Multiferroic PbZrxTi1−xO3/Fe3O4 epitaxial sub-micron sized structuresIonela Vrejoiu, Daniele Preziosi, Alessio Morelli, and Eckhard Pippel Citation: Appl. Phys. Lett. 100, 102903 (2012); doi: 10.1063/1.3692583 View online: http://dx.doi.org/10.1063/1.3692583 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i10 Published by the American Institute of Physics. Related ArticlesMagnetoelectric effect in AlN/CoFe bi-layer thin film composites J. Appl. Phys. 111, 07C720 (2012) Magnetoelectric manipulation of domain wall configuration in thin film Ni/[Pb(Mn1/3Nb2/3)O3]0.68-[PbTiO3]0.32(001) heterostructure Appl. Phys. Lett. 100, 092902 (2012) Multiferroic behavior of disordered bismuth-substituted zinc ferrite J. Appl. Phys. 111, 07D911 (2012) Nonlinearity in the high-electric-field piezoelectricity of epitaxial BiFeO3 on SrTiO3 Appl. Phys. Lett. 100, 062906 (2012) Domain structure and in-plane switching in a highly strained Bi0.9Sm0.1FeO3 film Appl. Phys. Lett. 99, 222904 (2011) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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Multiferroic PbZrxTi12xO3/Fe3O4 epitaxial sub-micron sized structures

Ionela Vrejoiu,a) Daniele Preziosi, Alessio Morelli, and Eckhard PippelMax Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany

(Received 14 December 2011; accepted 20 February 2012; published online 7 March 2012)

Large range well ordered epitaxial ferrimagnetic nominally Fe3O4 structures were fabricated by

pulsed-laser deposition and embedded in ferroelectric PbZrxTi1�xO3 (x¼ 0.2, 0.52) epitaxial films.

Magnetite dots were investigated by magnetic force microscopy and exhibited magnetic domain

contrast at room temperature (RT). Embedding ferroelectric PbZrxTi1�xO3 layers exhibit remnant

polarization values close to the values of single epitaxial layers. Transmission electron microscopy

demonstrated the epitaxial growth of the composites and the formation of the ferrimagnetic and

ferroelectric phases. Physical and structural properties of these composites recommend them for

investigations of stress mediated magneto-electric coupling at room temperature. VC 2012 AmericanInstitute of Physics. [http://dx.doi.org/10.1063/1.3692583]

Multiferroic materials are currently being devoted a

great deal of attention, in the intensive pursuit of sizable

magneto-electric effects that can be applied in future devi-

ces.1,2 Along with the investigation of possible magneto-

electric responses and mechanisms, both in single phase

multiferroic materials and composite type of multiferroics,

the demands for miniaturization require development of

methods for top-down patterning or bottom-up synthesis of

epitaxial nanostructures.3–5

We report on the fabrication of arrays of epitaxial nomi-

nally Fe3O4 structures of circular shape and size below

500 nm. Magnetite is a ferrimagnetic material with high Cu-

rie temperature, large room temperature (RT) magnetization

and, very important for magneto-electric composites where

the coupling should occur via elastic deformations, has large

positive linear saturation magnetostriction, kS¼ 40� 10�6.6

For reference, ferromagnetic metals such as Fe and Co have

negative linear saturation magnetostriction, kS¼�7� 10�6

and kS¼�60� 10�6, respectively.6 Other ferrimagnetic

ferrites with large (negative) linear saturation magnetos-

triction, such as CoFe2O4 (kS¼�110� 10�6) or NiFe2O4

(kS¼�25� 10�6),6 have been often employed in combina-

tion with piezoelectric materials, such as BaTiO3, BiFeO3, or

PbZrxTi1�xO3 (PZT).1,2,4,5 Moreover, Kato and Ida demon-

strated that Fe3O4 thin films exhibited ferroelectric switching

and, thus, ferroelectric order at low temperatures7 and, a

recent study8 reported relaxor ferroelectricity in magnetite

single crystals. Combining piezoelectric materials epitaxially

with a magnetostrictive material such as magnetite, Fe3O4,

with positive magnetostriction and relatively low magneto-

crystalline anisotropy (compared with CoFe2O4, for

instance), is nevertheless very appealing; the effects of me-

chanical stress may lead to the changes of the magnetization

chiefly in the direction of the applied stress. However, the

combination of magnetite dot-like structures with any of

these ferroelectrics has been neglected, most likely because

of the difficulties of fabricating high quality magnetite struc-

tures.9 The epitaxial growth of films or sub-micron structures

of the right phase, Fe3O4, which is in competition with the

a�Fe2O3 and c-Fe2O3 and FeO-like phases, requires special

precautions. Moreover, magnetite thin films and probably

structures should have low electrical resistance, which would

make piezoelectric/electrical measurements quite difficult.

We grew magnetite structures by pulsed-laser deposition

on a bottom epitaxial PbZrxTi1� xO3 (x¼ 0.2, 0.52, in brief

PZT20/80 and PZT52/48, respectively) layer, and then cov-

ered by a top PbZrxTi1� xO3 layer of the same composition.

Structural investigations by high resolution transmission elec-

tron microscopy (HRTEM) and high angle annular dark field

scanning transmission electron microscopy (HAADF-STEM)

combined with electron energy loss spectroscopy (EELS)

proved that these composite structures were epitaxial and

enabled the investigation of the oxidation state of iron in the

magnetite structures, respectively. Magnetic force microscopy

(MFM) investigations proved that the magnetite structures

were magnetic at room temperature. Piezoresponse force mi-

croscopy (PFM) performed simultaneously with MFM proved

that the embedding PZT layers had piezoelectric response.

SrRuO3 covered SrTiO3 (100)-oriented crystals were used

as substrates. For the growth of the magnetite dots, stencil

masks were applied on the substrates. The stencils consisted of

amorphous SiN membranes on Si wafers, with circular aper-

tures of about 400 nm diameter.10 Epitaxial dot-like structures

were grown in situ with the stencil attached mechanically to the

substrates, at temperatures of about 575 �C. After removing the

stencil mask, the substrate with the Fe3O4 dots was reheated to

585 �C and the top PZT film was deposited.10

MFM and PFM were performed with a MFP-3D Asylum

Research. For MFM, ASYMFM silicon cantilevers with

50 nm CoCr coating were used. Macroscopic ferroelectric

measurements were made with a Precision Multiferroic 200 V

Test System (Radiant Technology) and with a TF2000 Ana-

lyzer (AixaCCT). Cross section specimens for transmission

electron microscopy were prepared by focused ion beam mill-

ing (FEI Nova NanoLab 600) of thin lamellae. HAADF-

STEM and EELS analyses were performed in a FEI Titan 80-

300 microscope with a spherical aberration corrected probe

forming system.11

Figures 1(a) and 1(b) show topography and MFM phase

images (5 lm� 5 lm areas) of magnetite dots (of about

a)Author to whom correspondence should be addressed. Electronic mail:

vrejoiu@mpi-halle.de. Current address: Max Planck Institute for Solid

State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany.

0003-6951/2012/100(10)/102903/4/$30.00 VC 2012 American Institute of Physics100, 102903-1

APPLIED PHYSICS LETTERS 100, 102903 (2012)

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400 nm diameter and 60 nm height) grown on Nb-doped

SrTiO3(100), this sample being used as reference. The MFM

phase image (Fig. 1(b)) evidences that the magnetite dots

have multiple domain state at RT. Figures 1(c) and 1(d)

show topography and MFM phase images (5 lm� 5 lm

areas) of a sample in which magnetite dots (of about 400 nm

diameter and 50 nm height) are sandwiched between two epi-

taxial ferroelectric PZT20/80 layers. Despite the about

45 nm thick top PZT20/80 layer covering the magnetite dots,

the signal generated by the magnetic domains of the dots is

detectable and magnetic contrast is visible in the dot areas.

This indicates that the embedded Fe3O4 dots are strongly

magnetic at RT and can be probed by MFM through the cap-

ping PZT layer.

Figure 2 illustrates the HAADF-STEM, HRTEM, and

EELS analyses performed on the same sample as the one

investigated by MFM (Fig. 1). The micrographs show that

the heterostructures of 75 nm thick bottom PZT20/80/magne-

tite/40 nm thick top PZT20/80 have relatively sharp interfa-

ces and are epitaxial, despite the large in-plane lattice

mismatch between magnetite (abulk¼ 8.40 A at RT) (Ref. 6)

and PZT20/80 (abulk¼ 3.935 A, cbulk¼ 4.15 A at RT).12

HRTEM imaging (Fig. 2(d)) and the corresponding Fourier

transforms (FFTs) of the micrographs (see Fig. 2(e)) indi-

cated that the dot had spinel structure. Quantitative analysis

of the FFT yielded a lattice spacing of �0.290 nm for the

(220)-plane family, value that slightly deviates from

0.2967 nm (Ref. 9) for the same in the case of bulk Fe3O4.

However, the dot structures have extended structural defects,

formation of antiphase boundaries being notorious in the

case of epitaxial magnetite films.13 Also magnetic field

induced distortions affect the quality of both STEM and

HRTEM images of the magnetic dot structures (see for

instance, the dark contrast with no visible lattice fringes, in

Figs. 2(a) and 2(b)). Hence, evaluation of the lattice parame-

ter of the nominally magnetite dot is affected by large errors.

The EELS spectrum acquired on a magnetite dot, based on

the value of the intensity ratio of the Fe L3,2 white lines (i.e.,

�4.65, for our dots)14 and the features of the O K-edge fine

structure,15 indicated that the oxidation state of iron corre-

sponds most likely to Fe3O4 stoichiometry (see Fig. 2(c)).

However, we cannot rule out the possibility that the dots are

actually a rather disordered mixture of Fe3O4 and c-Fe2O3.

The spinel structure of ferrimagnetic c-Fe2O3 (abulk¼ 8.34 A

at RT) (Ref. 6) is quite similar to that of Fe3O4 in terms of

RT lattice parameters, so it is difficult to distinguish between

them by indexations of electron diffraction patterns or XRD,

especially when epitaxial strain imposed by the substrate and

extended structural defects may severely distort the lattice.

Refined EELS measurements with determination of exact

peak positions ought to be performed and shall enable the

discrimination, provided that EELS spectra are recorded ei-

ther in dual EELS mode or with a separate low loss spectrum

immediately after the core loss. A quite large energy shift

between c-Fe2O3 and Fe3O4 should be detected. Also x-ray

absorption spectroscopy (XAS) would give more reliable in-

formation about the valency of Fe ions, whether indeed

mixed Fe2þ and Fe3þ exist in the dot, as expected for

Fe3O4.16 However, one has to keep in mind that this cannot

be done through the capping PZT layer, therefore only

slightly different samples, with different fabrication history,

can be compared.

We measured ferroelectric hysteresis loops through plati-

num electrodes (about 60lm� 60 lm), sputtered on top of the

areas where the PZT-embedded magnetite dots are, as shown in

photograph inset in the right lower corner of Fig. 3. Figure 3

summarizes the results for two samples, one with PZT20/80/

magnetite/PZT20/80 (Fig. 3(a)) and one with PZT52/48/

magnetite/PZT52/48 (Fig. 3(b)) heterostructures that differ in

the composition of the epitaxial PZT layers. Among the PZT

solid solution compositions, PZT52/48 has the highest piezo-

electric coefficients and electromechanical coupling constants

and remnant polarization of �50 lC/cm2, whereas for tetrago-

nal fully c-axis grown PZT20/80 epitaxial films, we reported

FIG. 1. (Color online) Magnetic force microscopy

investigations: (a) topography image (5 lm� 5 lm) and

(b) its corresponding MFM phase image of magnetite

dot structures (� 400 nm diameter) grown on Nb-doped

SrTiO3(100) (the insets are zoomed in images of single

dots); (c) topography image (5 lm� 5 lm) and (d) its

corresponding MFM phase image of magnetite struc-

tures (�400 nm diameter) sandwiched between epitax-

ial PZT20/80 films.

102903-2 Vrejoiu et al. Appl. Phys. Lett. 100, 102903 (2012)

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record values of polarization of about 100lC/cm2.3,12 The rem-

nant polarization was about Pr¼ 70 lC/cm2 for the PZT20/80/

magnetite/PZT20/80 sample (Fig. 3(a)) and Pr¼ 40 lC/cm2 for

the PZT52/48/magnetite/PZT52/48 sample (Fig. 3(b)), both

measured at 1 kHz and RT. The hysteresis loops of both sam-

ples prove very good ferroelectric behavior and sharp polariza-

tion switching peaks and low leakage currents.

These nanostructured multiferroic composites with epi-

taxial interfaces hold promise for further studies, devoted to

investigate the effects of the out-of-plane polarization

switching in the embedding PZT layers upon the magnetic

domains of the embedded magnetite. This will be performed

in a consecutive MFM and PFM study17 with the MFP-3D

Asylum Research scanning probe microscope. We have per-

formed preliminary RT measurements of the out-of-plane

piezoelectric response, before (pristine state) and after an in-

plane magnetic field was applied (62500 gauss), using a

variable field module (Asylum Research) to apply the mag-

netic field. No striking changes in the piezoelectric response

of the ferroelectric PZT on top of the magnetic dots have

been observed after application of the magnetic field. Figure 4

summarizes our PFM investigations: images of the phase

(upper row) and amplitude (middle row) of the piezores-

ponse in the as-grown state (a and d), after application of

þ2500 gauss (b and e), and �2500 gauss (c and f) in-plane

magnetic field. However, no striking effects were observed

upon application of the in-plane static magnetic field. The

images in the lowest row are zoom-ins on a single structure.

Polarization switching in the ferroelectric PZT could be

achieved both on top of the magnetic dot and in between the

dots, as seen from the piezoelectric phase hysteresis loops

shown in the graph of Fig. 4(i). We shall continue the inves-

tigations by taking in situ MFM images of the magnetic

domains of the dots, while the structures are subjected to an

electric field. Interdigital top electrodes shall be deposited

for applying in-plane electric fields on the PZT52/48/magnetite/

PZT52/48 sample on the insulating substrate.

Summarizing, epitaxial multiferroic composites consist-

ing of ferrimagnetic magnetostrictive Fe3O4 dot structures

FIG. 2. (Color online) HAADF-STEM

((a) and (b)) and HRTEM (d) cross sec-

tion micrographs of a magnetic dot and

the bottom and top PZT20/80 films. In

(c), we show an electron energy loss

spectrum acquired on the same magnetic

structure as in HAADF-STEM images

(a) and (b), and (e) Fourier transform of

the HRTEM image.

FIG. 3. (Color online) Ferroelectric hysteresis loops measured through plati-

num top electrodes (as shown in the bottom right inset photograph) at 1 kHz

and room temperature. On two different PZT/magnetic dot structures: (a)

PZT20/80/magnetic dots/PZT20/80 and (b) PZT52/48/magnetic dots/

PZT52/48.

102903-3 Vrejoiu et al. Appl. Phys. Lett. 100, 102903 (2012)

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and ferroelectric PbZrxTi1� xO3 layers were fabricated by

pulsed-laser deposition. The magnetite dots were investi-

gated by magnetic force microscopy and proved to be ferro-

magnetic at room temperature. The embedding ferroelectric

PbZrxTi1�xO3 layers exhibit remnant polarization values

close to the values of the single epitaxial layers. The struc-

tural investigations by transmission electron microscopy

demonstrated the high structural quality of the composites

with epitaxial interfaces. Future studies will be dedicated to

the stress mediated magneto-electric coupling of these multi-

ferroic composites.

We thank Norbert Schammelt (MPI-Halle) for the fabri-

cation of the FIB-TEM specimens, Dr. Daniel Biggemann

(MPI-Halle) for HRTEM investigations, and Dr. Massimo

Ghidini (University of Cambridge, UK) for the kind assis-

tance with MFM and insightful discussions.

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FIG. 4. (Color online) Room tempera-

ture piezoresponse force microscopy of

PZT20/80/magnetic dots/PZT20/80 struc-

tures: phase (upper row) and amplitude

(middle row) images in the as-grown

state ((a) and (d)), and after application of

þ2500 gauss ((b) and (e)), and �2500

gauss ((c) and (f)) in-plane magnetic

field. There is a slight mechanical drift

that affected the scanned areas (see

marked dots in (d)–(f)) during these

consecutive measurements. Images in the

lowest row are zoom-ins on a single

structure. Polarization switching in the

PZT could be achieved both on top of the

magnetic dot and in between the dots, as

seen from the piezoresponse phase hys-

teresis loops in (i).

102903-4 Vrejoiu et al. Appl. Phys. Lett. 100, 102903 (2012)

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