical control of the direction of the spin accumulation and the generation of the giant pure spincurrent were demonstrated by the multiterminal spin injections. The author also shows that the
diffusion process of the spin current is strongly affected by the spin absorption and reabsorption
effects. The magnetization switching of the ferromagnetic nanodot due to the pure spin current
injection was demonstrated.

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DOI 10.1007/978-94-007-7604-3_55-1# Springer Science+Business Media Dordrecht 2014

A novel manipulation and sensitive detection techniques of the pure spin currents have beendeveloped recently by using the spin Hall effect [45–47]. Moreover, the spin current is found to begenerated from the heat ﬂow [48] and to transport even in electrical insulator [49]. These novel
techniques may open up possibilities for new spintronic devices with ultralow power consumptions.
However, the application feasibility of the pure spin currents is still low at moment because of the
low generation efﬁciency of the pure spin current. Further improvements should be achieved for thepractical application of the pure spin current. Very recently, a room-temperature magnetization
switching using the pure spin current induced by the spin Hall effect has been demonstrated[50]. This innovative demonstration may revolutionize the realization of more functional spin
devices utilizing pure spin currents.

Acknowledgment
The author would like to thank Prof. Otani for the valuable discussions and the ﬁnancial supports
from NEDO and CREST.

References
1. Wolf SA et al (2001) Spintronics: A Spin-Based Electronics Vision for the Future. Science
294:1488
2. Zutic I, Fabian I, Das Sarma S (2004) Spintronics: Fundamentals and applications. Rev Mod
Phys 76:323
3. Chappert C, Fert A, Van Dau Nguyen F (2007) The emergence of spin electronics in data
storage. Nat Mater 6:813–823
4. Maekawa S, Valenzuela SOV, Saitoh E, Kimura T (eds) Spin current. Oxford University Press
5. Pratt WP Jr, Lee S-F, Slaughter JM, Loloee R, Schroeder PA, Bass J (1991) Perpendicular Giant
Magne- toresistances of Ag/Co Multilayers. Phys Rev Lett 66:3060
6. Brataas A, Kent AD, Ohno H (2012) Current-induced torques in magnetic materials. Nature
Mater. Nat Mater 11:372–381
7. Johnson M, Silsbee RH (1987) Thermodynamic analysis of interfacial transport and of thethermomagne- toelectric system. Phys Rev B 35:4959
8. Jedema FJ, Filip AT, van Wees BJ (2001) Electrical spin injection and accumulation at room
temperature in an all-metal mesoscopic spin valve. Nature (London) 410:345
9. Valenzuela SO (2009) Nonlocal Electronic Spin Detection, Spin Accumulation and the Spin
Hall effect. Int J Mod Phys B 23:2413
10. Kimura T, Otani Y (2007) Spin transport in lateral ferromagnetic/nonmagnetic hybrid
structures. J Phys J Phys Cond Mat 19:165216; Otani Y, Kimura T (2011) Spin current related
phenomena in metallic nano-structures. Physica E 43:735
11. Hoffmann A (2007) Pure Spin-Currents. Phys Stat Sol (c) 4:4236
12. Bader SD, Parkin SSP (2010) Spintronics. Annu Rev Condens Matter Phys 1:71
13. Urech M, Korenivski V, Poli N, Haviland DB (2006) Direct Demonstration of Decoupling of
Spin and Charge Currents in Nanostructures. Nano Lett 6:871
14. Takahashi S, Maekawa S (2003) Spin injection and detection in magnetic nanostructures. Phys
Rev B 67:052409

Page 15 of 17

Handbook of Spintronics
DOI 10.1007/978-94-007-7604-3_55-1# Springer Science+Business Media Dordrecht 2014

15. Kimura T, Hamrle J, Otani Y (2005) Estimation of spin-diffusion length from the magnitude ofspin- current absorption: Multiterminal ferromagnetic/nonferromagnetic hybrid structures.
Phys Rev B 72:014461
16. Mihajlovic G et al (2010) Enhanced spin signals due to native oxide formation in Ni80Fe20/Ag
lateral spin valves. Appl Phys Lett 97:112502
17. Garzon S, Zutic I, Webb RA (2005) Temperature-Dependent Asymmetry of the Nonlocal SpinInjection Resistance: Evidence for Spin Nonconserving Interface Scattering. Phys Rev Lett
94:176601
18. van Staa A, Wulfhorst J, Vogel A, Merkt U, Meier G (2008) Spin precession in lateral all-metal
spin valves: Experimental observation and theoretical description. Phys Rev B 77:214416
19. Bass J, Pratt WP Jr (2007) Spin-diffusion lengths in metals and alloys, and spin-ﬂipping at
metal/metal interfaces: an experimentalist’s critical review. J Phys Condens Matter 19:183201
20. Kimura T, Otani YC, Levy Phys PM (2007) Electrical Control of the Direction of Spin
Accumulation. Rev Lett 99:166601
21. Nonoguchi S, Nomura T, Kimura T (2012) Electrical manipulation of spin polarization andgeneration of giant spin current using multi terminal spin injectors. J Appl Phys 111:07C505
22. Kimura T, Otani Y, Hamrle J (2006) Switching Magnetization of a Nanoscale Ferromagnetic
Particle Using Nonlocal Spin Injection. Phys Rev Lett 96:037201
23. Yang T, Kimura T, Otani Y (2008) Giant spin-accumulation signal and pure spin-currentinduced re- versible magnetization switching. Nat Phys 4:851 4
24. Zou H, Ji Y (2011) Temperature evolution of spin-transfer switching in nonlocal spin valves
with dipolar coupling. J Magn Magn Mater 323:2448; Zou H, Chen S, Ji Y (2012) Reversal andexcitations of a nanoscale magnetic domain by sustained pure spin currents. Appl Phys Lett
100:012404
25. Sun JZ et al (2009) Publisher’s Note: A three-terminal spin-torque-driven magnetic switch.
Appl Phys Lett 95:083506
26. Sun JZ (2000) Spin-current interaction with a monodomain magnetic body: A model study.
Phys Rev B 62:570–578
27. Kimura T, Hamrle J, Otani Y, Tsukagoshi K, Aoyagi Appl Y (2004) Spin-dependent boundary
resistance in the lateral spin valve structure. Phys Lett 85:3501
28. Zhang S, Levy PM, Fert A (2002) Mechanisms of Spin-Polarized Current-Driven Magnetization Switch- ing. Phys Rev Lett 88:236601
29. Stiles MD, Zangwill A (2002) Anatomy of spin-transfer torque. Phys Rev B 66:014407
30. Zhang J, Levy PM, Zhang S, Antropov V (2004) Identiﬁcation of Transverse Spin Currents in
Noncollinear Magnetic Structures. Phys Rev Lett 93:256602
31. Taniguchi T, Yakata S, Imamura H, Ando Y (2008) Penetration depth of transverse spin current
in ferromagnetic metals. IEEE Trans Magn 44:2636
32. Taniguchi T, Yakata S, Imamura H, Ando Y (2008) Determination of Penetration Depthof Transverse Spin Current in Ferromagnetic Metals by Spin Pumping. Appl Phys Exp
1:031301-1-3
33. Nonoguchi S, Nomura T, Kimura T (2010) Longitudinal and transverse spin current absorptions
in a lateral spin-valve structure. Phys Rev B 86:104417
34. Taniyama T, Nakatani I, Namikawa T, Yamazaki Y (1999) Resistivity due to Domain Walls in
Co Zigzag Wires. Phys Rev Lett 82:2780
35. van Son CP, van Kempen H, Wyder P (1987) Boundary resistance of the ferromagneticnonferromagnetic metal interface. Phys Rev Lett 58:2271–2273

Page 16 of 17

Handbook of Spintronics
DOI 10.1007/978-94-007-7604-3_55-1# Springer Science+Business Media Dordrecht 201436. Schmidt G, Ferrand D, Molenkamp WL, Filip TA, van Wees JB (2000) Fundamental obstacle
for electrical spin injection from a ferromagnetic metal into a diffusive semiconductor. Phys Rev
B 62:R4790–R4793
37. Balke B, Wurmehl S, Fecher HG, Felser C, Kubler J (2008) Rational design of new materials for
spintronics: Co2FeZ (Z ¼ Al, Ga, Si, Ge). Sci Technol Adv Mater 9:014102
38. Kimura T, Hashimoto N, Yamada S, Miyao M, Hamaya K, Room-temperature generation of
giant pure spin currents using Co2FeSi spin injectors. arXiv:1108.0156; Hamaya K,
Hashimoto N, Yamada S, Miyao M, Kimura T To be published in Phys Rev B
39. Bridoux G, Costache MV, Van de Vondel J, Neumann I, Valenzuela SO (2011) Enhanced spinsignal in nonlocal devices based on a ferromagnetic CoFeAl alloy. Appl Phys Lett 99:102107
40. Takahashi YK, Kasai S, Hirayama S, Mitani S, Hono K (2012) All-metallic lateral spin valvesusing Co2Fe(Ge0.5Ga0.5) Heusler alloy with a large spin signal. Appl Phys Lett 100:052405
41. Rashba EI (2000) Theory of electrical spin injection: Tunnel contacts as a solution of the
conductivity mismatch problem. Phys Rev B 62, R16267
42. Kimura T, Otani Y, Hamrle Phys J (2006) Enhancement of spin accumulation in a nonmagnetic
layer by reducing junction size. Rev B 73:132405
43. Tsoi M, Jansen AGM, Bass J, Chiang W-C, Seck M, Tsoi V, Wyder Phys P (1998) Excitation of
a Magnetic Multilayer by an Electric Current. Rev Lett 80:4281
44. Albert FJ, Emley NC, Myers EB, Ralph DC, Buhrman Phys RA (2002) Quantitative Study ofMagnetization Reversal by Spin-Polarized Current in Magnetic Multilayer Nanopillars. Rev
Lett 89:226802
45. Saitoh E, Ueda M, Miyajima H, Tatara G (2006) Conversion of spin current into charge current
at room temperature: Inverse spin-Hall effect. Appl Phys Lett 88:182509
46. Valenzuela SO, Tinkham M (2006) Direct electronic measurement of the spin Hall effect.
Nature (London) 442:176
47. Kimura T, Otani Y, Sato T, Takahashi S, Maekawa S (2007) Room-Temperature Reversible
Spin Hall Effect. Phys Rev Lett 98:156601
48. Slachter A, Bakker FL, Adam JP, van Wees BJ (2010) Thermally driven spin injection from aferromagnet into a non-magnetic metal. Nat Phys 6:879
49. Kajiwara Y, Harii K, Takahashi S, Ohe J, Uchida K, Mizuguchi M, Umezawa H, Kawai H,
Ando K, Takanashi K, Maekawa S, Saitoh E (2010) Transmission of electrical signals by spinwave interconversion in a magnetic insulator. Nature (London) 464:262
50. Liu L, Pai C-F, Li Y, Tseng H-W, Ralph DC, Buhrman RA (2012) Spin torque switching withthe giant spin Hall effect of tantalum. Science 336:555

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