# Modulation of Spin Seebeck Effect by Hydrogenation

K. Ogata  Department of Integrated Science, University of Tokyo, Meguro, Tokyo
153-8902, Japan  T. Kikkawa  Department of Applied Physics, University of
Tokyo, Bunkyo, Tokyo 113-8656, Japan  E. Saitoh  Department of Applied
Physics, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan  Institute for AI
and Beyond, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan  Y. Shiomi
<EMAIL_ADDRESS>Department of Integrated Science, University of
Tokyo, Meguro, Tokyo 153-8902, Japan  Department of Basic Science, University
of Tokyo, Meguro, Tokyo 153-8902, Japan

###### Abstract

We demonstrate the modulation of spin Seebeck effect (SSE) by hydrogenation in
Pd/YIG bilayers. In the presence of 3% hydrogen gas, SSE voltage decreases by
more than 50 % from the magnitude observed in pure Ar gas. The modulation of
the SSE voltage is reversible, but the recovery of the SSE voltage to the
prehydrogenation value takes a few days because of a long time constant of
hydrogen desorption. We also demonstrate that the spin Hall magnetoresistance
of the identical sample reduces significantly with hydrogen exposure,
supporting that the observed modulation of spin current signals originates
from hydrogenation of Pd/YIG.

††preprint: AIP/123-QED

Hydrogen is an energy carrier which can be produced by an environmentally
clean process and therefore has a positive impact on decarbonization
ma12121973 . To utilize hydrogen as a clean and renewable alternative to
carbon-based fuels, hydrogen safety sensors are also critical to assure the
development of hydrogen systems BUTTNER20112462 . Metal-hydride systems have
been widely studied for the potential of solid-state hydrogen storage and
sensing. In particular, Pd is frequently used as a catalyst for hydrogen
dissociation and adsorption. A hydrogen molecule decomposes into independent
hydrogen atoms when the molecule approaches a Pd surface due to the strong
interaction between Pd and H atoms. As the smallest single atom, a H atom can
easily diffuse into the interstices of the Pd lattice and cause lattice
expansion. As a result, hydrogen adsorption changes the density of states of
Pd near the Fermi energy PhysRevLett.42.476 ; PhysRevB.20.3543 , significantly
modulating its electrical and optical properties.

The hydrogenation of Pd films also impacts spintronic effects. Pd is known to
exhibit a strong spin-orbit coupling and has been used in many spintronic
experiments. Magnetic multilayers and super-lattices which include Pd layers
have been of particular interest. It was reported that in Co/Pd bilayers which
possess a strong interfacial perpendicular magnetic anisotropy, the magnetic
anisotropy and ferromagnetic resonance are reversibly modulated by hydrogen
exposure doi:10.1063/1.4800923 ; doi:10.1063/1.4893588 ; PhysRevB.83.094432 ;
OKAMOTO2002313 ; doi:10.1063/1.4812664 ; doi:10.1063/1.4800923 ;
doi:10.1063/1.4905463 ; AKAMARU2015S213 . Efficient hydrogen sensing based on
magnetization dynamics was also reported in similar materials
doi:10.1063/1.4800923 ; doi:10.1063/1.4985241 ; doi:10.1063/1.4993158 .
Moreover, the inverse spin Hall effect (ISHE) induced by spin pumping was
successfully modulated by hydrogen exposure in Co/Pd bilayers 8119549 ;
PhysRevB.101.174422 . Absorption of hydrogen gas at 3% concentration in the Pd
layer reduces the ISHE voltage by 20% PhysRevB.101.174422 . This decrease in
ISHE signals in the presence of hydrogen gas was attributed to the decrease in
spin diffusion length due to enhanced scatterings to hydrogen atoms in the Pd
layer. After the hydrogen gas is flushed out of the setup, the ISHE voltage
returns to the prehydrogenation value; hence the observed effect is
reversible.

The studies of hydrogen effects on spintronic materials have been carried out
for the combination of Pd layers with itinerant magnetic films. For the
measurement of ISHE in ferromagnetic/nonmagnetic metallic bilayers, however,
it is known that the precise estimation of ISHE voltage is difficult because
of spurious spin rectification effects such as anisotropic magnetoresistance
and anomalous Hall effect which couple the dynamic magnetization to microwave
currents in the ferromagnetic layer doi:10.7566/JPSJ.86.011003 . Hence
magnetic insulators such as Y3Fe5O12 (YIG) should be more suitable to
investigate hydrogen effects on pure ISHE signals kajiwara2010transmission .

In this letter, we demonstrate the reversible manipulation of the spin Seebeck
effect (SSE) by hydrogen exposure in Pd/YIG bilayers. The SSE is the
generation of a spin current as a result of a temperature gradient applied
across a junction consisting of a magnet and a metal doi:10.1063/1.3507386 ;
uchida2016thermoelectric . The spin current injected into a metal can be
converted into a voltage by ISHE. Since the ISHE induced by SSE is a
transverse thermoelectric effect, it can be employed to realize transverse
thermoelectric devices, which could potentially overcome the inherent
limitations of conventional thermoelectric devices D1EE00667C ;
kikkawa2021observation . Moreover, SSE is expected to be utilized for flexible
heat-flow sensors kirihara2016flexible . The manipulation of SSE by
hydrogenation demonstrated below may open up new device potentials in spin
caloritronics.

We used epitaxial YIG films with 2 micron thickness grown by liquid phase
epitaxy on Gd3Ga5O12 (111) substrates. The YIG surfaces were mechanically
polished, and then 5-nm-thick Pd films were sputtered at room temperature. The
Pd layer is 5 mm long and 0.5 mm wide. For the Pd/YIG bilayers, the SSE
measurements in the longitudinal configuration doi:10.1063/1.3507386  were
performed at room temperature using an electromagnet (3470 Electromagnet
System, GMW Associates). The bilayer sample was placed between sapphire and
copper plates. A 1-k${\rm\Omega}$ resistive heater was attached to the upper
sapphire plate and the lower copper plate is a heat sink. To facilitate
hydrogenation of the Pd layer, a breathable tape (TBAT-252, TRUSCO) was
inserted between the upper plate and the sample [Fig. 1(a)]. The temperature
gradient is generated by applying an electric current to the heater. Two-pairs
of leads were attached to the Pd layer to measure not only the SSE but also
the spin Hall magnetoresistance (SMR) PhysRevLett.110.206601  in the same
setup. The distance between the voltage terminals is 2.5 mm. The
thermoelectric voltage due to the ISHE induced by SSE was monitored with a
Keithley 2182A nanovoltmeter. SMR was measured by lockin detection using
Anfatec USB Lockin Amplifier 250; the frequency and amplitude of ac electric
current are 111 Hz and 0.8 mA. The sample was loaded into a small chamber to
control the atmosphere. For hydrogenation measurements, the samples were first
measured in pure Ar gas ($>$99.9999 vol.%) at atmospheric pressure followed by
a 3%/97% H2/Ar gas mixture. Before the measurements in Ar-H2 gas, we waited
20-40 minutes for the Pd layer to be completely hydrogenated
PhysRevB.101.174422 ; lederman2004magnetooptic  after the chamber was filled
with 1 atm Ar-H2 gas.

Figure 1: (a) Measurement setup of the SSE. A breathable tape was inserted
between Pd/YIG and the heater part to facilitate hydrogen absorption and
desorption in the Pd film. (b) Magnetic field ($H$) dependence of the SSE
voltage ($V_{SSE}$) measured in 1 atm of Ar. The heater power ($P$) was
changed from 0 mW to 100 mW. (c) Heater power ($P$) dependence of the SSE
voltage ($V_{SSE}$) at 200 mT. The raw data is shown in (b).

First, we measured SSE voltage $V_{SSE}$ of Pd/YIG in Ar atmosphere. Figure
1(b) shows the magnetic-field ($H$) dependence of $V_{SSE}$ measured at
several heater power levels. Here symmetric components of the output voltage
with respect to $H$ are subtracted and antisymmetric components are plotted;
note that the symmetric components which are not to be attributed to the
effect under study are almost independent of $H$ in our measurements. When the
heater is off, $V_{SSE}$ is almost zero in the entire $H$ range in Fig. 1(b).
As the heater power $P$ increases from zero, the clear SSE signals appear and
their magnitudes increase with $P$. The sign of $V_{SSE}$ is the same as that
reported for Pt/YIG doi:10.1063/1.3507386 . The saturated magnitude of
$V_{SSE}$ is plotted against $P$ in Fig. 1(c). The $V_{SSE}$ magnitude
increases linearly with the heater power, which indicates that $V_{SSE}$ is
proportional to temperature gradient generated across the Pd/YIG junction. The
temperature difference $\Delta T$ generated at $P=100$ mW is estimated to be
$\sim$1.5 K (see Fig. S1 in Supplementary Material).

Figure 2: Magnetic field ($H$) dependence of the SSE voltage ($V_{SSE}$)
measured before hydrogenation, during exposure to hydrogen gas, and after the
hydrogen gas is flushed out of the setup. The heater power is kept at $100$
mW.

Next, the effect of exposing the 3% H2 mixture on the Pd/YIG sample is
investigated in Fig. 2 (see also Fig. S2 in Supplementary Material for
additional experimental results). Here the heater power $P$ is kept constant
at 100 mW during the series of measurements. After the initial SSE measurement
in pure Ar gas at atmospheric pressure already shown in Figs. 1(b) and 1(c),
the sample chamber was filled with H2 3% H2/Ar mixture and the SSE measurement
was performed. As shown in Fig. 2, the magnitude of $V_{SSE}$ is found to be
reduced by more than 50% in the presence of H2 gas. After completing the SSE
measurement in Ar-H2 atmosphere, the sample was then remeasured in pure Ar.
The $V_{SSE}$ magnitude returned to the pristine value as shown in Fig. 2.
Note that this data was taken 2.5 days after the chamber was refilled with Ar.
The observed decrease in the SSE signal is safely ascribed to the presence of
hydrogen in Pd/YIG, and importantly, the change is reversible.

It is well known PhysRevB.101.174422 ; doi:10.1063/1.4986214  that upon
hydrogenation, Pd thin films undergo two stages of lattice expansion depending
on the hydrogen gas concentration. For light concentration levels up to 2-3 %,
the lattice constant grows by approximately 1% in the out-of-plane direction
only. This expansion is reversible. In the second stage, the lattice constant
grows by up to 4% in both out-of-plane and in-plane directions. These changes
are irreversible, causing structural changes to the Pd lattice. In our SSE
measurements under 3% hydrogen gas, the sample should undergo the first stage
of lattice expansion and the SSE is thereby reversible. Note that we confirmed
by x-ray diffraction that the Pd films are (111) oriented as in the literature
doi:10.1063/1.4986214  (see Supplementary Material).

Figure 3: (a) Magnetic field ($H$) dependence of the SSE voltage ($V_{SSE}$)
measured 1.7-66 hours after Ar gas is refilled in the measurement chamber. The
heater power is kept at $100$ mW. (b) Time dependence of the SSE voltage
($V_{SSE}$) at 210 mT measured after Ar gas is refilled in the measurement
chamber. The selected raw data is shown in (a). The black curve is a fit to
the experimental data (see text).

Though the modulation of SSE by hydrogen absorption/desorption is reversible,
the recovery time of the SSE signal due to hydrogen desorption is as long as
2.5 days. It was reported that the hydrogen desorption takes a long time in
contrast to the quick hydrogen absorption doi:10.1063/1.4812664 , and the
response time depends significantly on materials. The time for hydrogen
desorption is typically at most several tens of minutes for Co/Pd
doi:10.1063/1.4800923 ; doi:10.1063/1.4893588 ; PhysRevB.83.094432 ;
OKAMOTO2002313 ; doi:10.1063/1.4812664 ; doi:10.1063/1.4800923 ;
doi:10.1063/1.4905463 ; AKAMARU2015S213 , while the completion of the entire
desorption requires at least a few days at $10^{-3}$ mbar for FePd alloys
doi:10.1063/1.5142625 . Our Pd/YIG also includes Fe and Pd, and the situation
looks similar to FePd alloys.

We then take a closer look on the dehydrogenation process by the time
dependent measurement of SSE in Fig. 3. Figure 3(a) shows $V_{SSE}$ curves
measured at different times after the measurement chamber is refilled with
pure Ar gas. The $V_{SSE}$ magnitude is approximately 0.5 ${\rm\mu}$V just
after the gas is replaced with Ar, and increases monotonically with time.
After 50 hours, the $V_{SSE}$ magnitude is almost saturated at $\sim 1$
${\rm\mu V}$.

The time dependence of $V_{SSE}$ at 210 mT is plotted in Fig. 3(b). The
$V_{SSE}$ magnitude increases monotonically with time, as already shown in
Fig. 3(a). We fit the experimental data by a standard relaxation function:
$V_{SSE}\propto 1-e^{-t/\tau}$, where $t$ is the measurement time and $\tau$
is a time constant of hydrogen desorption. The fitting curve matches the
experimental data very well, meaning that the hydrogen desorption follows an
exponential function. The same function was adopted for the hydrogenation
effect on magneto-optical effects in Pd/Co/Pd films doi:10.1063/1.4812664 .
The fit in Fig. 3(b) yields $\tau\approx 25$ hour. Such a long time constant
was not observed in the spin pumping measurement for Pd/Co bilayers
PhysRevB.101.174422 .

In contrast to the spin pumping measurements, the attachment of the heater to
the Pd surface is required in the SSE measurements, which may adversely affect
the absorption/desorption of hydrogen because of small numbers of exposed
surface atoms. To confirm that spin current signals in the Pd layer is indeed
modulated by hydrogenation, we also perform the measurement of SMR (spin Hall
magnetoresistance) for the same sample in the same setup. The SMR is a
magnetoresistance effect related to a nonequilibrium proximity effect caused
by the simultaneous action of the SHE and ISHE PhysRevLett.110.206601 ;
PhysRevB.87.144411 ; the absorption/reflection of spin current at the
ferromagnet/metal interface results in magnetoresistance, since the spin-
dependent scattering at the metal/ferromagnet interface depends on the angle
between the polarization of spin Hall current and the magnetization of the
attached magnetic layer. The experimental setup is illustrated in the inset to
Fig. 4(a). Magnetic field is applied perpendicular to the electric-current
direction in the film plane.

Figure 4: Magnetic field ($H$) dependence of the magnetoresistance (MR) ratio
($\rho(H)/\rho(H=0)-1$) measured before hydrogenation (a), during exposure to
hydrogen gas (b), and after the hydrogen gas is flushed out of the setup (c).

Figure 4 shows the hydrogen effects on SMR in the Pd/YIG bilayer. Here, since
the size of SMR is very small, the magnetoresistance measurements were
repeated several times and averaged. The error bars stand for the standard
errors. Before hydrogenation [Fig. 4(a)], a negative magnetoresistance effect
is observed. The magnetic-field dependence of resistance change follows the
magnetization process of the YIG layer, consistent with the SMR
PhysRevLett.110.206601 . The size of SMR is about $1\times 10^{-3}$%. This
magnitude is about ten times smaller than that in Pt/YIG
PhysRevLett.110.206601 . A small SMR of about 10% compared to Pt/YIG was also
reported in the literature PhysRevB.98.224424 .

During the exposure to 3% hydrogen gas, the SMR magnitude decreases
significantly as shown in Fig. 4(b). Although quantitative analysis is
difficult because of the large error bars, the suppression of SMR ratio by
hydrogenation looks more than 50%, consistent with the modulation in SSE
voltages (Figs. 2 and 3). After the hydrogen gas is flushed out of the chamber
and pure Ar gas is refilled, we confirmed that the size of SMR returns to the
initial value [Fig. 4(c)].

An important finding in the SMR measurement is that the SMR ratio has already
returned to its original value 30 minutes after refilling Ar gas. Namely, the
time constant of hydrogen desorption in the SMR measurement is much shorter
than that in the SSE measurement. Since both the measurements were performed
for the same sample in the same setup, the long time constant of hydrogen
desorption in the SSE measurement cannot be attributed to impurities/defects
in the Pd layer, surface oxidation, surface morphology doi:10.1063/1.4812664 ,
or moisture which may trap hydrogen atoms and hinder the hydrogen desorption
JOSE20106804 ; KISHORE2005234 .

In our measurements of SSE and SMR, spin current signals are significantly
suppressed by hydrogen exposure as shown in Figs. 2-4. The decrease in the
spin Hall signals with hydrogen exposure is consistent with the previous spin
pumping measurements for Co/Pd 8119549 ; PhysRevB.101.174422 . Scatterings of
conduction electrons to hydrogen atoms in the Pd layer decrease the spin
diffusion length due to the enhanced Elliot-Yafet relaxation mechanism, and
result in the decrease in spin-pumping signals PhysRevB.101.174422 . This
mechanism should be also applicable to SSE and SMR. Since the theory has shown
that both effects depend on spin diffusion length and spin Hall angle of the
Pd layer uchida2016thermoelectric ; PhysRevB.87.144411 , the signal variation
by hydrogenation can be attributed to the decrease in the spin diffusion
length PhysRevB.101.174422 . On the other hand, it is notable that magneto-
optical Kerr signals are enhanced by hydrogenation in Co/Pd bilayers
doi:10.1063/1.4905463 ; doi:10.1063/1.4812664 , in contrast to the decrease in
the spin current signals 8119549 ; PhysRevB.101.174422 . In transport
measurements such as (inverse) spin Hall effects, enhanced electron
scatterings due to interstitial hydrogen impurities are likely to play a
dominant role in the hydrogenation effect. The significant scattering effect
due to hydrogen atoms is also evidenced by the reduction of the anomalous Hall
signal in hydrogenated CoxPd1-x films doi:10.1063/1.4985241 .

The decrease in the $V_{SSE}$ magnitude ($>50$%) by hydrogen exposure is
noticeably greater than the change in ISHE signals reported in the spin
pumping measurements PhysRevB.101.174422 ; the decrease in the spin-pumping
voltage in H2/Ar mixture with 3% of hydrogen was only 20%. The larger signal
change in our results suggests that there may be other factors for the
reduction of $V_{SSE}$ besides hydrogenation of the Pd layer. The first
possibility is imperfect separation of ISHE signals from spin rectification
effects in metallic Co/Pd bilayers 8119549 ; PhysRevB.101.174422 . Another
possible origin is different interfacial stresses to the Pd layer between YIG
and Co. It is known that electrical resistivity of single-layer Pd grown on Si
substrates increases upon hydorogenation, while it tends to decrease for
bilayer cases PhysRevB.101.174422  because of the interfacial compressive
stress from the underlying layer. The interfacial stress can also affect the
interface spin mixing conductance, modulating the injection efficiency of spin
currents. Note that the resistivity of the Pd film on YIG decreases by
hydrogenation, but the change in resistivity is as small as 1% (Fig. S3 in
Supplementary Material), which cannot explain the large variation ($>$ 50%) of
SSE voltage by hydrogen exposure.

Moreover, since the SSE also depends on bulk spin transport in the YIG layer
PhysRevB.95.174401 ; PhysRevX.6.031012  in contrast to the spin pumping,
hydrogen effects on YIG may contribute to the significant reduction in the
$V_{SSE}$ magnitude. Hydrogen diffusion in YIG was indeed reported for
annealed samples in H2 atmosphere doi:10.1063/1.333600 ; MILANI198573 ;
doi:10.1063/1.337383 . The hydrogen diffusion in the YIG layer can suppress
the magnon and phonon transport, which should reduce the $V_{SSE}$. Also the
interface spin-exchange coupling can be weakened by hydrogen around the
interface, leading to the decrease in the interface spin-injection efficiency.

The presence of hydrogen effects on the YIG layer is also suggested by the
different recovery time constants between SSE and SMR. Our measurements in
Figs. 3 and 4 showed that the time constant for the signal recovery of SSE is
much longer than that of SMR. The different recovery time constants are
attributable to different bulk sensitivity of these effects. In SMR, spin-
dependent scattering at the Pd/YIG interface is essential. In contrast, bulk
thermal spin current also plays an important role in the SSE voltage
PhysRevB.95.174401 ; PhysRevX.6.031012  in addition to the interfacial spin
coupling. Bulk properties of magnetic materials such as bulk magnetization,
thermal conductivity, and magnon transport coefficient contribute to the SSE
signals, but not to SMR or spin pumping. Also in the case of magneto-optical
effects of Pd/Co frequently studied before for hydrogenation effects, the
variation of perpendicular magnetic anisotropy originates from interface
effects doi:10.1063/1.4800923 ; doi:10.1063/1.4812664 . Hence the SSE is a
rare spintronic phenomenon that depends not only on interface properties but
also on bulk properties of magnons and phonons in the magnetic layer. Hydrogen
effects on YIG could be related to the reduction of $V_{SSE}$ and also the
long time constant for hydrogen desorption in SSE.

In conclusion, we experimentally demonstrated the reversible modulation of SSE
and SMR by hydrogenation in Pd/YIG bilayers. Absorption of hydrogen results in
the decrease in both SSE and SMR signals by more than 50%. Enhanced
scatterings of conduction electrons to hydrogen atoms in the Pd layer are
partly responsible for the decrease in the spin-current signals, as reported
in the previous spin pumping experiments. The modulation of SSE voltage is
reversible, but the time constant for the signal recovery is longer than 2
days. The long time constant for hydrogen desorption in the SSE measurement is
in contrast to the case of SMR, in which the SMR ratio already returned to the
prehydrogenation value 30 minutes after the chamber was refilled with pure Ar.
We speculate that the significant decrease in the SSE magnitude by hydrogen
exposure and the long time constant for hydrogen desorption in SSE are related
to the hydrogen modulation of bulk properties of the YIG layer, since the SSE
depends not only on interfacial spin couplings but also on bulk properties of
the magnetic layer. We hope that the present results will stimulate further
research on hydrogen effects on Pd films grown on insulating magnetic oxides.

See the supplementary material for additional SSE data, x-ray diffraction
data, and resistivity change by hydrogen exposure.

We thank Y. Miyazaki for the experimental help of sample preparation and Dr.
T. Yokouchi for the fruitful discussion. This research was supported by JST
CREST (JPMJCR20C1 and JPMJCR20T2), Institute for AI and Beyond of the
University of Tokyo, and JSPS KAKENHI Grant Numbers JP20H05153, JP20H02599,
JP20H04631, JP21K18890, JP19H05600, and JP19H02424.

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

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