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After decades of intensive research, perovskite oxides, more than ever, seem destined for a great future.

The interest in these transition-metal oxides arises from the fact that, even though they are based on the same simple cubic structure (Fig. 1a), perovskites with the nominal composition ABO3 can exhibit a wide diversity of behaviours. Large and tunable dielectric constants, piezoelectricity, ferroelectricity, colossal magnetoresistance, charge ordering, spin-dependent transport and superconductivity are examples of their most common properties, many of which are exploited in various technological applications for electronics, data storage or sensing. Lately, a tremendous amount of research has focused on multiferroics that combine ferroelectricity and magnetism. The search for strong coupling between these two properties has been fuelled by the dream of magnetically readable and electrically writable data storage. So far, bismuth ferrite (BiFeO3) has been the leading candidate among the oxides, being perhaps the only known single-phase room-temperature magnetoelectric multiferroic, but with a magnetic order that is not as easily controlled by an electric field as ideally required for applications. Reporting in Physical Review Letters, Benedek and Fennie1 now explore a new mechanism to achieve enhanced magnetoelectric coupling in perovskites.

In recent years, thanks to advances in deposition techniques, the interest in perovskites moved from bulk ceramics and single crystals to thin films, multilayers and superlattices. The functional properties of ABO3 compounds are strongly sensitive to external parameters and it was shown, for instance, that engineering of epitaxial strain is a powerful tool that can be used to tune the ferroelectric properties2 (Fig. 2) or induce multiferroism3. Going further and stacking different perovskites in epitaxial structures not only allows their properties to be combined but, sometimes, also totally new phenomena to be induced4. Recent examples include the metallic and superconducting interface found at the boundary between the two band insulators

LaAlO3 and SrTiO3 (ref. 4 and references therein) and the emergence of so-called improper ferroelectricity in ultrashort period PbTiO3/SrTiO3 superlattices5 (Fig. 1b). In the PbTiO3/SrTiO3 system, the ferroelectric polarization is not the primary driver of the phase transition, but arises from an unexpected coupling with other non-polar lattice instabilities.

The approach taken by Benedek and Fennie uses a similar type of coupling between lattice instabilities to achieve an unprecedented control over magnetization by an electric field. Their key idea is to consider compounds in which ferroelectricity and ferromagnetism are induced by the same lattice instability, which allows a robust and controllable

coupling of magnetization and ferroelectric polarization and, possibly, electric-field switching of the magnetization  — a dream for every researcher working on multiferroic materials.

The compound studied by the authors is Ca3Mn2O7, a naturally occurring layered perovskite of the Ruddlesden–Popper series (Fig. 1c). In their high-symmetry phase, ABO3 compounds typically develop two types of lattice instabilities, giving rise to a wide variety of phase transitions and ground states: a polar ferroelectric instability, and non-polar instabilities related to rotations and tilts of the oxygen octahedra. In simple perovskites, these instabilities usually compete with each other to produce a ground state with

MULTIFERROICS

Coupling of three lattice instabilitiesThe interaction between ferroelectric distortion and two rotational modes in some transition-metal oxides promises a strategy for strong magnetoelectronic coupling, possibly at room temperature.

Philippe Ghosez and Jean-Marc Triscone

O

a b c d

PbTiO3/SrTiO3superlattice

Ca3Mn2O7Ruddlesden–Popper

SrBi2Nb2O9Aurrivillius

Sr, Ca

Pb, Bi

Ti, Mn, Nb

ABO3perovskite

Figure 1 | Artificially and naturally layered perovskites. Schematic front view of a, the simple cubic perovskite structure, b, PbTiO3/SrTiO3 1/1 superlattice5, c, Ca3Mn2O7 Ruddlesden–Popper compound1 and d, SrBi2Nb2O9 Aurrivilius compound10.

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270 NATURE MATERIALS | VOL 10 | APRIL 2011 | www.nature.com/naturematerials

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ELECTRON MICROSCOPY

Hydrogen brightens upThe imaging mode of scanning transmission electron microscopy known as annular bright-field has reached enough sensitivity to image columns of the lightest of elements within a crystal.

Philip E. Batson

Since its invention in the late 1960s by Crewe and co-workers, annular dark-field (ADF) imaging in the scanning

transmission electron microscope (STEM) has been very successfully used to produce quantitative, atomic-level structure in thin sections of bulk materials1.With the recent introduction of aberration correction in the electron optics, ADF imaging routinely reveals single atoms of heavy elements in crystalline and amorphous materials2,3. A strength of this imaging mode is given

by a particular feature of ADF imaging, known as ‘Z-contrast’, which provides a strong sensitivity to the atomic number and therefore to the type of atoms in the material4. Unfortunately, when light and heavy elements are mixed in a solid, the weak ADF signal from the light element is completely hidden by the strong signal from the heavy element. Now, however, writing in Nature Materials, Ishikawa and co-workers show that through annular bright-field (ABF) imaging — which is

complementary to ADF — hydrogen atom columns in a crystal can clearly be resolved5. In particular they report results on a specimen of YH2, providing a detailed analysis of the imaging mode to explain its effectiveness. Their results augment similar work by others in VH2 (ref. 6), making clear how this technique will be valuable for more general use.

Annular dark-field STEM imaging was designed to suppress diffraction contrast in crystalline materials so that more subtle,

either ferroelectric polarization or oxygen distortions, with one of the only and most notable exceptions being BiFeO3 (ref. 6).

In Ca3Mn2O7, as in PbTiO3/SrTiO3 superlattices, the ground state includes both types of lattice distortions and arises from the condensation of one polar mode and two types of oxygen rotations. Moreover, the symmetry relationship between these three modes is such that it allows for an unusual trilinear energy coupling term, yielding improper ferroelectric behaviour7. In contrast to the example of epitaxial strain that can induce ferroelectricity by changing the shape of the energy versus polarization curve, here the rotations of the oxygen octahedra act as an effective field that induces a spontaneous polarization by shifting the energy versus polarization curve (Fig. 2). Benedek and Fennie coin the concept of hybrid improper ferroelectric for these systems, to highlight the fact that although in usual improper ferroelectrics a single primary order parameter induces the polarization8, here two independent rotational modes are required. In general, there is no guarantee that both these primary order parameters will condense at the same temperature to produce a unique phase transition. This can however eventually happen for sufficiently strong trilinear energy couplings9 and seems indeed to be the case in ultrashort period PbTiO3/SrTiO3 superlattices5 and, also in SrBi2Nb2O9, an Aurivillius layered compound (Fig. 1d), which exhibits a similar coupling between

polarization and two rotational modes, like SrBi2Ta2O9 (ref. 10).

Improper ferroelectrics are not just an academic curiosity as they can also lead to unusual and technologically useful properties, including large temperature-independent dielectric constants4 or strongly-reduced depolarization-field-driven size effects11. In their Letter, Benedeck and Fennie demonstrate on a concrete example that it is also a

fascinating practical pathway to realize a strong coupling between ferroelectricity and weak ferromagnetism12. This approach is general and could be transposed to other functional properties. Many efforts are now needed to validate these findings experimentally. This study also motivates the search of alternative systems taking full advantage of this concept. The wide diversity of naturally or artificially layered perovskites offers an amazing variety of possible materials combinations and essentially limitless, still mainly unexplored, possibilities. ❐

Philippe Ghosez is in the Department of Physics, Université de Liège, B-4000 Sart Tilman, Belgium. Jean-Marc Triscone is in the Department of Condensed Matter Physics, Université de Genève, CH-1211 Geneva, Switzerland. e-mail: Philippe.Ghosez@ulg.ac.be

References1. Benedek, N. A. & Fennie, C. J. Phys. Rev. Lett.

106, 107204 (2011). 2. Haeni, J. H. et al. Nature 430, 758–761 (2008).3. Lee, J. H. et al. Nature 466, 954–958 (2010).4. Zubko, P. et al. Annu. Rev. Condens. Matter Phys.

2, 141–165 (2011).5. Bousquet, E. et al. Nature 452, 732–736 (2008).6. Kornev, I. A. & Bellaiche, L. Phys. Rev. B 79, 100105(R) (2009).7. Levanyuk, A. P. & Sannikov, D. G. Usp. Fiz. 112, 561–589 (1974).8. Fennie, C. J. & Rabe, K. M. Phys. Rev. B 72, 100103 (2005).9. Etxebarria, I., Perez-Mato, J-M. & Boullay, P. Ferroelectrics

401, 17–23 (2010).10. Perez-Mato, J. M. et al. Phys. Rev. B 70, 214111 (2004).11. Sai, N., Fennie, C. J. & Demkov, A. A. Phys. Rev. Lett.

102, 107601 (2009).12. Fox, D. L., Tilley, D. L., Scott, J. F. & Guggenheim, H. J.

Phys. Rev. B 21, 2926–2936 (1980).

Ener

gy

Polarization

Non ferroelectricsProper ferroelectricsImproper ferroelectrics

Figure 2 | Typical energy versus polarization curves. Non-ferroelectric compounds can be made ferroelectric either by renormalizing the energy curvature at the origin to produce a double well as can be done via strain (red to blue curve) or by shifting the energy well through the linear coupling with primary non-polar mode(s) as it is done in (hybrid) improper ferroelectrics (red to green curve). In hybrid improper ferroelectrics, owing to the trilinear coupling term, switching the polarization implies the reversal of one of the rotational modes (green to dashed-green curve).

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