Letterpubs.acs.org/NanoLett

Single Molecule Magnetoresistance with Combined
Antiferromagnetic and Ferromagnetic Electrodes
Alexei Bagrets,*,†,‡ Stefan Schmaus,§,∥,■ Ali Jaafar,⊥ Detlef Kramczynski,§ Toyo Kazu Yamada,§,¶Mébarek Alouani,⊥ Wulf Wulfhekel,§,∥ and Ferdinand Evers†,#†

Institute of Nanotechnology and ‡Steinbuch Centre for Computing, Karlsruhe Institute of Technology,
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany§
Physikalisches Institut and ∥DFG-Center for Functional Nanostructures, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse
1, 76131 Karlsruhe, Germany⊥
Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS-ULP, 23 rue du Loess BP 43, 67034
Strasbourg, France¶
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan#
Institut für Theorie der Kondensierten Materie, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe,
Germany■
Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany
S Supporting Information*

ABSTRACT: The magnetoresistance of a hydrogen-phthalocyanine molecule placed on an antiferromagnetic Mn(001) surface andcontacted by a ferromagnetic Fe electrode is investigated using
density functional theory based transport calculations and lowtemperature scanning tunneling microscopy. A large and negative
magnetoresistance ratio of ∼50% is observed in combination with ahigh conductance. The eﬀect originates from a lowest unoccupied
molecular orbital (LUMO) doublet placed almost in resonancewith the Fermi energy. As a consequence, irrespective of the mutualalignment of magnetizations, electron transport is always dominated by resonant transmission of Mn-majority charge carries
going through LUMO levels.
KEYWORDS: Spintronics, molecular electronics, nanomagnetism, giant magnetoresistance, phtalocyanine, antiferromagnetic Mn ﬁlms

M

is nonmagnetic per se, the GMR can reach very large values,∼60%. The eﬀect has been understood as a consequence ofspin-selective hybridization, being relevant also in the case of
CoPc molecules adsorbed on ferromagnetic surfaces.4−6 A very
similar mechanism is active in magnetic ﬁlms, including
inorganic7,8 and organic materials.9−11
A nanoscopic GMR junction with a single molecule as thenonmagnetic spacer oﬀers the possibility to replace one of theferromagnetic electrodes by an antiferromagnet. Because of thelocal nature of the contact to the antiferromagnetic electrode,the junction samples only the local magnetic order and a GMR
can in principle be achieved. This structure has the advantagethat the antiferromagnet is ideally hard magnetic, so thatcomplex artiﬁcial antiferromagnets to pin the hard magnetic
layer are not required.
Motivated by this idea, in this work we investigate a spinselective hybridization of H2Pc to an antiferromagnetic

odern information storage largely relies on basic
spintronic devices such as the giant magnetoresistance(GMR) read heads.1,2 These structures consist of two
ferromagnetic layers separated by a nonmagnetic spacer. The
conduction across the spacer in “current perpendicular to theplane” (CPP) GMR sensors depends on the relative orientationof the magnetization of the two ferromagnetic layers. To
achieve high sensitivity for the magnetic stray ﬁelds of therecording medium, both a high change of the resistance and aneasy modiﬁcation of the relative magnetic orientation of thetwo layers by the external ﬁeld are required. The latter istypically achieved by designing one of the layers to be soft
magnetic and ﬁxing the magnetization of the second layer by
the exchange bias eﬀect to an antiferromagnet.
A strong drive exists to downscale these magnetic sensors to
the nanometer scale. Molecular electronics oﬀers a pathway toachieve this aim by using single molecules as nonmagnetic
spacer elements. Recently, it has been shown that a high GMR
eﬀect in combination with a low resistance can be achieved inelectron transport across single hydrogen-phthalocyanine(H2Pc) molecules contacted with two ferromagnetic Co
electrodes.3 It has been found that even though the molecule© 2012 American Chemical Society

Received: May 24, 2012
Revised: August 13, 2012
Published: September 18, 20125131

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Nano Letters

Letterposition of the resonance and the coupling matrix elements
drop out in the magnetoresistance ratio, so that it is essentiallyinsensitive to the exact energy of the resonance, that is, to thecharge transfer between the molecule and the contacts. Thiscase was relevant for explaining a recent experiment,3 where thecurrent was found to be carried by the LUMO with adisplacement from the Fermi energy Δ > (ΓS+ΓD).
Let us now focus on the opposite case, that is, on resonanttransport across a molecular level, Δ ≪ (ΓS + ΓD). On the basisof the model formulated above, the on-resonance situation
exhibits a magnetoresistance ratio, MRon = MRoff/2. It is still
positive, but MRon is reduced as compared to the oﬀ-resonancecase by a factor of 2. Our atomistic transport simulations
presented below show that resonant transport is realized for
H2Pc molecules adsorbed on the layer-wise antiferromagnet bct
Mn(100) surface and contacted by an iron-coated STM tip.
Furthermore, DFT calculations conﬁrm (see Figure 1a) that inMn(001) surface with help of density functional theory (DFT)
based transport simulations and low-temperature scanningtunneling microscopy (STM) measurements. In particular, weconsider the Mn surface to be an interesting research object for
two reasons. First, on the theoretical side, the work function of
Mn (≃4.1 eV) is below the Co one (≃5.0 eV). This opens up a
possibility to study resonant transport through the lowest
unoccupied molecular orbital (LUMO) of H2Pc opposed to oﬀresonant transport found previously for the same molecules
adsorbed at Co surface.3 Second, on the experimental side, theantiferromagnetic order of the Mn thin ﬁlms12 allows, asmentioned above, for an alternative realization of a GMR device
with only one ferromagnetic electrode.
Single Resonant Level Model. Charge and spin transport
in molecular junctions are governed by electron transfer
between the continuum of electronic levels in the electrodes
and the molecular states. When a molecule is brought into
contact to a metallic lead, ﬁrst the molecular levels may shift inenergy due to charge transfer to or from the molecule. Second,
due to the coupling to the leads, the molecular states arebroadened. A qualitative understanding of the spin selective
broadening Γσ follows from the “golden rule” type expression13
Γσ(E) = |tσ(E)|2 ρσ (E)Here, tσ(E) denotes the coupling matrix element of a molecularorbital with a state of the substrate at energy E; σ denotes thespin direction, and ρσ(E) is a spin-projected density of states inthe substrate. Usually, one ignores the spin dependency in the
coupling matrix, tσ(E) ≈ t(E), so that
Γσ(E) ≈ |t(E)|2 ρσ (E)The motivation behind is that the tunneling barrier separating
the molecule from the substrate should exhibit only a weak
dependency on the spin, because it is the interaction of theelectrons inside the substrate, only, that discriminates spin
directions. Hence, Γσ inherits its spin dependency predominantly from the surface magnetism that expresses itself asρ↑(EF) ≠ ρ↓(EF).
The consequences for the magnetoresistance ratio14
MR =Tmaj,maj + Tmin ,min − Tmaj,min − Tmin ,majFigure 1. Model geometries of Mn(001)/H2Pc/Fe junctions andcorresponding transmission curves computed assuming either parallel(P) or antiparallel (AP) alignment of magnetizations of the Mn (001)surface layer and the Fe tip. Atomic structures used for simulations areshown where the lower cluster models the Mn substrate, and theupper cluster the Fe tip. The molecule takes a slight bending whenconnecting the substrate with the tip. For computational details, seealso Supporting Information. Upper plot (a): a geometry with the“ﬂat” Fe tip exhibits a positive magnetoresistance ratio, MR = (TP(EF)− TAP(EF))/TAP(EF). Lower plot (b): in contrast, a geometry with the“sharp” Fe tip shows a reversed (negative) magnetoresistance ratio.

Tmaj,min + Tmin ,majmay be estimated for an isolated resonance at energy EM with
the Breit-Wigner expression for the transmission T(EF)TS , D =Γ SΓ D
Δ2 +(Γ S + Γ D)2
4(1)

where S,D = {maj, min} represent the spin states in the source(S) and drain (D) contacts, Δ = EM − EF, and ΓS and ΓD to beevaluated at the Fermi energy, EF. Here, we have assumed thatthe coupling of the molecule to the left and right leads, that is,substrate and STM tip, is symmetric, which implies Γmaj = Γmaj
tip
min
min
= Γmaj= Γminsubstr, and Γ
tip = Γsubstr.
In the oﬀ-resonant situation, Δ ≫ (ΓS + ΓD), this simple
model predicts a magnetoresistance ratio
MRoff =

(1 − rs)2
2rs

the case of Mn surface with a roughly symmetric coupling to
the STM tip the GMR is indeed reduced, compared to the caseof oﬀ-resonant transport. Namely, for our model geometry wehave found a value ≈50%, which implies the surface spinpolarization rs ≈ 0.27 and which is smaller as compared to
GMR ≈ 65% found earlier in the oﬀ-resonant case.3
Furthermore, the detailed DFT-studies suggest (see Figures
1a,b) that the magnetoresistance ratio is very sensitive to theatomistic details of the contact region. In the limit of a very
weak coupling to the tunneling tip it can even change sign,(2)

which is positive and controlled by a single parameter rs = Γ /Γmin, the surface spin polarization. Details on the energetic
maj

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Nano Letters

LetterFigure 2. Electronic structure and transmission for “ﬂat’ tip geometry displayed in Figure 1a. Lower row (b,d): Local density of states in the H2Pc
molecule (projected to all carbon, upper traces; all nitrogen, lower traces). Majority spins (α, red) and minority spins (β, blue). Upper row (a,c):
Associated, spin-resolved transmission functions. Left column shows the case of parallel (P) alignment of magnetizations of the Fe tip and of the Mn(001) surface, right column shows antiparallell (AP) alignment. Plot highlights the fact that transmission resonances exhibit the spin-dependent
hybridization. Near the Fermi energy there is a tendency for broad structures for minority spins and narrow lines for majority spins.

share the same square surface facette. In the “bridge” position,
two H atoms sit in neighboring facettes sharing the same link
between two Mn atoms, see also Supporting Information
Figure 2. For each one of these positions, we have determined
the optimum distance between the molecule and the surface.
We have found that when relaxing atoms within the LSDA, the“top” position was favored against the “center” one by 60 meVwith an optimum distance of 2.1 Å. By contrast, the “bridge”
position appeared to be discouraged with an additional energy
cost of 3.1 eV. The GGA+vdW approach did not change theadsorption site but reduced slightly the adsorption distance to1.93 Å.16
Electronic Structure and Transmission. The frontier
states of H2Pc are both ligand based with an associated highesto
occupied molecular orbital (HOMO)−LUMO gap of ∼2.0 eV(see Supporting Information, Section IIB). The HOMO islocated on the outer organic shell with wave function nodes on
the N atoms. By contrast, the LUMO is a doublet with wave
function nodes located on six out of the eight N atoms. Becauseof the strong electronegative character of the nitrogen, the
LUMO has a tendency to partially ﬁll in the presence of a metalelectrode. For this reason the LUMO of H2Pc tends to be thecurrent carrying orbital. This scenario has been found alreadyfor H2Pc on Co(111)3 and we expect it to be qualitatively valid
as well for Mn surface.
For more quantitative estimates of charge transfer and levelalignment we continue with our DFT-based analysis. Here, we
consider two model geometries, with a “ﬂat” (Figure 1a) and a“sharp” Fe tip (see Figure 1b), which account for the case ofstrong and weak coupling, respectively. We thus can investigate
to what extend the contact details inﬂuence the electronic
structure and hence also the transmission function.20 Morespeciﬁcally, we assume that the molecule is slightly bent along
the low-energy vibrational eigenmode (ω ≈ 15 meV) when it isbound both to the substrate and to the STM tip. This choice

MRon < 0. A generalization of the simpliﬁed model to this caseyields an estimate
MRon =

2
1

ra + r

a

−1
(3)

min
where now ra = Γmaj
substr/Γsubstr. The assumptions are Δ ≈ 0,
min/majΓmin/maj
≪
Γ
.
Using
an estimation ra ≈ 0.27 for thetip
substrsurface spin polarization obtained for the case of symmetriccoupling (see above), we get a negative magnetoresistanceratio, MRon ≈ −50%. This numerical estimate is qualitatively
consistent with the DFT-estimate MRon ≈ −15% (see Figure
1b).
We show below that our experimental observations indeed
conﬁrm these theoretical estimates: the spread of the
conductance data is substantially increased on Mn(001) ascompared to the Co(111) surface (Figure 5), which weinterpret as an indication of the enhanced contact sensitivity.
Importantly, also the sign reversal of MRon has been seen andthe spread of measured values is well consistent with thetheoretical limits.
Adsorption Geometry. To support our theoreticalestimates based on the simpliﬁed resonant level model, we
have performed accurate electronic structure and transport
calculations for the H2Pc molecules. The binding geometry of
H2Pc on the Mn/Fe(001) surface has been determined via
standard slab calculations employing density functional theory
within the local spin density approximation (LSDA), andgradient corrected approximation (GGA) amended by van der
Waals (vdW) interactions within the Grimme scheme15 (GGA
+vdW). Details of calculations are given in the Supporting
Information, Section I. In essence, symmetry considerations
suggest to consider three binding geometries. In the “top”
position two H atoms share the same Mn atom from thesubstrate, while in the “center” (or “hollow”) position they5133

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Nano Letters

LetterFigure 3. Figure analogous to previous one, Figure 2, now for the “sharp” tip geometry shown in Figure 1b.

was motivated by earlier STM experiments.3,22 Furthermore,
we suppose that d electrons of the low-coordinated Fe atoms
can be involved in the covalent like bond with π electrons of aside phenyl ring of the molecule. A plausible binding
mechanism, partly including also van der Waals interactions,is discussed in Section II of Supporting Information.
Case of “Flat” STM Tip. The results for the electronic
structure of the “ﬂat” tip geometry as encoded in the local
density of states have been displayed in Figure 2. We ﬁrst
discuss the H2Pc-projected (or local) density of state (LDoS)
for the case of parallel (P) alignment of the spins in Fe tip andthe Mn surface. As seen in Figure 2b, it exhibits the expectedfeatures. The majority spin resonance just below EF represents
the LUMO level partially ﬁlling with electrons supplied by thesurrounding metal. Despite of the coupling, the resonance isvery sharp due to the lack of available states for the majority
spin carriers near EF (for more information, see also Supporting
Information, Figures 6 and 7). Moreover, spectral weight isfound on N atoms and C atoms as well indicating thedelocalized nature of the LUMO orbitals. By contrast, theminority LUMO state is shifted below EF and has a signiﬁcantly
broader structure in the LDoS, which reﬂects the increased
hybridization with the metal states. In the case of antiparallel(AP) alignment, there is no longer a clear distinction between
minority and majority carriers any more, if the coupling to bothelectrodes is of a comparable strength. Indeed, as seen in Figure
2d, there is no well-developed distinction between the diﬀerent
spin channels, both exhibiting sharp LUMO resonances on
broad background features (see also Supporting Information,
Figure 7).
This situation readily carries over into the transmissionfunctions, Figure 2a,c. We encounter a relatively pronounced
LUMO resonance for parallel alignment in the majority
channel, Figure 2a. By contrast, the minority transmission
displays a strong broadening of the LUMO doublet that also
produces signiﬁcant transmission values above unity in a broad
energy window. The associated transmissions add up to GP =1.94 e2/h (Figure 1a). Again, nothing of this is seen in the

antiparallel conﬁguration, Figure 2c. It exhibits two similar
traces, both with peaked resonances on a broad background,irrespective of the spin direction; GAP = 1.27 e2/h (Figure 1a).
At this point our discussion shows that the “ﬂat” tip
conﬁguration exhibits generic physics in the present onresonance situation that is quite similar to the previously
discussed system, H2Pc on Co(111).3 Consequently, one may
expect that the simpliﬁed “toy” model could give a reasonableestimate for MRon, which should be smaller (roughly, up to the
one-half) than the value ≃65% found in ref 3. Our present
estimate for the “ﬂat” tip case supports this expectation, since
the magnetoresistance ratio MRon ≈ 53% is reduced bymagnitude.
Case of “Sharp” STM Tip. The characteristic feature of the“ﬂat” conﬁguration was the (roughly) symmetric way in which
both electrodes couple to the molecule. We now investigate theopposite situation where the tip is atomically sharp, see Figure
1b. In this case, the coupling of the molecule to the STM tip is
signiﬁcantly weaker than the one to the substrate. Hence theline width of all resonances and also the level ﬁlling is largely
determined by the substrate.
As a consequence one can formulate the following
expectations for the asymmetric case: the typical line width isreduced as compared to the symmetric coupling and there is acertain shift of the DFT-LUMO due to a slightly modiﬁed
charge transfer. Moreover, in the case of antiparallel alignmentof magnetizations, traces for up-spin and down-spin electrons
should be signiﬁcantly diﬀerent, since reversing majority andminority spins in substrate and tip no longer is an(approximate) symmetry operation (see also Supporting
Information Figure 8).
As may be inferred from Figure 3, the simulation data
conﬁrm all these expectations. In particular, they display a
LUMO transport resonance for the majority channel that isvery sharp and located at EF. For this reason, transport iscompletely dominated by majority spin carriers, which is inpronounced contrast to the symmetric, “ﬂat” Fe tip case. The
second important diﬀerence relates to the conductances. We
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Nano Letters

Letterwet etched W tip has been coated with a thin Fe ﬁlm resulting
in an in-plane magnetized tip. By measuring spatial resolved(dI/dV) maps at 200 mV the domains could be identiﬁed. By
applying a small external ﬁeld the relative orientation of the tip
and substrate magnetizations could be reversed. This has been
evidenced by the reversed spin contrast (see Figure 4c).
After magnetic contrast has been achieved, conductanceversus distance curves have been recorded on molecules independence of the mutual orientation of tip and surface
magnetizations. In Figure 5, a representative example ofconductance versus distance curves for the two magnetic
conﬁgurations (P versus AP) is shown. When the tip isapproaching the H2Pc molecule, a characteristic jump in the
conductance is observed, which is attributed to the formation ofa molecular junction driven by the excitation of a soft
vibrational mode of the molecule.22 After the molecularjunction is formed, its conductance strongly depends on themutual alignment of magnetizations in the electrodes. Whilethe conductance in the tunneling regime is higher for theparallel as compared to the antiparallel alignment, the reversed
situation is observed in the “contact” regime. In order to make areliable estimate for the GMR value, the measurements havebeen repeated several hundred times for both mutualalignments of magnetizations. Figure 5b shows the distributionof the conductance values obtained in the 161 transport
experiments. On average, the spin dependent molecular
conductances have been estimated as GP = (0.286 ± 0.013)
G0 and GAP = (0.441 ± 0.026) G0, where G0 = 2e2/h is the
conductance quantum. This results in the negative MR ratio for
the Mn/H2Pc/Fe junction, namely GMR = (GP − GAP)/GP =−(54 ± 16) %. While the absolute value of GMR is close tothat one measured previously on H2Pc in between the two Co
electrodes, the sign of the MR ratio is negative. This negative
MR ratio correlates with a prediction of the DFT-based
simulations for the atomically “sharp” tip geometry. We note,however, that the strong spread in measured conductances
observed in current experiment indicates the strong inﬂuence of
diﬀerent contact geometries on the on-resonance electrontransmission. While similar variations of the conductance havebeen found in nonmagnetic transport measurements,26 ourresults indicate that this eﬀect persists also in case of spin-

read oﬀ GP ≈ 1.1 e2/h and GAP ≈ 1.25 e2/h (see Figure 1b), so
the value for the antiparallel alignment of magnetizations in thecontacts does exceed the parallel one. Correspondingly, the
GMR is negative, MRon ≈ −13%. While the sign may beunderstood within the simpliﬁed model already, the magnitudeof the eﬀect is not predicted. This is because the “toy” model isvalid in the limit of an extremely asymmetric coupling, in which
all transmission peaks have values signiﬁcantly below unity. The
DFT-model calculation is not fully in this limit, yet, as the
DFT-conductances are still quite large. Furthermore, in DFTsimulation the precise value of the negative GMR ratio issensitive to the centering of the narrow majority spin LUMO
resonance about EF.21
STM Experiments. To support our theoretical ﬁndings, wehave performed as the second step electron transportmeasurements with the scanning tunneling microscope. Spinpolarized conductance versus distance curves3,22 have beenmeasured on top of H2Pc molecules adsorbed on an
antiferromagnetic Mn surface. The surface was prepared byevaporating a few layers of Mn (6−8 ML) on the clean Fe(001)surface of an home-grown Fe whisker. Depending on the exact
number of Mn layers diﬀerent magnetic domains areformed24,25 (see Figure 4b). For obtaining spin resolution, aFigure 4. (a) Topographical image of Mn grown on an Fe(001)whisker. (b) dI/dV map taken before reversing the external ﬁeld. (c)dI/dV map after reversing the external ﬁeld. The three arrows indicate
the main magnetic features of the Mn/Fe(001) surface. Green: a Mn
step edge at the surface, that is visible in the topography and the dI/dV
maps. White: a buried Fe step edge, that is only slightly visible in the
topography but can clearly be seen in the dI/dV map. Blue: coincident
Fe and Mn step edges. The magnetic eﬀect is compensated (V = 200
mV, T = 4.3 K).

Figure 5. (a) Exemplary conductance versus distance curves measured across H2Pc on Mn/Fe(001). The jump height in the antiparallel (AP) case ismuch higher than in the parallel (P) case. This results in the large and negative GMR ratio. Inset plot is a topographical STM image of H2Pc
molecules adsorbed on Mn/Fe(001). (b) Distribution of the conductance values, measured for the P and AP conﬁgurations, obtained in the 161transport experiments.
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Nano Letters

Letter(12) Wulfhekel, W.; Schlickum, U.; Kirschner, J. Microsc. Res. Tech.
2005, 66, 106−116.
(13) Bruus, H.; Flensberg, K. Many-body quantum theory in condensed
matter physics: an introduction; Oxford University Press: New York,
2004.
(14) We deﬁne the magnetoresistance ratio as MR = (GP − GAP)/
GAP, where GP and GAP are conductances across molecular junctions
with electrodes being magnetized either parallel (P) or antiparallel(AP) to each other.
(15) Grimme, S. J. Comput. Chem. 2006, 27, 1787−1799.
(16) It is interesting to mention that binding geometries are not very
sensitive to whether the LSDA or the GGA+vdW have been used. This
seems surprising, however, it is well-known that LSDA has a tendency
to overestimate and GGA to underestimate the binding energies. For
example, LDA reproduces the distance between graphite layers17
whereas GGA gives correct results only when the vdW interactions areincluded.18 A similar situation was also found for the adsorption of
CoPc on Co(111) surface.19(17) Spanu, L.; Sorella, S.; Galli, G. Phys. Rev. Lett. 2009, 103,
196401.
(18) Rydberg, H.; Dion, M.; Jacobson, N.; Schröder, E.; Hyldgaard,
P.; Simak, S. I.; Langreth, D. C.; Lundqvist, B. I. Phys. Rev. Lett. 2003,
91, 126402.
(19) Chen, X.; Alouani, M. Phys. Rev. B 2010, 82, 094443.
(20) Probably, none of the model geometries displayed in Figure 1 isparticularly close to the experimental situation, for example, due to thepresence of strain. Still, important insights will be gained about themolecular conductance since qualitative features, like contact
sensitivity, and also quantitative information, like the spread inmagnetoresistance values, will be properly reproduced by ourmodeling.
(21) We emphasize that the alignment of molecular levels with the
Fermi energy of the metal electrodes suﬀers from approximations in
the exchange-correlation functionals that one employs in DFT
calculations.23 Usually, DFT properly reproduces trends but notnecessarily the precise resonance position. Therefore, quantitative
deviations of the position of the true LUMO resonance from thevalues that one reads oﬀ Figures 1 and 2 are to be expected. However,as has already been pointed out in ref 3 the GMR is a ratio of two
conductances and therefore a tendency for error cancellation exists.
Hence, we believe that our estimates for the GMR could bequantitatively more robust against functional artifacts than thetransmission function itself.
(22) Takács, A. F.; Witt, F.; Schmaus, S.; Balashov, T.; Bowen, M.;
Beaurepaire, E.; Wulfhekel, W. Phys. Rev. B 2008, 78, 233404.
(23) Evers, F.; Burke, K. Pride, prejudice, and penury of ab initio
transport calculations for single molecules. In Nano and MolecularElectronics Handbook; Lyshevski, S. E., Ed.; CRC Press: Boca Raton,
FL, 2007.
(24) Yamada, T. K.; Bischoff, M. M. J.; Heijnen, G. M. M.;
Mizoguchi, T.; van Kempen, H. Phys. Rev. Lett. 2003, 90, 056803.
(25) Schlickum, U.; Janke-Gilman, N.; Wulfhekel, W.; Kirschner, J.
Phys. Rev. Lett. 2004, 92, 107203.
(26) Schull, G.; Frederiksen, T.; Arnau, A.; Sanchez-Portal, D.;
Berndt, R. Nat. Nanotechnol. 2011, 6, 23−27.

dependent transport across a single molecule in the ballistic
regime.
To summarize, we have investigated the magnetoresistanceof an H2Pc molecule adsorbed on an Mn(001) substrate bymeans of DFT-based simulations and low-temperature STM
measurements employing an Fe tip. A theoretical analysis
shows that due to spin-dependent hybridization of themolecule’s and electrodes’ electronic states, a nonmagnetic
molecule can exhibit a strong GMR eﬀect even when one of theelectrodes is an antiferromagnet, which is conﬁrmed by ourexperimental data. Interestingly, a negative GMR value wasobserved, which has been explained on the basis of a resonanttransport across the LUMO level of the molecule incombination with asymmetric coupling of the H2Pc withmagnetic electrodes.

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ASSOCIATED CONTENT

S Supporting Information*

Additional ﬁgures and information. This material is available
free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author*E-mail: Alexej.Bagrets@kit.edu.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
We thank M. Bowen and E. Beaurepaire for discussions on themolecule−metal interface and J. Arabski for purifying themolecules. T.K.Y. acknowledges funding by the Alexander-vonHumboldt foundation. A.B., S.S., F.E., and W.W. acknowledge
funding by the DFG (Deutsche Forschungsgemeinschaft), theCenter for Functional Nanostructures (CFN) and the BadenWürttemberg Stiftung.

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