ARTICLE
Received 27 Feb 2012 | Accepted 31 May 2012 | Published 3 Jul 2012

DOI: 10.1038/ncomms1940

Robust spin crossover and memristance acrossa single molecule
Toshio Miyamachi1,2, Manuel Gruber1,3, Vincent Davesne1,3, Martin Bowen3, Samy Boukari3,Loïc Joly3, Fabrice Scheurer3, Guillaume Rogez3, Toyo Kazu Yamada1,4, Philippe Ohresser5,
Eric Beaurepaire3 & Wulf Wulfhekel1,2

A nanoscale molecular switch can be used to store information in a single molecule. Although
the switching process can be detected electrically in the form of a change in the molecule′s
conductance, adding spin functionality to molecular switches is a key concept for realizing
molecular spintronic devices. Here we show that iron-based spin-crossover molecules canbe individually and reproducibly switched between a combined high-spin, high-conduction
state and a low-spin, low-conduction state, provided the individual molecule is decoupledfrom a metallic substrate by a thin insulating layer. These results represent a step to achieving
combined spin and conduction switching functionality on the level of individual molecules.

1 Karlsruhe Institute of Technology, Physikalisches Institut, Wolfgang-Gaede-Str. 1, 76131 Karlsruhe, Germany. 2 DFG-Center for Functional Nanostructures,

Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany. 3 Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 UdS-CNRS,
67034 Strasbourg Cedex 2, France. 4 Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan. 5 Synchrotron SOLEIL,
L′Orme des Merisiers, Saint-Aubin - BP 48, 91192 Gif-sur-Yvette, France. Correspondence and requests for materials should be addressed to T.M.
(email: toshio.miyamachi@kit.edu).
nature communications | 3:938 | DOI: 10.1038/ncomms1940 | www.nature.com/naturecommunications© 2012 Macmillan Publishers Limited. All rights reserved.

ARTICLE

nature communications | DOI: 10.1038/ncomms1940

Results
Fe-phen molecules on Cu(100). Figure 1c displays an STM imageof isolated Fe-phen molecules on Cu(100). Each molecule shows
two lobes. Because of the strong affinity of S to the Cu substrate, theNCS groups bind to the substrate and the two phen groups point
upwards and are seen with the STM as the two lobes. There exist two
molecular conformations for Fe-phen on Cu(100). Type I shows alarger inter-lobe distance than type II. To identify the spin state of
the two types, differential conductance spectra of the tunnellingcurrent (dI/dV) were measured (Fig. 1d). The spectrum measured
atop a type II molecule′s centre exhibits only minor differential
conductance variations that are likely related to the electronic
structure of the molecule adsorbed on Cu(100). In contrast, thespectrum atop a type I molecule′s centre shows a strong resonancenear the Fermi energy. The resonance exhibits a Fano line shape16,
indicating a Kondo resonance. A Fano fit to the spectrum reveals
a Kondo temperature TK of ≈317 K, a q-factor of 6.1 and a shift of
the Kondo resonance due to Coulomb repulsion of 5.7 meV. Thislarge TK underscores a strong electronic hybridization between the
Fe spin and the Cu substrate similar to that found in Fe2 + ions of
Fe-Phthalocyanine on Au(111) with Kondo temperatures between


a

b
Fe

eg

S =0

S=2

N

∆LS

C
S
H

c

d

II

∆HS

t2g

1.4

I
I

e

I

II

II

f

1.2
dI/dV (arbitrary units)Molecular electronics describes how the function of an electronic component is achieved using a single molecule1.
For instance, molecular switches2 may serve as highdensity memory devices. As a promising molecular class, spincrossover (SCO) complexes3 contain a transition metal ion that canbe switched between a low-spin (LS) and a high-spin (HS) state byexternal stimuli as temperature, light, pressure, magnetic or electricfields or charge flow4–10. The two configurations may lead to different conductances but, more importantly, the switchable spin of themetal ion has a high potential for molecular spintronics11. The SCO
property is related to a delicate energy balance between the HS andLS state associated with conformational changes of the molecule.
Maintaining this delicate balance for SCO molecules adsorbed onto
surfaces requires carefully controlling the molecule′s interaction
with the adjacent electrodes to sustain the SCO property.
As a prototypical SCO complex3, Fe(1,10-phenanthroline)2(NCS)2 molecules (Fe-phen) shown in Fig. 1a exhibits a LS→HS transition when heated above TC≈175 K that is accompanied by a change
in the Fe-N bond distances and angles12, leading to a reduction in
the ligand fields around the Fe site. At low temperatures, the splitting∆ between the t2g and the eg states is so large that Hund′s rules arebroken and a LS state with S = 0 is formed. At high temperatures, theligand field is reduced and Hund′s rules yield a HS state with S = 2(Fig. 1b)13. Thermally induced switching may occur in bulk3 or thickFe-phen films14. Yet investigations on the SCO of individual mole­cules require a fixed temperature and single-molecule selectivity. In
this sense, light-induced transitions are impractical because one cannot focus the stimulus by light on a single molecule within a dense
molecular matrix. Using scanning tunnelling microscopy (STM), thestimulus can, however, be focused on the area of current flow, which
is typically of nanometre size. Thus, with STM individual molecular switches can be addressed and reversibly switched, as well as themolecular origin of the switching event can be probed2.
Here we demonstrate how to electrically switch the spin state of,and the conductance across, individual Fe-phen molecules using
STM at low temperature (≈4.6 K) with high lateral and energy resolution. While Fe-phen molecules adsorbed onto a Cu(100) surface
exhibit a coexistence of HS and LS states, the strong coupling of the
NCS group to the substrate prevents a spin state from being written
electrically. Introducing an interfacial CuN layer, however, allows
switching between the HS and the LS state. The combined changes
in conductance and spin state demonstrate how to confer memristive to spintronic properties realising multifunctional spintronic
capacity in single molecules11,15.

I (HS)
1.0

0.8

0.6

II (LS)Cu(100)

–100

–50
0
50
Sample bias (mV)

100

Figure 1 | SCO Fe-phen molecules on bare Cu(100). (a) Threedimensional model of a Fe-phen molecule. (b) Simplified electronic
configuration of the Fe(II) 3d states in the ligand field. For large ligand
fields (∆LS), the electrons fully occupy the t2g states and the total spin S
is quenched, while for low ligand fields (∆HS), the Fe(II) ion is in the spinstate S = 2. (c) STM image of isolated Fe-phen molecules on the Cu(100)surface with two conformations denoted as I (HS) and II (LS). Image size
is 13×13 nm2. (d) dI/dV spectra recorded on the centre of type I (HS),type II (LS) molecules and Cu(100) surface. The colours indicate theposition, where the spectra were taken and are marked as coloured dotsof (e). The black dotted line denotes a Fano fit to the Kondo resonance.
(e) STM topography of a pair of type I (HS) and type II (LS) moleculesand (f) corresponding dI/dV map (bottom) obtained at + 10 mV showing
the position of the Kondo resonance at the centre of the HS molecule.
Image sizes are 6.7×3.7 nm2.

357 and 598 K17. A dI/dV map of the Kondo feature (Fig. 1e,f) reveals
that the Kondo cloud is centred around the expected position of
the Fe-ion for a type I molecular conformation, while there is noresonance for a type II molecular conformation.
A Kondo resonance is a signature of unpaired spins and is caused
by a many-body state including the substrate electrons. Thus, these
spectroscopic experiments ascribe a type I molecular conformation
as the HS and type II as the LS state. As further evidence, we have carried out X-ray absorption spectroscopy (XAS) measurements of ≈2
molecular layers (ML) of Fe-phen on Cu(100) at low temperatures.
As XAS is an element-specific spectroscopic method, thisapproach is very effective to elucidate the electronic structure of Fe
ions, that is, to resolve the HS and LS states. During the measurements, no external illumination except for the X-rays was applied to
avoid light-induced excited spin state trapping18. The surface structure of 2 ML thin films as verified by STM (Supplementary Fig. S1a,b)
exhibits an ordered structure along the four-fold symmetry of theCu(100) surface. A closer look at the film reveals the existence of two
types of molecular conformations that are similar to the type I andtype II conformations of individual molecules on Cu(100) shown in
Fig. 1c. We first present XAS spectra of Fe-phen powder acquired
above and below the SCO transition at T≈175 K14, as references toanalyse thin film data. Top and middle of Fig. 2a display the XAS
spectra of the HS state and the LS state measured at 200 K and
100 K, respectively. Clear differences exist between the spectra,
such as a dominant peak located at ≈706 eV for the HS state andat ≈708 eV for the LS state of the Fe 2p L3 absorption edge, in goodnature communications | 3:938 | DOI: 10.1038/ncomms1940 | www.nature.com/naturecommunications© 2012 Macmillan Publishers Limited. All rights reserved.

ARTICLE

nature communications | DOI: 10.1038/ncomms1940b

XAS (arbitrary units)

L2

710
705
Photon energy (eV)�

XAS(arbitrary units)

Fe 2p

XAS
(arbitrary units)

L2

700

b
715

e
CR
CL

L3

L2

720
725
Photon energy (eV)

Fe 2p

CR
CL

L3
L2

710

720

Photon energy (eV)

730

700

710

720

730

Photon energy (eV)Figure 2 | Coexistence of HS and LS states of Fe-phen molecules on
Cu(100). (a) Fe 2p XAS spectra of Fe-phen powder reference sample
obtained at 200 K (HS state: top) and 100 K (LS state: middle), andtwo monolayers of Fe-phen thin film on Cu(100) obtained at 50 K
(bottom). Reproduction of Fe 2p XAS L3 (b) and L2 (c) edges of twomonolayers Fe-phen using the reference spectra for the HS and LS statesobtained on the powder sample. Green circles represent the experimental
data of the thin film, black lines the reproduced spectra obtained fromthe linear combination of the powder spectra of the HS state (red dottedline) and LS state (blue dotted line). The black dotted line represents the
background. XAS and XMCD spectra of (d) two monolayers Fe-phen thinfilm on Cu(100) measured at 1.5 K and 6.5 T, and (e) Fe-phen powder
reference sample measured at 4 K and 6.5 T. To obtain dichroic signal,
the helicity of the X-ray was reversed between circular right (CR)and circular left (CL). The black dotted line shows the zero line of
the XMCD signal.

agreement with previous studies19. The bottom of Fig. 2a displays
the XAS spectrum of the molecular film on Cu(100) measured at
50 K. In agreement with the STM experiments, the XAS spectrum
exhibits both peaks indicating the coexistence of the two spin states.
To confirm this interpretation, we have reproduced the XAS 2 ML
spectrum through a linear combination of the HS (46% weight) andLS state (54% weight) reference spectra at the L3 and the L2 edge
(Fig. 2b,c). The validity of the mixed spin state in 2 ML Fe-phenthin film is further reinforced by X-ray magnetic circular dichroism(XMCD) measurements showing nearly identical spectra for thethin film sample (Fig. 2d) and the powder sample (Fig. 2e). Note thatsome of the LS state molecules are switched to the HS state owing to
the soft X-ray-induced excited spin state trapping19, which can beseen in the XAS spectra. A spectrum change of XAS with respect tocircular right- and left- polarized X-ray was observed only near the
peak at ≈706 eV and no change was observed at ≈708 eV. This clearly
proves the coexistence of the two spin states, as the polarization of the
X-ray only affects the magnetic HS state spectrum mainly positioned at ≈706 eV and the non-magnetic LS state spectrum mainly
positioned at ≈708 eV does not react to the polarization of the X-ray.
To conclude this part, XAS and XMCD measurements obtained on

0.9
�(HS)

0.8

�(LS)

0.7

CuN

�
�

200
100
0
0.0

XMCD
(arbitrary units)

Fe 2p

730

XMCD
(arbitrary units)

XAS(arbitrary units)

d

710
720
Photon energy (eV)

1.0

�

c

700

c

L3

dI/dV (arbitrary units)

L3

a

Fe 2p

XAS
(arbitrary units)

Fe 2p

Height (pm)

a

1.0
2.0
Length (nm)3.0

–40 –20 0
20 40
Sample bias (mV)

Figure 3 | SCO Fe-phen molecules on CuN/Cu(100). (a) STM image(3×5.5 nm2) of isolated Fe-phen molecules on the CuN/Cu(100)surface with two conformations denoted as α (HS) and β (LS) and(b) line scans across the long axis of the molecules showing the differencein their appearance. (c) dI/dV spectra taken on the centre of the two
configurations of the Fe-phen molecules together with a Fano fit (dottedblack line) to the Kondo resonance of type α. The black line is the
spectrum recorded on bare CuN with the same tip.

thin films on Cu(100) show that the spectra are split into two contributions very similar to those obtained for the HS and the LS statesin bulk samples. Thus, the two molecular states observed in STM
can be assigned to the HS (S = 2) and LS (S = 0) states.
Despite applying high local electric fields (up to several
GV m − 1) in the junction of the STM, high tunnelling voltages( ± 4 V) and high current densities (up to 1 µA per molecule), we wereunable to electrically switch between the two types (Supplementary
Fig. S2a–d). Switching between the states was only possible by moving molecules laterally with the STM tip. We infer that the strong
S-Cu bond impairs spin switching with the tunnelling conditions
given above.
Fe-phen molecules on CuN/Cu(100). To reduce the chemical interaction between molecule and substrate, we inserted a monoatomic
CuN layer, which significantly reduces the adsorption energy andensuing hybridization20. Moreover, the bond of S to the oxidized Cu
atoms in the CuN network is expected to be weaker. Figure 3a shows
an STM image of isolated Fe-phen molecules on CuN/Cu(100). As
with Cu(100), the STM images suggest that the molecules adsorb
with the NCS groups onto the CuN surface, and two types of molecular conformations denoted as type α and type β are observed (forlarge-scale images of the CuN surface and the molecules on the
CuN surface see Supplementary Fig. S3 a–d). The difference in themolecular shape as seen by STM is significantly smaller when compared with that on bare Cu, which might be due to the electronic
decoupling of the molecules. The two types reveal slight differencesin the height profiles (Fig. 3b): the central region of type α is higher
than that of type β. dI/dV spectra near the Fermi energy recordedon the centre of both types show a clear Kondo resonance only ontype α and a weak spectroscopic feature on type β (Fig. 3c). A similarly weak feature is also frequently found in the spectrum recordedon bare CuN. We associate it with inelastic vibronic excitationsof the CuN layer. From the spectroscopic data, we therefore ascribe
the type α (β) as the HS (LS) conformational state in analogy tonature communications | 3:938 | DOI: 10.1038/ncomms1940 | www.nature.com/naturecommunications© 2012 Macmillan Publishers Limited. All rights reserved.

Tunneling current (pA)

ARTICLEnature communications | DOI: 10.1038/ncomms1940

Before0

After– 1.0 V to + 1.4 V+ 1.4 V to – 1.0 V

–500

–1.0

b

–0.5

0.0
0.5
Sample bias (V)c� (HS)

1.0

e

1.5

� (HS)

� (LS)
dI/dV (arbitrary units)

1.4

Height (pm)d200
� (HS)

100

� (LS)

1.2
� (LS)
1.0
CuN
0.8

0
0

1
2
3 0
1
2
3
Length (nm)
Length (nm)

–80

–40
0
40
Sample bias (mV)

80

Figure 4 | Spin and junction conductance switching of single molecules.
(a) I(V) curves of isolated Fe-phen molecules on the CuN/Cu(100) surface
recorded on the centre of the molecule showing a hysteretic switching
behaviour (tunnelling conditions for tip stabilization: + 1 V, 100 pA). The
high (red) and low (blue) junction conductance states are visualized using
STM images (b for HS state and c for LS state, each 3.7×3.7 nm2) andcorresponding height profiles (d) along the molecular long axis acquired
before and after the switching event. (e) dI/dV spectra recorded on thecentre of the molecule before and after a switching event from the highconductance to the low-conductance state and spectrum of the bare CuN
surface recorded with the same tip. (b) to (e) indicate that the switching
event is a transition from the HS to the LS state.

the molecules on Cu(100). As further confirmation, spectral data
recorded on the organic ligands reveal strong electronic differences
between the two types (Supplementary Fig. S4), in agreement withtheoretical predictions of electronic differences between the HS andLS molecules12. The weaker electronic hybridization expected on
CuN compared with Cu results in a lowering of the fitted Kondo
temperature to TK≈105 K (q = 4.9, Coulomb shift 5.5 meV). This
reduced hybridization also results in molecular orbitals that areenergetically better defined (Supplementary Fig. S4). Note, however,that the molecular states are still rather broad. From the observed
widths of the molecular states, lifetimes of these excited states
between 2 and 5 fs can be estimated. These short lifetimes indicate
an efficient hybridization of the molecular states with the substrateand exclude the possibility that the observed peak of HS molecules
at the Fermi level is caused by a charged state of the molecule.
Contrary to Fe-phen on bare Cu(100), Fe-phen on CuN/Cu(100)can be electrically switched between the HS and LS states reproducibly,
leading to a single-molecule device whose state can be deterministically selected. When performing cyclic I(V) curves atop the molecule′scentre with the STM feedback loop opened, conformational switching
can be seen as an abrupt jump in the I(V) curve2. Figure 4a displays atypical cyclic I(V) sweep with arrows indicating the sweep direction.
An abrupt current increase (decrease) is seen on the forward (reverse)
sweep at ≈ + 1.2 V (≈−0.8 V). The cyclic I(V) curves depict a hysteretic behaviour at intermediate voltages indicated by two branches of
the I(V) curve (high conduction branch in red and low conduction
branch in blue). Repeated forward and reverse I(V) sweeps reveal theswitching between the two states to be fully reversible.
This switching behaviour falls into the general category of
memristive effects21, which generically describe how it is electrically

possible to reversibly alter a device′s conductance. In our case, thetwo branches of the I(V) curve are related to the HS and LS states of
the Fe-phen molecule as seen by comparing STM images and dI/dV
spectra of the molecule in each of the two branches. When ramping
the sample bias up, the molecule in the HS-state (type α) is foundto change its conformation after the abrupt drop in the tunnellingcurrent to the LS-state (type β), as shown in Fig. 4b and c with corresponding height profiles in Fig. 4d. This observation is reinforcedby the associated changes in the dI/dV spectra (Fig. 4e). 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 behaviour. We exclude the possible
artifact of STM tip changes, as the switching event does not alter
the STM images and dI/dV spectra of the surrounding molecules.
The observed combined switching of the spin and conduction
state of the molecule is in line with recent predictions for similarmolecules22. Note that SCO is not caused by charge transfer or
unpaired spins on the organic ligands as in the case of switchable
Kondo effect23,24. This is clearly evidenced by the fact that the
Kondo resonance is only observed on the position of the central
Fe ion and not on the organic ligands. Thus, the spin of the molecule
is associated with a magnetic moment on the metal ion3.
As the HS (LS) state of a single Fe-phen molecular junction
can be prepared by applying a suitably large applied bias of negative (positive) polarity (Fig. 4), we can test the bistable deterministic nature of this memory device by applying a suitable train ofbias pulses. As shown in Fig. 5a, alternating positive and negative
bias pulses lead to molecular junctions with a current level measured at + 0.1 V that is associated with each of the desired LS and HS
states, respectively. We emphasize that it is not only the amplitude of
the bias pulse but also its polarity that determines the state after thepulse. After the molecule has been switched from the HS to the LS
state by a positive voltage, it is not switched back by a second positive pulse to the HS state. Only applying a negative pulse switches
the molecule back to the HS state (Fig. 5b). The same holds for the
opposite switching process (not shown). This proves that addressing
the memory device is fully deterministic. Furthermore, the lifetimeof each spin state is remarkably long at the observation temperature:no spontaneous switches were observed even after tens of hours of
STM observation at low bias voltages ( <  ± 0.5 V).

Discussion
To gain further insight into the switching mechanism, we haveexamined the real-time traces of the tunnelling current during
the bias pulse used to perform switching (feedback loop opened).
Within the bias pulse, the current from a typical trace exhibitsa small stepwise reduction that occurs in the typical example
shown in Fig. 5c after a time delay of 1.4 s. Thus, the switch does
not occur instantaneously but after a significant delay. By performing many switching events, we have extracted the average switchingrate as a function of the current during the pulse. As the linear fits
(dotted lines) of the HS→LS and LS→HS switching rates versus
current in Fig. 5d with logarithmic x-y scales show, a simple powerlaw relates the rate R to the current I as R∝IN. We find that N≈8 for
the LS→HS transition, suggesting a higher-order process involving
vibrational heating due to inelastic scattering of the tunnelling electrons2,25, in competition with a heat loss to the substrate. For the
HS→LS transition, we find that the switching rate scales linearlywith current (N ≈ 1). This indicates that, although the switchingprobability per electron is on the order of P ~ 10 − 9, the switching is caused by single electron processes. We note that the HS→
LS conformational switch at voltages above ≈ + 1.1 V could involve
injecting charge onto the state that is present at + 1.1 eV (Supplementary Fig. S4). In the following, we discuss the broad featuresof a model for electrical molecular switching in analogy to that of
optical switching.

nature communications | 3:938 | DOI: 10.1038/ncomms1940 | www.nature.com/naturecommunications© 2012 Macmillan Publishers Limited. All rights reserved.

ARTICLE

nature communications | DOI: 10.1038/ncomms1940

b
3s

LS

10

20

30
Time (s)

1.5
1.0
3s
V=
0.5
+ 0.1 V
0.0
3
Delay
2
1
0.18 HS
0.15
LS
0.12
0.09
1 2 3 4 5Time (s)

40

50

100
80

60

LS

5

0

d

10

e
10

15
Time (s)25

20

Excited state~ 100 fs

HS to LS
LS to HSHS to LS
LS to HS

EnergyVoltage(V)
Current
(nA)

3s

HS

130
0

c

1.0
0.0
–1.0

HS

Current(pA)

Current(pA)

150

Voltage
(V)

1.0
0.0
–1.0

Switching rate (s–1)

Voltage(V)a1

P ~ 10–9
N~8~ 5 fs

8
7
6
2
1
0

0.1

LS

1
Current (nA)HS

10DistanceFe-Ligand

Figure 5 | Robust SCO-based single-molecule memristance. (a) Demonstration of reversible switching by voltage pulses of + 1.5 V inducing HS to
LS switching and − 1.2 V inducing LS to HS switching. Corresponding to the polarity of the voltage pulse, the tunnelling current for V =  + 0.1 V is clearlyswitched between two levels corresponding to the HS and LS states (150 pA and 130 pA, respectively). The tip was initially stabilized at + 100 mV and150 pA. (b) Demonstration of deterministic switching by the application of two consecutive pulses of the same polarity. The first positive pulse switches
the molecules from the HS to the LS state. The second positive pulse does not alter the state, while a third negative pulse switches back the molecule to
the HS state. The tip was initially stabilized at + 100 mV and 100 pA. (c) Real-time current trace of an isolated HS state molecule on the CuN/Cu(100)surface. A voltage pulse of + 1.5 V is applied for 3 s and the transition to the LS state is witnessed by a small decrease in current after a delay of 1.4 s(red arrow). (d) Observed switching rates for HS to LS states and vice versa by voltage pulses of ± 1.5 V. The dotted lines indicate power law fits of theswitching as function of the absolute tunnelling current. (e) Proposed model for the switching mechanism in both directions. While the switching from the
LS to the HS state is a high order process (N ≈ 8) involving heating by the tunnelling electrons, the transition from the HS to the LS state only requires one
electron (N ≈ 1) and involves an intermediate short-lived excited state.

When exciting the molecule from the HS state with electrons to
the molecular state at + 1.1 V, the positions of the atomic cores are
initially not changed owing to their slow reaction. This excited state
can favour ligand’s distances to the central Fe ion that better match
those of the LS state. If the lifetime of the excited state were long,the excited state would relax into its vibronic ground state. Electronic deexcitation would then result in the LS state of the molecule(Fig. 5e). The spectroscopic data, however, indicate that the electronically excited state has a lifetime of only 5 fs. Yet typical vibration times of the vibronic excitations are in the range of 10 THz, thatis, periods of a tenth of a ps. Thus, only in the low-probability casethat the electronically excited state survives for 0.1 ps is a transition
possible. A simple calculation for 5 fs electronic lifetime and 10 THz
vibration frequency results in a switching probability of P = 2×10 − 9.
This naturally explains the low success rate for switching and agrees
with the observed order of the process of N = 1. Interestingly, theenergy of the excited state in STM experiments lies close to thatneeded for optical switching (1.49 eV), which is associated to thetransition of the molecule from the 5T2 to the 5E state4. In this sense,a thicker insulating layer could increase the lifetime of the excitedstate and thus the switching probability. Further experiments, suchas time-resolved optical pump probe measurements, could clarify
the switching mechanism.
In conclusion, once the molecule–electrode interaction is
reduced, individual Fe-phen SCO molecules can be reversibly anddeterministically switched by a STM to a specific spin state, leading toa large difference in junction conductance. More broadly, strategies
within the past decade to combine memristive21,26 and spintronic15
functionalities in devices have relied on extrinsic27,28 or nominallyintrinsic29 properties of the materials and interfaces within thedevice. Similar yet separated memristive2 and spintronic11 functionalities have arisen in the ultimate limit of single-molecule junctions. Our results show how to harness an unambiguously intrinsic
property—SCO—to obtain memristive functionality. This functionality is robust, as it shows deterministic and reproducible switchingdepending on the polarity of the applied current, shows hysteresisand no spontaneous switches were observed for low tunnelling voltages. Relative to the SCO energy barrier, by appropriately tuning the
hybridization strength of a SCO molecule adsorbed onto a suitablespin injector, one could also achieve spintronic functionality on asimilar, single-molecule, magnetomemristive-based device potentially in spatially dense, multifunctional memory applications. During the refereeing of this manuscript, we became aware of a closely
related work, which has been published in the meantime30.

Methods

STM experiments. STM measurements were carried out in a home-built STM31 in
ultra high vacuum (p < 2.0×10 − 10 mbar) at low temperature (4.6 K). The STM tips
were prepared by chemical etching of a tungsten wire and flashing in ultra-highvacuum. The clean and flat Cu(100) surface was prepared by several cycles of Ar +sputtering (1.5 keV) and subsequent annealing up to 450 °C. Fe(phen)2(NCS)2
molecules were sublimed at 180 °C in ultra-high vacuum, while maintaining theCu(100) and CuN/Cu(100) surfaces at room temperature. The differential conductance dI/dV spectra were obtained with a lock-in technique using a modulation biasof 3 mV and a frequency of 723 Hz. When ramping the voltage, the feedback loop
was opened.
X-ray absorption experiments. Soft XAS and XMCD measurements were
performed at the DEIMOS beamline of SOLEIL in France in total electron-yieldmode at several temperatures and magnetic fields. The measurement chamber isnature communications | 3:938 | DOI: 10.1038/ncomms1940 | www.nature.com/naturecommunications© 2012 Macmillan Publishers Limited. All rights reserved.

ARTICLE

nature communications | DOI: 10.1038/ncomms1940

connected to chambers allowing in situ preparation and characterization (Auger
electron spectroscopy, STM) of the samples.
Preparation of CuN layers. To reduce the interaction between Fe-phen molecules
and the substrate, one monolayer decoupling CuN film was introduced on Cu(100)surface. After obtaining a clean Cu(100) surface, the CuN layer was preparedby ionic bombardment at room temperature for 25 min. using 4×10 − 4 mbar of
N2, a beam energy of 500 eV and subsequent annealing up to 330 °C. With these
preparation parameters, the entire Cu(100) surface is found to be covered with a
CuN layer.

References

1. Reed, M., Zhou, C., Muller, C. J., Burgin, T. P. & Tour, J. M. Conductance ofa molecular junction. Science 278, 252–254 (1997).
2. Qiu, X., Nazin, G. & Ho, W. Mechanisms of reversible conformationaltransitions in a single molecule. Phys. Rev. Lett. 93, 196806 (2004).
3. König, E. & Madeja, K. Unusual magnetic behaviour of some Iron(II)Bis-(1,10-phenanthroline) complexes. Chem. Commun. 3, 61–62 (1966).
4. Gütlich, P., Garcia, Y. & Goodwin, H. Spin crossover phenomena in Fe(II)complexes. Chem. Soc. Rev. 29, 419–427 (2000).
5. Hauser, A. Reversibility of light-induced excited spin state trapping in theFe(ptz)6(BF4)2 and the Zn1xFex(ptz)6(BF4)2 spin-crossover systems. Chem.
Phys. Lett. 124, 543–548 (1986).
6. Marchivie, M. et al. Structural characterization of a photoinduced molecularswitch. J. Am. Chem. Soc. 124, 194–195 (2001).
7. Kato, K., Takata, M., Moritomo, Y., Nakamoto, A. & Kojima, N. On-off opticalswitching of the magnetic and structural properties in a spin-crossovercomplex. Appl. Phys. Lett. 90, 201902 (2007).
8. Bousseksou, A. et al. Triggering the spin-crossover of Fe(phen)2(NCS)2 by apressure pulse: Pressure and magnetic field induced mirror effects. Comptes
Rendus Chimie 6, 329–335 (2003).
9. Baadji, N. et al. Electrostatic spin crossover effect in polar magnetic molecules.
Nature Mater. 8, 813–817 (2009).
10. Meded, V. et al. Electrical control over the Fe(II) spin crossover in a single
molecule: Theory and experiment. Phys. Rev. B 83, 245415 (2011).
11. Schmaus, S. et al. Giant magnetoresistance through a single molecule. Nature
Nanotech. 6, 185–189 (2011).
12. Reiher, M. Theoretical study of the Fe(phen)2(NCS)2 spin-crossover complex
with reparametrized density functionals. Inorg. Chem. 41, 6928–6935 (2002).
13. Lebègue, S., Pillet, S. & Ángyán, J. Modeling spin-crossover compounds by
periodic DFT+U approach. Phys. Rev. B 78, 024433 (2008).
14. Shi, S. et al. Study of molecular spin-crossover complex Fe(phen)2(NCS)2 thinfilms. Appl. Phys. Lett. 95, 043303 (2009).
15. Fert, A. Origin, development, and future of spintronics (Nobel Lecture). Angew.
Chem. Int. Ed. 47, 5956–5967 (2008).
16. Madhavan, V., Chen, W., Jamneala, T., Crommie, M. & Wingreen, N. Tunnelinginto a single magnetic atom: spectrocopic evidence of the Kondo resonance.
Science 280, 567–569 (1998).
17. Gao, L. et al. Site-specific Kondo Effect at Ambient Temperatures in Iron-based
Molecules. Phys. Rev. Lett. 99, 106402 (2007).
18. Gütlich, P. & Hauser, A. Thermal and light-induced spin crossover in iron(II)complexes. Coord. Chem. Rev. 97, 1–22 (1990).
19. Collison, D. et al. Soft X-ray induced excited spin state trapping and soft
X-ray photochemistry at the iron L2,3 edge in [Fe(phen)2(NCS)2] and[Fe(phen)2(NCSe)2] (phen = 1,10- phenanthroline). J. Chem. Soc. Dalton
Trans. 4371–4376 (1997).
20. Hirjibehedin, C., Lutz, C. & Heinrich, A. Spin coupling in engeneered atomic
structures. Science 312, 1021–1024 (2006).



21. Strukov, D., Snider, G., Stewart, D. & Williams, R. The missing memristor
found. Nature 453, 80–83 (2008).
22. Baadji, N. & Sanvito, S. Giant resistance change across the phase transition in
spin-crossover molecules. Phys. Rev. Lett. 108, 217201 (2012).
23. Komeda, T. et al. Observation and electronic current control of a local spinin a single-molecule magnet. Nat. Commun. 2, 217 (2009).
24. Mugarza, A. et al. Spin couig and relaxation inside molecule-metal contacts.
Nat. Commun. 2, 490 (2011).
25. Stipe, B., Rezeai, M. & Ho, W. Single-molecule vibrational spectroscopy andmicroscopy. Science 280, 1732–1735 (1998).
26. Auwäter, W. et al. A surface-anchored molecular four-level conductance switch
based on single proton transfer. Nat. Nanotech. 7, 41–46 (2012).
27. Bowen, M. et al. Bias-crafted magnetic tunnel junctions with bistable spindependent states. Appl. Phys. Lett. 89, 103517 (2006).
28. Teixeira, J. et al. Resonant tunneling through electronic trapping states in thin
MgO magnetic junctions. Phys. Rev. Lett. 106, 196601 (2011).
29. Garcia, V. et al. Ferroelectric control of spin polarization. Science 327,
1106–1110 (2010).
30. Gopakumar, T. G., Matino, F., Naggert, H., Bannwarth, A., Tuczek, F. & Berndt, R.
Electron-induced spin crossover of single molecules in a bilayer on gold.
Angew. Chem. Int. Ed. 51, 6262–6266 (2012).
31. Zhang, L., Miyamachi, T., Tomani´c, T., Dehm, R. & Wulfhekel, W. A compact
sub-Kelvin ultrahigh vacuum scanning tunneling microscope with high energy
resolution and high stability. Rev. Sci. Instrum. 82, 103702 (2011).

Acknowledgements

We thank the beamline Deimos staff at Synchrotron SOLEIL, B. Muller, J. Arabski and
C. Kieber for technical assistance with XAS experiments and M. Alouani for fruitful
discussions. We acknowledge funding from the Deutsche Forschungsgemeinschaft(DFG) within the Center for Functional Nanostructures (CFN) and the BadenWürttemberg Stiftung in the framework of the Kompetenznetz für Funktionale
Nanostrukturen (KFN), from the Agence Nationale de la Recherche ANR-06-NANO033-01 and ANR-09-JCJC-0137, from the Frontiers of Research in Chemistry, and fromthe Franco-German University. T.K.Y. acknowledges funding from the Alexander von
Humboldt Foundation.

Author contributions

T.M., W.W., M.B. and E.B. conceived and designed the experiments. G.R. synthesized
the molecules. T.K.Y., T.M. carried out initial sublimation experiments. T.M., M.G.,
V.D. carried out STM experiments. T.M., V.D., M.B., S.B., F.S., L.J., P.O. and E.B.
carried out XAS experiments. T.M., M.G., V.D. analysed the data. T.M., W.W., M.B.
and E.B. co-wrote the paper. All authors discussed the results and commented on
the manuscript.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/
naturecommunications
Competing financial interests: The authors declare no competing financial interests.
Reprints and permission information is available online at http://npg.nature.com/
reprintsandpermissions/
How to cite this article: Miyamachi, T. et al. Robust spin crossover and memristanceacross a single molecule. Nat. Commun. 3:938 doi: 10.1038/ncomms1940 (2012).
License: This work is licensed under a Creative Commons Attribution-NonCommercialShare Alike 3.0 Unported License. To view a copy of this license, visit http://
creativecommons.org/licenses/by-nc-sa/3.0/

nature communications | 3:938 | DOI: 10.1038/ncomms1940 | www.nature.com/naturecommunications© 2012 Macmillan Publishers Limited. All rights reserved.