Joint research project

Study and Development of Single-Phase Multiferroic Perovskite Ceramic and Thin Films for Multifunctional Devices

Project leaders
Carmen Galassi, Victor Fruth-oprisan
ROMANIA - RA - The Romanian Academy
CNR/RA 2014-2016
Chemical sciences and materials technology
Thematic area
Chemical sciences and materials technology
Status of the project

Research proposal

Multiferroics are materials that simultaneously possess two or more ferroic order parameters: ferroelectricity, ferromagnetism and/or ferroelasticity. The coexistence and coupling of magnetic and electric ordering parameters in a single-phase material is of huge interest for novel exciting applications within the framework of spintronics, information storage and communication, multiple energy harvesting devices, multifunctional wireless sensors etc [1]. This is confirmed also by statistics of the number of publications on multiferroics (as cited by INSPEC, SCIRUS and ISI) which shows that it increased from less than 100/year (in the period 1975-2000) to about 300/year in 2005, 700/year in 2010 and more than 1000 articles/year in 2011 and 2012.
Among single-phase multiferroics, some of the most studied are perovskite oxides (ABO3). The ferroelectric state, which occurs below the Curie temperature TC, is characterized by the off-center shift of A or B cations relative to the oxygen anions which gives rise to an electric dipole moment and a spontaneous polarization. The mechanism for stabilizing the ferroelectric displacement of B cations (generally a transition metal) is given by B 3d-O 2p orbitals hybridization. This requires empty d0 state to hybridize with O 2p orbital. However this is in contradiction with the condition for the occurrence of a magnetic state which requires filled d states. An alternative ferroelectric mechanisms occurs when A-cation sublattices are occupied by ions with a stereochemically active "lone pair", which brings the empty 6p orbital closer in energy to O 2p orbital and leads to their hybridization. Sometimes both mechanisms occur in the same material, like in PbTiO3 (PT), which makes this material very interesting for combining ferroelectric and magnetic cations on the different sublattices and thus obtaining multiferroicity.
A first way of combination of A and B cations in a perovskite structure for obtaining a ferromagnetic ferroelectric is to let B cation to be magnetic and to introduce an A cation with a stereochemically active ns2 lone pair. To this category belong materials like BiFeO3, BiMnO3, Pb(Fe1/2Nb1/2)O3 (PFN), Pb(Fe1/2Ta1/2)O3 (PFT) etc [2]. Following a second path, which consists in letting the B-site occupied by a ferroelectric d0 ion and substituting the A-site with a magnetic R ion (like e.g. EuTiO3), we propose to explore yet a different possibility which consists in adding also a transition element cation with large ionic radius, like Fe2+ (r = 1.21 ú), on the A site. The idea is that the 3d orbitals of transition elements are less localized than 4f orbitals of R ions and their superexchange interaction through O 2p or Ti 3d orbitals are stronger than R-O-R and R-Ti-R. We have recently reported the first encouraging results on (Pb,Sm,Fe)-TiO3 multiferroic ceramics [3]. We have found that this material has a good spontaneous ferroelectric polarization and weak ferromagnetism which persists even at room temperature. The remanent polarization and magnetization are comparable to those measured on similar multiferroic perovskites with dilute content of magnetic ions. On the base of these first results, we propose to explore in the framework of this joint research project, by exploiting the previous experience and research collaborations among our groups as well as our available infrastructures, different (Pb,R,Fe)TiO3 compositions with the nominal formula (Pb1-3x/2-yRxFey)TiO3, by varying the type of rare earth R-ion and the amount of R and Fe ions, with the aim to improve their properties. Moreover we propose to extend the research to multiferroic epitaxial thin films with the same compositions. Thin film growth techniques provide the ability to vary the film-substrate lattice mismatch and the epitaxial strain in the thin film materials, which has a strong impact on their properties. In multiferroic thin films it has been predicted [4] that epitaxial strain can drive a system into the region of the phase diagram where a spin-phonon-driven destabilization of the lattice occurs. In this way a single experimental parameter, strain, can simultaneously control multiple order parameters and it can be an additional tuning parameter for creating or improving multiferroic properties.
The Italian partners have a consolidated experience in the development of bulk materials and their characterization. Thus the CNR-ISTEC group has experience in the processing of ceramic powders, study of sintering parameters and of chemical/physical phenomena dealing with densification, microstructural, morphological, textural, chemical, electrical characterisations; equipments are available for the whole processing of bulk ceramic materials (from powder synthesis to densification, electroding and poling). The CNR-ISC group is active in the study of the microscopic mechanisms at the origin of the electromechanical and dielectric properties of perovskites like PZT, PMN-PT and recently also of multiferroic PT doped with magnetic impurities. A distinctive feature of their research is the combination of the dielectric and piezoelectric characterisation with the anelastic one. The latter allows also the non-polar degrees of freedom to be easily probed. Thus the phase transitions and phase diagram can be investigated [5-8].
The Romanian partners include researchers active in synthesis of ferroelectric perovskites and study by structural and spectroscopic methods, SEM, TEM, chemical analysis, magnetic measurements (ICF and collaborations with NIMP). Collaborations of ICF with NILPRP provide access to Pulsed Laser Deposition Laboratory (dr. Maria Dinescu) which has a consolidated experience in the field of film deposition. A long term collaboration exists between NILPRP and CNR team [9,10].
Thus the two national research teams have complementary know-how, experiences and experimental facilities, and the collaborations of our groups for the study of multiferroic perovskites appears very promising.

1. N. A. Spaldin and M. Fiebig, Science 309, 391 (2005).
2. D. A. Sanchez, N. Ortega, A. Kumar, R. Roque-Malherbe, R. Polanco, J. F. Scott, and R. S. Katiyar, AIP Advances 1, 042169 (2011).
3. F. Craciun, E. Dimitriu, M. Grigoras and N. Lupu, Appl. Phys. Lett 102, 242903 (2013).
4. C. J. Fennie and K. M. Rabe, Phys. Rev. Lett. 97, 267602 (2006).
5. F. Cordero, F. Craciun and C. Galassi, Phys. Rev. Lett. 98, 255701 (2007).
6. F. Cordero, F. Craciun, F. Trequattrini, E. Mercadelli and C. Galassi, Phys. Rev. B 81, 144124 (2010).
7. F. Cordero, F. Trequattrini, F. Craciun and C. Galassi, Phys. Rev. B 87, 094108 (2013).
8. F. Craciun, C. Galassi and R. Birjega, J. Appl. Phys. 112, 124106 (2012).
9. M. Cernea, C. Galassi, B. S. Vasile, C. Capiani, C. Berbecaru, I. Pintile, L. Pintilie, J. Eur. Ceram. Soc. 32 (10) , pp. 2389-2397
10. N. D. Scarisoreanu, A. Andrei, R. Birjega, R. Pascu, F. Craciun, C. Galassi, D. Raducanu, M. Dinescu, Thin Solid Films 520, 4568 (2012).

Research goals

- General objectives:
- investigation of the multiferroic properties and the phase transitions of (Pb1-3x/2-yRxFey)TiO3 ceramic and thin films;
- identification of composition region which allows to obtain good ferroelectric and magnetic properties at ambient temperature;
- growth of epitaxial thin films with these compositions

- Specific objectives and tasks
- synthesis of ceramic samples with nominal composition (Pb1-3x/2-yRxFey)TiO3 by conventional solid state ceramic processing;
- investigation of the crystallographic structure by x-ray diffraction (XRD);
- determination of the microstructure and composition of the samples by using scanning electron microscope (SEM) with EDS;
- ferroelectric domains dependence on composition, presence of nanodomains, check of secondary phases and phase separation by TEM and HRTEM;
- chemical states of the elements, identification of valence states of the magnetic ions and of Ti (XPS?);
- identification of Fe valence (2+/3+) and site of incorporation (Mossbauer spectroscopy?);
- piezoelectric and ferroelectric measurements of the piezoelectric coefficients, ferroelectric polarization and their variation with temperature for each composition;
- dielectric and anelastic spectroscopy measurements for the identification of phase transitions and the obtaining of phase diagram;
- magnetic measurements;
- synthesis of thin films by pulsed laser deposition, on selected substrates and electrode materials for epitaxial growth;
- influence of O pressure on the stoichiometry of thin films and valence states of the elements: check for the change in oxidation state of Fe from Fe2+ to Fe3+ in different oxygen pressure conditions;
- dielectric, piezoelectric and ferroelectric characterization of thin films;
- magnetic measurements on thin films.

Last update: 07/10/2022