PHYSICAL REVIEW APPLIED 8, 054031 (2017)

Dynamical Spin Injection Based on Heating Effect due to Ferromagnetic ResonanceKazuto Yamanoi,1 Yuki Yokotani,1 and Takashi Kimura1,2,*
1

Department of Physics, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan
2
Research Center for Quantum Nano-Spin Sciences, Kyushu University,
744 Motooka, Fukuoka 819-0395, Japan(Received 7 September 2016; revised manuscript received 18 September 2017; published 15 November 2017)
Dynamical spin injection based on the ferromagnetic resonance is an attractive and powerful method
for generating and manipulating the spin current. To date, in a ferromagnet/nonmagnet bilayer system, thespin-pumping effect from the ferromagnet into the nonmagnet has been considered only as the mechanismof the dynamical spin injection. Here, we examine another method for the dynamical spin injection basedon the heating effect due to the ferromagnetic resonance in a ðCo; FeÞB=Pt bilayer system. The (Co,Fe)B
film heated by the ferromagnetic resonance produces the temperature gradient across the ðCo; FeÞB=Ptinterface, resulting in the thermal spin injection from the (Co,Fe)B layer into the Pt film. The obtained
electrical signal clearly shows the spin Hall signature and exceeds 20 mV Ω−1 m−1 , which is relatively
large compared to the values reported in similar bilayer structures driven by spin pumping. The structuraldependence of the inverse spin Hall signal and its power dependence provide consistent results with the
dynamical thermal-spin-injection model due to the ferromagnetic resonance.
DOI: 10.1103/PhysRevApplied.8.054031

I. INTRODUCTION
There has been a great deal of interest in spintronic
devices consisting of ferromagnetic-nonmagnetic hybrid
structures since the discovery of spin-dependent transports[1]. In operations of the spintronic devices, a spin current, aflow of spin angular momentum, is a key ingredient rather
than the electric current in charge-based electric devices
because the magnetization can be controlled and detectedby the spin current. Spin current is, in general, created byapplying an electric field across the ferromagnet (FM) andnonmagnet (NM) interface, resulting in an injection of thespin-polarized electrons from the FM into the NM. Thismethod is known as electrical spin injection, which is a
powerful means for generating the spin current because of its
high selectivity and flexibility [2]. Moreover, by extending
the electrical spin injection to a multiterminal FM-NM
hybrid nanostructure, we are now able to create pure spincurrent, a flow of spin angular momentum without anaccompanying charge current. This pure spin current enables
us to observe various intriguing spin-related phenomena
sensitively because of the absence of the charge current andcould be employed to transport the spin information withlow power consumption [3–8]. The origin of the electricalspin injection is due to the spin dependence of the electricalconductivity. Since the Seebeck coefficient in the ferromagnet also shows the spin dependence, it is possible to inject the
spin current into the nonmagnet by using the temperature
gradient instead of the electric field. This method is known
as thermal spin injection, where the generation efficiency*

t-kimu@phys.kyushu-u.ac.jp

2331-7019=17=8(5)=054031(6)often shows significant enhancement in several ferromagnetic metals, depending on its band structure.
Apart from the aforementioned static spin injection, thedynamical spin injection induced by microwave irradiationis also an attractive mean for generating and manipulating
the spin current because of its simple experimental scheme.
To date, the mechanism of the dynamical spin injection
has been attributed to the spin pumping induced by themagnetization precession of ferromagnetic resonance(FMR) [9,10] in the FM/NM bilayer structure. The magnitude of the spin current induced by the spin pumping
is evaluated from the change of the linewidth of the
FMR spectra. However, the linewidth of the FMR spectra
is strongly affected by the crystal and geometrical inhomogeneity, namely, inhomogeneous broadening, whichmakes it difficult to estimate the intrinsic and extrinsic
contribution of the damping constant [11,12]. In addition,
recent studies pointed out the importance of the influenceof the ferromagnetic proximity effect [13] and the
Dzyaloshinskii-Moriya interaction [14] on the magnetization dynamics in the bilayer system. Therefore, the linewidth of the FMR is modified not only by spin pumping
but also by other additional effects. This uncertainty leads
to complications for the quantitative estimation of theinjected spin current due to the spin pumping. Moreover,the magnitude of the ac spin current with the precession
frequency is much larger than the dc spin current in the
spin-pumping mechanism [15]. However, the contributionof the ac spin current has not been considered intensively
because of the difficulty of the systematic analysis. Thus,
the dynamical spin injection based on spin pumping still
poses various problematic issues.

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PHYS. REV. APPLIED 8, 054031 (2017)

Here, we propose another mechanism for dynamical spin
injection based on the FMR-heating effect. We recently
found that heat dissipation due to FMR, which is causedby the imaginary part of the magnetic susceptibility,
increases the temperature of the ferromagnet [16]. This
fact indicates that the FMR-heating effect in the FM/NM
bilayer structure produces the temperature gradient acrossthe FM-NM interface, which could be a driving force ofthe spin current, namely, thermal spin injection [17,18].
Since we have demonstrated that CoFe-based ferromagnetic alloys show high thermal-spin-injection efficiency
because of its large spin-dependent Seebeck coefficientwith a favorable band structure [19], we may develop ahighly efficient spin-injection technique by combining
FMR heating with the appropriate material. In this paper,
we evaluate the performance of dynamical thermal spin
injection based on FMR heating.
II. EXPERIMENTAL PROCEDURES
In this study, we adapt a Co40 Fe40 B20 film as the FMRheating layer because the large electrical resistivity for the(Co,Fe)B film with a relatively small anisotropic magnetoresistance effect suppresses the spurious signals for the
spin-current transport [20]. First, we evaluate the temperature increase due to the FMR of the (Co,Fe)B. We preparea (Co,Fe)B strip underneath a Cu waveguide on a glass
substrate. Here, the (Co,Fe)B strip, whose width, length,
and thickness are 7.5 μm, 500 μm, and 30 nm, respectively,
is prepared by using an ultra-high-vacuum magnetronsputtering system under a base pressure of 10−6 Pa. The
electrical resistivity for (Co,Fe)B is 110 μΩ cm, indicating
an amorphous structure. The Cu waveguide, 8.5 μm wide,
500 μm long, and 100 nm thick, is deposited by Jouleevaporation. Figure 1(a) shows schematic illustrations of
the device structure for the temperature evaluation. Weemphasize that the electrical resistivity for Cu is approximately 2.3 μΩ cm at room temperature, which is approximately 50 times smaller than that for (Co,Fe)B. Thismeans that the current flowing in the (Co,Fe)B layer of thebilayer system is negligibly small because of the large
difference between the electrical resistivity and the film
thickness.
III. RESULTS AND DISCUSSIONS
A. Evaluation of the FMR-heating effectWe measure the resistance of a Cu waveguide on a
(Co,Fe)B strip under microwave irradiation as a function of
the static magnetic field along the (Co,Fe)B strip by using astandard lock-in technique. Figure 1(b) shows a representative result of the field dependence of a resistance under a12-GHz microwave irradiation with a power of 100 mW.
We can see clear peak signatures around the FMR fields,
depending on the microwave frequency, thus confirming
the presence of the FMR-heating effect of the (Co,Fe)B

FIG. 1. FMR-heating effect. (a) Circuit diagram for the resistancemeasurement under rf current injection together with a schematic
illustration of the FMR-heating effect. (b) Representative resistance spectrum due to the FMR-heating effect. (c) Maximum
resistance change ΔRM and the temperature change ΔT M due to
FMR heating as a function of the input power P with a frequencyof 11 GHz.

strip. From the analysis of the resistance spectra with thetemperature dependence of the Cu resistance, we are quantitatively able to estimate the temperature change during the
FMR [16]. Here, we define ΔRM as the maximum value of theresistance change due to the FMR. Here, we can convert ΔRM
to the temperature change ΔT M by using the equation
ΔT M ≈ 885ΔRM =RRT , with the room-temperature resistance
RRT . Figure 1(c) shows ΔRM as a function of the input rf
power. Although the linewidth of the resistance spectra for(Co,Fe)B is much smaller than that for Co43 Fe54 Al3, the ΔT M
value for (Co,Fe)B is larger than that of (Co,Fe)Al [16].
This is the case because the precession amplitude (the coneangle or dynamic susceptibility) increases with a decreasing
damping constant, as described later [21,22]. Thus, theferromagnet with the lower damping constant is suitablefor inducing efficient FMR heating with high-frequency
selectivity, while the larger damping constant contributes
to the broad linewidth of the resistance peak.
B. Dynamical thermal spin injection
due to FMR heating
We then experimentally investigate the dynamical
thermal spin injection due to the FMR heating in aðCo; FeÞB=Pt bilayer system. As schematically shown in
Fig. 2(a), the Cu waveguide, which is a microwave
generator for the FMR, is prepared on the ðCo; FeÞB=Pt
bilayer film. Since the temperature of the ferromagnetic
metal (Co,Fe)B increases under the FMR condition, theheat flow perpendicular to the interface between the(Co,Fe)B and Pt bilayers is generated. This action resultsin the thermal spin injection from the (Co,Fe)B layer into
the Pt one. The spin current injected from the (Co,Fe)B
layer into the Pt layer is converted to the electrical charge

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DYNAMICAL SPIN INJECTION BASED ON HEATING …

PHYS. REV. APPLIED 8, 054031 (2017)
FIG. 2. Dynamical thermal spin
injection. (a) Schematic illustrationof a spin Hall device operated bydynamical thermal spin injection
due to FMR heating. (b) Representative result of the field dependenceof the electric voltage induced by
the inverse spin Hall voltage under
microwave irradiation. (c) Angulardependence of the normalized signal change due to the inverse spin
Hall signal. (d) Power dependenceof the output voltage due to theinverse spin Hall effect. The dottedline represents a linear variation of
the output voltage.

current via the inverse spin Hall effect (SHE) of the Pt layer[4,23]. The ðCo; FeÞB=Pt bilayer film is prepared on a
FZ-Si substrate by using an ultra-high-vacuum magnetronsputtering system. The lateral dimension of the bilayer film
patterned by the stencil mask is 200 μm wide and 600 μm
long. The thicknesses of the (Co,Fe)B and Pt layers are 40
and 10 nm, respectively. The waveguide and the bilayer are
electrically separated by inserting a 100-nm-thick SiO2
film. Here, the resistance of the bilayer film is as low
as 33 Ω.
When the spin direction of the injected spin current isaligned with the x axis, the injected spin current flowing
along z axis is converted to the electrical charge current
along the Pt wire (x axis) through the inverse spin Hall
effect, as is schematically shown in Fig. 2(a). With an opencircuit condition, we can detect spin current as the electricalvoltage along the Pt wire. Figure 2(b) shows the fielddependence of the electrical voltage along the Pt wire underthe microwave irradiation of 6 GHz. A clear voltage peak isobserved at μ0 H ¼ 20 mT, which is exactly same as the
FMR field under 6-GHz microwave irradiation.
To obtain more definite evidence, the angular dependence of the overall signal change is investigated bychanging the direction of the static magnetic field. Here,
we define the angle ϕ, which is the difference between the
direction of the ac current and the static magnetic field. The
angular dependence of the inverse spin Hall effect is known
to show cosine dependence because the direction of theinduced charge current is given by the vector product s × Is ,where s and Is are the electron spin and the spin current,
respectively [4,6,23], We note that, in the present device
configuration, the direction of the ac magnetic field is fixedalong the x axis. Therefore, when the direction of the static
magnetic field is rotated by an angle ϕ, the effective
magnitude of the ac magnetic field should show cos ϕdependence. In such a situation, the output voltage due tothe spin Hall effect is proportional to j cos ϕj cos ϕ. As
can be seen in Fig. 2(c), the angular dependence ofthe normalized signal change is well reproduced byj cos ϕj cos ϕ, strongly supporting the spin Hall nature of
the voltage. However, the observed characteristics can also
be understood by the dynamical spin injection based on thespin pumping. Therefore, we have to find out the specificsignature of the thermal spin injection in order to identify
the proper mechanism of the dynamical spin injection in a
FM/NM bilayer system.
Figure 2(d) shows the maximum voltage change ΔV vs
the input power Pin. ΔV increases linearly below the input
power of 200 mW, but it starts to deviate above the input
power of 200 mW. This kind of saturation tendency was notobserved in previous reports on spin-pumping experiments
because the efficiency of the spin pumping increases anincreasing precession angle. On the other hand, in thethermal-spin-injection case, since the spin-injection efficiency is proportional to the longitudinal component of
the magnetization, increasing the precessional cone angle
contributes to reducing the spin-injection efficiency. Since
we can see a similar power dependence in the FMR-heating
effect shown in Fig. 1(d), this unique signature suggeststhat FMR heating is the main contribution to the output
voltage. Here, the threshold power is much larger than thatfor the FMR-heating device. This is the case because theexcitation efficiency for the spin Hall device is muchsmaller than that of the FMR-heating device. It also should
be noted that the voltage change due to the inverse spin
Hall effect exceeds 100 μV, which is relatively large
compared to the values obtained in the similar metallic
bilayer system based on the spin pumping. This result
implies that the thermal spin injection is more efficient than
the spin pumping in a material with a large spin-dependent
Seebeck effect.
As mentioned before, the bilayer sample structure usedin our study is the same as that for the spin-pumping
experiments [4,23–26]. Therefore, both the thermal spin
injection and the spin pumping are possible mechanisms
for generating the spin current in this structure. To
distinguish the dominant mechanism of the spin-current
generation in the present experiment, we analyze the054031-3

YAMANOI, YOKOTANI, and KIMURA

PHYS. REV. APPLIED 8, 054031 (2017)

experimental data more quantitatively and perform additional experiments. First, to properly evaluate the performance of the present spin Hall device, we introduce thenormalized spin Hall voltage η, which is given by theoverall change of the spin Hall voltage under the opencircuit condition divided by the series resistance of thebilayer system for a unit width Rw, namely, V=ðRwÞ. Thisis the case because the output spin Hall voltage depends onthe geometry and the device dimension. According to thesimple spin-diffusion model, the output spin Hall voltage V
is given by λαSHE iS0 Rw. Here, R is the resistance along thez axis for the Pt layer underneath the waveguide, λ is the
spin-diffusion length for the Pt layer, αSHE is the spin Hall
angle for the Pt layer, and iS0 is the injected spin current
density at the interface. Therefore, we obtain η ¼ λαSHE iS0 ,which is determined by the material parameter without any
geometrical parameters. Consequently, we can fairly compare the spin-injection efficiency by using the normalized
spin Hall voltage η.
Figure 3(c) shows the representative curve of the fielddependence of the normalized spin Hall voltage η under amicrowave irradiation of 6 GHz with an input power
100 mW. We note that the input power is 100 times larger
than the absorption power, which is the effective excitation
power for the FMR, as we address later. The obtained
amplitude of the signal is as high as 23 mV Ω−1 m−1 . Thisvalue is relatively high compared to Py=NM bilayer
systems (0.3–5 mV Ω−1 m−1 for Py=Pt bilayers, approximately 1 mV Ω−1 m−1 for Py=Ta bilayers and about
7 mV Ω−1 m−1 for Py=Pd bilayers [24–27]) and
0.1–1 mV Ω−1 m−1 for yttrium iron garnet/NM bilayer
systems [28–30], where the spin pumping can be considered the main mechanism.
To obtain a more clear signature for the thermal spin
injection, we also investigate the inverse spin Hall effectunder the dynamical spin injection in a Pt=ðCo; FeÞB bilayer

system, where the (Co,Fe)B layer and the Pt film aredeposited on the substrate with a configuration opposite
to that in the previous experiment. From the vector-networkanalyzer measurements, as can be seen in Figs. 3(a) and 3(b),we confirm that the resonant frequency and the frequency
linewidth are not significantly different between the two
samples. In the Pt=ðCo; FeÞB=Si substrate system, since apartial heat flow from the (Co,Fe)B layer diffuses into thesubstrate directly without passing through the Pt film in thisconfiguration, the magnitude of the thermally excited spincurrent injected into the Pt film should decrease, resulting
in the reduction of the inverse spin Hall voltage from theprevious experiment. On the other hand, if the main
mechanism for the spin injection is spin pumping, the spin
Hall voltage should not change, because of negligibly small
spin pumping into the Si substrate.
Figure 3(d) shows the normalized inverse spin Hall
voltage in the Pt=ðCo; FeÞB system. The sign of the voltage
is also reversed from the previous experiment because theflowing direction of the spin current is the opposite of theprevious experiment using a ðCo; FeÞB=Pt=Si substrate
system. Moreover, we can clearly confirm the significant
reduction of the induced voltage. By separating thesymmetric and antisymmetric contributions, we can obtaina more clear signature of the thermal spin injection. As canbe seen in Figs. 3(e) and 3(f), the symmetric signal showsa significant reduction, although the antisymmetric signal
does not change so much. This is the case because the heatflow from the (Co,Fe)B layer into the Pt wire is reduced.
These stacking-order dependences cannot be understoodby the spin-pumping model. The observed tendency is the
opposite of the previous study on stacking-order dependences [23], where the eddy current produces an additionalrf magnetic field. This fact may be due to the thinner
thickness of the Pt film and the smaller overlap area
between the waveguide and the ðCo; FeÞB=Pt bilayer.
FIG. 3. Distinguishing between thermal
spin injection and spin pumping. FMR
spectra under various magnetic fields in(a) the ðCo; FeÞB=Pt=Si substrate sampleand (b) the Pt=ðCo; FeÞB=Si substrate sample.
(c) Field dependence of the normalized spin
Hall signal induced in ðCo; FeÞB=Pt=Si substrate sample under microwave irradiation.
The vertical axis is the voltage normalized
by the resistance and the width of the bilayer.
(d) Field dependence of the normalized spin
Hall signal induced in Pt=ðCo; FeÞB=Si substrate sample under microwave irradiation.
(e) Separation of symmetric and antisymmetric contributions to the induced electric voltage in the ðCo; FeÞB=Pt=Si substrate sample.
(f) Separation of symmetric and antisymmetric contributions to the induced electric voltage in the Pt=ðCo; FeÞB=Si substrate sample.

054031-4

DYNAMICAL SPIN INJECTION BASED ON HEATING …
It also should be noted that these results exclude thepossibility of the anomalous Nernst effect (ANE) from the(Co,Fe)B layer because the ANE should not show stackingorder dependence. Although the spin pumping also contributes to the generation of the spin current, these results
strongly suggest that the dominant contribution of the
dynamical spin injection in our ðCo; FeÞB=Pt system is thethermally excited spin current associated with the large
spin-dependent Seebeck effect. We would emphasize thatthe non-negligible contribution from thermal spin injection
should be considered even in a NiFe=Pt bilayer system,
which is widely utilized in spin-pumping experiments[24–26]. The estimated spin Hall angle based on a spinpumping mechanism may be overestimated because thethermally excited spin current is neglected.
IV. CONCLUSION
In this work, we show that the FMR-heating effect iseffectively excited by the microwave irradiation under thebias magnetic field in a (Co,Fe)B film. We demonstratethat the inverse spin Hall effect is induced by the thermal
spin injection driven by the FMR-heating effect in aðCo; FeÞB=Pt bilayer system. The obtained voltage dueto the inverse spin Hall effect is relatively large compared toa similar bilayer system based on spin pumping, indicating
the efficient thermal spin injection from (Co,Fe)B. The
stacking-order dependences strongly support the fact thatthe spin current is generated by thermal spin injection due
to the FMR heating of the (Co,Fe)B film.
ACKNOWLEDGMENTS
This work is partially supported by Grant-in-Aid for
Scientific Research on Innovative Area, “Nano Spin
Conversion Science” (JP26103002) and that for Scientific
Research (S) (JP25220605), and Canon Foundation.

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