Functional Oxides

Functional oxides are a class of highly correlated materials displaying many different phenomena such as ferromagnetism, antiferromagnetism, ferroelectricity, piezoelectricity and superconductivity, with a strong interplay between them. For this reason they have attracted a lot of attention, in view of the possibility to introduce novel functionalities in electronic devices, beyond those provided by conventional semiconductor materials. In this framework the NaBiS group is currently working on the study of phenomena and architectures for the implementation of novel non-volatile memory devices involving ferroelectric and ferromagnetic materials.

A relevant research topic deals with the investigation of artificial multiferroic systems, made of ferromagnetic and ferroelectric materials with some coupling between the ferroelectric polarization and the magnetization, allowing to control the magnetic properties via an electric field or the ferroelectric state via a magnetic field. Thanks to the so-called “magnetoelectric coupling” it is then possible to design novel magnetic memories where the writing of the information is performed by applying an electric field. This represents a viable route to solve the well known problem of the energy dissipation during the magnetic writing, even beyond the current spin transfer torque technology, still requiring current densities as high as 106 A/m2.

Our research focuses on the magnetoelectric coupling of thin magnetic films (few nanometers thick) of 3d-metals and alloys (such as Fe and CoFeB), with in-plane or out-of-plane magnetization, grown by molecular beam epitaxy and magnetron sputtering, on BaTiO3 (100) films, grown by pulsed laser deposition in a MBE/PLD cluster tool. This constitutes a prototypical interface between a ferromagnet and a well-known piezo-ferroelectric material. Our group has been the first capable of growing high quality epitaxial Fe films on BaTiO3 to be used for detailed investigations of the interfacial magnetoelectric coupling. By employing magneto-electric and magneto-optic characterization tools, also in collaboration with external institutions (Elettra – Trieste, and ICMAB – Spain), we explored the interplay between the ferroelectric polarization of BaTiO3 and the magnetic properties (magnetic moment, coercive field, magnetic order) of the ferromagnetic side of the Fe/ BaTiO3 interface. In particular we demonstrated the room temperature, reversible electric switching of the interfacial Fe magnetization. This result recently lead to the electrical control of the TMR in magnetic tunnel junctions grown onto BaTiO3 films. In case of CoFeB films on BaTiO3, we have showed the possibility of reversibly and electrically switching the magnetization of the CoFeB films displaying perpendicular magnetic anisotropy, in bias magnetic fields as low as 10 Oe.

A second topic deals with the investigation of ferroelectric tunnelling junctions (FTJs), i.e. tunnelling junctions where the tunnelling barrier is ferroelectric, so as to display a huge variation of the resistance for opposite states of the ferroelectric polarization. This phenomenon, called Tunneling Electroresistance (TER), has been proposed to realize non-volatile memories and also memristive devices for neuromorphic computing.
In collaboration with the group of Prof. Fontcuberta in ICMAB, we have recently demonstrated the capability of fabricating micron sized FTJs with record values of the TER (up to 106) and clarified the role of an interfacial n-doped layer in the BaTiO3 barrier to understand the voltage modulation of the device resistance. More recently we have also contributed to the demonstration of the optical reading of the ferrolectric state in n-doped BaTiO3 films embedded in microcapacitors.

(a) Layout of the capacitors used for XMCD measurements under electric-bias condition, adapted from (G. Radaelli et al., NATURE COMMUNICATIONS, 2014). (b) STEM HAADF image (atomic resolution Z-contrast) and chemical color map in a RG overlay, (Co-L2,3 map in green and the Fe-L2,3 map in red) from the area marked with a green rectangle, acquired on the heterostructure used for experiments.

(a) Layout of the capacitors used for XMCD measurements under electric-bias condition, adapted from (G. Radaelli et al., NATURE COMMUNICATIONS, 2014). (b) STEM HAADF image (atomic resolution Z-contrast) and chemical color map in a RG overlay, (Co-L2,3 map in green and the Fe-L2,3 map in red) from the area marked with a green rectangle, acquired on the heterostructure used for experiments.

References

  • [DOI] S. Brivio, D. Petti, R. Bertacco, and J. C. Cezar, “Electric field control of magnetic anisotropies and magnetic coercivity in Fe/BaTiO3 (001) heterostructures,” Appl. Phys. Lett., vol. 98, iss. 9, 2011.
    [Bibtex]
    @Article{Brivio2011,
    Title = {Electric field control of magnetic anisotropies and magnetic coercivity in Fe/BaTiO3 (001) heterostructures},
    Author = {Brivio, S. and Petti, D. and Bertacco, R. and Cezar, J.C.},
    Journal = {Appl. Phys. Lett.},
    Year = {2011},
    Number = {9},
    Volume = {98},
    __markedentry = {[User:6]},
    Abstract = {Magnetoelectric effects in Fe/BaTiO3 (001) heterostructures have been investigated via magneto-optical Kerr effect as a function of temperature. We find major modifications of magnetic anisotropies and magnetic coercivity induced by the application of electric fields perpendicular to the interface. Changes in the coercive magnetic field, on the order of 100% at 250 K (in the orthorhombic phase of BaTiO3) and 40% at 300 K (in the tetragonal phase), which are linked to the ferroelectric behavior of BaTiO3, are reported. © 2011 American Institute of Physics.},
    Address = {European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France},
    Booktitle = {Applied Physics Letters},
    Comment = {Cited By :26
    Export Date: 26 June 2015},
    Doi = {10.1063/1.3554432},
    Owner = {Christian Rinaldi},
    Timestamp = {2015.06.26},
    Url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-79952389818&partnerID=40&md5=9310671eb4e4cea516ad858505a1d8ae}
    }
  • [DOI] S. Brivio, C. Rinaldi, D. Petti, R. Bertacco, and F. Sanchez, “Epitaxial growth of Fe/BaTiO3 heterostructures,” Thin Solid Films, vol. 519, pp. 5804-5807, 2011.
    [Bibtex]
    @Article{Brivio2011b,
    Title = {Epitaxial growth of Fe/BaTiO3 heterostructures},
    Author = {Brivio, S. and Rinaldi, C. and Petti, D. and Bertacco, R. and Sanchez, F.},
    Journal = {Thin Solid Films},
    Year = {2011},
    Pages = {5804-5807},
    Volume = {519},
    __markedentry = {[User:1]},
    Abstract = {The realization of epitaxial heterostructures involving ferroelectric (FE) and ferromagnetic (FM) materials is one of the possible routes towards the realization of devices exploiting sizable magnetoelectric effects. In this paper we demonstrate the epitaxial growth of Fe on BaTiO3(001) as this system represents a prototypical example of interface between well known FE and FM materials with bcc and perovskite structure respectively, both with Curie temperature well above 300K. Fe grows on BaTiO3 with 45 rotation of its cubic lattice with respect to that of the substrate in order to reduce the lattice mismatch. Negligible interdiffusion of Ba and Ti cations or Fe atoms is found by X-ray photoemission spectroscopy, while a sizable Fe oxidation occurs within an interfacial layer with thicknesses thinner than 3nm.},
    Doi = {10.1016/j.tsf.2010.12.193},
    File = {:C\:\\Users\\christian\\Documents\\Spintronica\\Articoli\\Personali\\[ThinSolidFilms_Brivio] Epitaxial growth of FeBaTiO3 heterostructures.pdf:PDF},
    ISSN = {0040-6090},
    Owner = {christian},
    Timestamp = {2011.02.11},
    Url = {http://dx.doi.org/10.1016/j.tsf.2010.12.193}
    }
  • [DOI] G. Radaelli, S. Brivio, I. Fina, and R. Bertacco, “Correlation between growth dynamics and dielectric properties of epitaxial BaTiO3 films,” Appl. Phys. Lett., vol. 100, iss. 10, 2012.
    [Bibtex]
    @Article{Radaelli2012,
    Title = {Correlation between growth dynamics and dielectric properties of epitaxial BaTiO3 films},
    Author = {Radaelli, G. and Brivio, S. and Fina, I. and Bertacco, R.},
    Journal = {Appl. Phys. Lett.},
    Year = {2012},
    Number = {10},
    Volume = {100},
    __markedentry = {[User:6]},
    Abstract = {We report on the effect of the substrate temperature on the growth dynamics and dielectric properties of BaTiO 3 epitaxial films on Nb:SrTiO 3 (001) substrates. By real-time reflection high energy electron diffraction analysis, we quantitatively estimated the adatoms surface diffusivity as a function of temperature. The increase of the substrate temperature promotes surface diffusion, which reflects in the improvement of BaTiO 3 ferroelectric properties up to 680 C. Above this temperature, Ba re-evaporation occurs, leading to a rapid deterioration of the dielectric and ferroelectric properties. This work shed light on thermally activated physical mechanisms which determine the ferroelectric properties of BaTiO 3 films. © 2012 American Institute of Physics.},
    Address = {Institut de Cincia de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain},
    Booktitle = {Applied Physics Letters},
    Comment = {Cited By :5
    Export Date: 26 June 2015},
    Doi = {10.1063/1.3692732},
    Owner = {christian},
    Timestamp = {2015.06.26},
    Url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84858415915&partnerID=40&md5=67eb5b7f87ec7d591ba0e5ecac4e1c06}
    }
  • [DOI] G. Radaelli, M. Cantoni, L. Lijun, M. Espahbodi, and R. Bertacco, “Two dimensional growth of ultrathin Fe films on BaTiO3 with sharp chemical interface,” J. Appl. Phys., vol. 115, iss. 6, 2014.
    [Bibtex]
    @Article{Radaelli2014b,
    Title = {Two dimensional growth of ultrathin Fe films on BaTiO3 with sharp chemical interface},
    Author = {Radaelli, G. and Cantoni, M. and Lijun, L. and Espahbodi, M. and Bertacco, R.},
    Journal = {J. Appl. Phys.},
    Year = {2014},
    Number = {6},
    Volume = {115},
    __markedentry = {[User:6]},
    Abstract = {The Fe/BaTiO3 interface is a prototypical artificial multiferroic system displaying purely electronic magnetoelectric effects at room temperature. As magneto-electric coupling is essentially localized at the interface, the properties of the very first Fe layers in contact with BaTiO 3 play a major role. In this paper, we investigate, by using X-ray photoemission spectroscopy and photoelectron diffraction, the in-situ growth, by molecular beam epitaxy, of ultrathin Fe films (∼7 monolayers) on a BaTiO3/SrTiO3(001) template. We found that growing the Fe films above room temperature (373K) is essential in order to avoid island growth and obtain a continuous film. Post-annealing up to 473K improves the film crystallinity but prevents chemical interdiffusion and roughening. Just an interfacial monolayer of oxidized iron is detected in these conditions, which appears as an unavoidable consequence of the Fe/BaTiO3 chemical interaction. Its active role in magnetoelectric coupling must be carefully taken into account to correlate theoretical predictions and experiments. © 2014 AIP Publishing LLC.},
    Address = {Institute of Solid State Physics, Key Laboratory of Materials Physics, Chinese Academy of Sciences, Hefei 230031, China},
    Booktitle = {Journal of Applied Physics},
    Comment = {Cited By :2
    Export Date: 26 June 2015},
    Doi = {10.1063/1.4864375},
    Owner = {christian},
    Timestamp = {2015.06.26},
    Url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84903775795&partnerID=40&md5=3c0314d10b76a900ee2a54ad931db433}
    }
  • [DOI] G. Radaelli, D. Gutiérrez, F. Sánchez, R. Bertacco, M. Stengel, and J. Fontcuberta, “Large room-temperature electroresistance in dual-modulated ferroelectric tunnel barriers,” Adv. Mater., vol. 27, iss. 16, p. 2602–2607, 2015.
    [Bibtex]
    @Article{Radaelli2015,
    Title = {Large room-temperature electroresistance in dual-modulated ferroelectric tunnel barriers},
    Author = {Radaelli, G. and Gutiérrez, D. and Sánchez, F. and Bertacco, R. and Stengel, M. and Fontcuberta, J.},
    Journal = {Adv. Mater.},
    Year = {2015},
    Number = {16},
    Pages = {2602--2607},
    Volume = {27},
    __markedentry = {[User:6]},
    Abstract = {Pt/BaTiO3/La0.7Sr0.3MnO3 tunnel junctions, at negative voltage bias, for two polarization directions are represented. It is demonstrated that reversing the polarization direction of a ferroelectric barrier in a tunnel junction leads to a change of junction conductance and capacitance, with concomitant variations on the barrier height and effective thickness, both contributing to produce larger electroresistance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.},
    Address = {Instituto Italiano di Tecnologia, Smart Materials-Nanophysics Department, Via Morego 30, Genoa, Italy},
    Booktitle = {Advanced Materials},
    Comment = {Export Date: 26 June 2015},
    Doi = {10.1002/adma.201405117},
    Keywords = {depletion width, electroresistance, ferroelectric tunnel junction, Schottky barrier},
    Owner = {christian},
    Timestamp = {2015.06.26},
    Url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-84928012913&partnerID=40&md5=6bafc7d539beb474e161656964c1c836}
    }
  • [DOI] G. Radaelli, D. Petti, M. Cantoni, C. Rinaldi, and R. Bertacco, “Absence of strain-mediated magnetoelectric coupling at fully epitaxial Fe/BaTiO3 interface (invited),” J. Appl. Phys., vol. 115, iss. 17, 2014.
    [Bibtex]
    @Article{Radaelli2014a,
    Title = {Absence of strain-mediated magnetoelectric coupling at fully epitaxial Fe/BaTiO3 interface (invited)},
    Author = {Radaelli, G. and Petti, D. and Cantoni, M. and Rinaldi, C. and Bertacco, R.},
    Journal = {J. Appl. Phys.},
    Year = {2014},
    Number = {17},
    Volume = {115},
    __markedentry = {[christian:2]},
    Doi = {10.1063/1.4870915},
    Eid = {172604},
    File = {:2014\\[JAppPhys_Radaelli] Absence of strain-mediated magnetoelectric coupling at fully epitaxial Fe-BaTiO3 interface (invited).pdf:PDF},
    Owner = {christian},
    Timestamp = {2014.04.10},
    Url = {http://scitation.aip.org/content/aip/journal/jap/115/17/10.1063/1.4870915}
    }
  • [DOI] G. Radaelli, D. Petti, E. Plekhanov, I. Fina, P. Torelli, B. R. Salles, M. Cantoni, C. Rinaldi, D. Gutirrez, G. Panaccione, M. Varela, S. Picozzi, J. Fontcuberta, and R. Bertacco, “Electric control of magnetism at the Fe/BaTiO3 interface,” Nat. Commun., vol. 5, 2014.
    [Bibtex]
    @Article{Radaelli2014,
    Title = {Electric control of magnetism at the Fe/BaTiO3 interface},
    Author = {Radaelli, G. and Petti, D. and Plekhanov, E. and Fina, I. and Torelli, P. and Salles, B. R. and Cantoni, M. and Rinaldi, C. and Gutirrez, D. and Panaccione, G. and Varela, M. and Picozzi, S. and Fontcuberta, J. and Bertacco, R.},
    Journal = {Nat. Commun.},
    Year = {2014},
    Month = mar,
    Volume = {5},
    __markedentry = {[User:1]},
    Abstract = {Interfacial magnetoelectric coupling is a viable path to achieve electrical writing of magnetic information in spintronic devices. For the prototypical Fe/BaTiO3 system, only tiny changes of the interfacial Fe magnetic moment upon reversal of the BaTiO3 dielectric polarization have been predicted so far. Here, by using X-ray magnetic circular dichroism in combination with high-resolution electron microscopy and first principles calculations, we report on an undisclosed physical mechanism for interfacial magnetoelectric coupling in the Fe/BaTiO3 system. At this interface, an ultrathin oxidized iron layer exists, whose magnetization can be electrically and reversibly switched on and off at room temperature by reversing the BaTiO3 polarization. The suppression/recovery of interfacial ferromagnetism results from the asymmetric effect that ionic displacements in BaTiO3 produces on the exchange coupling constants in the interfacial-oxidized Fe layer. The observed giant magnetoelectric response holds potential for optimizing interfacial magnetoelectric coupling in view of efficient, low-power spintronic devices.},
    Comment = {Supplementary information available for this article at http://www.nature.com/ncomms/2014/140303/ncomms4404/suppinfo/ncomms4404_S1.html},
    Doi = {10.1038/ncomms4404},
    File = {:2014\\[JAppPhys_Radaelli] Absence of strain-mediated magnetoelectric coupling at fully epitaxial Fe-BaTiO3 interface (invited).pdf:PDF},
    Owner = {christian},
    Publisher = {Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.},
    Timestamp = {2014.03.04},
    Url = {http://dx.doi.org/10.1038/ncomms4404}
    }

Involved People

Related projects

This research has been supported by the Cariplo Foundation via the project EcoMag and by MIUR via the project FIRB “Ossidi nano strutturati: multifunzionalità e applicazioni” (RBAP115AYN).