2012 12th tEEE International Conference on Nanotechnology (IEEE- NANO)
The International Conference Centre Birmingham
20-23 August 20112, Birmingham, United Kingdom

Spintronics with single molecules
w_ Wulfhekel, T. Miyamachi, S. Schmaus, T.K. Yamada, A.F. Takacs, A. Bagrets, F. Evers, T. Balashov,M. Gruber, V. Davesne, M. Bowen, and E. Beaurepaire

I. INTRODUCTION
Abstract-We demonstrate that with the help of Scanning
Tunneling Microscopy(STM),

spintronic

functions can berealized with single molecules.
First, spin transport across single organic molecules wasinvestigated and a molecular giant magnetoresistance (GMR)
junction was realized. For this, single phthalocyanine molecules(Pc) were contacted by two ferromagnetic electrodes or by an
antiferromagnetic and a ferromagnetic electrode. As substrates,
ferromagneticConano-islandsgrown

Cu(ll1)oranti­

ferromagnetic Mn films on Fe(IOO) were used, onto which thePcmolecules

were

deposited.

The

magnetic

state

of

the

substrate was determined by spin-polarized STM with Co or Fe
tips. Then, the tip of the STM was approached in a controlled
way

to contact the molecule.

Below 0.4

nm distance,

anattractive interaction between the tip and the molecule leads toa jump to contact of one of the side groups of the molecule andto a well defined molecular junction. Through the contacted
molecule, a GMR of 60% was observed in case of Co substrates
and Co tips. In case of Mn surfaces and Fe tips, a negative
GMR of -50% was seen. These results are explained on basis of
ab initio calculations showing a selective hybridization of themolecular states with states of the electrodes.
Second, we demonstrate that single spin-crossover molecules
can be switched between a non-magnetic and a magnetic state
reversibly and deterministically by the application of local
tunneling currents and that the lifetimes of both states exceed
practical measuring times of STM. Thus, we demonstrate amagnetic memory device containing a single magnetic moleculeand which can be read and written entirely by electric currents.

li

Manuscript received June 20, 2012. This work was supported in partby the Deutsche Forschungsgemeinschaft (DFG WU 349/3-1), the Center
for Functional Nanostructures (CFN), the Alexander-von-Humboldt
Foundation, the French-German University, the Argence Nationale de laRecherche (ANR-06-NANO-033-01 and ANR-09-JCJC-0137) and theBaden-WOrttemberg Stiftung (KFN).
W. Wulfhekel is with the Karlsruhe Institute of Technology, 76131
Karlsruhe, Germany (phone: +49-721-60843440; e-mail: wulf.wulfhekel@
kit.edu).
T Miyamachi is with the Karlsruhe Institute of Technology, 76131
Karlsruhe, Germany.
S. Schmaus was with the Karlsruhe Institute of Technology, 76131
Karlsruhe, Germany and is now at the Max-Planck Institute for Solid State
Research, 70569 Stuttgart, Germany.
TK. Yamada was with the Karlsruhe Institute of Technology, 76131
Karlsruhe, Germany and is now at Chiba University, Chiba 263-8522,
Japan.
A.F. Takacs was with the Karlsruhe Institute of Technology, 76131
Karlsruhe, Germany and is now at the Babes-Boyai University, 400084
Cluj-Napoca, Romania.
A. Bagrets, F. Evers and T Balashov are with the Karlsruhe Instituteof Technology, 76131 Karlsruhe, Germany.
M. Gruber, Y. Davesne, M. Bowen and E. Beaurepaire are with the
IPCMS, 67034 Strasbourg, France.

S

pin electronics is used today mainly in read heads in
hard disk drives and random access memory. To
increase the information density, the devices need tobe scaled down in size which has become challenging in the
recent past. Although tunneling magnetoresistance (TMR)
devices offer large values of magnetoresistance, the large
areal resistance puts a limit to downsizing the devices, as inthese the current through the devices becomes too small anda fast read out is hampered [1]. In contrary to that, current
perpendicular to the plane GMR junction show a very low
areal resistance, such that devices using this principle sufferfrom comparably large lead resistances reducing the overallmagnetoresistance of the complete device [2]. A promising
approach to extreme downsizing is to form a single
molecular contact between the two magnetic electrodes of a
GMR junction.
An even more promising molecular class are spin­crossover (SCQ) complexes containing a transition metal ionthat can be switched between a low-spin (LS) and a high­spin (HS) state by external stimulus as temperature, light,
pressure, magnetic or electric fields or charge flow [3-5 ].
The two configurations may lead to different conductances
but, more importantly, their switchable spin of the metal ion
has a high potential for magnetic storage within individual
molecules [6].
In this work, we will address all these functions using low
temperature scanning tunneling microscopy (STM) [7] torealize and characterize molecular functions down to theindividual molecule.

II. GIANT MAGNETORESISTANCE ACROSS SINGLE MOLECULES
Contacting single phthalocyanine molecules with an
STM

A.

Recently phthalocyanine molecules attracted interest withregard to their electronic properties on different metal
substrates [8]. The behavior of this group of molecules onmagnetic (Co) and non-magnetic (Cu) surfaces were
investigated. Figure Ia displays the chemical structure of aphthalocyanine molecule. These molecules are used to bind
metal cations in the center, and by incorporation of different
metals their properties can be changed. In our experiments
hydrogen (H2PC) and cobalt phthalocyanine (CoPc) werestudied. As substrate we use the (llI) surface of a copper
single crystal, which was cleaned by cycles of Ar-sputteringand annealing. The molecules were sublimed from aKnudsen cell at 5 5 0 K. Additional experiments were carriedout on substrates, where a sub-monolayer amount of Co was
deposited on Cu(ll1) leading to the formation of double
layer islands of Co [9].
Topographic measurements show that both H2Pc and
CoPc are adsorbed flat and as single molecules when only asmall amount is deposited (see Fig. Ib). In both cases a non­magnetic tungsten tip was used for imaging.

B.

Fig. 1. a) Model of cobalt phthalocyanine. The molecule consists of four
benzopyrrol units (B,-B4)' which are connected by nitrogen bonds (Nb). In
its center the cobalt cation is surrounded by the nitrogen atoms of thepyrrol (Np) and a metal organic complex is formed. Topographic image ofb) H,Pc on Cu (8 nm x 8 nm) , c) CoPc on Cu (6 nm x 6 nm). In all cases
one can see the single molecules and four lobes from the benzol rings.
H,Pc shows an empty space in the middle while CoPc displays aprotrusion.

To study the transport properties of the molecules
distance curves were recorded, in which the tunnelingcurrent 1 was measured as function of the distance between
tip and sample at constant sample bias V. The distancecurves are measured in the following way. After detecting amolecule, the STM tip was moved atop of it at a low voltage(typically 10 mY). Then the feedback loop of the STM wasswitched off. By ramping the voltage on the z-piezo, the tip
was approached towards the surface in a controlled way.
Afterwards, the tip was retracted back to the initial position
and another approach was begun. The tunneling current wasrecorded during the approach phase.
In Fig. 2 we present the results for H2Pc on copper. On thebare Cu(111) an exponential increase of the tunnelingcurrent with approaching of the tip is found. This reflects the

reduction of the tunneling barrier width. The current
recorded upon approaching the molecule, however, rises
faster. The super-exponential behavior can be explained by apartial lifting of the molecule due to the close proximity ofthe tip. Ultimately, the molecule jumps into contact with the
tip and bridges the tunneling junction [10]. The bond which
is formed remains intact after retracting the tip. This can beobserved in the nearly constant current for the following
distance curves. In this bridge position of the molecule, a
relatively high conductance of 0. 1 Go was observed, whichmost likely reflects the conductance via the delocalized 7t­
orbitals of the molecule. W hen the topographic scan iscontinued (see insert of Fig. 2), the molecule disappears. The
molecule was transferred to the tip. This can be explained by
the phthalocyanine forming a more stable bond to the W tip
than to the copper surface. Similar results with identicalconductances have been achieved for CoPc on Cu.
Measurements of H2Pc and CoPc on the Co islands show
a similar jump into contact behavior like on Cu. In contrast
to the experiments on Cu the subsequent distance curves
measured on Co start at the original current set point. Thismeans that the molecule-tip bond that establishes during thejump to contact is broken again when retracting the tip. In
agreement to that, the molecule is also seen in the following
topographic scan. We conclude that the bond of the molecule
to cobalt island is stronger than to the W tip. Interestingly,
also the conduction in the bridged position is with about 0. 3
Go higher than that in the Cu case. This holds for both CoPc
and H2Pc. This finding can be explained by the different
adsorption mechanism. While on Cu(ll1) the molecules are
only physisorbed, on Co(111) they are chemisorbed. The
charge transfer in the latter case leads to a shift of the lowest
occupied molecular orbital (LUMO) to the Fermi energy
thus leading to resonance transport. This mechanism also
explains the stronger binding of the molecule to Co inrespect to Cu [10, 11].
B.

Spin transport across H]Pc molecules

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0

2
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v
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0
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0.01

0.001

0.0001 ........,_.,..--.---,..._.--..,-_--,.=;:=:::;::::=;===;:=/
o0.1
offset [nm]0.2

Fig. 2. Distance curves for H,Pc on bare copper. The black triangles show
the measurements on bare copper. One can see the perfect exponential
behavior, which is expected for tunneling currents. The light gray circles
are measured during the first approach, where an super-exponential
increase to a saturation current is observable. The dark grey diamonds
show a nearly constant current. In the inset one can see in the topographic
image (5 nm x 5 nm) that the molecule has disappeared after the distancecurves were recorded. Go�2e/h.

By using Co coated tips STM becomes sensitive to thespin. To detect the direction of magnetization of individual
Co islands on Cu(111), dlldV curves were measured on thebare islands with the lock-in technique. The differential
conductivity is proportional to the local density of states(LDOS) within the Tersoff-Hamann model. Especially at
surface states or bulk band edges appearing as peaks in the
dlldV spectrum the LDOS is highly spin-polarized. As aresult the dlldV recorded with spin-polarized tips shows adependence of the peak intensities on the relative orientationof the sample and tip magnetization. Figure 3b shows thedlldV curves for two different islands, one magnetizedparallel the other antiparallel to the tip. Particularly large
differences in the spectra are found at -350 meV stemmingfrom the surface state of Co [9]. This difference is used todetect the local magnetization direction of Co islands byrecording maps of the local differential conductivity at thisvoltage. The brightness of Figure 3a represents this local
differential conductivity, indicatingareas magnetizedparallel in bright and antiparallel to the tip in dark. The
tunneling magneto resistance (TMR) defined as thedifference of conductivity over the smaller conductivity ofthe tunneling junction formed by the tip and sample isstrongly energy dependent as depicted in Fig. 3c. Note thatat small bias the TMR is only of the order of S%.

·4.5nAN

Sample bias M

Fig. 3. a) Topographic image of two Co islands on the Cu(lll) surface.
The brightness indicates the measured dlldV signal at -310 mY. On candistinguish between parallel (bright) and antiparallel (dark) magnetizationof the islands with respect to the tip. b) shows the dJldV spectra taken on
two oppositely magnetized islands. In c) the TMR ratio calculated fromthe dlldV spectra is plotted. The highest value is measured at around -350
meY. This is used to distinguish between the two differently magnetizedislands.

To perform spin transport measurements across single
molecules, we positioned the magnetic tip above a moleculeat the aromatic side groups, opened the feed-back loop, andapproached the tip towards the molecule as before. The
current after the jump only depends slightly on the distanceand represents the conductance of the molecule. This
conductance in contact depends on the relative orientation of
the magnetization of tip and sample (see Fig. 4). For theparallel case the conductance is found to be around 0. 30 Go
while in the antiparallel case it is smaller (0. 17 Go).

two Co islands with opposite perpendicular magnetization
were recorded in the same scan. This measurement has been
repeated several hundred times and the distribution of theobserved molecular conductance is depicted in Figure Sa.
The observed scatter of the conductance, be it due to noiseof the current measurement or to slightly different binding
geometries of the molecule to the tip, is not negligible.
Nevertheless, one can clearly see the difference between thetwo magnetization directions. A lower tendency is observed
for the anti parallel measurement. To determine the GMR
from the statistical measurements Gaussian fits were used. In
the parallel case we measured an averaged conductancevalue of Gp=(0.2S3±0.00S) Go and in the anti parallel case
Gap=(0. 148±0.00S) Go. These values result in an optimistic
GMR ratio of 61 %. This is one order of magnitude larger
than the TMR for direct tunneling. This can be explained by
the fact that the molecular states couple to specific highly
spin polarized Co minority states. In the parallel case, theelectron wave function in the minority channel extends over
the whole junction causing a high conductance (see Fig. Sb).
In the antiparallel case, the wave functions of both spinchannels only extend over the molecule and one of theelectrodes leading to a low conductance [11].
a)

:J
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�

_ antiparaliel

30

� 20
II
E
-l 10
0.2

0.4

Molecular conductance Gmol [Golb)parallelmajorityminority

antiparalielminority��omajority

(j)
u
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50
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majorityminority

Fig. 5. a) Histogram of the molecular conductances (381 times parallel and
366 times antiparallel). A Gaussian fit is used to determine theconductance and calculate the GMR ratio. b) Calculated electron wave
functions for parallel and antiparallel oriented magnetizations for the two
spin channels.

10"2

C
o
U

10.3 +--�--'--�----r-�--'r---'
0.10
0.15
0.20
0.00
0.05
Offset [nmjFig. 4. Two typical single conductance distance curves measured at twocontrariwise magnetized islands with a constant tunneling voltage of 10
mY. Below a certain tip-surface separation the curves shows a jump of the
conductance to higher values. This can be identified by a jump of themolecules into contact. After the jump there are two contributions to thetransport.

To ensure measurements with the same tip, the data on

Similar GMR experiments have been carried out for H2Pc
on Mn films grown on Fe(lOO) contacted with Fe-coatedtips. In this case, the bottom electrode is antiferromagnetic.
As the hybridization of the molecular states with thesubstrate is only local, this configuration still leads to welldefmed spin of the bottom electrode and as a consequence toa GMR. As will be shown elsewhere [12], the GMR isnegative in this case but of large size (SO%). The use of
antiferromagnets as one of the electrodes has the advantage,that the antiferromagnet is the ideal hard magnetic layer thus
simplifying the structure of a GMR sensor.

III. SWITCHJNG INDIVIDUAL SPIN-CROSSOVER MOLECULES
Molecular spintronic devices may also be based on spins
located within the molecule. As the directions of these spinsare usually unstable even at cryogenic temperatures,information cannot be stored in the direction of the spin.
Spin-crossover (SCO) molecules show relatively stable spin
configurations which differ in the size of the magnetic
moment. These are much more promising for storing
magnetic information in individual molecules.

a)b)

Low spin
5=0High spin
5=2

c)
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-5,100
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2.0

Length (nm)3.0-a{H5)

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0 .7....'-:'-'-�....
....,....JL.....w..,........J�d-40 -20 0 20 40

energy recorded on the center of both types show a clear
Kondo resonance only on type a and a weak spectroscopic
feature on type p (see Fig. 6e). A Kondo resonance is asignature of unpaired spins and is caused by a many-bodystate including the substrate electrons. Thus, these
spectroscopic experiments ascribe a type a molecularconformation as the HS and type p as the LS state. A Fano fit
to the spectrum reveals a Kondo temperature of about 105 K.
Fe-phen molecules on CuN/Cu(lOO) can be electricallyswitched between the HS and LS states reproducibly, leading
to a single-molecule device whose state can be
deterministically selected. When performing cyclic J(V)curves atop the molecule's center with the STM feedback
loop opened, conformational switching can be seen as anabrupt jump in the J(V) curve. Figure 7a displays a cyclic
J(V) sweep with arrows indicating the sweep direction. An
abrupt current increase (decrease) is seen on the forward(reverse) sweep at about + 1.2 V (about -0.8 V). The cyclic
J(V) curves depict a hysteretic behavior at intermediate
voltages indicated by two branches of the J(V) curve.
Repeated forward and reverse J(V) sweeps reveal theswitching between the two states to be fully reversible.

a)�
�

�

-

0

As a prototypical SCO complex, Fe( 1, 1 0-phenanthroline)2(NCS)2 (Fe-phen) molecules (see Fig. 6a) exhibit a low spin(LS) to high spin (HS) transition when heated above 175 K
that is accompanied by a change in the Fe-N bond distances
and angles [3], leading to a reduction in the ligand fields
around the Fe site. At low temperatures, the splitting Llbetween the hg and the eg states is so large that Hund's rules
are broken and a LS state with s=o is formed. At hightemperatures, the ligand field is reduced and Hund's rules
yield a state with S=2 (see Fig. 6b).
Here we demonstrate how to electrically switch the spinstate of, and the conductance across, individual Fe-phenmolecules on CuN/Cu(lOO) surface using STM.
Figure 6c shows an STM image of isolated Fe-phenmolecules on CuN/Cu(lOO). Due to the strong affinity of S
to the CuN surface, the STM image suggests that themolecules adsorb with the SCN groups onto the CuN
surface, and two types of molecular conformations denoted
as type a and type p are observed. The two types reveal
slight differences in the height profiles (cf. Fig. 6d): thecentral region of type a is higher than that of type p. The
differential conductance (dJ/dV) spectra near the Fermi

After

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U

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�c
::J
t-

_

-1.0Vlo+1.4 V+---+ 1 .4 V 10 - 1.0 V

0.0

5arrple bias (mY)

Fig. 6. a) Three dimensional model of an Fe-phen molecule. b) Simplified
electronic configuration of the Fe(TJ)-3d states in the ligand field. c) STM
image of isolated Fe-phen molecules on the CuN/Cu(IOO) surface and d)hne scans across the long axis of the molecules. e) dl/dV spectra taken on
the center of the Fe-phen molecules.

Before+---0.5

Sarrple bias (V)

1.0

1.5

-40
0
40
Sarfllie bias (mV)

80

d)

b)

:iD
§ 1.2
e
3

c)

� 1.0
'5

1

2

Length (nm)

3 0

1

2

Length (nm)-80

Fig. 7. a) l(V)-curves of isolated Fe-phen molecules recorded on the centerof the molecule. b) STM images c) and corresponding height profiles
acquired before and after the switching event. d) dJldV spectra recorded on
the center of the molecule before and after a switching event.

This switching behavior falls into the general category of
memristive effects, which generically describe how it iselectrically possible to reversibly alter a device's
conductance. In our case, the two branches of the J(V) curve
are related to the HS and LS state of the Fe-phen molecule
as seen by comparing STM images and dl/dV spectra of themolecule in each of the two branches.
When ramping the sample bias up, the molecule in the
HS-state (type a) is found to change its conformation after
the abrupt drop in the tunneling current to the LS-state (typeP) as shown in Figs. 7b and 7c. This observation isreinforced by the corresponding changes in the dlldV spectra(cf. Fig. 7d). Equally, switching from the LS to the HS state
has been verified.
This unambiguously ascribes the intrinsic SCO property
of Fe-phen as the origin of this nanoscale memristive
behavior [13]. We emphasize that it is not only the amplitudeof the sample voltage but also its polarity that determines thestate of the molecule. After the molecule has been switchedfrom the HS to the LS state by a positive voltage, it is notswitched back by high positive voltage to the HS state. Only
applying a negative voltage switches the molecule back to
the HS state. The same holds for the opposite switchingprocess. This proves that addressing the memory device isfully deterministic.

L. Zhang, T. Miyamachi, T. Tomanic, R. Dehm, and W. Wulfhekel, "A
compact sub-Kelvin ultrahigh vacuum scanning tunneling microscopewith high energy resolution and high stability", Rev. Sci. Instrum. vol.
82, 103702, October 20II.
[8] A. Zhao et al."Controlling the Kondo Effect of an Adsorbed Magnetic
ion Through Its Chemical Bonding", Science vol. 309, pp. 1542-1544,
September 2005.
[9] O. Pietzsch, A. Kubetzka, M. Bode, and R. Wiesendanger, "Spin­polarized scanning tunneling spectroscopy of nanoscale cobalt islands
on Cu(111)", Phys. Rev. Lett. vol. 92, 057202, February 2004.
[10] A. F. Takacs, et al. "Electron transport through single phthalocyanine
molecules studied using scanning tunneling microscopy", Phys. Rev.
B vol. 78, 233404, December 2008.
[II] S. Schmaus, et al. "Giant magnetoresistance through a single
molecule" Nature Nanotechnology vol. 6, pp. 185-189, March 2011.
[12] A. Bagrets et al. submitted.
[13] T. Miyamachi, et al. "Robust Spin Crossover and Memristance acrossa Single Molecule" Nature Communication in press.
[7]

CONCLUSION

In conclusion, we have shown that single molecules
adsorbed onto surfaces can be addressed individually by the
tip of an STM to show a large molecular GMR effect. Using
SCO molecules, individual molecules can be switcheddeterministically between a HS and a LS state that not only
differ in their magnetic moment but also in theirconductance. These functions are the elementary functions
for molecular spintronic devices, i. e. , sensing a magnetic
configuration of the metallic leads and storing digital
information in the spin state of single molecules. While theseare only demonstrations of the working principles, real
devices need to be assembled as solid state circuits. For
these, a single molecule or few molecules can be integrated
in thin film devices probed by local currents and voltages.
ACKNOWLEDGMENT

We thank F. Witt, Y. Nahas, A. Bork, S. Boukari, L. Joly,
F. Scheurer, G. Rogez and P. Ohresser for their contributions
to the results presented, here. Further, are thankful for the
stimulating scientific atmosphere in the Center for
Functional Nanostructures (CFN) and the graduate school of
the French German University.
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