Spin currents injected electrically and thermally from highly spin polarized Co2MnSi
Alexander Pfeiffer, Shaojie Hu, Robert M. Reeve, Alexander Kronenberg, Martin Jourdan, Takashi Kimura, andMathias Kläui
Citation: Applied Physics Letters 107, 082401 (2015); doi: 10.1063/1.4929423
View online: http://dx.doi.org/10.1063/1.4929423
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APPLIED PHYSICS LETTERS 107, 082401 (2015)

Spin currents injected electrically and thermally from highly spin polarized
Co2MnSi
Alexander Pfeiffer,1 Shaojie Hu,2 Robert M. Reeve,1 Alexander Kronenberg,1€ui1,a)Martin Jourdan,1 Takashi Kimura,2,3 and Mathias Kla
1

Institut f€
ur Physik, Johannes Gutenberg-Universit€
at Mainz, 55099 Mainz, Germany
Research Center for Quantum Nano-Spin Sciences, Kyushu University, 6-10-1 Hakozaki,
Fukuoka 812-8581, Japan3
Department of Physics, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan
2(Received 19 May 2015; accepted 6 August 2015; published online 24 August 2015)
We demonstrate the injection and detection of electrically and thermally generated spin currents
probed in Co2MnSi/Cu lateral spin valves. Devices with different electrode separations arepatterned to measure the non-local signal as a function of the electrode spacing and we determine a
relatively high effective spin polarization a of Co2MnSi to be 0.63 and the spin diffusion length of
Cu to be 500 nm at room temperature. The electrically generated non-local signal is measured as afunction of temperature and a maximum signal is observed for a temperature of 80 K. The thermally generated non-local signal is measured as a function of current density and temperature in asecond harmonic measurement detection scheme. We find different temperature dependences forthe electrically and thermally generated non-local signals, which allows us to conclude that thetemperature dependence of the signals is not just dominated by the transport in the Cu wire, butthere is a crucial contribution from the different generation mechanisms, which has been largely
C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4929423]
disregarded till date. V
Recently, pure spin currents have been receiving a great
deal of attention as an exciting and efficient new means ofmanipulating the magnetic state of a device, while potentially reducing disadvantageous Joule heating and Oersted
field effects at the position of the ferromagnet that is manipulated. Non-local spin valves, consisting of two spatially separated magnetic nano-structures bridged by a nonmagnetic
channel, have been intensively studied as an easy possibility
to generate and detect pure spin currents via spin injection
from the injector into the conduit.1,2 This leads to a spin
accumulation which diffuses away from the injection pointand comprises a pure diffusive spin current in the directionof the second ferromagnetic electrode, where the spin current
can be detected and manipulate the local magnetization due
to the spin transfer torque.3,4 A large efficiency for a domain
wall displacement assisted by a pure spin current4 and evenpure spin current induced domain wall displacement wasreported.5 Furthermore, non-local spin valves have recently
started to be intensely investigated as a geometry for futuremagnetic read-head devices.6 Scientifically, the non-local
technique allows for the determination of key parameters of
the spin transport, namely, the spin polarization a of the ferromagnetic and the spin diffusion length of the nonmagnetic
material3 and based on these parameters the ratio between
spin and charge current.7 To generate larger spin currents
and improve the efficiency of magnetization manipulation
but also to obtain larger signals for use in read-heads, recent
studies used Heusler based ferromagnetic electrodes andfound large spin signals.8–14
One promising material for non-local spin valves is the
Heusler compound Co2MnSi for which recently 100% spinpolarization at room temperature was observed.15a)Electronic mail: klaeui@uni-mainz.de

0003-6951/2015/107(8)/082401/5/$30.00

In addition to these electrically generated spin currents,
it has recently been demonstrated that it is also possible togenerate thermal spin currents in non-local spin valves via
the spin dependent Seebeck effect due to the thermal gradient that is established at the injector-conduit interface as aresult of Joule heating.12,13,16–18 While potentially very
useful, for instance, for waste-heat recovery, for this generation method, spin current efficiencies currently tend to belower than the electrical injection method, calling for
improvements. Hu et al. made recent progress in this regard
by employing a CoFeAl thermal spin current injector electrode.13 The resulting dramatic improvements to the spin
injection efficiency were partly attributed to the large spindependent Seebeck coefficient which arises from thechange in sign in the Seebeck coefficient for spin up anddown electrons due to the large difference in DOS for thedifferent spin-subbands at the Fermi level and partly attributed to the low spin resistance leading to reduced backflow
spin absorption.19
In the present work, we study Co2MnSi/Cu non-local
spin valves. By measuring the electrically generated nonlocal signal as a function of the electrode separation we areable to extract the key parameters for our devices at room
temperature finding a spin diffusion length of 500 nm in the
Cu conduit and a spin polarization of 0.63 at room temperature. The reduction of the measured spin polarization ascompared to the previously measured intrinsic spin polarization of the Heusler material is attributed to the lower interface spin polarization, highlighting the importance of theinterface properties on the measured signals. We compare
the electrically generated non-local signals as a function oftemperature with thermally generated signals in the same
sample. These measurements show a significantly different
temperature behaviour, which can be explained by different

107, 082401-1

C 2015 AIP Publishing LLC
V

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Pfeiffer et al.

temperature dependencies of the injection mechanisms at theCo2MnSi/Cu interface. Such contributions have been largely
overlooked in previous studies, where mostly spin relaxation
mechanisms in the non-magnetic material are proposed toaccount for the temperature evolution of the signals, which
are clearly not sufficient to explain our results.20–23
A schematic depiction and a scanning electron microscope
image of the patterned nanowires are shown in Figure 1.
A 47 nm thick Co2MnSi and 42 nm thick Ag thin film isgrown via rf sputtering and then nano-structured to obtainabout 200 nm wide ferromagnetic electrodes with a 200 nm
thick and 170 nm wide Cu nanowire on top by two-steplift-off electron beam lithography. We have confirmed
the 100% intrinsic spin polarization of the grown Co2MnSi
as described in Ref. 15. Before depositing the Cu nanowire,
30 s Arþ-ion milling is performed to provide clean interfaces. Different devices, fabricated from the same film andmade in the same batch, were measured with a range inelectrode separations from 200 nm to 1000 nm. In order tocharacterize the devices, we perform two sets of measurements. First, for the study of spin currents via electrical spininjection we apply a small 220 lA alternating current
between the injector and the end of the Cu wire which acts
as the non-magnetic (N) spin current conduit (contact 6 andcontact 5 as shown in Figure 1). This generates a spin accumulation which diffuses as a pure spin current towards theinterface between the Cu wire and the detector. If the spin resistance is low,19 the pure spin current is then strongly
absorbed in the detector. Depending on the relative orientation of the magnetization of the two ferromagnetic electrodes, a high non-local voltage for a parallel alignment and a
low non-local voltage for an antiparallel alignment of themagnetization of the nanowires are measured between thedetector and the conduit (contact 7 and contact 1) in the firstharmonic measurement via a standard lock-in technique.
Note that for the current densities used, the Oersted field is
negligible compared to the switching fields and points furthermore perpendicular to the plane. As a second effect, astronger applied current leads to significant Joule heating of

FIG. 1. (a) Schematic illustration of a non-local spin valve consisting of
two nanowires made of Co2MnSi bridged by a Cu nanowire. (b) High resolution scanning electron microscope image with numbered contacts andthe nanowires labeled FM1, FM2, FM3, and N. Three nanowires provide
the possibility to measure two non-local signals with one device using
FM2 as detector and FM1 or FM3 as injector.

Appl. Phys. Lett. 107, 082401 (2015)

the FM1/N interface. The resulting temperature gradient
between the hot junction and the cold sample environment
gives rise to a thermally generated spin current due to thespin dependent Seebeck effect.12,13,16 Since the measured
voltage scales quadratically with the applied charge current
due to Joule’s law, we measure the thermally generated nonlocal voltage as the second harmonic signal with the sameprobe configuration as for the electrically generated signal.
Temperature dependent measurements are carried out for
certain temperatures between 5 K and 300 K in the same
sample, where we keep the applied charge current and thetemperature constant during the measurement of the nonlocal signal. For a quantitative comparison in the case ofelectrically generated spin currents, we define a non-local resistance via VNL/Iac where VNL is the measured non-local signal and Iac the applied charge current of about 220 lA.
A non-local curve with its characteristic two states while
sweeping the field along the easy axes of the wires is shown
inside in Figure 2(a). Furthermore, we present in this figure
the non-local resistance as a function of the electrode distance where the red dots are the measured data points andthe orange curve is a fit using an analytical solution7,24 of thespin diffusion problem in 1-D
DRNL ðdÞ ¼

a2 R2F RNexpðd=kN Þ 2RN RF þ 2R2F þ R2N sinhðd=kN Þ:(1)


FIG. 2. (a) Observed non-local signal as a function of the electrode distance measured at room temperature and the corresponding fit based on
Equation (1). (b) Normalized non-local signal as a function of temperature
where we observe a maximum signal at a temperature of 80 K and a reduction of the signal of 40% between 5 K and 80 K.

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082401-3

Pfeiffer et al.

In this formula, a is the effective spin polarization of
Co2MnSi and kN is the spin diffusion length of Cu. RN and
RF are the spin resistances of the nonmagnetic and ferromagnetic materials which are defined as RS;i ¼ 2qi  ki =ðSð1  ai Þ2 Þ with q as the resistivity and S as the effective
cross sectional area where the spin current flows. kF is
assumed to be 1 nm, the average effective cross section S is
determined to be 33 000 nm2 while the resistivities are measured as 3.2  107 X m for Co2MnSi and 1.8  108 X m for
Cu. We determine the spin diffusion length kN of Cu to be
500 nm which is in agreement with other reports20,21,25,26
and determine a to be 0.63 at room temperature. Whilehigher than for typical 3d metals, this spin polarization value
is lower than the recently reported intrinsic spin polarizationof Co2MnSi15 which implies significant spin relaxation
occurs when the current is injected across the Co2MnSi/Cu
interface. For transparent contact sources of such spin relaxation can include absorption of the spin accumulation by theferromagnet due to a low spin resistance,20 increased backscattering of electrons from the start of the conduit and spin
flip scattering of electrons as they cross the interface or at
impurities in the vicinity of the interface.27 However, for a
perfect spin polarized material the spin relaxation within the
ferromagnet should be suppressed.11 In our case, the junction
resistances are relatively high for an all metallic system (onthe order of 5 X), which means that indeed spin flip at theinterface can occur. In this case, we are probing the interfacial spin polarization in the measurement which can be
expected to be lower than the intrinsic spin polarization.28
To circumvent this problem in the future, dedicated materials
combinations have to be developed where the interfacial
polarization of the spins that enter the non-magnetic spin
conduit is 100%, which can possibly be achieved by bandstructure matching and using an all-Heusler stack, which isthough beyond the scope of the current work.
In Figure 2(b), we show the non-local signal as a functionof temperature in the range of 5 K and 300 K for the 350 nm
separation device. Initially on reducing the temperature thesignal displays a monotonic increase, as expected from the
Elliott-Yafet spin relaxation mechanism in the case of a monotonic decrease in resistivity on cooling.29 However, around
80 K a maximum in the signal is observed and at lower temperatures the signal decreases significantly. Whilst an obvious
low temperature decrease in signal is not apparent in the
recently studied Heusler non-local spin valves,10,11 the occurrence of a maximum of the non-local signal at a certain temperature is well studied in literature.20–23 The physical origin
for this nonmonotonic behaviour is still an open question,with a variety of explanations having been put forward including surface scattering in the conduit, surface oxidation, grainboundaries, magnetic impurities, and a manifestation of a
Kondo effect.20–23
To shed light onto the origin of the temperature dependence, we compare further below the electrically injected spincurrent to a thermally generated spin current. If the temperature dependence is dominated by an effect of the spin current
propagation in the spin current conduit, such as surface oxidation of the conduit, different injection methods should
yield the same temperature dependence.

Appl. Phys. Lett. 107, 082401 (2015)

So to check this, a thermally generated non-local curve,
measured with an electrode distance of 350 nm and an
applied current density of 1.8  1011 A/m2 while sweeping
the field, is shown inside in Figure 3(a) in addition to theobserved thermal signal as a function of current density. A
similar switching behaviour to the electrically generated signals is observed. Compared to the electrically generated nonlocal curve, the voltage drops are observed for a narrower
field range instead of a wide field plateau for the antiparallelstate for the electrically generated spin currents with lessthermal excitation. Nevertheless, we can identify the voltage
drop with the antiparallel alignment and average over 50
field value data points in the lower state resulting in a low
statistical error for the thermal signal as the difference
between parallel and antiparallel alignment allowing us to
extract the amplitude of the signal with high confidence.
In Figure 3(b), we show the normalized thermal generated non-local signal. Please note that the applied current
density is 1.2  1011 A/m2 for these measurements, which is
low enough to prevent any significant increase of the resistance between contact 6 and contact 5, meaning that the cryostat temperature is indeed the sample temperature. The
thermally generated signal shows a very different low temperature behaviour than the electrically generated one. We
do not observe a maximum but a constant signal at temperatures between 5 K and 65 K and also the size of the relative

FIG. 3. (a) Thermal signal as a function of the current density. We find the
expected quadratic behaviour as a consequence of Joule heating showingthat the spin current is indeed due to thermal spin injection. (b) Normalized
thermal signal as a function of temperature with a wide increase of the signal
between 300 K and 70 K and a constant signal at very low temperatures.

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082401-4

Pfeiffer et al.

signal differs significantly. We measure in the temperature
range between 295 K and 75 K an increase in the signal of
350% while for the electrically generated signal we justobserve an increase of 220%.
These observations motivate a comparison between bothtypes of spin currents, their temperature dependence andtheir physical origin.
In both cases, the resulting signal depends both on themagnitude of the spin accumulation initially generated in the
conduit and the decay of that spin accumulation as it
approaches the detector. At higher temperatures, the spin
relaxation in the conduit is expected to be dominated by the
Elliott-Yafet spin relaxation mechanism,29 since additionallyproposed contributions to spin relaxation such as surface
scattering along the conduit length,20,21,23 or the Kondo
effect in the vicinity of the interface22 are only expected topotentially play a role at low temperatures. In the ElliottYafet mechanism, the spin-flip scattering rate is proportional
to the momentum scattering rate and therefore should scalewith the resistivity of the conduit identically, regardless of
the initial generation mechanism.29 This explains the generaltrend for the increase in signal with decreasing temperaturein both cases in the temperature range between 300 K andabout 80 K. In the case of the thermal spin injection, the temperature gradient in the vicinity of the injector will complicate the picture since the spin diffusion length may not be
uniform along the length of the conduit. This may partly
explain why above 100 K the electrical signal is welldescribed by a linear function, whereas the thermal spin signal has a changing slope.
At lower temperatures, a qualitative difference isobserved between the two curves. For temperatures below
80 K, we observe a maximum in the electrically generated
non-local signal and for lower temperatures a strong reduction of 40% to the maximum signal whilst in the case of thermally generated spin currents, we observe a plateau between
65 K and 5 K. In order to explain the nonmonotonic behaviour in this regime, additional temperature dependent spin
relaxation pathways are usually invoked in part or all of the
conduit.20,22,23 However, if these were the only dominant
effects, we would expect no difference for the two cases. We
therefore deduce that the two different temperature dependences must include effects from the different generation
mechanisms and cannot purely originate from the temperature dependence of the spin transport in the spin current conduit. This is a key result since such effects are largely
disregarded in the existing literature where the temperaturedependence was explained only by changes in the transport
properties of the nonmagnetic channel.20–23 Yet, for bothspin current sources a temperature dependence of the generated spin current may be expected with changes to the polarization of electrically injected carriers and variations in thetemperature gradients in the device, in addition to an evolution in the junction resistance.
In conclusion, we studied electrically and thermally generated spin currents in Co2MnSi/Cu spin valve structures asa function of temperature. In the case of electrical injection
we determine the spin transport parameters of the devices bymeasuring the non-local signal as a function of electrode separation. From our fit, we find the effective spin polarization

Appl. Phys. Lett. 107, 082401 (2015)of Co2MnSi and the room temperature spin diffusion lengthof Cu, which are determined to be 63% and 500 nm, respectively. While this is a relatively high spin polarization, a significant contribution to spin relaxation due to the interface
properties is suggested, calling for future care in interface
engineering and choice of materials combinations to achieve
the desired maximum spin accumulation in the conduit.
By direct comparison between the electrically and thermally generated signals as a function of temperature in thesame device, we study the temperature dependence of theinjection and spin transport. Since the spin transport in thenonmagnetic channel is diffusive for both types of spin currents,13 the same temperature behaviour would be expected
if effects in the conduit dominated both measurements.
However, a qualitative difference in the behaviour isobserved, demonstrating that in contrast to most existingstudies, the temperature dependence of the spin current generation mechanism must be taken into account and will havea decisive effect on the device behaviour.
This work was funded by the German Ministry for
Education and Science (BMBF), the German Research
Foundation (DFG), the Graduate School Material Science in
Mainz (DFG/GSC 266), EU 7th Framework Programme(WALL FP7-PEOPLE-2013-ITN 608031; MAGWIRE, FP7ICT-2009-5), DFG SpinCat (SPP1538) the European
Research Council through the Starting Independent
Researcher Grant MASPIC (ERC-2007-StG 208162), andthe Research Center of Innovative and Emerging Materials
at Johannes Gutenberg University (CINEMA). A. Pfeiffer
and M. Kl€aui thank the participants of SpinMechanics 2
Workshop for valuable discussions and are grateful for thefinancial support from the German Academic Exchange
Service (DAAD) via the SpinNet Program (56268455).
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