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Interfacial Magnetoelectric Switching in Multiferroic Heterostructures

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  • University of Duisburg-Essen, Duisburg, Germany

Abstract and Figures

Novel methods of switching magnetism with electric fields and vice versa, and aiming at magnetoelectric (ME) data processing are reported. First, the patented MERAM @ uses the electric field control of exchange bias via an epitaxial Cr 2 O 3 layer and exchange coupling to a Pt/Co/Pt trilayer. It promises to crucially reduce Joule energy losses in RAM devices. Second, magnetic switching of the electric polarization by a transverse magnetic field in a 3-1 composite of a vertically poled BaTiO 3 thick film embedding CoFe 2 O 4 nanopillars produces a regular surface pola-rization pattern with rectangular symmetry. Its possible use for data processing is discussed.
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Interfacial Magnetoelectric Switching in Multiferroic Heterostructures
Wolfgang Kleemann
1,a
, Pavel Borisov
2,b
, Xi Chen
3,c
,
and Carolin Schmitz-Antoniak
1,d
1
Fakultät Physik, Universität Duisburg-Essen, 47048 Duisburg, Germany
2
Department of Physics, University of West Virginia, Morgantown WV 26506, USA
3
Physics Department, South China University of Technology, Guangzhou 510640,
P.R. China
a
wolfgang.kleemann@uni-due.de,
b
pavel.borisov@mail.wvu.edu,
c
xichen@scut.edu.cn,
d
carolin.schmitz-antoniak@uni-due.de
Keywords: Magnetoelectric Effect, Exchange Bias, Multiferroic Composites, Magneto-electric
Switching
Abstract. Novel methods of switching magnetism with electric fields and vice versa, and aiming at
magnetoelectric (ME) data processing are reported. First, the patented MERAM
@
uses the electric
field control of exchange bias via an epitaxial Cr
2
O
3
layer and exchange coupling to a Pt/Co/Pt
trilayer. It promises to crucially reduce Joule energy losses in RAM devices. Second, magnetic
switching of the electric polarization by a transverse magnetic field in a 3-1 composite of a
vertically poled BaTiO
3
thick film embedding CoFe
2
O
4
nanopillars produces a regular surface pola-
rization pattern with rectangular symmetry. Its possible use for data processing is discussed.
1. Introduction
Switching of magnetism with electric fields and magnetic control of electric polarization have been
the big challenges of magnetoelectric (ME) interaction ever since its early prediction [1]. For
applications in micro- and nanoelectronic devices the voltage control of magnetic states is of
primary importance in order to defeat Joule heating [3]. A most promising contemporary suggestion
for non-volatile data storage is the electric field control of exchange bias in the ME random access
memory (MERAM
[4]), in which a voltage switches the surface magnetization of a ME Cr
2
O
3
layer
and - via exchange coupling - the magnetization of the attached Pt/Co/Pt trilayer FM1 (Fig. 1a) [5].
Its giant magnetoresistance (GMR) in conjunction with the hard ferromagnetic layer FM2
(permanently up-magnetized) and the nonmagnetic conductive spacer layer NM is carrying the
information R
±
corresponding to binary “1” and “0”, which is actually controlled by the voltage
±V
0
.
Magnetic switching of the electric polarization in composites of magnetostrictive and piezoelectric
layers is a challenging task in contemporary sensorics. We have investigated the classic 3-1
composite of a BaTiO
3
(BTO) thick film embedding CoFe
2
O
4
(CFO) nanopillars by using element
sensitive X-ray linear dichroism (XLD). In a transverse magnetic field H
y
a rectangular surface
polarization pattern is observed (Fig. 1a), which is due to shear stress transferred from magneto-
strictive ferrimagnetic CFO onto the piezo- and ferroelectric BTO environment [2].
Materials Science Forum Vols. 783-786 (2014) pp 1623-1627
© (2014) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/MSF.783-786.1623
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 80.141.25.26, Universität Duisburg-Essen, Duisburg, Germany-25/04/14,14:32:28)
Fig. 1 (a) MERAM cell for electric switching of the magnetization of Pt/Co/Pt
(FM1) via exchange biased magnetoelectric Cr
2
O
3
[4]. (b) Shear stress-induced
staggered polarisation ±P
x
and
±P
y
in nanopillar composited CoFe
2
O
4
/BaTiO
3
on
SrTiO
3
substrate [3].
2. Experimental Procedure
The heterostructure in Fig. 1a (e-beam grown under UHV conditions) consists (from top) of a metal
(e.g. Au) cap layer, the (0001) orientated substrate crystal Cr
2
O
3
, the 'free layer' FM1 =
Pt(0.5nm)/Co(0.35nm)/Pt(3nm) with perpendicular magnetic anisotropy, the non-magnetic coupling
layer NM, e.g. Cu(5 nm), and the 'fixed' hard-magnetic layer FM2, e.g. NdFeB (200 nm), with
perpendicular anisotropy and fixed emerging magnetic field
µ
0
H
0
~ 0.3 T [5].
By applying a DC voltage ±V
0
to the ME 'master' layer the emerging electric field and the fixed
magnetic field H
0
will create altogether an AF and ME single domain in Cr
2
O
3
, e.g. under field-
cooling conditions. Its surface magnetization [7] is able to shift the hysteresis loop of the attached
'slave' layer FM1 by exchange bias to both positive and negative magnetic fields [8].
The mutual orientation of the magnetization in the layers FM1 and FM2 is sensed by their parallel
resistance R
±
±±
±
via the giant magnetoresistance effect [9]. Hence, the primary electric bit information,
±V
0
, is transformed into a non-volatile magnetic one, R
±
±±
±
. Thus an energy saving novel data storage
concept [4] has emerged, the technological potential of which is actually tested.
Self-assembled CoFe
2
O
4
(CFO) nanopillars of height t ~ 400nm and cross section ~ 70×70 nm
2
were grown by pulsed laser deposition (PLD; Lambda Physik: P = 1 Jcm
-2
,
λ
= 248 nm) from a
target consisting of 65% BTO and 35% CFO (Kurt J. Lesker Co.) via spontaneous phase separation
in a BaTiO
3
(BTO) matrix on a SrTiO
3
(001) substrate (STO) [6]. Phase purity features and strict
(001) orientation of both CFO and BTO were demonstrated by X-ray diffraction and absorption
spectroscopy [2]. Shape anisotropy warrants the magnetic easy axis of CFO to be (001)-directed,
while the same property holds for the polar c-axis of the BTO matrix in consequence of epitaxial
strain at the STO subtrate.
Application of an intraplanar magnetic field,
µ
0
H
y
= 3T, breaks the tetragonal symmetry of the CFO
nanopillars. They become orthorhombically distorted, where the y-edges gets shrunk and the x- and
z-edges become elongated following the magnetostriction rules of spinel-type CFO with
λ
||
< 0 and
λ
> 0 [6]. As a reaction, the BTO matrix becomes deformed, while being compressed along the
local x-directions, but expanding along the y-directions. This gives rise to global rectangular
symmetry of the BTO surface as verified by Ti
4+
based X-ray linear dichroism [2].
1624 THERMEC 2013
3. Results
3.1. Electric switching of magnetic bits in MERAM
The exchange bias shifted hysteresis curves of the Pt/Co/Pt trilayer FM1 (Fig. 1a) under parallel and
antiparallel ME field cooling (H
fr
± E
fr
) to room temperature are shown in Fig. 3. Positive and
negative magnetization ±M
r
remains at the permanent background field H
0
(Fig. 1b; circles in
Fig.3). In practice the magnetic field will never be changed, i.e. H
fr
H
0
, and the AF domain
switching can even be accomplished at room temperature without any intermittent heating [7, 11].
As a consequence, the magnetization switching of the 'free' layer will give rise to a significant drop
of the joint resistivity of the layers FM1 and FM2 between R
-
and R
+
for antiparallel and parallel
magnetization vectors, respectively (GMR [9]).
Detailed studies of MERAM demonstrators have evidenced compatibility with existing RAM
principles. This concerns the criteria of rapid switch time (< 100ns at cell size < 130 nm), long
retention time ( 10 ys for 2 monolayer thickness of Co), and low power consumption (10
-12
Ws
for switching and reading at room temperature under |H
fr
·E
fr
| >10mT·2V/40nm). Existing leakage
problems with excessive intergrain conductivity of the Cr
2
O
3
'master layer' were overcome by
annealing and/or evaporation at T > 500°C. However, still the low Néel temperature of Cr
2
O
3
(T
N
308K) cannot yet compete with other materials, since thermal stability of commercial devices
should be warranted up to 400°C. We expect filling this gap by alloying Cr
2
O
3
with a related AF
oxide at higher T
N
even when risking deteriorated ME properties.
Fig. 2 Magnetization hysteresis curves of
Pt/Co/Pt 'free' trilayer (= FM1 in Fig. 1a) nega-
tively and positively exchange bias shifted
after field cooling to T = 295K in antiparallel
and parallel freezing fields H
fr
and E
fr
,
respectively. Positive and negative magneti-
zation remains at H
fr
(= H
0
in Fig. 1a; black
circles).
3.2. Magnetic switching of polarization patterns via strain coupling
The magnetostrictive rectangular shaping of the CFO nanopillars by an intra-planar magnetic field
(Fig. 1b) has important consequences onto the ferroelectric properties of the embedding BTO
matrix. Since the nanopillars are clamped to the STO substrate surface, shear stress components of
different direction and sign, ±σ
13
and ±σ
23
, is encountered at the edges of the pillars. They give rise
to in-plane polarization components P
x
and P
y
by virtue of the off-diagonal piezoelectric coupling
coefficient d
15
,
P
x,y
= ±d
15
σ
13,23
. (1)
Materials Science Forum Vols. 783-786 1625
This transverse contribution has been shown to be quite sizeable in BTO because of the vicinity of
the tetragonal-to-orthorhombic phase transition at room temperature [10].
The polarization pattern forming around the superficial cross sections of the nanopillars breaks the
quadratic symmetry of the surface into a rectangular one as viewed in Fig. 3a. Here a quasi-regular
pattern has been 'formatted' into a square array of 'bits' by current pulses δI driven along nanowires.
Their Oersted fields, H, are uniformly directed thus defining a horizontal axis, which may define
logic '0' throughout. In order to write a bit with value '1' at position (m,n) a two-step procedure
might be envisaged: first, one column is written at position n by a vertical current pulse δI, which
turns all long axes by 90°; second, again all lines but number m are refreshed by horizontal current
pulses δI such that only bit (m,n) keeps its particular '1' direction (Fig. 3b).
.
Fig. 3 (a) Formatting a CFO nanopillar array under current (δI) induced Oersted
fields (H) into the horizontally stretched logic '0' ground state. (b) Vertically
stretched logic state '1' of bit (m,n) with amplified planar polarization pattern δP of
the BTO environment.
It is seen (Fig. 3b) that the polarization pattern around the logic '1' appears amplified due to
constructive superposition of in- and outgoing polarization vectors with those of their of their '0'
valued neighborhood. This property might be utilized when reading the bit values, e.g. by
noncontact nanoellipsometry via the local birefringence of the BTO matrix.
Non-volatility of the written information is warranted by the stable remanence of the distorted
ferrimagnetic nanopillars [2]. Reasonably fast write and read procedures of this novel spatially
dense ME data storage device appear possible. They might compete with those of FERAM devices,
where ferroelectric nanocapacitors are at the heart of the data storage principle.
4. Conclusion
Magnetoelectric switching with converse fields remains one of the most challenging task to gain
those technological performances, which have motivated researchers in the field of multiferroics
from the beginning. In two different scenarios we have shown how to switch electric polarization
with a magnetic field and, conversely, switching magnetism with mere electric fields. The latter
option is definitely the most fascinating because of its advanced power strategy, which appears
crucial in contemporary spintronics. Despite remarkable success we are still aware of materials
issues, which hopefully will become surmounted in the near future.
1626 THERMEC 2013
Acknowledgment
Thanks for fruitful cooperation are due to C. Binek (Univ. Nebraska Lincoln) and H. Wende
(Universität Duisburg-Essen).
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Materials Science Forum Vols. 783-786 1627
... Nd 2 Fe 14 B thick film), with permanent perpendicular stray field H 0 . Reproduced by permission from [43] via Copyright Transfer Agreement with Trans Tech Publications Ltd. logic states as given by the connection matrix and truth table in table 2. ...
... They may serve as a polarization memory being magnetically written in crossbar technique and read out, e.g. optically via local dichroic reflectivity [43]. ...
... MERAM cell for switching of the magnetization of Pt/ Co/Pt (FM1) via exchange biased magnetoelectric Cr 2 O 3 (1 1 1) with voltages ±V 0 and sensing GMR values R ±[12]. Reproduced by permission from[43] via Copyright Transfer Agreement with Trans Tech Publications Ltd. ...
Article
Full-text available
Recent progress in preparing and understanding composite magnetoelectrics is highlighted. Apart from optimized standard solutions no vel methods of switching magnetism with electric fields and vice versa with focus on magnetoelectric (ME) data processing in multiferroic and magnetoelectric nanocomposites deserve particular interest. First, we report on the patented MERAM, which uses the electric field control of exchange bias in a layered composite via an epitaxial magnetoelectric Cr2O3 layer exchange coupled to a Pt/Co/Pt trilayer . It promises to crucially reduce Joule energy losses in RAM devices. Second, magnetic switching of the electric polarization by a transverse magnetic feld in a composite of CoFe2O4 nanopillars embedded in a v ertically poled BaTiO3 thick flm produces a regular surface polarization pattern with rectangular local symmetry. Its possible use for data processing is discussed. Third, in the relaxor ferroelectric single-phase compound (BiFe0.9Co0.1O3)0.4– (Bi1/2K1/2TiO3)0.6 polar nanoregions emerging from ferrimagnetic Bi(Fe,Co)O3 regions embedded in a Bi1/2K1/2TiO3 relaxor component transform into ferroelectric clusters and simultaneously enable congruent magnetic clusters. The local polarization and magnetization couple with record-high direct and converse magnetoelectric coupling coeffcients, α ≈ 1.0 × 10−5 s m−1. These ‘multiferroic’ clusters are promising for applications in data storage or processing devices.
... Nd 2 Fe 14 B thick film), with permanent perpendicular stray field H 0 . Reproduced by permission from [43] via Copyright Transfer Agreement with Trans Tech Publications Ltd. logic states as given by the connection matrix and truth table in table 2. ...
... They may serve as a polarization memory being magnetically written in crossbar technique and read out, e.g. optically via local dichroic reflectivity [43]. ...
... MERAM cell for switching of the magnetization of Pt/ Co/Pt (FM1) via exchange biased magnetoelectric Cr 2 O 3 (1 1 1) with voltages ±V 0 and sensing GMR values R ±[12]. Reproduced by permission from[43] via Copyright Transfer Agreement with Trans Tech Publications Ltd. ...
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Recent progress in preparing and understanding composite magnetoelectrics is highlighted. Apart from optimized standard solutions novel methods of switching magnetism with electric fields and vice versa with focus on magnetoelectric (ME) data processing in multiferroic and magnetoelectric nanocomposites deserve particular interest. First, we report on the patented MERAM, which uses the electric field control of exchange bias in a layered composite via an epitaxial magnetoelectric Cr2O3 layer exchange coupled to a Pt/Co/Pt trilayer. It promises to crucially reduce Joule energy losses in RAM devices. Second, magnetic switching of the electric polarization by a transverse magnetic field in a composite of CoFe2O4 nanopillars embedded in a vertically poled BaTiO3 thick film produces a regular surface polarization pattern with rectangular local symmetry. Its possible use for data processing is discussed. Third, in the relaxor ferroelectric single-phase compound (BiFe0.9Co0.1O3)0.4-(Bi1/2K1/2TiO3)0.6 polar nanoregions emerging from ferrimagnetic Bi(Fe,Co)O3 regions embedded in a Bi1/2K1/2TiO3 relaxor component transform into ferroelectric clusters and simultaneously enable congruent magnetic clusters. The local polarization and magnetization couple with record-high direct and converse magnetoelectric coupling coefficients, α ≈ 1.0 × 10⁻⁵ s m⁻¹. These 'multiferroic' clusters are promising for applications in data storage or processing devices.
... This was the starting point of extensive research based on ME-AFM Cr 2 O 3 aiming at spintronic applications, which led to several proposals of futuristic spintronic devices for storage, memory, and logic use. [24][25][26][27][28][29][30][31][32] Recently, the nonvolatile electric manipulation of FMs has received increasing attention, 33) reflecting the demand for ultralow-energy-consumption data-processing devices. How- ever, only few possible materials remain for the realization of nonvolatile 180° switching of the ferromagnetic moment by electrical means. ...
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