ARTICLESPUBLISHED ONLINE: 20 FEBRUARY 2011 | DOI: 10.1038/NNANO.2011.11

Giant magnetoresistance through a single molecule
Stefan Schmaus1,2, Alexei Bagrets2,3, Yasmine Nahas1,2, Toyo K. Yamada1,4, Annika Bork1,
Martin Bowen5, Eric Beaurepaire5, Ferdinand Evers3,6 and Wulf Wulfhekel1,2 *Magnetoresistance is a change in the resistance of a material system caused by an applied magnetic ﬁeld. Giantmagnetoresistance occurs in structures containing ferromagnetic contacts separated by a metallic non-magnetic spacer,and is now the basis of read heads for hard drives and for new forms of random access memory. Using an insulator (forexample, a molecular thin ﬁlm) rather than a metal as the spacer gives rise to tunnelling magnetoresistance, which
typically produces a larger change in resistance for a given magnetic ﬁeld strength, but also yields higher resistances,which are a disadvantage for real device operation. Here, we demonstrate giant magnetoresistance across a single, nonmagnetic hydrogen phthalocyanine molecule contacted by the ferromagnetic tip of a scanning tunnelling microscope. We
measure the magnetoresistance to be 60% and the conductance to be 0.26G0 , where G0 is the quantum of conductance.
Theoretical analysis identiﬁes spin-dependent hybridization of molecular and electrode orbitals as the cause of thelarge magnetoresistance.

Spin-polarized transport through a single molecule has attracted
interest, because it combines the goals of downward size scalingof electronic components (to lower power consumption while
increasing speed and density) and increasing computational bandwidth by manipulating spin in addition to charge. The concept ofspin electronics1,2 was pioneered with the discovery that the resistance of two ferromagnetic layers separated by a thin non-magnetic
spacer may be driven by an external magnetic ﬁeld or the ﬂow ofcharge. In the case of a thin metallic spacer, this effect is termed
giant magnetoresistance (GMR)3,4, and for ultrathin insulating
layers it is called tunnelling magnetoresistance (TMR)5,6. Acrossthin organic semiconducting layers, transport may occur via tunneling or sequential tunneling, that is, diffusive hopping7,8, demonstrating a magnetoresistance thanks to the low values of spin–orbit
coupling, despite the presence of defects in these imperfectly
mastered semiconductors9,10.
Spin electronics has led to new generations of read heads in harddisk drives and random access memory, but further progress indownsizing spintronic devices has been challenging. Indeed,although TMR values may dwarf those of their GMR counterparts
through a judicious selection of ferromagnets11 and inorganic12 ororganic13,14 tunnel barriers, downsizing such devices can lead tohigh resistances8 and shot noise, which are incompatible withhigh-frequency applications15. Separately, as the device size is
reduced, the GMR bias output shrinks below the bias drop of theleads, which in turn requires one to artiﬁcially lower the current
densities to achieve a high magnetic sensitivity16.
A promising approach to extreme downsizing has been to study
electronic single-molecule devices such as diodes17 and transistors18–20. Turning now to molecular spintronics, recent studies
have described the spintronic impact of chemisorption of molecules21 onto ferromagnets. This highly conductive interface22 candrive spintronic properties8,21, due in part to ferromagnetic coupling
between the transition metal site of the molecule and the ferromagnetic surface21,23,24.

Here, we present a combined experimental and theoretical study
on spin-polarized charge transport across a single molecule in the
GMR regime that takes advantage of these novel molecular spintronic properties. The junction, built with the help of a scanning tunnelling microscope (STM), consists of two ferromagnetic electrodes
bridged by a hydrogen phthalocyanine (H2Pc ¼ C32H18N8) molecule. Our study shows that, due to charge transfer towards the molecule and the hybridization of molecular orbitals near the Fermi
level with bulk electronic states of the electrodes, transport acrossthe molecule is nearly resonant in the minority channel with anultralow areal resistance product. Surprisingly, the molecule cansustain a large current density with substantial spin-polarization,
thus validating the concept of nanoscale spintronic devices.
Indeed, despite the absence of transition metal sites on the moleculethat could contribute to spin ﬁltering, we observe a high differential
GMR value of 60% across the molecular junction, with an areal
resistance product of only 70 mV mm2. Our ab initio theoreticalanalysis explicitly identiﬁes this effect as a strong, generic, spindependent hybridization mechanism that can also dominate
the magnetoresistance in the TMR regime, as was reported
phenomenologically8.

Sample characterization
STM25–27 and break-junction28,29 techniques have generally provento be powerful and versatile tools with which to study the transport
properties of single-molecule junctions. In this work, we use magnetic
electrodes to measure the magnetoresistance of single molecules using
spin-polarized STM (Sp-STM). Experiments were carried out in ahome-built STM instrument working in ultra-high vacuum at 4 K(ref. 30) on individual H2Pc molecules sandwiched between
cobalt-coated tungsten tips and ferromagnetic cobalt nano-islands
on Cu(111) single crystals (see Supplementary Information).
Figure 1a presents the STM topography of the sample. The cobalt
nano-islands exhibit a spontaneous out-of-plane magnetization due
to a strong interfacial anisotropy31,32. With cobalt-coated tips

1
Physikalisches Institut, Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe, Germany, 2 DFG-Center for Functional Nanostructures, Karlsruhe Instituteof Technology (KIT), 76128 Karlsruhe, Germany, 3 Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), 76128 Karlsruhe, Germany,
4
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan, 5 Institut de Physique et Chimie des Matériaux de Strasbourg,
UMR 7504 UdS-CNRS, 67034 Strasbourg Cedex 2, France, 6 Institut Für Theorie der Kondensierten Materie, Karlsruhe Institute of Technology (KIT),
D-76128 Karlsruhe, Germany. * e-mail: wulf.wulfhekel@kit.edu

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NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2011.11ba
13.5 (nA V−1)

Antiparallel

Parallel60
40
20

dl/dV (nA V−1)

Parallel
Antiparallel

0
−1.0

−0.5

0.0

0.5

1.0

Sample bias (V)
150

c

50
0

5 nm
−4.5 (nA V−1)
−1.0

−0.5

0.0

0.5

TMR (%)

100

−50
1.0

Sample bias (V)Figure 1 | H2Pc molecules adsorbed on cobalt islands with different out-of-plane magnetic orientations. a, Topographic image of H2Pc molecules adsorbed
onto two cobalt islands on the Cu(111) surface. Colour code: measured dI/dV at 2310 mV. The two island species can be distinguished by the magnetization
parallel (in yellow) and antiparallel (in red) to the tip magnetization. b, Typical dI/dV spectra taken on parallel and antiparallel oriented cobalt islands (marked
by red and blue crosses in a) clearly reveal spin-polarized density of states below the Fermi edge. c, Energy dependence of the optimistic TMR ratio
calculated from the dI/dV spectra. The highest value is measured at approximately 2350 meV, and is used to distinguish between the magnetic orientationof the islands.

(10 monolayers) showing an out-of-plane magnetization, we can use
the sensitivity of the Sp-STM technique to the spin-polarized
density of states33 to determine the orientation (parallel or antiparallel) of the magnetization of individual islands relative to that of thetip. Figure 1b shows typical differential conductance (dI/dV ) curves
measured on top of cobalt islands of parallel and antiparallel orientation. Particularly large differences in the spectra are found at
2350 meV, which corresponds to the surface state of cobalt34. Byrecording maps of the differential conductance at this bias voltage(Fig. 1a), we can thus identify the spin orientation of the cobalt
islands and determine the differential TMR. The latter is deﬁned
as the difference in differential conductance (dI/dV) divided by
the smaller differential conductance of the tunnel junction formedby the tip and cobalt island for ensembles of junctions with parallel
and antiparallel oriented cobalt islands. As can be seen in Fig. 1c,the differential TMR for such cobalt/vacuum/cobalt junctions isstrongly energy-dependent, as reported previously32. Remarkably,
the differential TMR reaches only 5% at low bias voltage.

Spin-dependent conductance across a single molecule
The H2Pc molecules that were also deposited adsorb lying ﬂat on thecobalt surface24, and are therefore easily recognized in Fig. 1 by theirfour aromatic isoindole (BzPy) side groups35. Although the local
spin-polarization in the tunnelling regime has been imaged with
Sp-STM for phthalocyanine molecules with 3d metal centres24,36,
we here focus on GMR transport measurements across non-magnetic H2Pc molecules in the contact regime. To observe the transition from the TMR regime to the GMR regime across themolecule, we alter the junction geometry and achieve tip-to-molecule contact by positioning the STM tip above the aromatic sidegroups of a H2Pc molecule, opening the feedback loop and decreasing the tip-to-molecule distance by a small offset (1 Å s21) whilemeasuring the tunnelling current25–27,37,38. We ﬁrst observe an exponential increase of the conductance due to the decreasing tunnellingbarrier width (Fig. 2a). Below a certain tip-to-surface separation(typically 3–4 Å), the conductance abruptly increases and thereafter
depends only weakly on the distance. As reported regarding alkanedithiol molecular wires26, this underscores the ‘jump-to-contact’ of a2

molecular segment. For H2Pc molecules, the transition between theﬂat adsorption and the contact geometry has been ascribed to the
20 meV vibrational bending mode of the aromatic side groups of
the H2Pc molecule. Note that when voltages higher than thisvibrational mode are applied in the contact geometry, the vibration
can be excited by the current and the molecular contact becomes
unstable (see Supplementary Information).
The conductance after the jump-to-contact encompasses twocontributions: the current across the molecule Gmol and a leakage
current Gtun due to direct tunnelling between the tip and sample26(Fig. 2c). To obtain a reasonable estimate of the molecular conductance Gmol , we therefore subtract the conductance Gtun measured
before the jump from the conductance Gcont measured after the
jump, that is, Gmol ≈ Gcont 2 Gtun. Because measurements on twoparallel and antiparallel oriented cobalt islands are required,using identical tip conditions, to extract the GMR correctly, werecord the traces from two such islands in the same topographic
scan. This method notably eliminates possible magnetostriction
effects between the two ferromagnetic electrodes, because neither
the tip nor the island magnetization need be switched by an external
magnetic ﬁeld.
In the particular measurement at 10 mV in Fig. 2a, we ﬁnd a conductance of GP ¼ 0.26G0 for the parallel and GAP ¼ 0.19G0 for the
antiparallel magnetic conﬁguration in units of G0 ¼ 2e 2/h. Theserather large conductance values underscore a strong hybridization
between the electronic levels of the metallic electrodes and thoseof the molecule21. This is in sharp contrast to the orders of magnitude lower values reported for almost insulating molecular TMR
devices8,13 and single-molecule devices based on C60 (ref. 39).
Each such measurement was repeated several hundred times on
the four side groups of about 10 molecules adsorbed onto the twoparallel and antiparallel oriented islands, leading to a distributionof the conductances across these parallel-type and antiparallel-typejunctions as depicted in Fig. 2d. Compared to previous results26,
the width of the resulting conductance distribution is relatively
narrow. The broadening and possible structural elements of these
distributions could reﬂect variation in the contact and binding geometries, but also ﬂuctuations in the spin-polarized density of states

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ARTICLES

NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2011.11b

Parallel
Antiparallel

Gtun

Number of measurements

d

0.05

b

Co(111) tip

0.10
0.15
Offset (nm)Surface

c

Tip
Gmol
Gtun

Co(111) surface

Surfacec

0.20

Parallel
Antiparallel

50
40
30
20

Conductance, 2e2 h−1

0.01

0.00

a

Tip
Gtun

0.1

Gcont = Gmol + Gtun

Conductance, G0a“Contact” , parallel“Contact” , antiparallel“Flat” , parallel“Flat” , antiparallel1

0.1

Contact

Tunnelling

10
–8

0
0.2
0.4
Molecular conductance, Gmol (G0)

Figure 2 | Current–distance traces and magnetoresistance measured
across single H2Pc molecules. a, Typical set of conductance-distance curves
measured on top of a H2Pc molecule adsorbed onto parallel and antiparallel
magnetized islands with a constant tunnelling voltage of 10 mV(G0 ¼ (2e 2/h)). b, As the tip approaches the molecule, the tunnel barrier
width decreases, so the conductance increases exponentially. c, Below acertain tip-to-surface separation (typically 3–4 Å) the conductance abruptly
increases as the molecule jumps into contact, and then varies only slightly
upon further reducing the distance. Transport across the contacted molecule
reﬂects both tip-to-surface tunnelling and conduction across the molecule.
d, Histogram of corrected molecular conductances (381 parallel and 366
antiparallel). A Gaussian ﬁt is used to determine the statistical conductancein the parallel and antiparallel conﬁgurations, and thus the GMR ratio. Error
bars indicate statistical errors in the conductance distribution.

on the islands32,40. Gaussian ﬁts were used to determine the average
conductances and the GMR from the measurement statistics. We
ﬁnd GP ¼ (0.253+0.005)G0 and GAP ¼ (0.158+0.005)G0. These
values result in an optimistic GMR ratio at V ¼ 10 mV of
GMR =GP − GAP= (61 + 9)%
GAP

Surprisingly, this GMR ratio is one order of magnitude larger
than the differential TMR ratio found for direct tunnelling
between the tip and the cobalt surface.

Transport calculations
To understand what causes this large value of GMR, we performed
transport calculations based on density functional theory (DFT)
using the non-equilibrium Green’s function (NEGF) formalism andthe TURBOMOLE package41 (see Supplementary Information).
The atomic structure of the molecular junction was found by optimizing the H2Pc geometry on a Co(111) surface that was modelledusing a 65-atom cluster. Our analysis suggests that H2Pc adsorbs
preferentially in the bridge position onto Co(111) (Fig. 3a), due toa binding energy 28.17 eV that is larger than the one found ineither the hollow site position (28.06 eV) or the atop site position(27.45 eV), which is consistent with earlier ﬁndings24,42.
We have calculated spin-polarized transport in the linear
response at low bias voltage for two junction geometries, schematically shown in Fig. 3a,b, corresponding to the TMR and GMR

–7.5
–7
–6.5
Surface-to-tip distance (Å)

Figure 3 | Ab initio simulations of current–distance traces and themagnetoresistance effect across a H2Pc molecule. a,b, Contact geometry
used in the transport calculation: for a H2Pc molecule adsorbed on the
Cobalt island (a), and in simultaneous contact with the tip and the cobalt
surface through a lifting of the aromatic group (b). Cobalt sites, grey;hydrogen, white; carbon, green; nitrogen, cyan. c, Conductance of H2Pc
sandwiched between two parallel or antiparallel aligned Co(111) surfaces inthe tunnelling (before contact) and ballistic (after contact)junction geometries.

transport regimes (see Supplementary Information). Qualitatively,
these calculations (Fig. 3c) reproduce very well our experimentalﬁndings (Fig. 2a). We conﬁrm the exponentially increasing conductance in the tunnelling regime, Gtun(d ) / e2bd, for which the distance d between the two electrodes is still large. The slope isindependent of the relative alignment of electrode magnetizations,and the computational values of b theo ¼ 1.87 Å21 (and the work
function Wtheo ¼ 3.24 eV) are in agreement with experiment (b ¼
1.9+0.3 Å21; W ¼ 3.2 eV).
Once contact between the tip and the molecule has been established, the variation of Gcont (d) with the contact distance d is very
weak, just as observed in the experiment. It is, however, still sensitive
to the relative orientation of the magnetization of the electrodes. We
ﬁnd that GAP(d ) is always much lower than GP(d). To quantitatively
compare with experiment, we consider the theoretical GMR ratio at
the distance d at which the ratio r ¼ Gtun/Gcont matches the valuer ≈ 4 found experimentally. We thus ﬁnd that GMR ≈ 65% and is
only weakly dependent on d.
We now discuss the basic conduction mechanism that underscores the large GMR measured. Pc molecules are characterized
by the energetically isolated highest occupied molecular orbital(HOMO) and the nearly doubly degenerate lowest unoccupied molecular orbital (LUMO)43,44. We note that the HOMO* levels corresponding to the aromatic group hybridize only very weakly, withalmost no amplitude on the bridging nitrogen (Nb)4. In contrast,
the LUMO states are located on two of the four aromatic groups,
with a strong hybridization to all nitrogen atoms forming theinner macrocycle (see Supplementary Information). Because thenitrogen bond to cobalt includes states at the Fermi energy EF ,(refs 21,45), transport should occur via the (quasi-degenerate)
LUMO level. We conﬁrm this fact by examining in Fig. 4a the transmission probability per spin direction T,(E) near the Fermi energy
across a parallel oriented junction. We ﬁnd that, for both spin

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NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2011.11b

Parallel
1

T(E) per spin, e2 h−1

T(E) per spin, e2 h−1

a

0.1

−2

−1.5

−1

−0.5
0
0.5
E − EF (eV)

1

1.5

2

Antiparallel
1

0.1

−2

−1.5

−1

0
−0.5
0.5
E − EF (eV)1Majority

Minority

Minority

Majority

Majority

Minority

Majority

Minority

1.5

2Figure 4 | Molecular orbitals and transmission curves reveal the spin-selective LUMO broadening and the impact on GMR across the H2Pc molecule.
a, Transmission probability T(E) of an electron with energy E through a parallel oriented molecular junction for the majority (red) and minority (orange) spinchannels and the current-carrying (LUMO) orbitals of the molecular junction at the Fermi energy. Hybridization of the LUMO orbitals with the cobalt states
is weak in the majority channel and strong in the minority channel. b, Corresponding data for an antiparallel oriented junction. Majority spin electrons are
injected from the cobalt surface to the minority spin band of the cobalt tip (violet), and minority spin electrons are injected from the cobalt surface to themajority spin band of the cobalt tip (blue). In this conﬁguration, the LUMO orbitals are hybridized in an asymmetric way between the cobalt surface and thetip, thus accounting for the conductance decrease relative to that found for parallel oriented junctions.

channels, GP near EF is indeed dominated by a peak centred slightly
above EF indicating transmission through the LUMO level. We ﬁnd
a similar origin of electron transport for an antiparallel oriented
junction (Fig. 4b).
The peak width is determined by the amount of hybridization of
the molecular LUMO orbital (through N–Co binding) with thestates of the cobalt electrodes. Because the surface density of statesof the cobalt electrodes at EF is enhanced for minority spin electrons
as compared to majority spin electrons, the LUMO broadening
found for a parallel oriented junction is stronger in the minoritytransport channel than in the majority channel (Fig. 4a). This
broadening asymmetry is, of course, less pronounced for an antiparallel oriented junction (Fig. 4b). Note that our experiments operate
at low voltage, 10 meV, in the regime where the current response to
the bias voltage is still linear. In this regime, the conductanceresembles transmission at the Fermi energy.
The dependence of LUMO broadening on a given spin channel
and the relative alignment of electrode magnetizations both, in turn,have a direct impact on the GMR obtained across the molecularjunction. Indeed, the conductance across a single level of thequasi-degenerate LUMO generically takes on the Breit–Wigner
form46 (here a factor of 2 accounts for the degeneracy of the
LUMO: G ≈ 2GsubstrateGtip/((ELUMO 2 EF)2 þ (Gsubstrate þ Gtip)2/4).
This expression considers the energy separation between the
LUMO and EF, as well as the LUMO broadenings (inverse lifetimes)
Gsubstrate and Gtip due to hybridization to the substrate and tip,
respectively. Each is, in turn, split into Gmin(maj) depending on
the spin channel considered. Because transport is off-resonant,that is, |ELUMO 2 EF| ≫ Gmin,maj, we have GP ≈ 2 (GminGmin þ
GmajGmaj )/(ELUMO 2 EF)2, while GAP ≈ 4GminGmaj/(ELUMO 2 EF)2.
Introducing the ratio @ ¼ Gmaj/Gmin, we thus ﬁnd
GMR ≈4(Gmin − Gmaj )2 (1 − @)2=2@
2Gmin Gmaj

This simple formula implies two important rules of thumb for spinpolarized transport off-resonance across a molecule. First, the GMR
is insensitive to the precise location of the resonance energy provided that EF lies reasonably within the level broadening. Second,
the GMR is mainly indicative of the ratio @ of minority and majority
molecular orbital broadenings due to hybridization. As such, theGMR may reach larger (here order-of-magnitude) values than its
TMR counterpart without molecules.
Several papers have recently described means of tuning this spinpolarized broadening of molecular orbitals near EF, and the resulting amplitude/sign of interfacial spin polarization useful for spin
injection into organic materials. For instance, Barraud andco-workers invoke disorder to explain, within a phenomenologicalmodel, the presence at EF of a localized molecular state that promotes spin-polarized resonant transport in the tunnelling regime8.
Other reports have considered the impact of intrinsic molecularstates near EF (ref. 22) on spin polarization21,36,45. Relative to thesereports, our results explicitly reveal the fundamental and appliedinterest in the direct coupling of two such ferromagnet–moleculeinterfaces. In this case, the spin selective broadening of the
LUMO leads to a highly conductive situation in the parallel conﬁguration with electronic states delocalized across the wholejunction, much like the classical and metallic GMR junctions3,4.
The resulting single-molecule spintronic junction, operatingin the contact regime, combines a low resistance (52 kV) for
high-frequency applications, a low resistance–area product(70 mVmm2) for increased density, and a large spintronic response(60%) for enhanced data-processing capabilities. This effect should
also be present in nanoscale solid-state devices consisting of two ferromagnetic electrodes separated by a monolayer of organic molecules useful for applications.

Methods

The Cu(111) crystal was cleaned by several cycles of Arþ sputtering and annealing.
The molecules were evaporated in situ from a Knutsen cell heated to 500 K.
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NATURE NANOTECHNOLOGY DOI: 10.1038/NNANO.2011.11
During the deposition process the sample was maintained at 270 K to reduce
thermal diffusion of the deposited molecules. dI/dV curves were measured on thebare islands with a lock-in technique.
DFT-based transport calculations were carried out with a homemade codebuilding upon the NEGF formalism and the TURBOMOLE package41. Our
implementation enabled us to perform transport simulations with free boundary
conditions, which for the present case were extended to account for the spinpolarized electronic structure of the magnetic electrodes (for further details, see
Supplementary Information). The gradient-corrected approximation (GGA) DFT
energy was amended by empirical corrections47 to account for dispersive van der
Waals interactions between the molecule and the surface.

Received 22 November 2010; accepted 14 January 2011;
published online 20 February 2011

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Acknowledgements
The authors thank O. Hampe, J. Kortus, K. Fink, S. Boukari, Xi Chen, M. Alouani,
R. Mattana, J. van Ruitenbeek and P. Seneor for useful communications. The authors also
acknowledge support from the Deutsche Forschungsgemeinschaft (WU 349/3-1 and
SPP1243), the Center for Functional Nanostructures, the French–German University, theAlexander von Humboldt foundation, and the Agence Nationale de la Recherche (ANR-06NANO-033-01) as well as from the Yamada Science Foundation and the Asahi GlassFoundation.

Author contributions
S.S. and W.W. conceived and designed the experiments. S.S., Y.N., T.K.Y. and An.B.
performed the experiments. S.S., Y.N. and An.B. analysed the data. Al.B. and F.E. designed
and performed the calculations. M.B. and E.B. provided puriﬁed molecules. S.S., Al.B.,
T.K.Y., F.E., M.B., E.B. and W.W. co-wrote the paper. All authors discussed the results andcommented on the manuscript.

Additional information
The authors declare no competing ﬁnancial interests. Supplementary informationaccompanies this paper at www.nature.com/naturenanotechnology. Reprints andpermission information is available online at http://npg.nature.com/reprintsandpermissions/.
Correspondence and requests for materials should be addressed to W.W.

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