References of "Ghosez, Philippe"
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See detailLarge elasto-optic effect and reversible electrochromism in multiferroic BiFeO3
Sando, D.; Yang, Yurong; Bousquet, Eric ULg et al

in Nature Communications (2016), 7

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous ... [more ▼]

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology—a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption—reminiscent of piezochromism—which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses. [less ▲]

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See detailThermoelectric properties of the unfilled skutterudite FeSb3 from first principles and Seebeck local probes
Lemal, Sébastien ULg; Ngoc, Nguyen; de Boor, Johannes et al

in Physical Review. B: Condensed Matter and Materials Physics (2015), 92

Using a combination of first-principles calculations and experimental transport measurements, we study the electronic and magnetic structure of the unfilled skutterudite FeSb3. We employ the hybrid ... [more ▼]

Using a combination of first-principles calculations and experimental transport measurements, we study the electronic and magnetic structure of the unfilled skutterudite FeSb3. We employ the hybrid functional approach for exchange-correlation. The ground state is determined to be anti-ferromagnetic with an atomic magnetic moment of 1.6 μB/Fe. The Néel temperature Tn is estimated at 6 K, in agreement with experiments which found a paramagnetic state down to 10 K. The ground state is semiconducting, with a small electronic gap of 33 meV, also consistent with previous experiments on films. Charge carrier concentrations are estimated from Hall resistance measurements. The Seebeck coefficient is measured and mapped using a scanning probe at room temperature that yields an average value of 38.6 μV/K, slightly lower than the theoretical result. The theoretical conductivity is analyzed as a function of temperature and concentration of charge carriers. [less ▲]

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See detailCoupling and electrical control of structural, orbital and magnetic orders in perovskites
Varignon, Julien ULg; Bristowe, Nicholas ULg; Bousquet, Eric ULg et al

in Scientific Reports (2015), 5

Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional ... [more ▼]

Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional electronic properties exhibited by perovskites is often the so-called Jahn-Teller distortion. For applications, an electrical control of the Jahn-Teller distortions, which is so far out of reach, would therefore be highly desirable. Based on universal symmetry arguments, we determine new lattice mode couplings that can provide exactly this paradigm, and exemplify the effect from firstprinciples calculations. The proposed mechanism is completely general, however for illustrative purposes, we demonstrate the concept on vanadium based perovskites where we reveal an unprecedented orbital ordering and Jahn-Teller induced ferroelectricity. Thanks to the intimate coupling between Jahn-Teller distortions and electronic degrees of freedom, the electric field control of Jahn-Teller distortions is of general relevance and may find broad interest in various functional devices. [less ▲]

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See detailOptimization of Thermoelectric Properties of MgAgSb-Based Materials: A First-Principles Investigation
Miao, Naihua ULg; Ghosez, Philippe ULg

in Journal of Physical Chemistry C (2015)

Recently, MgAgSb-based materials (MAS) have been proposed as promising candidates for room-temperature thermoelectric applications with a ZT larger than unity. In this work, we present a comprehensive ... [more ▼]

Recently, MgAgSb-based materials (MAS) have been proposed as promising candidates for room-temperature thermoelectric applications with a ZT larger than unity. In this work, we present a comprehensive theoretical study of the structural, electronic, and thermoelectric properties of MAS by combining first-principles calculations and Boltzmann transport theory. The predicted Seebeck coefficients are compared with available experimental data. The effects of crystal structure and volume on the electronic and thermoelectric properties of MAS are discussed. The thermoelectric quantities are optimized with respect to the chemical potential tuned by doping carriers. It is suggested that the thermoelectric performance of the α phase of MAS can be enhanced by hole doping and strain engineering. Our work intends to provide a theoretical support for future improvement on the thermoelectric performance of MAS and related materials. [less ▲]

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See detailUnderstanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments
Benedek, Nicole; Rondinelli, James; Djani-Ait, Hania ULg et al

in Dalton Transactions (Cambridge, England : 2003) (2015), 44(23), 10527-10890

ABO3 perovskites have fascinated solid-state chemists and physicists for decades because they display a seemingly inexhaustible variety of chemical and physical properties. However, despite the diversity ... [more ▼]

ABO3 perovskites have fascinated solid-state chemists and physicists for decades because they display a seemingly inexhaustible variety of chemical and physical properties. However, despite the diversity of properties found among perovskites, very few of these materials are ferroelectric, or even polar, in bulk. In this Perspective, we highlight recent theoretical and experimental studies that have shown how a combination of non-polar structural distortions, commonly tilts or rotations of the BO6 octahedra, can give rise to polar structures or ferroelectricity in several families of layered perovskites. We discuss the crystal chemical origin of the polarization in each of these families – which emerges through a so-called ‘trilinear coupling’ or ‘hybrid improper’ mechanism – and emphasize areas in which further theoretical and experimental investigation is needed. We also consider how this mechanism may provide a generic route for designing not only new ferroelectrics, but also materials with various other multifunctionalities, such as magnetoelectrics and electric field-controllable metal-insulator transitions. [less ▲]

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See detailModel of two-dimensional electron gas formation at ferroelectric interfaces
Aguado-Puente, P.; Bristowe, Nicholas ULg; Yin, B. et al

in Physical Review. B : Condensed Matter (2015), 92

The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it ... [more ▼]

The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it gives rise to. Here, we studyunderwhatcircumstancessimilarprocessescanalsotakeplaceunderneathferroelectricthinfilms.Weusea simpleLandaumodeltodemonstratethatintheabsenceofextrinsicscreeningmechanisms,amonodomainphase can be stabilized in ferroelectric films by means of an electronic reconstruction. Unlike in the LaAlO3/SrTiO3 heterostructure, the emergence with thickness of the free charge at the interface is discontinuous. This prediction is confirmed by performing first-principles simulations of free-standing slabs of PbTiO3. The model is also used to predict the response of the system to an applied electric field, demonstrating that the two-dimensional electron gas can be switched on and off discontinuously and in a nonvolatile fashion. Furthermore, the reversal of the polarization can be used to switch between a two-dimensional electron gas and a two-dimensional hole gas, which should, in principle, have very different transport properties. We discuss the possible formation of polarization domains and how such configuration competes with the spontaneous accumulation of free charge at the interfaces. [less ▲]

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See detailLow-Dimensional Transport and Large Thermoelectric Power Factors in Bulk Semiconductors by Band Engineering of Highly Directional Electronic States
Bilc, Daniel ULg; Hautier, Geoffroy; Waroquiers, David et al

in Physical Review Letters (2015), 114

Thermoelectrics are promising for addressing energy issues but their exploitation is still hampered by low efficiencies. So far, much improvement has been achieved by reducing the thermal conductivity but ... [more ▼]

Thermoelectrics are promising for addressing energy issues but their exploitation is still hampered by low efficiencies. So far, much improvement has been achieved by reducing the thermal conductivity but less by maximizing the power factor. The latter imposes apparently conflicting requirements on the band structure: a narrow energy distribution and a low effective mass. Quantum confinement in nanostructures andtheintroductionofresonantstatesweresuggestedaspossiblesolutionstothisparadox,butwithlimited success. Here, we propose an original approach to fulfill both requirements in bulk semiconductors. It exploits the highly directional character of some orbitals to engineer the band structure and produce a type of low-dimensional transport similar to that targeted in nanostructures, while retaining isotropic properties. Using first-principle calculations, the theoretical concept is demonstrated in Fe2YZ Heusler compounds, yielding power factors 4 to 5 times larger than in classical thermoelectrics at room temperature. Our findings are totally generic and rationalize the search of alternative compounds with similar behavior. Beyond thermoelectricity, these might be relevant also in the context of electronic, superconducting, or photovoltaic applications. [less ▲]

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See detailFerromagnetism induced by entangled charge and orbital orderings in ferroelectric titanate perovskites
Bristowe, Nicholas ULg; Varignon, Julien ULg; Fontaine, Denis et al

in Nature Communications (2015), 6

In magnetic materials, the Pauli exclusion principle typically drives anti-alignment between electron spins on neighbouring species resulting in antiferromagnetic behaviour. Ferromagnetism exhibiting ... [more ▼]

In magnetic materials, the Pauli exclusion principle typically drives anti-alignment between electron spins on neighbouring species resulting in antiferromagnetic behaviour. Ferromagnetism exhibiting spontaneous spin alignment is a fairly rare behaviour, but once materialized is often associated with itinerant electrons in metals. Here we predict and rationalize robust ferromagnetism in an insulating oxide perovskite structure based on the popular titanate series. In half-doped layered titanates, the combination of Jahn–Teller and oxygen breathing motions opens a band gap and creates an unusual charge and orbital ordering of the Ti d electrons. It is argued that this intriguingly intricate electronic network favours the elusive inter-site ferromagnetic (FM) ordering, on the basis of intra-site Hund's rules. Finally, we find that the layered oxides are also ferroelectric with a spontaneous polarization approaching that of ​BaTiO3. The concepts are general and design principles of the technologically desirable FM ferroelectric multiferroics are presented. [less ▲]

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See detailNovel magneto-electric multiferroics from first-principles calculations
Varignon, Julien ULg; Bristowe, Nicholas ULg; Bousquet, Eric ULg et al

in Comptes Rendus Physique (2015), 16

Interest in first-principles calculations within the multiferroic community has been rapidly on the rise over the last decade. Initially considered as a powerful support to explain experimentally observed ... [more ▼]

Interest in first-principles calculations within the multiferroic community has been rapidly on the rise over the last decade. Initially considered as a powerful support to explain experimentally observed behaviours, the trend has evolved and, nowadays, density functional theory calculations have become also an essential predicting tool for identifying original rules to achieve multiferroism and design new magneto-electric compounds. This chapter aims at highlighting the key advances in the field of multiferroics, to which first-principles methods have contributed significantly. The essential theoretical developments that made this research possible are also briefly presented. [less ▲]

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See detailFirst principles study of the structural, electronic and thermoelectric properties of misfit cobaltite
Lemal, Sébastien ULg; Varignon, Julien ULg; Bilc, Daniel ULg et al

Poster (2014, December 03)

In the context of environmental issues that become more and more prevalent in our society, there has been recently an increase of interest for thermoelectric (TE) materials, which have the property to ... [more ▼]

In the context of environmental issues that become more and more prevalent in our society, there has been recently an increase of interest for thermoelectric (TE) materials, which have the property to convert heat into electricity, and vice-versa. Although they do not display exceptional thermopower (in comparison to best thermoelectric like bismuth telluride), oxide materials have attracted some attention for high-temperature TE applications, due to their high stability. Amongst them, CoO2-layered compounds were proposed as good p-type TE candidates. Still, these compounds have been only poorly characterized both theoretically and experimentally. In this work we report a first-principles study of misfit calcium cobaltite (Ca2CoO3)(CoO2)1.618 based on density functional theory and an hybrid functional. The computed structural, electronic and magnetic properties match well the avalaible experimental data. Then the thermoelectric properties can be deduced using the Boltzmann transport formalism within the constant relaxation time approximation and will be discussed. [less ▲]

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See detailPhase diagram of BiFeO3/LaFeO3 superlattices studied by x-ray diffraction experiments and first-principles calculations
Rispens, Gijsbert; Ziegler, Benedikt; Zanolli, Zeila ULg et al

in Physical Review. B, Condensed Matter and Materials Physics (2014), 90

Combining structural and functional measurements, we have mapped the phase diagram of BiFeO3/LaFeO3 superlattices grown by off-axis sputtering on (110)o DyScO3 substrates. The phase diagram displays three ... [more ▼]

Combining structural and functional measurements, we have mapped the phase diagram of BiFeO3/LaFeO3 superlattices grown by off-axis sputtering on (110)o DyScO3 substrates. The phase diagram displays three distinct regions as a function of BiFeO3 fraction, with a BiFeO3-like ferroelectric phase and a LaFeO3-like paraelectric phase at its extremities, and a complex intermediate region, as supported by first-principles calculations. This intermediate region shows unusual, mixed functional behavior, most likely due to competing phases driven by substitution with a same-size central ion and the specific boundary conditions imposed by the superlattice structure. In the BiFeO3 rich superlattices, scaling of the ferroelectric-to-paraelectric transition temperature with the BiFeO3 thickness could provide an alternate route for studying ferroelectric size effects in BiFeO3. [less ▲]

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See detailFirst-principles study of PbTiO3 under uniaxial strains and stresses
Sharma, Henu; Kreisel, Jens; Ghosez, Philippe ULg

in Physical Review. B : Condensed Matter (2014), 90

The behavior of PbTiO3 under uniaxial strains and stresses is investigated from first-principles calculations within density functional theory. We show that irrespective of the uniaxial mechanical ... [more ▼]

The behavior of PbTiO3 under uniaxial strains and stresses is investigated from first-principles calculations within density functional theory. We show that irrespective of the uniaxial mechanical constraint applied, the system keeps a purely ferroelectric ground state, with the polarization aligned either along the constraint direction (FEz phase) or along one of the pseudocubic axes perpendicular to it (FEx phase). This contrasts with the cases of isotropic and biaxialmechanical constraints for which novel phases combining ferroelectric and antiferrodistortive motions have been previously reported. Under uniaxial strain, PbTiO3 switched from an FEx ground state under compressive strain to an FEz ground state under tensile strain beyond a critical strain ηc zz ≈ +1%. Under uniaxial stress, PbTiO3 exhibits either an FEx ground state under compression (σzz < 0) or an FEz ground state under tension (σzz > 0). Here, however, an abrupt jump of the structural parameters is also predicted under both compressive and tensile stresses at critical values σzz ≈ +2 and −8 GPa. This behavior appears to be similar to that predicted under negative isotropic pressure and might turn out to be practically useful for enhancing the piezoelectric response in nanodevices. [less ▲]

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See detailReactive Walls
Ghosez, Philippe ULg; Triscone, Jean-Marc

in Nature (2014), 515

Domain walls are natural borders in ferromagnetic, ferroelectric or ferroelastic materials. It seems that they can also be reactive areas that produce crystallographic phases never before observed in bulk ... [more ▼]

Domain walls are natural borders in ferromagnetic, ferroelectric or ferroelastic materials. It seems that they can also be reactive areas that produce crystallographic phases never before observed in bulk materials. [less ▲]

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See detailAtomically precise interfaces from non-stoichiometric deposition
Nie, Y. F.; Zhu, Y.; Lee, C.-H. et al

in Nature Communications (2014), 5

Complex oxide heterostructures display some of the most chemically abrupt, atomically precise interfaces, which is advantageous when constructing new interface phases with emergent properties by ... [more ▼]

Complex oxide heterostructures display some of the most chemically abrupt, atomically precise interfaces, which is advantageous when constructing new interface phases with emergent properties by juxtaposing incompatible ground states. One might assume that atomically precise interfaces result from stoichiometric growth. Here we show that the most precise control is, however, obtained by using deliberate and specific non-stoichiometric growth conditions. For the precise growth of Srnþ1TinOnþ1 Ruddlesden–Popper (RP) phases, stoichiometric deposition leads to the loss of the first RP rock-salt double layer, but growing with a strontium-rich surface layer restores the bulk stoichiometry and ordering of the subsurface RP structure. Our results dramatically expand the materials that can be prepared in epitaxial heterostructures with precise interface control—from just the n¼N end members (perovskites) to the entire RP homologous series—enabling the exploration of novel quantum phenomena at a richer variety of oxide interfaces. [less ▲]

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See detailFirst-principles characterization of the P21ab ferroelectric phase of Aurivillius Bi2WO6
Djani-Ait, Hania ULg; Hermet, Patrick ULg; Ghosez, Philippe ULg

in Journal of Physical Chemistry C (2014), 118

The structural, dielectric, dynamical, elastic, piezoelectric and nonlinear optical (second-order susceptibility and Pockels tensors) properties of Bi2WO6 in its P21ab ferroelectric ground state are ... [more ▼]

The structural, dielectric, dynamical, elastic, piezoelectric and nonlinear optical (second-order susceptibility and Pockels tensors) properties of Bi2WO6 in its P21ab ferroelectric ground state are determined using density functional theory. The calculation of infrared and Raman spectra on single crystal allowed us to clarify the assignment of experimental phonon modes, considering the good agreement between the calculated and the experimental Raman spectra obtained on polycrystal. The calculation of the elastic constants con rms the elastic stability of the crystal and allow us to estimate the Young and shear moduli of polycrystalline samples. The piezoelectric constants have signi cant intrinsic values comparable to those of prototypical ABO3 ferroelectrics. The electro-optic response is strongly dominated by the ionic contribution of transverse optic modes, yielding sizable Pockels coe cients around 9 pm/V along the polar direction. [less ▲]

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See detailThe origin of two-dimensional electron gases at oxide interfaces: insights from theory
Bristowe, Nicholas ULg; Ghosez, Philippe ULg; Littlewood, P.B. et al

in Journal of Physics : Condensed Matter (2014), 26

The response of oxide thin films to polar discontinuities at interfaces and surfaces has generated enormous activity due to the variety of interesting effects that it gives rise to. A case in point is the ... [more ▼]

The response of oxide thin films to polar discontinuities at interfaces and surfaces has generated enormous activity due to the variety of interesting effects that it gives rise to. A case in point is the discovery of the electron gas at the interface between LaAlO3 and SrTiO3, which has since been shown to be quasi-two-dimensional, switchable, magnetic and/or superconducting. Despite these findings, the origin of the two-dimensional electron gas is highly debated and several possible mechanisms remain. Here we review the main proposed mechanisms and attempt to model expected effects in a quantitative way with the ambition of better constraining what effects can/cannot explain the observed phenomenology. We do it in the framework of a phenomenological model constructed to provide an understanding of the electronic and/or redox screening of the chemical charge in oxide heterostructures. We also discuss the effect of intermixing, both conserving and not conserving the total stoichiometry. [less ▲]

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See detailDoping-dependent band structure of LaAlO3/SrTiO3 interfaces by soft x-ray polarization-controlled resonant angle-resolved photoemission
Cancellieri, C.; Reinle-Schmitt, M.L.; Kobayashi, M. et al

in Physical Review B (2014), 89

Polarization-controlled synchrotron radiation was used to map the electronic structure of buried conducting interfaces of LaAlO3/SrTiO3 in a resonant angle-resolved photoemission experiment. A strong ... [more ▼]

Polarization-controlled synchrotron radiation was used to map the electronic structure of buried conducting interfaces of LaAlO3/SrTiO3 in a resonant angle-resolved photoemission experiment. A strong polarization dependence of the Fermi surface and band dispersions is demonstrated, highlighting different Ti 3d orbitals involved in two-dimensional (2D) conduction. Measurements on samples with different doping levels reveal different band occupancies and Fermi-surface areas.The photoemission results are directly compared with advanced first-principles calculations, carried out for different 3d-band filling levels connected with the 2D mobile carrier concentrations obtained from transport measurements, with indication of charge localization at the interface. [less ▲]

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See detailLinear electro-optic effect in multiferroic BiFeO3 thin films
Sando, D.; Hermet, Patrick ULg; Allibe, J. et al

in Physical Review B (2014), 89

Multiferroics are materials with coexisting magnetic and ferroelectric orders, which show potential for electrically controlled spintronic devices. A common application of ferroelectrics is in electro ... [more ▼]

Multiferroics are materials with coexisting magnetic and ferroelectric orders, which show potential for electrically controlled spintronic devices. A common application of ferroelectrics is in electro-optical modulators exploiting their electric-field-dependent optical indices. The coupling of optical and magnetic degrees of freedom is attractive for designing multifunctional devices, but to date the electro-optical response of multiferroics has hardly been explored. Here we report a joint experimental and theoretical study of this effect in multiferroic BiFeO3 thin films. We confirm the large birefringence present in single crystals and determine the electro-optic coefficients r13 and r33. We present approaches to increase the obtained coefficients, for instance, by using tetragonal-like BiFeO3, and expand the potential of multiferroics to optical applications. [less ▲]

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See detailFirst-principles study of the lattice dynamical properties of strontium ruthenate
Miao, Naihua ULg; Bristowe, Nicholas ULg; Xu, Bin et al

in Journal of Physics : Condensed Matter (2014), 26

By means of first-principles calculations, various properties of SrRuO3 are investigated, focusing on its lattice dynamical properties. Despite having a Goldschmidt tolerance factor very close to 1, the ... [more ▼]

By means of first-principles calculations, various properties of SrRuO3 are investigated, focusing on its lattice dynamical properties. Despite having a Goldschmidt tolerance factor very close to 1, the phonon dispersion curves of the high-temperature cubic phase of SrRuO3 show strong antiferrodistortive instabilities. The energetics of metastable phases with different tilt patterns are discussed, concluding that the coupling of oxygen rotation modes with anti-polar Sr motion plays a key role in stabilizing the Pnma phase with respect to alternative rotation patterns. Our systematic analysis confirms previous expectations and contributes to rationalizing better why many ABO3 perovskites, including metallic compounds, exhibit an orthorhombic ground state. The zone-center phonon modes of the Pnma phase have been computed, from which we propose partial reassignment of available experimental data. The full dispersion curves have also been obtained, constituting benchmark results for the interpretation of future measurements and providing access to thermodynamical properties. [less ▲]

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See detailElectric control of the magnetization in BiFeO3/LaFeO3 superlattices
Zanolli, Zeila ULg; Wojdeł, Jacek; Iniguez, Jorge et al

in Physical Review B (2013), 88

First-principles techniques are used to investigate the behavior of BiFeO3/LaFeO3 perovskite oxide superlattices epitaxially grown on a (001)-SrTiO3 substrate. The calculations show that 1/1 superlattices ... [more ▼]

First-principles techniques are used to investigate the behavior of BiFeO3/LaFeO3 perovskite oxide superlattices epitaxially grown on a (001)-SrTiO3 substrate. The calculations show that 1/1 superlattices exhibit a Pmc21 ground state combining a trilinear coupling of one polar and two oxygen rotational lattice modes, and weak ferromagnetism. The microscopic mechanism allowing one to manipulate the magnetization with an electric field in such systems is presented and its dependence on strain and chemical substitution is discussed. BiFeO3/LaFeO3 artificial superlattices appear to be good candidates to achieve electric switching of magnetization at room temperature. [less ▲]

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