Multiferroics are a relatively rare class of multifunctional materials that simultaneously exhibit several ferroic orders among ferromagnetic, ferroelectric and ferroelastic (ferrotoroidic ordering is also sometimes included). Given the scarcity of compounds that present two or more strictly ferroic orders, antiferroic orders (e.g. antiferromagnetic) are often considered. Most of the currently investigated multiferroics are generally magnetic and ferroelectric, 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.
The figure presents a classification of insulating oxides. The largest circle represents all insulating oxides among which one finds electrically polarizable materials (green ellipse) and magnetically polarizable materials (orange ellipse). Within each ellipse, the circle represents materials with a finite polarization (ferroelectrics) and/or a finite magnetization (ferro- and ferrimagnets). Depending on the definition, multiferroics correspond to the intersection between the ellipses or the circles. The small circle in the middle denotes systems exhibiting a magnetoelectric coupling: their magnetic properties can be controlled by an electric field, and reciprocally their ferroelectric response can be influence by a magnetic field. It is this possibility to control magnetism by purely electrical means that makes multiferroics attractive for spintronics (e.g. to write magnetic information by a voltage). This classification is inspired from the review of Eerenstein, Mathur and Scott.
BiFeO3 thin films
The growth of single phase BFO films is not easy as Bi is very volatile and because several very stable Fe oxide phases can form in addition to the perovskite phase. Some years ago we studied in detail the influence of growth conditions on the properties of BFO films. The figure shows the pressure-temperature phase diagram of BFO films grown by pulsed laser deposition. We found that single phase BFO can be formed in a narrow pressure-temperature window. Then the films are insulating, ferroelectric and antiferromagnetic: the center panel on the left shows a neutron diffraction (1/2 1/2 1/2) peak of a 70 nm BFO film grown on STO(001), indicating G-type antiferromagnetic order without the cycloidal modulation present in the bulk.
Outside of this window, Bi- or Fe-rich parasitic phases form. The top right inset shows scanning electron microscopy and conductive-tip AFM images of a BFO film with square and conductive Bi2O3 impurities. The bottom right inset presents a magnetic hysteresis curve of a BFO film with ferrimagnetic gamma-Fe2O3 impurities.
This research was carried out in collaboration with the CEA Saclay (D. Colson) and the Laboratoire Léon Brillouin (F. Ott, S. Petit, A. Bataille).
Some related publications :
H. Béa, M. Bibes, A. Barthélémy, K. Bouzehouane, E. Jacquet, A. Khodan, J.-P. Contour, S. Fusil, F. Wyczisk, A. Forget, D. Lebeugle, D. Colson, M. Viret
Influence of parasitic phases on the properties of BiFeO3 epitaxial thin films
Appl. Phys. Lett. 87, 072508 (2005)
H. Béa, M. Bibes, S. Fusil, K. Bouzehouane, E. Jacquet, K. Rode, P. Bencok, A. Barthélémy
Investigation on the origin of the magnetic moment of BiFeO3 thin films by advanced x-ray characterizations
Phys. Rev. B, 74, 020101(R) (2006)
H. Béa, M. Bibes S. Petit, J. Kreisel and A. Barthélémy
Structural distortion and magnetism of BiFeO3 epitaxial thin films: A Raman spectroscopy and neutron diffraction study
Philos. Mag. Lett. 87, 165 (2007)
Strain engineering of multiferroic thin films
Strain engineering has recently 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%), we found that the 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. Theory indicates that this is due to an interplay of polar and oxygen tilting instabilities, absent in most other systems. Here, the ferroelectric Tc gets very close to the antiferromagnetic ordering temperature, which should bring about giant magnetoelectric responses.
The figure shows the strain dependence of the ferroelectric Curie temperature Tc, the Néel temperature Tn and the activation temperatures for oxygen octaedra tilts. The solid lines corresponds to theory (effective Hamiltonien calculations) and the symbols to experimental data.
For even large compressive strain levels, we have found that 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), thus showing room-temperature physical properties very similar to those of bulk BFO, despite a very different crystalline structure. Interestingly, the Néel temperature is concomitant to a ferroelectric phase transition at about 380K, which should be accompanied by a strong magnetoelectric coupling.
This research is carried out in 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).
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)
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. The basic operation of such magnetoelectric random access memories (MERAMs) 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.
The figure shows a sketch of a possible MERAM element. The binary information is stored by the magnetization direction of the bottom ferromagnetic layer (blue), read by the resistance of the magnetic trilayer (Rp when the magnetizations of the two ferromagnetic layers are parallel), and written by applying a voltage across the multiferroic ferroelectric–antiferromagnetic layer (FE -AFM; green). If the magnetization of the bottom ferromagnetic layer is coupled to the spins in the multiferroic (small white arrows) and if the magnetoelectric coupling is strong enough, reversing the ferroelectric polarization P in the multiferroic changes the magnetic configuration in the trilayer from parallel to antiparallel, and the resistance from Rp to antiparallel (Rap). A hysteretic dependence of the device resistance with voltage is achieved (blue curve).
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.
After a lot of materials and device optimisation, 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 :
H. Béa, M. Bibes, F. Ott, B. Dupé, X.-H. Zhu, S. Petit, S. Fusil, C. Deranlot, K. Bouzehouane, and A. Barthélémy
Mechanisms of exchange bias with multiferroic BiFeO3 epitaxial thin films
Phys. Rev. Lett. 100, 017204 (2008)
M. Bibes and A. Barthélémy
Towards a magnetoelectric memory
Nature Mater. 7, 426 (2008)
J. Allibe, I. C. Infante, S. Fusil, K. Bouzehouane, E. Jacquet, C. Deranlot, M. Bibes, and A. Barthélémy
Coengineering of ferroelectric and exchange bias properties in BiFeO3 based heterostructures
Appl. Phys. Lett. 95, 182503 (2009)
J. Allibe, S. Fusil, K. Bouzehouane, C. Daumont, D. Sando, E. Jacquet, C. Deranlot, M. Bibes and A. Barthélémy
Room temperature electrical manipulation of giant magnetoresistance in spin valves exchange-biased with BiFeO3
Nano Lett. 12, 1141 (2012)