University of Nebraska - Lincoln
DigitalCommons@University of Nebraska - Lincoln
)0)++%+!1(#(%-*.3"+)#!2).-1 %1%!0#(!/%01)-(71)#1!-$120.-.,7
Robust isothermal electric control of exchange bias
at room temperature
Xi He
University of Nebraska-Lincoln
Ning Wu
University of Nebraska-Lincoln
Anthony N. Caruso
University of Missouri-Kansas City#!031.!-3,*#%$3
Elio Vescovo
Brookhaven National Laboratory4%1#.4."-+'.4
Kirill D. Belashchenko
University of Nebraska-Lincoln"%+!1(#(%-*.3-+%$3
See next page for additional authors
.++.52()1!-$!$$)2).-!+5.0*1!2 (:/$)')2!+#.,,.-13-+%$3/(71)#1"%+!1(#(%-*.
9)102)#+%)1"0.3'(22.7.3&.0&0%%!-$./%-!##%11"72(%%1%!0#(!/%01)-(71)#1!-$120.-.,7!2)')2!+.,,.-1-)4%01)27.&%"0!1*!
)-#.+-2(!1"%%-!##%/2%$&.0)-#+31).-)-)0)++%+!1(#(%-*.3"+)#!2).-1"7!-!32(.0)8%$!$,)-)120!2.0.&)')2!+.,,.-1-)4%01)27.&
%"0!1*!)-#.+-
% )3)-'!031.-2(.-7%1#.4.+).%+!1(#(%-*.)0)++.5"%-%2%0!-$)-%*(0)12)!-."312
)1.2(%0,!+%+%#20)##.-20.+.&%6#(!-'%")!1!20..,2%,/%0!230% Kirill Belashchenko Publications
(:/$)')2!+#.,,.-13-+%$3/(71)#1"%+!1(#(%-*.
ARTICLES
PUBLISHED ONLINE: 20 JUNE 2010 | DOI: 10.1038/NMAT2785
Robust isothermal electric control of exchange
bias at room temperature
Xi He
1
, Yi Wang
1
, Ning Wu
1
, Anthony N. Caruso
2
, Elio Vescovo
3
, Kirill D. Belashchenko
1
,
Peter A. Dowben
1
and Christian Binek
1
*
Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power
consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high-
speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization.
From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles
calculations, we show that the (0001) surface of magnetoelectric Cr
2
O
3
has a roughness-insensitive, electrically switchable
magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr
2
O
3
single crystal, we
achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values
by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk
antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly
at the bulk Néel temperature.
S
pintronics strives to exploit the spin degree of freedom of
electrons for an advanced generation of electronic devices
1,2
.
In particular, voltage-controlled spin electronics is of vital
importance to continue progress in information technology.
The main objective of such an advanced technology is to
reduce power consumption while enhancing processing speed,
integration density and functionality in comparison with present-
day complementary metal–oxide–semiconductor electronics
3–6
.
Almost all existing and prototypical solid-state spintronic devices
rely on tailored interface magnetism, enabling spin-selective
transmission or scattering of electrons. Controlling magnetism at
thin-film interfaces, preferably by purely electrical means, is a key
challenge to better spintronics
7–10
.
The absence of direct coupling between magnetization and
electric field makes the electric control of collective magnetism
in general, and surface and interface magnetism in particular,
a scientific challenge. The significance of controlled interface
magnetism started with the exchange-bias effect. Exchange bias is
a coupling phenomenon at magnetic interfaces that manifests itself
most prominently in the shift of the ferromagnetic hysteresis loop
along the magnetic-field axis and is quantified by the magnitude
µ
0
H
EB
of the shift
11
. The exchange-bias pinning of ferromagnetic
thin films is employed in giant magnetoresistance and tunnelling
magnetoresistance structures of magnetic-field sensors and modern
magnetic read heads
12
.
Electric control of exchange bias has been proposed for various
spintronic applications that go beyond giant magnetoresistance and
tunnelling magnetoresistance technology
5
. One approach to such
voltage control requires a reversible, laterally uniform, isothermal
electric tuning of the exchange-bias field at room temperature,
which remains a significant challenge.
Early attempts in electrically controlled exchange bias tried
to exploit the linear magnetoelectric susceptibility of the
antiferromagnetic material Cr
2
O
3
as an active exchange-bias
1
Department of Physics & Astronomy and the Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588-0111,
USA,
2
Department of Physics, 257 Flarsheim Hall, University of Missouri, 5110 Rockhill Road, Kansas City, Kansas 64110, USA,
3
Brookhaven National
Laboratory, National Synchrotron Light Source, Upton, New York 11973, USA. *e-mail: cbinek2@unl.edu.
pinning system
13
. In a magnetoelectric material an applied
electric field induces a net magnetic moment, which can
be used to electrically manipulate the magnetic states of an
adjacent exchange-coupled ferromagnetic film
14
. The small
value of the maximum parallel magnetoelectric susceptibility
α
me
k
(T = 263 K) ≈ 4.13 ps m
−1
of Cr
2
O
3
(ref. 15) led many
researchers to the conclusion that multiferroic materials are
better suited for this purpose. Such materials have two or more
ferroic order parameters, such as ferroelectric polarization and
(anti)ferromagnetic order
16
.
The potential for an increased magnetoelectric response, for
the multiferroic materials, was dictated by the maximum possible
value of α
me
ij
. It is determined by the geometric mean of the
ferroic susceptibilities, both of which can individually be very
high in multiferroics
17–20
. Coupling between these order parameters
has been demonstrated
21
. However, it is typically weak, and the
theoretical upper limit of α
me
ij
is rarely reached
16
.
Artificial two-phase multiferroics have been studied extensively.
Such piezoelectric/ferromagnetic heterosystems allow for electric
control of anisotropy
22,23
. However, strain-induced non-hysteretic
magnetoelastic effects are often not stable (persistent) in the
absence of an applied field (that is, volatile). Removing the electric
field from a linear piezoelectric element releases the strain in the
ferromagnetic component and hence restores the anisotropy of
the piezoelectrically unstrained film. When using a ferroelectric
material, to induce piezoelectric strain control, one may take
advantage of the ferroelectric hysteresis to impose some residual
strain that will persist after removing the electric field. In contrast
to this electric control of magnetic anisotropy in two-phase
multiferroics, we report on a non-volatile electric control of
unidirectional magnetic anisotropy.
The most promising multiferroic single-phase materials used
for electrically controlled exchange bias are YMnO
3
and BiFeO
3
(refs 24,25). Complete suppression of the exchange bias has been
NATURE MATERIALS | VOL 9 | JULY 2010 | www.nature.com/naturematerials 579
This document is a U.S. government work and
is not subject to copyright in the United States.
ARTICLES
NATURE MATERIALS DOI: 10.1038/NMAT2785
achieved at 2 K in an YMnO
3
/NiFe (permalloy) heterostructure.
This effect, however, is irreversible. Moreover, the limitation of
low temperatures makes YMnO
3
unsuitable for applications. The
situation is better with BiFeO
3
. In BiFeO
3
/CoFe heterostructures,
local magnetization reversal on a lateral length scale of up to
2 µm has been demonstrated
25,26
. However, global magnetization
reversal, which could be revealed in macroscopic magnetic
hysteresis, has not been achieved. Global, but not isothermal
magnetoelectric switching has been achieved in the pioneering
Cr
2
O
3
/CoPt heterostructure
13
. However, each sign reversal of the
exchange-bias field required a new magnetoelectric annealing
procedure, in which the pinning layer is cooled from T > T
N
to T < T
N
in the presence of both electric and magnetic fields.
Isothermal electric control of exchange bias has been attempted
by various groups, but with only marginal success
27,28
. The result
was that reversible and global electrically controlled exchange bias
carried out isothermally at room temperature remained elusive.
Here we reveal an unconventional ferromagnetism at the
(0001) surface of the magnetoelectric antiferromagnet Cr
2
O
3
and
demonstrate its suitability for electrically controlled exchange bias
and magnetization. New insights were achieved by combining
first-principles calculations, general symmetry arguments, spin-
resolved photoemission spectroscopy and magnetometry (see
Supplementary Information) for the Cr
2
O
3
(0001) surface and
its interface in an exchange-bias heterostructure. We used a
molecular beam epitaxy (MBE)-grown chromia thin film (see the
Methods section) for the spin- and energy-resolved ultraviolet
photoemission spectroscopy (UPS), whereas the isothermal electric
control of exchange bias was done on a heterostructure involving
an oriented chromia single crystal with (0001) surface. The choice
of a high-quality single crystal for the exchange-bias system
completely rules out sample heating induced by leakage currents
because of the virtually perfect insulating properties of single-
crystalline chromia. The UPS measurements have been carried
out in zero electric field after magnetoelectric initialization of
the antiferromagnetic domain state. The non-zero conductivity
of thin films is a well-known experimental advantage used for
the photoemission investigation of samples that otherwise are
virtually perfectly insulating in the bulk. The finite conductivity of
the thin film prevents charge accumulation, which could lead to
misrepresented photoelectron energies.
On the basis of the understanding of the surface ferromagnetism
of Cr
2
O
3
(0001), a new concept of Cr
2
O
3
(0001)-based exchange
bias is implemented. As a result, a reversible, isothermal and
global electric control of exchange bias is demonstrated at room
temperature by reproducible electrically induced discrete shifts of
the global magnetic hysteresis loop along the magnetic-field axis
(see Supplementary Movie).
Magnetically uncompensated surfaces of antiferromagnetically
ordered single crystals have been a subject of intense investigations,
in particular in the framework of exchange bias
11
. The surface
magnetization of an uncompensated antiferromagnetic surface
with roughness usually averages out, so that only a small non-
equilibrium statistical fluctuation remains for exchange coupling
with the adjacent ferromagnet
29
.
The surfaces of single-domain antiferromagnetic magneto-
electrics, such as the (0001) surface of the antiferromagnetically
long-range ordered Cr
2
O
3
, are remarkable exceptions. The free
energy of this system, with a boundary, depends on the polar vector
n (external normal) as a macroscopic parameter. The existence
of the magnetization at the boundary can be deduced from the
reduction of the bulk magnetic point group by the presence of
an invariant vector n. As both n and E are polar vectors, the
boundary reduces the symmetry in a similar way to the electric
field, E, in the bulk. An equilibrium magnetization must therefore
exist at the surface of a magnetoelectric antiferromagnet, or at
its interface with another material. This argument automatically
includes equilibrium surface roughness; a more detailed analysis
will be published elsewhere.
Both the bulk single crystal and the thin-film sample are
confirmed to be (0001) oriented by X-ray diffraction. The surface
topography of the bulk and the thin-film sample are mapped using
atomic force microscopy (AFM). Figure 1 is organized in such
a way that structural data of the bulk sample are shown in the
upper panels of a and b. The corresponding data of the thin-film
sample are shown in the lower panels of Fig. 1a,b. The (0001)
orientation of the bulk surface is independently corroborated by
the hexagonal reflection pattern obtained in low-energy electron
diffraction. The prominent (0006) and (00012) X-ray peaks of the
bulk sample are virtually identically reproduced in the thin film
(compare peak positions in the upper and lower panels of Fig. 1a).
The surface topography of the samples reveals a plateau with a
root-mean-squared (r.m.s.) roughness of 0.88 nm for the surface of
the bulk crystal (upper panel of Fig. 1b) and an even lower r.m.s.
roughness of 0.19 nm (lower panel of Fig. 1b) for the thin-film
sample measured along selected lines.
Figure 1c illustrates a configuration of the Cr
2
O
3
(0001) surface.
It is seen that the particular antiferromagnetic domain has an
uncompensated surface magnetic monolayer with aligned moments
on all surface Cr
3+
ions, even if the surface is not atomically flat.
Two features conspire to produce this property. First, the corundum
lattice of Cr
2
O
3
can be imagined as a layered arrangement of
buckled Cr
3+
ions sandwiched between the triangular layers of O
2−
ions
30
. The electrostatically stable charge-neutral surface of this
crystal is terminated by a semi-layer of Cr; this termination can be
viewed as the cleavage of the crystal in the middle of the buckled
Cr
3+
layer
31
. Second, Cr ions, which are structurally similar with
respect to the underlying O layer, have parallel spins. As a result,
a single-domain antiferromagnetic state has all surface Cr spins
pointing in the same direction. Note that although we have shown
the surface Cr ions in bulk-like positions in Fig. 1, this assumption is
immaterial for the existence of the surface magnetization, as follows
from the general symmetry argument.
In single-crystalline Cr
2
O
3
, the antiferromagnetic order allows
two degenerate 180
◦
antiferromagnetic domains
14
(see Fig. 1
and Supplementary Fig. S1). These two domains have surface
magnetizations of opposite sign. If the degeneracy of the two
domain types is not lifted, the system develops a random multi-
domain state with zero net surface magnetization when it is
cooled below T
N
. However, magnetoelectric annealing allows for
preferential selection of one of these 180
◦
domains by exploiting
the free-energy gain 1F = αEH (ref. 14). As a result, even a
rough Cr
2
O
3
(0001) surface becomes spin-polarized when an
antiferromagnetic single-domain state is established. Evidence
of this roughness-insensitive surface magnetism is revealed by
magnetometry (Supplementary Fig. S2 and Discussion) as well
as spin-resolved UPS. Interpretation of the latter is supported by
calculations of the site-resolved density of states (DOS) revealing
a spin-polarized surface band above the valence-band maximum,
in agreement with experimental findings. The UPS carried out
on our MBE-grown Cr
2
O
3
(0001) sample is sensitive to occupied
surface electronic states.
Figure 2a shows the spin-polarized photoelectron intensity
versus binding energy measured at 100 K. First, the MBE-grown
Cr
2
O
3
(0001) thin film has been cooled from T > T
N
in
a small magnetic field of 30 mT alone, into a multidomain
antiferromagnetic state. Spin-up and spin-down photoelectron
intensities /
↑,↓
(red circles and blue squares) are virtually identical,
indicating negligible net surface magnetization and polarization.
Furthermore, multiple measurements were undertaken for the
single-antiferromagnetic-domain states, each with a fresh sample
preparation. Subsequent sample preparations involve alternating
580 NATURE MATERIALS | VOL 9 | JULY 2010 | www.nature.com/naturematerials
NATURE MATERIALS DOI: 10.1038/NMAT2785
ARTICLES
Cr
2
O
3
(0006)
Cr
2
O
3
(0006)
Cr
2
O
3
(00012)
Al
2
O
3
(0006)
Al
2
O
3
(0006)
Al
2
O
3
(00012)
Al
2
O
3
(0009)
Al
2
O
3
(00012)
Cr
2
O
3
(00012)
*
*
2 (°)
Intensity (arb. units)
X (μm)
200 nm
r.m.s. within (0.15 μm, 0.81 μm)
0.88 nm
r.m.s. within (0.04 μm, 0.50 μm)
0.19 nm
Height (nm)
200 nm
30 40 50 60 70 80 90
10
4
10
3
10
2
10
1
Intensity (arb. units)
30 40 50 60 70 80 90
10
4
10
3
10
2
10
1
0 0.5 1.0 1.5 2.0
X (μm)
0 0.5 1.0 1.5 2.0
¬5
0
5
10
Height (nm)
¬5
0
5
10
θ
2 (°)
θ
Cr
3+
O
2¬
a
c
b
Figure 1 | Structural characterization. a, θ–2θ X-ray diffraction pattern of chromia bulk single crystal (upper panel) and thin film (lower panel) showing the
chromia (0006) and (00012) peaks, respectively. The film is deposited on a sapphire (0001) substrate, giving rise to (0006), (00012), K
α
and K
β
(∗)
peaks and a weak structure-factor-forbidden (0009) peak. The inset shows a room-temperature low-energy electron diffraction pattern of the hexagonal
chromia (0001) surface measured at an electron energy of 140 eV. b, Real-space topography of the chromia (0001) surface of bulk single crystal (upper
panel) and thin film (lower panel) measured by AFM. The respective main frames show cross-sectional analysis along indicated lines. A r.m.s. roughness of
0.88 nm is calculated in the region between scanning position 0.15 and 0.81 µm for the bulk single crystal. The r.m.s. roughness of 0.19 nm of the thin film is
measured between 0.04 and 0.50 µm. c, The spin structure of a Cr
2
O
3
single crystal with a stepped (0001) surface is shown for one of its two
antiferromagnetic single-domain states. Up (red) and down (dark blue) spins of the Cr
3+
ions (green spheres) point along the c axis.
NATURE MATERIALS | VOL 9 | JULY 2010 | www.nature.com/naturematerials 581