Multiferroics are a relatively rare class of multifunctional materials that simultaneously exhibit several ferroic orders among ferromagnetic, ferroelectric and ferroelastic. Most of the currently investigated multiferroics are generally magnetic and ferroelectric, but very few showing a finite large magnetization (corresponding to ferro- or ferrimagnetic ordering). Practically, the vast majority of multiferroics are thus ferroelectric antiferromagnets or weak ferromagnets. We focus here on insulating multiferroic oxides.
We started to work on multiferroics with BiMnO3 thin films for spin-filtering and then moved toward BiFeO3 (BFO) . BFO is the archetypal room temperature multiferoic: it is both ferroelectric and antiferromagnetic with high ordering temperatures (1100K for the ferroelectric Curie temperature and 640K for the magnetic Néel temperature). BiFeO3, by far the most studied, has outstanding ferroelectric properties (polarisation of 100 µC/cm², the highest of all known ferroelectrics) , a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Artificial multiferroic heterostructures with embedded ferromagnets, ferroelectrics or piezoelectrics are also considered.
Strain engineering of multiferroic films and heterostructures
Strain engineering has emerged as a powerful way to tune the various remarkable properties of perovskite oxide thin films. The strong coupling between degrees of freedom make BFO an exciting system for strain-driving phase transitions and discover novel physical properties. We grew epitaxial BFO films on substrates spanning a broad range of lattice mismatches. From LSAT (-2..5% strain) to NdScO3 (NSO, +0.8%). BFO distorts monoclinically from the bulk rhombohedral R3c phase which causes a surprising decrease of the ferroelectric Curie temperature. This is highly unexpected as strain, either compressive or tensile, increases this transition temperature in conventional ferroelectrics. The ferroelectric Tc gets very close to the antiferromagnetic ordering temperature, which should bring about giant magnetoelectric responses.
We investigated the rich spin physics of BiFeO3 in a detailed study of the static and dynamic magnetic response of strain-engineered films. Using Mössbauer and Raman spectroscopies combined with Landau–Ginzburg theory and effective Hamiltonian calculations, we show that the bulk-like cycloidal spin modulation that exists at low compressive strain is driven towards pseudo-collinear antiferromagnetism at high strain, both tensile and compressive. For moderate tensile strain we also predict and observe indications of a new cycloid. Finally, we reveal that strain progressively drives the average spin angle from in-plane to out-of-plane, a property we use to tune the exchange bias and giant-magnetoresistive response of spin valves.
For even large compressive strain levels, BFO transforms into a highly distorted pseudo-tetragonal structure with a giant ratio of the long to the short perovskite axes (c/a = 1.23). This new “T”-like phase is also ferroelectric and antiferromagnetic (G-type). Its Néel temperature is concomitant to a ferroelectric phase transition at about 380K, which should be accompanied by a strong magnetoelectric coupling. This is an example of artificial material allowed by strain engineering.
Collaboration with the Ecole Centrale Paris (B. Dkhil), the University of Arkansas (L. Bellaiche), the Laboratoire Léon Brillouin (S. Petit, A. Bataille), the University of Rouen (J. Juraszek) and the MPQ Lab of University Paris Denis Diderot (M. Cazayous), Helmholtz-Zentrum Berlin für Materialen und Energie (S. Valencia), Univ. Sidney (D. Sando).
Some related publications :
H. Béa, B. Dupé, S. Fusil, R. Mattana, E. Jacquet, B. Warot-Fonrose, F. Wilhelm, A. Rogalev, S. Petit, V. Cros, A. Anane, F. Petroff, K. Bouzehouane, G. Geneste, B. Dkhil, S. Lisenkov, I. Ponomareva, L. Bellaiche, M. Bibes, and A. Barthélémy
Evidence for room-temperature multiferroicity in a compound with a giant axial ratio
Phys. Rev. Lett. 102, 217603 (2009)
B. Dupé, I. C. Infante, G. Geneste, P.-E. Janolin, M. Bibes, A. Barthélémy, S. Lisenkov, L. Bellaiche, S. Ravy, and B. Dkhil
Competing phases in BiFeO3 thin films under compressive epitaxial strain
Phys. Rev. B 81, 144128 (2010)
I. C. Infante, S. Lisenkov, B. Dupé, M. Bibes, S. Fusil, E. Jacquet, G. Geneste, S. Petit, A. Courtial, J. Juraszek, L. Bellaiche, A. Barthélémy and B. Dkhil
Bridging multiferroic phase transitions by epitaxial strain in BiFeO3
Phys. Rev. Lett. 105, 057601 (2010)
I. C. Infante, J. Juraszek, S. Fusil, B. Dupé, P. Gemeiner, O. Diéguez, F. Pailloux, S. Jouen, E. Jacquet, G. Geneste, J. Pacaud, J. Íñiguez, L. Bellaiche, A. Barthélémy, B. Dkhil and M. Bibes
Multiferroic phase transition near room temperature in BiFeO3 films
Phys. Rev. Lett. 107, 237601 (2011)
D. Sando, A. Agbelele, D. Rahmedov, J. Liu, P. Rovillain, C. Toulouse, I. C. Infante, A. P. Pyatakov, S. Fusil, E. Jacquet, C. Carrétéro, C. Deranlot, S. Lisenkov, D. Wang, J-M. Le Breton, M. Cazayous, A. Sacuto, J. Juraszek, A. K. Zvezdin, L. Bellaiche, B. Dkhil, A. Barthélémy and M. Bibes
Crafting the magnonic and spintronic response of BiFeO3 films by epitaxial strain
Nature Mater. 12, 641 (2013)
Giant magnetoelectric coupling at room temperature
We explored the combination of FeRh, which undergoes a first order antiferromagnetic-ferromagnetic transition near room temperature, with a ferroelectric material. The ferroelectricity of BaTiO3 substrates was used to modify the metamagnetic transition of epitaxial thin films of FeRh, thus toggling electrically between the antiferromagnetic and magnetic states slightly above room temperature. Combining structural, magnetic, and ab initio studies, we concluded that strain modifications related to the electric control of ferroelastic domains in the BaTiO3 substrate are mainly governing this magnetic phase transition. These results correspond to a magnetoelectric coefficient one order of magnitude larger than any other reports and open the path to the use of ferroelectric materials for magnetic storage or spintronics.
Selected publications :
R.O. Cherifi, V. Ivanovskaya, L.C. Phillips, A. Zobelli, I.C. Infante, E. Jacquet, V. Garcia, S. Fusil, P.R. Briddon, N. Guiblin, A. Mougin, A.A. Ünal, F. Kronast, S. Valencia, B. Dkhil, A. Barthélémy & M. Bibes
Electric-field control of magnetic order above room temperature
Nature Mater. 13, 345 (2014)
L. C. Phillips, R. O. Cherifi, V. Ivanovskaya, A. Zobelli, I. C. Infante, E. Jacquet, N. Guiblin, A. A. Ünal, F. Kronast, B. Dkhil, A. Barthélémy, M. Bibes and S. Valencia
Local electrical control of magnetic order and orientation by ferroelastic domain arrangements just above room temperature
Sci. Rep. 5, 10026 (2015)
Magneto-electric spin valves
The electrical control of magnetization via the magnetoelectric coupling offers the opportunity of combining the respective advantages of ferroelectric memories (FeRAMs) and magnetic memories (MRAMs) in the form of non-volatile magnetic storage bits that are switched by an electric field (MERAM). The basic operation of such magnetoelectric random access memories combines the magnetoelectric coupling with the interfacial exchange coupling between a multiferroic and a ferromagnetic to switch the magnetization of the ferromagnetic layer by using a voltage.
To realize a MERAM element, a key prerequisite is the observation of an exchange coupling between the multiferroic and a ferromagnetic layer. We have carried out a systematic investigation of this effect and found that the exchange field between the two layers is correlated to the size of the ferroelectric domains in the multiferroic. This behavior is reminiscent of the Malozemoff’s model of exchange bias, and provides clues on how to control magnetization reversal by control the ferroelectric domain size. We succeeded to fabricate spin-valves exchange coupled to an underlying BFO film, and control the exchange bias and the giant magnetoresistance with an electric field, at room temperature. The effect is however not reversible, which we now understand as due to the way the domain structure of BFO is modified upon voltage cycling.
Some related publications :
M. Bibes and A. Barthélémy
Towards a magnetoelectric memory
Nature Mater. 7, 426 (2008)
Julie Allibe, Stéphane Fusil, Karim Bouzehouane, Christophe Daumont, Daniel Sando, Eric Jacquet, Cyrille Deranlot, Manuel Bibes and Agnès Barthélémy
Room Temperature Electrical Manipulation of Giant Magnetoresistance in Spin Valves Exchange-Biased with BiFeO3
Nano Lett. 12, 1141 (2012)
S. Fusil, V. Garcia, A. Barthélémy and M. Bibes
Magnetoelectric devices for spintronics
Annu. Rev. Mater. Res. 44, 91 (2014)
V. Garcia, M. Bibes and A. Barthélémy
Artificial multiferroic heterostructures for an electric control of magnetic properties
Comptes Rendus Phys. 16, 168 (2015)
Ferroelectric tunnel junctions
In tunnel junctions incoporating a ferroelectric as the barrier material, switching the ferroelectric polarization leads to different resistance state, producing a so-called tunnel electroresistance (TER) effect. The TER can reach values of more than 10^6% and thus ferroelectric tunnel junctions (FTJs) emerge as interesting non-volatile memory devices, not suffering from the destructive read-out issue of FeRAMs. In 2009, using scanning probe microscopy, we demonstrated a giant TER effect for the first time. We then fabricated solid-state tunnel junctions displaying a large TER, and operating with ~10 ns voltage pulses. We also found that FTJs can be not only be used as binary memories but also as analogue devices, i.e. memristors. In contrast to most other memristors that operate through the displacement of matter at the nanoscale, we have shown that in FTJs the resistance state is determined by the ferroelectric domain configuration within the barrier. Most recently, we have also shows that thes ferroelectric tunnel memristors can be used to implement learning rules (e.g. spike-timing dependent plasticity), believed to be key for unsupervised learning in articial intelligence systems.
Selected publications :
V. Garcia, S. Fusil, K. Bouzehouane, S. Enouz-Vedrenne, N. D. Mathur, A. Barthélémy and M. Bibes
Giant tunnel electroresistance for non-destructive readout of ferroelectric states
Nature 460, 81 (2009)
André Chanthbouala, Arnaud Crassous, Vincent Garcia, Karim Bouzehouane, Stéphane Fusil, Xavier Moya, Julie Allibe, Bruno Dlubak, Julie Grollier, Stéphane Xavier, Cyrile Deranlot, Amir Moshar, Roger Proksch, Neil D. Mathur, Manuel Bibes and Agnès Barthélémy
Solid-state memories based on ferroelectric tunnel junctions
Nature Nanotech. 7, 101 (2012)
André Chanthbouala, Vincent Garcia, Ryan O. Cherifi, Karim Bouzehouane, Stéphane Fusil, Xavier Moya, Stéphane Xavier, Hiroyuki Yamada, Cyrile Deranlot, Neil D. Mathur, Manuel Bibes, Agnès Barthélémy and Julie Grollier
A ferroelectric memristor
Nature Mater. 11, 860 (2012)
H. Yamada, V. Garcia, S. Fusil, S. Boyn, M. Marinova, A. Gloter, S. Xavier, J. Grollier, E. Jacquet, C. Carrétéro, C. Deranlot, M. Bibes and A. Barthélémy
Giant Electroresistance of Super-tetragonal BiFeO3-Based Ferroelectric Tunnel Junctions
ACS Nano 7, 5385 (2013)
S. Boyn, J. Grollier, G. Lecerf, B. Xu, N. Locatelli, S. Fusil, S. Girod, C. Carrétéro, K. Garcia, S. Xavier, J. Tomas, L. Bellaiche, M. Bibes, A. Barthélémy, S. Saïghi and V. Garcia
Learning through ferroelectric domain dynamics in solid-state synapses
Nature Commun. 8, 14736 (2017).
Multiferroic tunnel junctions
In spintronics, a key building block is the magnetic tunnel junction where two ferromagnetic electrodes sandwich an ultrathin layer of dielectric. In these devices, the tunnel resistance has two distinct states corresponding to the antiparallel or parallel configurations of the magnetic electrodes (which are controllable by a magnetic field). This effect, called tunnel magnetoresistance (TMR), is based on spin-dependent electron tunneling that is extremely sensitive to the spin-dependent electronic properties of the ferromagnet/dielectric interface. Hence, measurements of TMR can be used to probe interfacial changes of the magnetic properties. The interplay between ferroelectricity and ferromagnetism can be investigated through transport measurements in artificial multiferroic tunnel junctions where a ferroelectric tunnel barrier is sandwiched between two ferromagnetic electrodes. The two ferroic order parameters (ferromagnetism and ferroelectricity) give rise to four distinct resistance states due to the combined TMR and TER effects. In addition, interface magnetoelectric coupling between the ferroelectric and the ferromagnetic can be probed by measuring the modulation of the spin-dependent tunneling current (through the TMR).
M. Gajek, M. Bibes, S. Fusil, K. Bouzehouane, J. Fontcuberta, A. Barthélémy, A. Fert
Tunnel junctions with multiferroic barriers
Nature Mater., 6, 296 (2007)
V. Garcia, M. Bibes, L. Bocher, S. Valencia, F. Kronast, A. Crassous, X. Moya, S. Enouz-Vedrenne, A. Gloter, D. Imhoff, C. Deranlot, N. D. Mathur, S. Fusil, K. Bouzehouane, A. Barthélémy
Ferroelectric Control of Spin Polarization
Science 327, 1106 (2010)
S. Valencia, A. Crassous, L. Bocher, V. Garcia, X. Moya, R. O. Cherfi, C. Deranlot, K. Bouzehouane, S. Fusil, A. Zobelli, A. Gloter, N. D. Mathur, A. Gaupp, R. Abrudan, F. Radu, A. Barthélémy and M. Bibes
Interface-induced room-temperature multiferroicity in BaTiO3
Nature Mater. 10, 753 (2011)
Laura Bocher, Alexandre Gloter, Arnaud Crassous, Vincent Garcia, Katia March, Alberto Zobelli, Sergio Valencia, Shaïma Enouz-Vedrenne, Xavier Moya, Neil D. Marthur, Cyrile Deranlot, Stéphane Fusil, Karim Bouzehouane, Manuel Bibes, Agnès Barthélémy, Christian Colliex and Odile Stéphan
Atomic and Electronic Structure of the BaTiO3/Fe Interface in Multiferroic Tunnel Junctions
Nano Lett. 12, 376 (2012)
Ferroelectric field-effect transistors
Ferroelectric-field effect is somewhat equivalent to a switchable chemical doping. The large ferroelectric polarization in the supertetragonal phase of BiFeO3 makes it, in principle, possible to accumulate or deplete carrier densities on the order of 3 × 1014 cm-2 in a non-volatile way. This is two orders of magnitude higher than what can be achieved with standard dielectric gate oxides. In compressively strained (Ca,Ce)MnO3 thin films grown on YAlO3, very low carrier densities are required to induce an insulator to metal transition. Thus, in BiFeO3/(Ca,Ce)MnO3 bilayers, a purely electrostatic and reversible metal-insulator transition by ferroelectric polarization reversal is accessible. Our first experiments evidenced large modulations of the resistivity of up to 10 at 200 K and 4 at 300 K.
These results show the great potential of Mott insulators such as CaMnO3 as this is the largest non-volatile room-temperature modulation of resistivity achieved in a ferroelectric field-effect transistor (Fe-FET). However, progress must be made in the control and understanding of the interface properties in order to enhance the gating efficiency.
Selected publications :
H. Yamada, M. Marinova, P. Altuntas, A. Crassous, L. Bégon-Lours, S. Fusil, E. Jacquet, V. Garcia, K. Bouzehouane, A. Gloter, J.E. Villegas, A. Barthélémy and M. Bibes
Ferroelectric control of a Mott insulator
Sci. Rep. 3, 2834 (2013)
Arnaud Crassous, Rozenn Bernard, Stéphane Fusil, Karim Bouzehouane, David Le Bourdais, Shaïma Enouz-Vedrenne, Javier Briatico, Manuel Bibes, Agnès Barthélémy and Javier E. Villegas
Nanoscale Electrostatic Manipulation of Magnetic Flux Quanta in Ferroelectric/Superconductor BiFeO3/YBa2Cu3O7-δ Heterostructures
Phys. Rev. Lett. 107, 247002 (2011)