PAPER www.rsc.org/materials | Journal of Materials Chemistry

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Room temperature ferrimagnetic thin films of the magnetoelectricGa2�xFexO3†

Morgan Trassin,a Nathalie Viart,*a Gilles Versini,a Sophie Barre,a Genevi�eve Pourroy,a Jihye Lee,b William Jo,b

Karine Dumesnil,c Catherine Dufourc and Sylvie Robertc

Received 7th July 2009, Accepted 22nd September 2009

First published as an Advance Article on the web 16th October 2009

DOI: 10.1039/b913359c

(0k0) oriented films of the magnetoelectric material Ga2�xFexO3 (0.8 # x # 1.4) have been grown by

pulsed laser deposition on various substrates: the non conducting yttrium stabilized zirconia (YSZ)

(001) and the conducting indium tin oxide (ITO) buffered YSZ(001) and single crystalline Pt(111)

buffered YSZ(111). The films are ferrimagnetic for all compositions and their Curie temperature

increases with x. For x ¼ 1.4, their Curie temperature is above room temperature (370 K) and their

room temperature saturation magnetization is 90 emu/cm3. The effect of the conducting substrates on

both the crystalline and electrical properties of the films has been studied. The single crystalline Pt(111)

buffered YSZ(111) substrates allow substantial improvements both on the crystallographic and

electrical points of view with a reduction of the number of in-plane variants down to 3 and a decrease of

the leakage current down to 10�5 A at 10 V. This work opens new perspectives for the integration of

a room temperature ferrimagnetic magnetoelectric material in spintronic devices.

Introduction

Magnetoelectric materials, thanks to the coupling existing

between their magnetic and electric properties, allow the

manipulation of magnetization by an electric field. Such mate-

rials currently receive considerable interest for they open new

perspectives in terms of memory devices. Indeed, a magneto-

electric memory would combine the best of both FeRAM and

MRAM worlds.1 At the present time, the development of

magnetoelectric memories appears to be limited by the scarcity

of magnetoelectric materials and even more by the scarcity of

magnetoelectric materials presenting a magnetic order at room

temperature. BiFeO3 (BFO) is the only material considered so far

in the literature presenting both a magnetoelectric coupling and

a magnetic order at room temperature. The magnetoelectric

manipulation of magnetization has recently been proved exper-

imentally possible at room temperature with this ferroelectric

antiferromagnet.2 However, since the magnetic order is antifer-

romagnetic, the actual use of BFO requires the use of an extra

layer to which it is magnetically coupled. This makes the fabri-

cation of devices more complicated and the device itself subject

to failure. It is therefore essential to make efforts in developing

magnetoelectric materials presenting a non zero magnetization at

room temperature. With this aim in view, we have considered

gallium ferrite compounds Ga2�xFexO3 (0.8 # x # 1.4) (GFO)

with great interest. It indeed appears as the perfect alternative

aInstitut de Physique et Chimie des Mat�eriaux de Strasbourg (UMR 7504CNRS-UDS), BP 43, 23, rue du Loess, 67034 Strasbourg Cedex 2, FrancebDepartment of Physics and Division of Nanosciences, Ewha WomansUniversity, Seoul 120 750, Republic of KoreacInstitut Jean Lamour (UMR 7198), Universit�e H. Poincar�e - Nancy I, BP239, 54506 Vandoeuvre les Nancy Cedex, France

† Electronic supplementary information (ESI) available: EDX spectrumand Monte Carlo simulation. See DOI: 10.1039/b913359c

8876 | J. Mater. Chem., 2009, 19, 8876–8880

material to BFO in magnetoelectric memories: it is ferrimagnetic

above room temperature for x ¼ 1.43 and pyroelectric with

a strong magnetoelectric coupling.4,5 Bulk GFO adopts an

orthorhombic structure, crystallizing in the space group Pc21n

with a ¼ 0.87512 � 0.00008 nm, b ¼ 0.93993 � 0.00003 nm and

c ¼ 0.50806 � 0.00002 nm.6 The Ga3+ and Fe3+ cations are

distributed on four types of sites labeled Fe1, Fe2, Ga1 and Ga2.

The Ga1 site is tetrahedral and mainly occupied by Ga3+ cations.

The three other sites are octahedral and can be occupied by both

Fe3+ and Ga3+ cations. The electric polarization in GFO is

observed along b and may be due to either the distortion of the

Ga1 tetrahedra6 or to the non compensated displacements of the

Fe1 and Fe2 octahedra centers along b.5 Due to the different

number of Fe3+ cations at Fe1 sites on the one hand, and at

Fe2 and Ga2 sites on the other hand, the antiferromagnetic coupling

between these sites results in an overall ferrimagnetic behavior along

the c axis with, for x ¼ 1, a Curie temperature of 240 K and a low

temperature spontaneous moment of 136 emu/cm3.7

Although the bulk properties have been well established, GFO

has been seldom studied in thin films. Kundaliya et al.8 have

reported the elaboration of oriented polycrystalline GFO thin

films by pulsed laser deposition on non conducting yttrium-

stabilized zirconia (YSZ) substrates. Sun et al.9 reported the

preparation of GFO on conducting (111) SrTiO3 : Nb (STON)

substrates. But the large lattice mismatch between GFO and

STON (ca. 8%) leads to a poor crystallographic quality of the

elaborated films.

We report here on the pulsed laser deposition of GFO (0k0)

oriented thin films with a restricted number of in-plane variants.

First, we show that thin films of high crystalline quality can be

obtained through the optimization of the experimental deposi-

tion conditions (temperature, oxidizing deposition pressure).

Second, we demonstrate that it is possible to observe ferrimag-

netism at room temperature in thin films by tuning the iron to

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gallium ratio, as previously shown for bulk single crystals only.3

The possibility to elaborate high crystalline quality thin films on

a crystallographically matching conducting electrode is

addressed in the third part. This last point is of high importance

when it comes to the electrical characterization of the films.

Fig. 1 XRD patterns of GaFeO3 thin films grown on YSZ (001)

(a) under 1 mbar O2 : N2 at 700 �C, 800 �C and 900 �C, (b) at 900 �C

under 1 mbar and 0.2 mbar O2 : N2.

Experimental

Films have been elaborated using a KrF excimer laser (l¼ 248 nm)

with a 10 Hz repetition rate and a fluence of 1 J cm�2. The

Ga2�xFexO3 (0.8 < x < 1.4) targets were obtained by sintering

a pellet of corresponding mixtures of high purity Ga2O3 (99.99%,

Fluka A.G.) and Fe2O3 (99%, Prolabo) powders at 1400 �C for

24 h. The commercially available YSZ substrates (Crystal

GmbH) did not undergo any cleaning process prior to use. The

buffer indium tin oxide (ITO) conducting layer (70 nm) was

deposited from a sintered In2O3:SnO2 (90:10 wt%) target under

7.10�5 mbar O2 : N2 pressure at 80 �C on YSZ (001) substrates.

The films were annealed in situ at 600 �C under the same partial

pressure for 2 h. The deposition of the single crystalline Pt(111)

electrode (25 nm) on YSZ(111) substrates has been described

elsewhere by the authors.10 The GFO depositions (200 nm) were

performed exploring a wide range of substrate temperatures

(700–900 �C) and reactive gas partial pressures (0.2–1 mbar

O2 : N2). After the deposition, the samples were cooled to room

temperature under the gas deposition pressure. The Ga/Fe ratio

was checked by energy dispersive X-ray spectrometry (EDX)

coupled to a scanning electron microscope (SEM) (JEOL

6700F). The roughness of the films was quantified by atomic

force microscopy (Digital Instruments Dimension 3100 AFM).

The measurements were performed at a scan rate of 0.5 Hz with

a 125 mm long tapping mode Nanosensors Si cantilever having

a force constant of 42 N/m and a resonance frequency of

300 kHz. The structure was characterized by X-ray diffraction

through q–2q scans and rocking curves using a Siemens D5000

diffractometer equipped with a monochromatic copper radiation

(Ka1 ¼ 0.154056 nm). Phi-scans and reciprocal space mappings

were performed using a Panalytical X’Pert Pro MRD diffrac-

tometer, also equipped with a monochromatic copper radiation

(Ka1 ¼ 0.154056 nm). The magnetic properties were studied

using a superconducting quantum interference device magne-

tometer (SQUID MPMS XL, Quantum Design).

Results and discussion

Growth of stoichiometric GaFeO3 on YSZ (001): optimization of

the deposition conditions

Growth of stoichiometric GaFeO3 on YSZ (001) has been per-

formed in a 1 mbar reactive gas pressure, varying the deposition

temperature between 700 �C and 900 �C. The q–2q mode X-ray

diffraction diagrams of these films are shown in Fig. 1a. All the

films exhibit a pure b-axis growth of GFO. The increase of the

peaks’ sharpness with the deposition temperature shows that

the higher the deposition temperature, the higher the degree of

crystallisation. When the reactive gas partial pressure is

decreased, Fe3+ reduces into Fe2+ and a parasitic phase corre-

sponding to the (001) oriented cubic magnetite Fe3O4 appears in

the films.13

This journal is ª The Royal Society of Chemistry 2009

The optimal deposition conditions of GFO on YSZ(001)

appear to be at 900 �C under 1 mbar O2 : N2 gas pressure. In

these conditions, we have shown11 that the films present an

epitaxial growth along the [010] axis and 6 in-plane variants

located every 30�. The different orientations of the GFO

variants can be understood when considering the different

matching possibilities between the GFO and YSZ lattices:13

cGFO ¼ 0.5076 nm z aYSZ ¼ 0.5139 nm (1.2% mismatch), on the

one hand, and

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2

GFO þ c2GFO

qz2aYSZ (1.6% mismatch), on the

other hand. The angle between aGFO and the diagonal of the ac

plane of the GFO cell is a ¼ cos�1 aGFO=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2

GFO þ c2GFO

q� �z30�.

This explains the existence of the six GFO variants located every

30�. The magnetic properties of the films are close to those of the

bulk with an out-of-plane [010] hard direction and a Curie

temperature of ca. 200 K.3

Growth of Ga2�xFexO3 on YSZ (001): shifting the Curie

temperature above room temperature by tuning the Ga to Fe

ratio

Ga2�xFexO3 thins films (0.8 < x < 1.4) were deposited on YSZ

(001) substrates at 900 �C under a 1 mbar O2 : N2 pressure. The

quantifications resulting from the EDX analyses are summarized

in Table 1. They show a perfect matching between the observed

and nominal Fe/Ga ratios.

The q–2q X-ray diffraction patterns of the films deposited

directly on YSZ (001) (Fig. 2a) demonstrate that the GFO

orthorhombic structure is conserved for all considered x values.

Pure b-axis growth of Ga2�xFexO3 layer is observed for all

x values and no parasitic phase is observed. The rocking curve

around the GFO (040) peak exhibits a FWHM of 0.5� for all

films. As shown by F scans of the GFO {062} planes, the 30�

periodicity already observed for x ¼ 1 is still present for all

J. Mater. Chem., 2009, 19, 8876–8880 | 8877

Table 1 Cationic ratios observed by EDX for the different compositionsx in the studied Ga2�xFexO3 200 nm films (0.8 < x < 1.4)

x x ¼ 0.8 x ¼ 1.0 x ¼ 1.2 x ¼ 1.4

Fe/Ganominal ratio

0.67 1 1.5 2.33

Fe/Ga ratioobserved by EDX

0.67 � 0.05 0.97 � 0.05 1.53 � 0.05 2.36 � 0.05

Fig. 2 a) XRD patterns of Ga2�xFexO3 thin films grown on YSZ (001)

(0.8 < x < 1.4) under 1 mbar O2 : N2 at 900 �C, (b) F scans of the

Ga2�xFexO3 {062} reflection.

Fig. 3 Reciprocal lattice mappings of the GFO reflections {062} and

{570} for different Ga2�xFexO3 compositions (0.8 < x < 1.4).

Table 2 Evolution of the Ga2�xFexO3 lattice parameter with the ironcomposition (0.8 < x < 1.4)

x x ¼ 0.8 x ¼ 1.0 x ¼ 1.2 x ¼ 1.4

a/�A 8.733(9) 8.741(5) 8.7535(2) 8.788(8)b/�A 9.370(1) 9.3898(3) 9.4145(7) 9.424(6)c/�A 5.070(1) 5.0761(3) 5.0795(7) 5.088(8)

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studied x. Reciprocal lattice maps of the GFO thin films were

performed around reflections {062} and {570} (Fig. 3) for all

studied compositions. These measurements, combined to the

observation of the {0k0} in the q–2q mode, allow us to determine

the lattice parameters (Table 2). The isotropic increase of the

lattice parameters is in total agreement with a study performed

on the bulk.5 This increase is mainly due to the difference in ionic

radii between Fe3+ and Ga3+ ions, respectively 65 and 62 pm.

Magnetization measurements were performed on 3 � 3 mm2

samples of Ga2�xFexO3 (200 nm)/YSZ films. As was already

observed for the x ¼ 1 sample,8 the films present a strong

anisotropy, the out-of-plane direction (GFO b axis) being a hard

magnetization direction. The six crystallographic variants sepa-

rated by only 30�, result in the absence of in-plane anisotropy.

The in-plane magnetization versus temperature measurements

(Fig. 4a), performed for all films in a 200 Oe applied magnetic field,

revealed the increase of the Curie temperature with x (Table 3).

A Curie temperature superior to room temperature was obtained

for x ¼ 1.4 (TC ¼ 370 K). The hysteresis cycles were measured at

5 K for all samples and shown only for the x¼ 1.4 composition in

Fig. 4b. The saturation magnetization values are in perfect

agreement with the values calculated (Table 3) when considering

the Fe3+ magnetic moments and occupancies of the Ga1, Ga2, Fe1

and Fe2 sites observed by Arima et al. by neutron diffraction

measurements on bulk single crystals.5 One can thus consider

8878 | J. Mater. Chem., 2009, 19, 8876–8880

that, in the elaborated thin films as in single crystals, the Ga1 site

is almost exempt of Fe, while the Fe1 and Fe2 sites are prefer-

entially filled with Fe. The amount of Fe in the Ga2 site increases

with x. The in-plane coercive field reaches the relatively high

value of about 3200 Oe at 5 K for x ¼ 1.4. A hysteresis cycle is

still observed when performing M-H measurements on the

x ¼ 1.4 sample at room temperature, as awaited from the Curie

temperature of 370 K for this composition (Fig. 4a). The

observed saturation magnetization is of 90 emu/cm3 and the

coercive field of ca. 600 Oe at 300 K.

The bulk-like behaviour of the GFO thin films, and especially

this room temperature ferrimagnetic character of the Ga2�xFexO3

thin films, allows great hopes for potential uses in electronics.

This journal is ª The Royal Society of Chemistry 2009

Fig. 4 (a) Magnetization versus temperature for Ga2�xFexO3 thin films

with 0.8 < x < 1.4, (b) M-H hysteresis loops of a Ga0.6Fe1.4O3 thin film at

5 K and at 300 K (in-plane measurements).

Fig. 5 XRD patterns of GFO films grown on two different conducting

electrodes.

Fig. 6 AFM characterization of a 200 nm GaFeO3 film on Pt(111)/

YSZ(111).

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Growth of GaFeO3 on a conducting bottom electrode: towards

the electric characterization

Since the GFO b axis (electric polarisation axis) is pointing out of

plane for depositions on YSZ (001) substrates, the films’ electric

polarisation may be measured in a transverse configuration. For

that purpose, GFO films must be grown on a conducting bottom

electrode. Because of the rather unusual GFO cell parameters, an

adequate bottom electrode with the right matching parameters is

difficult to find. Such an electrode is needed to keep the b-axis

growth of high crystalline quality GFO. Two different con-

ducting bottom electrodes which offer small lattice mismatch

have been chosen: ITO (001) buffered YSZ (001) and Pt (111)

buffered YSZ (111), for which the lattice mismatch is 0.4% and

4.9%, respectively.13 We have optimized the deposition condi-

tions of both the ITO and Pt electrodes in order to obtain ultra-

flat single crystalline layers. The conditions for the growth of the

single crystalline Pt (111) conducting electrodes on YSZ (111) are

described elsewhere.10 No parasitic phase is observed in the q–2q

X-ray diffraction patterns of the films deposited on any of these

different bottom conducting electrodes. The pure b-axis growth

Table 3 Evolution of the Curie temperature, measured saturation magnetizcomparison between the saturation magnetization measured at 5 K and theobserved at 4 K by Arima et al.5

Composition x 0.

TC (K) 13Ms meas (emu/cm3) (5 K) 12Hc (Oe) (5 K) 22Site occupation Ga1 (Ga/Fe) 0.

Ga2 (Ga/Fe) mFe ¼ �3.9 mB/Fe 0.Fe1 (Ga/Fe) mFe ¼ +4.5 mB/Fe 0.Fe2 (Ga/Fe) mFe ¼ +4.7 mB/Fe 0.

Ms calc (emu/cm3) (4 K) 13

This journal is ª The Royal Society of Chemistry 2009

of GFO orthorhombic structure is conserved for both bottom

electrodes (Fig. 5). The surface of the GFO films deposited onto

such electrodes has been observed by atomic force microscopy.

Fig. 6 shows the AFM characterization of the GaFeO3/Pt(111)/

YSZ(111) film, as representative of all the studied samples. The

root mean square (rms) value of the roughness is not significantly

increased by the presence of an underlying conducting electrode,

and kept to a sufficiently low value (ca. 1.6 nm) to allow

a potential future insertion of the stack into multilayered devices.

GFO crystalline quality is kept high as shown by rocking

curves FWHM of 1.5� and 0.6� observed for the (040) peak on

ations and coercive fields of Ga2�xFexO3 thin films with 0.8 < x < 1.4—value estimated from the sites occupation and Fe3+ magnetic moments

8 1.0 1.2 1.4

0 215 280 3600 146 220 25500 2400 2500 320093/0.07 0.90/0.10 0.74/0.25 N/A80/0.20 0.76/0.24 0.55/0.45 N/A35/0.65 0.16/0.84 0.14/0.86 N/A32/0.68 0.17/0.83 0.15/0.85 N/A0 142 230 N/A

J. Mater. Chem., 2009, 19, 8876–8880 | 8879

Fig. 7 I–V curves of the GFO thin films deposited on the two studied

conducting electrodes.

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ITO buffered YSZ(001) and Pt(111)/YSZ(111), respectively.

Since the b-axis growth of the GFO films is maintained on the

conducting electrodes and results in an out of plane polar b axis,3

it is possible to measure the electric polarization by simply

depositing 300 mm diameter Pt dots on top of the films.

Fig. 7 shows the I–V characteristics of GFO thin films of

constant thicknesses (200 nm) deposited on ITO buffered

YSZ(001) and Pt(111)/YSZ(111) electrodes. It clearly shows the

influence of the bottom electrode on the measured leakage

current. The ITO bottom electrode allows good crystallization of

the GFO films and the conservation of the six GFO in-plane

variants. However the leakage currents measured with that

electrode are rather high. The GFO thin films deposited on single

crystalline Pt(111)/YSZ(111) electrodes exhibit excellent crys-

tallization and the number of GFO in-plane variants is reduced

from six to three. While the ITO(001) surface is cubic, the

hexagonal symmetry exhibited by Pt(111) surface results in the

reduction of the matching possibilities. The measured leakage

current on this latter electrode decreases by 4 orders of magni-

tude in comparison to the use of an ITO bottom electrode

(Fig. 7). High leakage is responsible for artifacts in the

measurement of polarization–electric field (P–E) loops resulting

in cigar-shaped loops that are misinterpreted as ferroelectric

loops.12 Growth on a single crystalline Pt(111) electrode there-

fore leads to substantial improvement of both the crystalline

quality and the dielectric properties of the GFO films.

Conclusions

The optimal elaboration conditions for the deposition of

epitaxial Ga2�xFexO3 (0.8 # x # 1.4) have been determined.

High crystalline quality (FWHM ¼ 0.5�), b-axis oriented GFO

thin films were obtained for a high substrate temperature

(900 �C) and oxidizing atmosphere pressure (PO2:N2¼ 1 mbar).

The variation of the Ga/Fe atomic ratio allowed the elaboration

8880 | J. Mater. Chem., 2009, 19, 8876–8880

of room temperature ferrimagnetic thin films. Ga0.6Fe1.4O3 thin

films present a Curie temperature of 370 K and their room

temperature saturation magnetization is of 90 emu/cm3. The

films were grown on different conducting bottom electrodes in

order to allow their electrical characterization. Growth on

a single crystalline Pt(111) electrode greatly reduces both the

number of GFO in-plane crystallographic variants (divided by 2)

and the electrical leaky behavior of the GFO films (a leakage

current of 10�5 A is observed at 10 V). In conclusion we have

obtained highly crystalline, perfectly (0k0) oriented GFO thin

films, showing ferrimagnetism at room temperature, on a con-

ducting bottom electrode. This opens wide perspectives for this

attracting room temperature magnetoelectric material in the field

of the magnetoelectric memories.

Acknowledgements

This work was supported by a Centre National de la Recherche

Scientifique (CNRS)/Korea Science & Engineering Foundation

(KOSEF) Joint Research Program through the KOSEF Grant

No. F01-2008-000-10156-0. The authors also wish to thank C.

Leuvrey for the SEM EDX analyses and A. Derory for his

assistance on the SQUID measurements.

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