IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 6, JUNE 2011

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Electrical Control of the Magnetic State of Fe
Lukas Gerhard , Toyo Kazu Yamada , Timofey Balashov , Albert F. Takács , Rien J. H. Wesselink ,Markus Däne , Michael Fechner , Sergey Ostanin , Arthur Ernst , Ingrid Mertig , and Wulf Wulfhekel
Physikalisches Institut, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
Faculty of Physics, Babes-Bolyai University, 400084 Cluj-Napoca, Romania
Faculty of Science and Technology and MESA+ Institute for Nanotechnology,University of Twente,
7500 AE Enschede,The Netherlands
Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA
Martin-Luther-Universität Halle-Wittenberg, Institut für Physik, 06099 Halle, Germany
Magneto-electric coupling offers a new pathway to information storage in magnetic memory devices. This phenomenon has been observed in various materials ranging from insulators to semiconductors. In bulk metallic systems, magneto-electric coupling has been
disregarded as the electric field cannot enter bulk metals. In this work, we show that a substantial magneto-electric coupling exists
in metallic Fe nano-islands grown on Cu(111). Using the electric field in the tunnel junction of a scanning tunneling microscope, themagnetic order parameter and the crystal structure of Fe was changed on the nanometer scale. This allows high density nonvolatileinformation storage by means of magneto-electric coupling in a simple metallic system.
Index Terms—Hard disks, magnetoelectric effects, magnetic memory, scanning probe microscopy.

I. INTRODUCTION

II. THE MODEL SYSTEM Fe/Cu(111)

HE possibility to change the magnetic state of a nanostructure in a controlled way by means other than a magnetic field attracted significant interest in the last years. Magneto-electric coupling (MEC) links the magnetic state to theelectric field. The ultimate dream is to use MEC in magnetic
data storage devices to write magnetic bits. Since the discoveryof the first multiferroic materials almost thirty years ago [1],only little attention has been payed to MEC in metals. Most
materials showing MEC are multiferroic insulators, where ferroelectric and ferromagnetic order parameters coexist [2]–[4].
In these materials, the electric polarization can be changed byapplying an external electric field [5]–[7]. The change of theelectric polarization is caused by a displacement of the positive
and negative ions with respect to each other. This displacementin turn may influence the magnetic state. The microscopic processes that lead to MEC are rather complex and still lacking acomplete theoretical understanding. As in metals any external
electric field is screened by the formation of a surface charge,
MEC cannot be found in a bulk metallic system. This surface
screening charge, however, is not only formed by the motionof the mobile electrons, but also by a slight displacement of thepositively charged atomic cores in the top atomic layer [8]. Supposed in a metallic system the magnetic configuration changes
upon lattice deformations on the picometer scale, the possibility of MEC in metallic nanostructures would be opened. In
this combined theoretical and experimental work, we show that
MEC is present in magnetic nano-structures and magnetic information can be written, stored, and read on the nanometer scale.

Fe is a suitable material for MEC as it exhibits remarkable
phase properties: the magnetic order of Fe delicately depends
on the unit cell volume [9]. Here, self-organized Fe nano-structures were used as a model system for MEC in metals. At room
temperature, a sub-monolayer (ML) amount of Fe was depositedon atomically clean Cu(111) by molecular beam epitaxy in ultrahigh vacuum (UHV) leading to the formation of 2 ML highFe islands [10]. MEC of these samples was then studied witha scanning tunneling microscopy (STM) operating at 4.5 K in
UHV. For this particular system of Fe on Cu(111), Biedermann
and colleagues showed that two different phases coexist [11].
The center of the islands exactly follows the face-centered cubic(fcc) stacking of the Cu(111) substrate while the rim adopts abody-centered cubic (bcc) like structure similar to the groundstate of bulk Fe. Our STM measurements revealed the sameatomic configuration. Especially a perfectly hexagonal structure
was observed in the center and a bcc-like stacking at the borders that is accompanied by a misalignment of the atomic lines
by small angels with respect to the fcc lattice [11], [12]. As thebcc structure appears higher than the fcc structure by about 30
pm, the two phases can easily be distinguished in STM images(see inset in Fig. 1). The magnetic order of the two phases is
not directly accessible experimentally. Performing spin-polarized STM experiments we excluded magnetic structures that arelaterally inhomogeneous. In order to distinguish the remaining
two laterally homogeneous magnetic orders (ferromagnetic andlayer-wise antiferromagnetic), we performed spatially resolved
scanning tunneling spectroscopy (STS) measurements on theisland center and rim. In an extended Tersoff-Hamann model,
the differential conductance measured by STS can be identified
with the local density of states (LDOS) [13]. Best agreement isfound when normalizing the measured
signal over thetunneling matrix element[14]. The resulting experimental
spectra are shown in Fig. 1 (solid lines) [12]. These spectra

T

Manuscript received January 03, 2011; accepted January 11, 2011. Date ofcurrent version May 25, 2011. Corresponding author: W. Wulfhekel (e-mail:wulf.wulfhekel@kit.edu).
Digital Object Identifier 10.1109/TMAG.2011.2107506

0018-9464/$26.00 © 2011 IEEE

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Fig. 1. Scanning tunneling spectra (solid lines) at positions indicated in theinset in comparison with ab-initio calculations of the LDOS (dashed lines).
Spectra taken on the fcc phase (left side) only match with the calculated LDOSof an antiferromagnetic structure, while spectra taken on the bcc phase (rightside) only match with the LDOS of a ferromagnetic bcc structure.

were compared to theoretical calculations of the LDOS. We performed the first principles calculations of the LDOS assuming
different magnetic configurations of a fcc and a bcc bilayer Fe
double-layer on a semi-infinite Cu(111) substrate. The experimental spectrum of the fcc center of the Fe islands matches
only with a theoretical LDOS of a layer-wise antiferromagnetic
fcc structure and an interlayer distance of 2.00 (see Fig. 1,
left side, dashed line). The rims of the island show experimental
spectra that exclusively fit the LDOS of a ferromagnetic Fe bilayer and an interlayer distance of 2.12 (see Fig. 1, right side,dashed line). All other combinations did not fit (not shown, see[12]). This identifies the darker phase in the STM images of the
Fe islands as an antiferromagnetic fcc structure and the brighter
phase as a ferromagnetic bcc structure.
III. SIMULATION OF MEC
As theoretical calculations and STM measurements show asmall height difference between the antiferromagnetic fcc andthe ferromagnetic bcc phase, the possibility to change the interatomic distance between first and second Fe layer by applying
an electric field would allow to influence the magnetic structure
via MEC. We therefore performed first principles calculations
using the Vienna ab initio simulation package (VASP) within
the density functional theory approach [12]. In a first step only
vertical relaxations of a bilayer Fe slab on a Cu(111) substrate
were considered in a plate capacitor simulating an electric fieldof 1 GV/m. In accordance to the simple model presented in the
introduction, the simulations show the formation of a surface
charge by the displacement of the electrons and the atomic coresin the top Fe layer as can be seen in Fig. 2, [12]. In the vicinityof the positive electrode the atomic cores are repelled thereby
reducing the Fe interlayer distance [see Fig. 2(a)]. A negativeelectrode attracts the atomic cores thereby increasing the interlayer distance [see Fig. 2(b)]. Additionally, the calculations of
the total energy show an antiferromagnetic alignment of the two
Fe layers for reduced interlayer distance in a positive electricfield and a ferromagnetic alignment in the case of an increased
interlayer distance in a negative electric field. As the atomicallyresolved STM measurements for Fe islands on Cu(111) revealeda coexistence of fcc and bcc structures [11], also lateral displacements of the topmost Fe atoms were considered being responsible for the transitions from fcc to bcc. This diffusionless transi-

Fig. 2. Simulation of electric field-induced structural changes of a bilayer Fe
slab on a Cu(111) substrate [12]. (a), (b) Charge density of 2 Fe layers and 2 Culayers of the substrate shows a reduced interlayer distance for a positive electricfield (a) and an increased interlayer distance for negative electric field (b). The
structural changes lead to an antiferromagnetic alignment for the reduced (a)and to a ferromagnetic alignment for the enhanced interlayer distance (b). (c),(d) Total energy per unit cell as function of a lateral displacement of the top layer
Fe atoms along the martensitic transition from fcc to bcc (from left to right).
The energetically most favorable configurations are antiferromagnetic fcc for apositive electric field (c) and ferromagnetic bcc for a negative electric field (d).

tion is called martensitic, which consists basically of an in-plane
displacement of the top Fe atoms from the fcc threefold hollow
sides to the bcc twofold bridge positions [11]. Therefore, in a

GERHARD et al.: ELECTRICAL CONTROL OF THE MAGNETIC STATE OF Fe

Fig. 3. STM images of the same Fe island before (a) and after (b), (c) applying
negative electric field pulses at indicated positions. (d) Applied gap voltage andheight underneath the STM tip as function of time demonstrating the reproducibility of the switching.

second step, the total energy was calculated for the top layer Fe
atoms in an electric field, vertically relaxed at each lateral position following the path from the fcc to the bcc position [12].
Both ferromagnetic and antiferromagnetic configurations wereconsidered, each in positive and negative electric fields. This results in an energy landscape shown in Fig. 2(c), (d). In a positiveelectric field the antiferromagnetic fcc configuration is lowest inenergy per unit cell and is well separated by a barrier of about 20
meV from the bcc configuration that is unstable [see Fig. 2(c)].
In an applied electric field of negative polarity, the fcc configuration is unstable and the ferromagnetic bcc configuration isfavored [see Fig. 2(c)]. This implies a coupling of the magnetic
order to the electric field and a stability of the two phases withrespect to a transition to the other one.
IV. EXPERIMENTAL OBSERVATION OF MEC WITH STM
As the theoretical calculations predict a MEC in the Fe islands, the influence of an electric field in the tunneling gap of
an STM was also experimentally studied. Thus, voltage pulses
with the tip positioned above the island were given to induce a
local and large electric field. In this way, STM can be used at
low bias voltages to image the state of the island and at highbias voltages to manipulate the magnetic state. In this sense an
STM is a unique tool to investigate MEC, as it offers the opportunity to control huge electric fields on the nanometer scale incombination with its ability of imaging with atomic resolution.
Fig. 3(a) shows an STM image of an Fe island recorded at low
electric field. We applied an electric field pulse of 1 GV/mlasting 50 ms. Afterwards another STM image of this islands
is recorded at low electric field, shown in Fig. 3(b). It can beclearly seen that in full accordance to the theoretical predictions
the area underneath the tip was switched from the ferromagnetic
bcc to the antiferromagnetic fcc state. The original state is regained by applying an electric field of negative polarity. This
switching is a reproducible process as can be seen in Fig. 3(d).
The apparent height of the structure underneath the tip at a fixed
position can be switched back and forth. When applying positiveelectric fields an antiferromagnetic fcc structure was induced,
when applying negative electric fields a ferromagnetic bcc structure was obtained. Measuring STS spectra before and after the1621

switching, we confirmed that not only the crystallographic structure changes, but also the magnetic state is switched in accordance. Thus, it is indeed possible to write single magnetic bitson the nanometer scale by electric fields. As the bcc Fe structure exhibits a smaller in-plane lattice constant than that of theCu(111) substrate, the bcc structure only occurs at the edges of
the triangular islands where the in-plane stress is lower. An increase of the bcc area leads to an increase of the in-plane stressin the island and thus favors the fcc configuration. There are
two consequences for the switching. First, a bistability of antiferromagnetic fcc and ferromagnetic bcc can only be foundclose to the island edge. Second, the overall stress in the Fe island is limited. As can be seen in the two consecutive images
Fig. 3(b) and (c), switching two edges, the third edge switchesfrom ferromagnetic bcc to antiferromagnetic fcc.
V. VERIFICATION OF MEC AS SWITCHING MECHANISM
In the ab-initio calculations, the tunneling junction was modeled by a plate capacitor in order to understand the influenceof the electric field. In the STM measurements; however, a tunneling current is present as well. The influence of the tunnelingcurrent has to be considered to exclude other mechanisms than
MEC. We therefore investigated the relation between the twobasic parameters of the STM setup, the applied bias voltage andthe tunneling current. At a fixed lateral position of the tip, a tunneling current was chosen and the applied bias voltage is ramped
while recording the height of the sample underneath the tip. The
inset in Fig. 4(a) shows that at a certain critical voltage the structure suddenly changes from the lower fcc to the higher bcc. The
voltage ramp is repeated at a series of different tunneling currents, resulting in different critical voltages. The pairs of critical voltages and corresponding tunneling currents are plottedin Fig. 4(a). This graph shows the phase diagram of the Fe bilayer islands as function of the measured critical voltage andthe tunneling current. This relation between voltage and current
can be used to compare the switching process in the experimentwith different possible models for the phase transition.
Under the assumption, that the transition is caused by the energy of the tunneling electrons, for example by inducing an inelastic excitation the a phase transition only depends on the applied bias voltage. As can be seen in Fig. 4(a), thismodel does not fit this data. Another possibility is a mechanism
depending on the tunneling current, e.g., due to the spin torque[15], spin accumulation [16], or electro-migration. The corresponding straight line in Fig. 4(a)fails in reproducing the experimental results. A third possibility for a phasetransition might relate to local heating by the tunneling current.
In this model the dissipated power would be the critical value,
but as can be seen in Fig. 4(a), this canalso be excluded. Finally we consider the overlap of the wave
functions of tip and sample, i.e., a mechanical contact or partial
chemical bond formation. This is directly linked to the distance
between tip and sample. In order to relate the distance between
tip and sample with the applied bias voltgage and the tunnelingcurrent, we use a simple well known approximation for the tunneling current in an STM setup:, whereis the apparent barrier height [17] and is the distance between tip and sample. This model also clearly deviates from theexperimental findings as can be seen in Fig. 4(a)
.

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tric field, only the absolute value necessary to induce the transition is slightly lower. In the range of intermediate electric fields,
we find a hysteretic behavior. This finally provides a complete
set of experiments, proving that MEC in Fe nano-islands can beused to reproducibly write and read nonvolatile magnetic bitswith electric fields.
In summary, we have shown that magnetic information can be
written, stored and read only by electric fields. While the results
demonstrate large MEC at metallic surfaces, for applications,the vacuum barrier should be replaced by an oxide layer to allow
integration into solid state memories.
ACKNOWLEDGMENT
T. K. Yamada acknowledges funding by the Alexandervon-Humbold foundation, A. Ernst and W. Wulfhekel by theDeutsche Forschungsgemeinschaft (projects ER 340/4-1, WU
349/8-1), A. F. Takács for the Sectoral Operational Program for
Human Resources Development 2007-2013 and the European
Social Fund (POSDRU 89/1.5/S/60189).

Fig. 4. Systematic study of the switching mechanism via MEC. (a) Relation
between the critical voltage and the tunneling current [12]. For each tunnelingcurrent, the bias voltage was ramped up, until the critical voltage is reached andthe phase transition from fcc to bcc is observed (see inset). This results in pairsof tunneling current and critical voltage corresponding to the phase boundary(black triangles) and offers the possibility to exclude other mechanisms than
MEC. (b) Phase diagram with the critical voltage plotted against the tip sample
distance.

We are left with the switching to be driven by MEC. To evaluate the scenario of a constant electric field, we used
the approximation for the tunneling current as function of thebias voltage mentioned above. This model fits the experimental
data well [black line in Fig. 4(a)] and hence explains the phasetransition. At a closer look, we find two parameters that haveto be determined in this relation: the resistance in contact, for, and the apparent barrier height . The latter can be determined experimentally by approaching the STM tip to the surface in a controlled way. Thus, the increase of is measured asfunction of . From the observed rise of the effective barrierheight is determined. Additionally, we assume a resistanceoffor the metallic point contact at zero distance. With this, we can estimate from the bias voltage andtunneling current and re-plot the critical bias voltage against[Fig. 4(b)]. As expected, the voltage can be fitted as a linear
function of the distance. Thus the slope gives the value of thecritical electric field, that is determined to values between 3
and, similar to those used in the calculations. In
order to provide a full phase diagram we extended this voltage
distance relation to positive electric fields. As can be seen in
Fig. 4(b), also the switching from bcc to fcc is driven by an elec-

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