REVIEW PAPER

Japanese Journal of Applied Physics 50 (2011) 08LA03
DOI: 10.1143/JJAP.50.08LA03

Electric Field Control of Fe Nano Magnets: Towards Metallic Nonvolatile Data Storage Devices
Toyo Kazu Yamada1;2 , Lukas Gerhard1 , Timofey Balashov1 , Albert F. Takács1;3 ,Rien J. H. Wesselink1 , and Wulf Wulfhekel1
1

Physikalisches Institut, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
3
Faculty of Physics, Babes-Bolyai University, 400084 Cluj-Napoca, Romania
2

Received January 24, 2011; accepted May 26, 2011; published online August 22, 2011
Magnetoelectric coupling at metal surfaces opens up a new possibility for metallic nonvolatile magnetic data-storage devices, in which themagnetic bits are controlled by an electric field. We studied the atomic and magnetic order in bilayer Fe nano-islands grown on a Cu(111)substrate with a scanning tunneling microscopy setup in ultra high vacuum at 4.5 K. Electric field pulses (108 –109 V/m) were found to be able tocause a displacement of the Fe atoms, switching simultaneously the crystalline and the magnetic order, which is the prove of magnetoelectric
coupling at the metallic Fe surface. We also succeeded in controlling the direction of the switching by the polarity of the electric field.
# 2011 The Japan Society of Applied Physics

1. Introduction

The construction of a ubiquitous network society relieson the drastic development of data-storage devices withlow power consumption, increasing speed and bit density.
The realization of such near-future devices will be possible
if reading, writing, and storing information processes,
such as in hard disks, are performed by electric ﬁelds.
Currently magnetic ﬁelds used to controlling each nanomagnet inside data-storage devices are energy consumptive
due to the current that is ﬂowing through the coils.
Furthermore, the magnetic stray ﬁeld limits the size of themagnetic bits to be written and the magnetic anisotropy
needed for small stable bits cannot be overcome bymagnetic ﬁelds anymore. These limitations can be avoided
when using magnetoelectric coupling in magnetic storage
devices.
Materials in which magnetic order can be controlled byelectric ﬁelds have been already studied for a long time,
while they are usually insulators such as TbMnO3 ,1)
HoMnO3 ,2) BaTiO3 ,3) BiFeO3 ,4) and have complicate
structures, making it diﬃcult to fabricate practical devices.
If one ﬁnds such phenomena in simple metals, this will bea big step towards a realization of electrically controlled
magnetic devices as metals have been widely used in recent
magnetic data-storage devices.
The electric ﬁeld can induce charges on the surface of
metals, which may shift the atomic position. If this subtle
displacement changes the magnetic order, the material
displays a magnetoelectric coupling. In general, electricﬁelds cannot penetrate bulk metals, because they arescreened by the conduction electrons. However, theinﬂuence of an electric ﬁeld becomes detectable if the
thickness of the metal is reduced to the nanometer scale.
Recently, controlling the magnetic anisotropy by the electricﬁeld was reported for the Fe/MgO junctions.5)
A necessary condition to ﬁnd magnetoelectric coupling in
metals is that the metal possesses a close interaction between
crystalline and magnetic structure. Iron is one of the metallic
magnets in which the magnetic order is very sensitive to the
crystalline structure. A subtle displacement of the interatomic distances causes a wide variety of spin conﬁgurations
even in bulk.6,7)
E-mail address: toyoyamada@faculty.chiba-u.jp

Iron in bulk is a typical bcc-metal at room temperature,but it takes the fcc -phase at 1185–1667 K and the bcc
-phase at 1667–1811 K, indicating a small energy barrier
between bcc and fcc phases.8) Bcc Fe in bulk is ferromagnetic, but fcc Fe in bulk does not have a stable ferromagnetic phase.6,7) The energy balance of the phases can be
modiﬁed. Especially at surfaces, interfaces and nanometerscale regimes, the phase diagram of iron is drasticallychanged due to the break of symmetry, causing uniquefeatures. Recently, a surface reconstruction on the surface
of a bcc-Fe(110) whisker single crystal was found,9) where
the top atomic layer has a fcc(111)-like quasi-hexagonal
symmetry and interacts with the sub-surface bcc(110)symmetry, causing a Moiré-like pattern. In Fe nano-islands
grown on a Cu(111) substrate, the coexistence of bcc(110)and fcc(111) phases was found.10) This indicates that thebarrier between the phases becomes smaller at the surface orin nanometer scale regimes, especially along the bcc h110i
orientation which is the most dense-packed in bcc.
In this study, we applied electric ﬁeld to the nanometerscale bilayer Fe islands grown on a Cu(111) single crystal
with a scanning tunneling microscopy (STM) setup in ultrahigh vacuum at 4.5 K. We studied the crystalline andmagnetic structure of the two phases in the Fe nano-islands
and succeeded to characterize them: (1) The bcc structure
is ferromagnetic and (2) the fcc structure is layer-wise
antiferromagnetic. Applying a high electric ﬁeld to the Fe
nano-islands induces subtle displacements of the atomic
positions, changing the crystalline and the magnetic ordersimultaneously.11,12) Statistical experiments showed that thepolarity of the electric ﬁeld allows selective switching fromone phase to the other and vice versa. This new ﬁnding ofa magnetoelectric coupling at the metallic Fe surface will rise
new hope towards an electric ﬁeld controlled magnetic datastorage device.13)
2. Experimental Methods

Fe nano-islands were grown in a ultrahigh vacuum (UHV)
preparation chamber. Its base pressure is below 5  108 Pa.
The Cu(111) single crystal was sputtered by Ar ions at 300 K
and subsequently annealed up to 450 K. Repetitions of the
sputtering and annealing cycles result in a clean and atomically ﬂat Cu surface. Afterwards 0.2 monolayers of Fe,
purity 99.995%, were deposited from an electron-bombardment type evaporator onto the Cu(111) substrate at 300 K.

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Jpn. J. Appl. Phys. 50 (2011) 08LA03

The fabricated samples were studied in our scanningtunneling microscope at 4.5 K in UHV. Topographic images
and atomically resolved STM images were obtained using
non-magnetic tungsten tips. Local density of states (LDOS)
and magnetic information at each position were obtainedby scanning tunneling spectroscopy (STS) as well as spinpolarized STM/STS techniques with W tips and spinpolarized Fe/W tips.14–17) Experimentally obtained diﬀerential tunneling conductivity (dI=dV ) curves were normalized by its ﬁtted tunneling probability functions (T ) torecover the LDOS.11,18–20)
3. Results and Discussion

Fig. 1. An STM topographic image (80  50 nm2 ) obtained on the surface

3.1 Atomic and spin configurations in the Fe nano islandsof 0.2 MLs of Fe on a clean Cu(111) single crystal substrate. Brightness
denotes height. The white and black circles indicate areas with the fcc(111)
and the bcc(110)-like structure, respectively. The height diﬀerence between
the areas is about 30 pm.

Figure 1 shows a topographic STM image (80  50 nm2 )obtained on the surface of 0.2 MLs of Fe deposited on theclean Cu(111) single crystal substrate. Fe forms bilayer
islands with sizes of 5–20 nm. Due to the symmetry of the
fcc substrate, the islands preferentially grow in a triangularlike shape.21) These nano-islands do not consist of a single
phase, but exhibit a coexistence of bcc and fcc structures.10)
The bcc area appears brighter (black circle in Fig. 1) and the
fcc area appears darker (white circle in Fig. 1). The heightdiﬀerence is about 30 pm.
Figure 2 shows an atomically resolved STM image (4  4nm2 ) at the boundary between bcc and fcc areas. The whiteline indicates the boundary. The red lattice indicates thehexagonal symmetry of the fcc(111) substrate, which ﬁts
well the atomic positions at the fcc area, but not at the bcc
area. Sphere models at the bcc and the fcc areas, depicted inFig. 2, show that the top-layer atoms of a fcc(111) surface
area sit on the three-fold hollow sites of the sub-surface Felayer, while the atoms of a bcc(110) surface sit on the bridge
sites. This is why there is not only an in-plain atomic displacement but also an out-of-plain displacement necessary tocomplete the transition from fcc to bcc. The in-plain displacement can be observed as a 5.3 tilt of the atomic latticedirections while the out-of-plain displacement results in theheight diﬀerence of the two phase of about 30 pm. Hence our
STM images directly give the information about a possible
phase transition that might be induced by an electric ﬁeld.
To identify the magnetic order of the two phases,
however, need a more sophisticated approach. As we willsee, the magnetic order does not remain the same as in thebulk ground state, i.e., ferromagnetic, but is diﬀerent for thetwo phases. In order to investigate the spin conﬁguration of
the bcc and fcc phase in the bilayer Fe nano-islands a
combination of experimental STS, spin-polarized STM
measurements and theoretical ab-initio calculations11) was
performed.
Our spin-polarized STM measurements conﬁrmed that
there is no spin variation in the top layer, i.e., we have tofocus on possible magnetic structures in which the spins of
the top layer point the same direction.
Normalized diﬀerential tunneling conductivity [ðdI=dV Þ=T ] curves, which correspond to the LDOS, obtained on
the bcc and the fcc area are shown in Fig. 3. These showthat the LDOS near the Fermi energy is quite sensitive tothe crystalline structure. The bcc(110)-like area has a sharp
LDOS peak around 0:2 V, but the fcc(111)-like area has
three peaks at 0:4 V, near the Fermi energy, and at +0.3 V.
Our theoretical calculations revealed that the LDOS peaks
strongly depend on the spin conﬁguration between the top
and sub-surface layers.11) For example, the layer-wise
ferromagnetic coupling in the fcc scheme produces a strong
LDOS peak at +0.2 V, which does not ﬁt our experimentallyobtained LDOS at the fcc area, i.e., the spins in the fcc area
don’t couple ferromagnetically between layers. A compar-

Fig. 2. (Color online) An atomically resolved STM image (4  4 nm2 ) obtained at the boundary between the fcc and the bcc areas. The red lattice is a
guides to the eyes, showing the hexagonal symmetry of the fcc(111) Cu surface. The white line indicates the boundary, where the atomic lattices tile about 5
degrees. Sphere models show a top view of the atomic positions at each area.

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Jpn. J. Appl. Phys. 50 (2011) 08LA03

Fig. 3. (Color online) Upper panels show diﬀerential tunnelingconductivity (dI=dV ) curves normalized by its ﬁtted tunneling probability(T ) curves, corresponding to the local density of states (LDOS), obtained atbcc(110)- and fcc(111)-like areas on the Fe nano-islands. Insets indicate thecross section views of sphere models.11) Arrows indicate magnetic momentsof each Fe atom. Lower panels show LDOS obtained on the same area after
introducing a does of 2 L of hydrogen gas (see Supplementary Informationin ref. 11).

ison between the experimentally obtained LDOS and thecalculated LDOS with various spin conﬁgurations allows us
to recover the true spin conﬁguration of the bcc and the fcc
areas: The bcc area has a ferromagnetic state and the fcc
area has a layer-wise antiferromagnetic state, i.e., the bcc
and the fcc area of the Fe nano-islands have diﬀerent spin
conﬁgurations.
Before performing any experiments related to magnetoelectric coupling, one has to rule out inﬂuence on the Fesurfaces by impurities since the 3d metal surfaces are
relatively active compared to the noble metals, binding
easily with impurities such as oxygen22) or hydrogen11)
atoms. Oxygen atoms were usually observed as dark
depressions (5–10 pm in depth) in the STM images due tothe quench and slight shift of the 3d surface states.22) In our
STM experiments, we observed such dark depression rarely
and estimated the concentration much lower than 0.1%.
Concerning possible hydrogen impurities, our experimental
data of hydrogen atoms on Pt(111) surface and the sticking
coeﬃcient diﬀerence between Fe and Pt11) give us aconcentration of hydrogen of less than 0.1%. We also
checked the consequences of a deliberate hydrogen contamination of the iron surface by introducing a dose of 2 L(L = Torrs) hydrogen into the UHV chamber. An STM
image shows the resulting H-(2  2) superstructure on thesurface and the drastic change of the experimental LDOS
(Fig. 311)). With this, we are sure that the surface of the Fe
nano-islands is clean in the sense, that we can performreliable switching experiments without regarding a possible
inﬂuence due to contaminations.
3.2 Electric field controlling the Fe nano magnets

Our theoretical calculations predict that a huge electric ﬁeldof the order of 109 V/m is needed to trigger the crystal-lographic and magnetic phase transition.11,12) An STM
setup can produce such large electric ﬁelds: Since the gap
distance between the STM probe tip and the sample surface
is smaller than 1 nm, a bias voltage of the order of 1 V
generates an electric ﬁeld of more than 109 V/m. In thisstudy, we applied electric ﬁelds of 106 –1010 V/m to the Fe
nano-islands.
Figure 4 shows a Fe nano-island including ferromagnetic
bcc and antiferromagnetic fcc areas. First, we obtained an
STM image under a low electric ﬁeld (107 V/m) (leftpanel), where the center area of the island is in the
ferromagnetic (FM) bcc state. Subsequently, the STM probe
was moved to the center of the island (cross in the sketch)and a voltage pulse (109 V/m) within 50 ms was applied.
Then again, a STM image was taken under a low electricﬁeld (right panel). Now the center area of the Fe nano-island
is in the antiferromagnetic fcc state, i.e., we experimentally
succeeded to switch the magnetic states of the metallic Fe
nano-islands by an electric ﬁeld from ferromagnetic bcc to
antiferromagnetic fcc which is the ﬁrst prove of magnetoelectric coupling at metal surfaces. Depending on the size ofelectric ﬁeld, we succeeded to switch a whole island or avery small area of about 1 nm2 (see the demonstrationin ref. 11). If each area of 1 nm2 represents one bit of
information, this will give us a data-storage device with anextremely high density of more than 1012 bits/inch2 . So far,we conﬁrmed that the switching is possible with a pulse as
short as 0.06 ms, which is the time resolution of our STM
setup (see Supplementary Information in ref. 11). Much
faster response time for the switching is expected, which isan exciting aspect in the view of future applications.
Using an STM setup, we must take into account theinﬂuence of the tunneling current. In Fig. 5, the experimentally obtained relation between critical voltages and tunneling current values, where the phase switching occurs, isplotted as black triangles (details of the measuring method
are shown in ref. 12). We tried to ﬁt this by equations for thecritical voltage as a function of the tunneling current basedon diﬀerent possible models for the switching. This way
we want to make sure whether the origin of the switchingis the electric ﬁeld or not. Five diﬀerent possibilities forthe switching mechanism are considered: (1) spin torqueeﬀect23) or spin accumulation,24) or electro-migration, (2)inelastic spin scattering,25) (3) local heating, (4) mechanicalinteraction or chemical bonding with the tip, and ﬁnally (5)the electric ﬁeld.11,12)(1) Spin torque, spin accumulation, and electro-migration
are determined by the number of electrons, thus theswitching occurs at a certain constant tunneling current(I ¼ Ic , dashed line in Fig. 5). (2) Inelastic tunneling
electrons could excite the spin state of the Fe nano-clusters
at a particular bias voltage, which may change the spinorder. Then the switching occurs at a constant bias voltage(V ¼ Vc , red line in Fig. 5). (3) It could be that the switching
relates to a thermal phase transition due to very local heating
under the tip. In such case the switching occurs at a constant
power (P ¼ Pc , i.e., V is proportional to I 1 , green line in
Fig. 5). (4) The mechanical or chemical interaction between
the tip and the sample is due to the overlap of the wave
functions that is proportional to the tip–sample distance (z).
Since the tunneling current can be described as I /08LA03-3# 2011 The Japan Society of Applied Physics

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Jpn. J. Appl. Phys. 50 (2011) 08LA03

Fig. 4. (Color online) STM topographic images obtained on the same single Fe nano-island before and after the application of a electric ﬁeld pulse.
The ﬁeld was applied at the center of the island (). The area colored in red changes the phase, showing magnetoelectric coupling at the Fe surface.

Fig. 5. (Color online) A systematic study to ﬁnd the origin of the phaseswitching. Experimentally obtained critical tunneling current and bias
voltages, where the phase switch occurs, are plotted as triangles. Fourpossible origins where the switching occurs at (1) constant tunneling current(I ¼ Ic , dashed line), (2) constant bias voltage (V ¼ Vc , red line),(3) constant power (P ¼ Pc , green line), and (4) constant tip–sample
distance or resistance (R ¼ Rc , blue line) do not ﬁt to the experimental data.
Only, (5) the constant electric ﬁeld model ﬁts the experimental data well,proving that the origin of the switching is the electric ﬁeld (E ¼ Ec , blackline).

V expð2zÞ, where  is the decay constant,19) a constantdistance corresponds to a constant resistance R ¼ V =I /expð2zÞ / Rc (blue line in Fig. 5). In summary, Fig. 5clearly shows that these four possible origins: (1)–(4) do not
ﬁt to the experimentally obtained values (triangles), excluding these possibilities as the origin of the phase switching.
(5) If the phase switching occurs at a constant electric ﬁeld
(E ¼ Ec ), the tunneling current formula I / V expð2zÞ
can be described as I / V expðcV Þ with E ¼ Ec ¼ V =z,i.e., V / z (c: coeﬃcient). With this we succeeded to ﬁt ourexperimental data well (black line in Fig. 5), proving theorigin of the switching to be the electric ﬁeld.
Quantitatively, the amplitude of the electric ﬁeld causing
the phase switching is able to get from the experimental data
since the tunneling conductance, I / V expð2zÞ, can be
described as I ¼ G0 V when the tip contacts to the Fe nanoislands (z ¼ 0), where G0 denotes the quantum conductanceof 2e2 =h ¼ 1=ð12:9 kÞ. Thus the tunneling current can be
described as I ¼ G0 V expð2zÞ at z 6¼ 0. Since experimentally we know the switching values of I and V , we canget z at each set of data. If we re-plot the data in the V –z
plane,11) its slope corresponds to the critical electric ﬁeld.
We got Ec ¼ ð3{9Þ  108 V/m.
For controlling the phase switching one to the other, thevector quantity of the electric ﬁeld is one of the mostimportant key issues. Depending on the polarity of theelectric ﬁeld, negative or positive charges are induced on the
tip apex, which pull or push the Fe atomic nucleus andtherefore determine whether the ferromagnetic bcc or the
antiferromagnet fcc state is favoured. Theoretical calculations found that atomic and spin orders strongly depend on
the polarity of the electric ﬁeld.11,12) The energetically moststable conﬁguration was the antiferromagnetic fcc state forthe positive electric ﬁeld and the ferromagnetic bcc state forthe negative electric ﬁeld. This theoretical prediction can be
conﬁrmed experimentally by statistical switching experiments. We repeated very similar experiments as Fig. 4, so
positive or negative electric ﬁeld pulses were applied to the
Fe nano-islands and checked whether the state changed after
the pulse (64 times with positive ﬁeld and 49 times withnegative ﬁeld).
The theoretical calculations11,12) predict the energy landscape of the bilayer Fe nanoislands. Under a positive electric
ﬁeld, the fcc antiferromagnetic (AFM) state is energeticallystable, so the barrier from this state to the bcc FM state is40 meV higher than that from the bcc FM to the fcc AFM
state. We tried to switch the bcc FM area to the fcc AFM
state 40 times and we succeeded to switch 38 times byapplying positive electric ﬁeld pulses, but only two out of 24
trials succeeded in the case of switching from the fcc AFM
to the bcc FM state by a positive electric ﬁeld (see upper
panel in Fig. 6), which ﬁts pretty well to the theory.11,12)This also shows that the fcc AFM state is practically stable
under the positive electric ﬁeld. The same procedures werealso performed for the case of a negative electric ﬁeld (lower
panel in Fig. 6). The calculated energy landscape showedthat the energy barrier from the bcc FM to the fcc AFM stateis more than 100 meV,11,12) which was conﬁrmed by ourexperiments, so under the negative ﬁeld, we succeeded toswitch from the fcc AFM to the bcc FM state 22 times out of
24 trials and never succeeded to switch from the bcc FM to
the fcc AFM state (0 out of 25 trials), which indicates thatexperimentally the bcc FM state is stable under a negativeelectric ﬁeld.

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Jpn. J. Appl. Phys. 50 (2011) 08LA03

Promotion of Science, Chiba University Global COE
Program ‘‘Advanced School for Organic Electronics’’, andthe Rumanian funding: ‘‘Sectoral Operational Program for
Human Resources Development 2007-2013, co-ﬁnancedby the European Social fund, under the project number
POSDRU 89/1.5/S/60189’’.

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Fig. 6. (Color online) Statistical experiments proved that the fcc
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‘‘38/40’’ indicates that the switching succeeded 38 times out of 40 trials.
Insets show a sketch of the STM tunnel junction, where + and  denote theinduced positive and negative charges.

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These statistical experiments give us important information about how to control the switching of the magnetic
states in the Fe nano-islands by the polarity of the electric
ﬁeld.

6)
7)
8)
9)

4. Conclusions

We studied bilayer Fe nano-islands (5–20 nm in size) grown
on a Cu(111) single crystal by means of STM and spinpolarized STM/STS in UHV at 4.5 K. We experimentally
conﬁrmed that the magnetic state in the Fe nano-islands
can be controlled by electric ﬁelds of 108 –109 V/m, i.e.,magnetoelectric coupling exists at the metal surface. A
positive electric ﬁeld favors the transition from theferromagnetic bcc to the antiferromagnetic fcc state, anda negative electric ﬁeld favors the transition from theantiferromagnetic fcc to the ferromagnetic bcc state. These
new ﬁndings make it possible to read, write and store
information in metallic magnets by an electric ﬁeld. This isa breakthrough towards the realization of new classes ofelectric ﬁeld controlled magnetic data-storage devices.
Acknowledgments

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We acknowledge support by the Alexander von Humboldt
foundation, the CNCSIS-UEFISCSU, the DFG (SFB762),
and the DFG (WU349/8-1), as well as from the Yamada
Science Foundation, the Asahi Glass Foundation, the MEXT
JST ‘‘Special Coordination Funds for Promoting Science andTechnology — Improvement of Research Environment for
Young Researchers’’, Grants-in-Aid for Scientiﬁc Research(22810005 and 23681018) from the Japan Society for the

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