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D. Sando,
A. Agbelele,
D. Rahmedov,
J. Liu,
P. Rovillain,
C. Toulouse,
I. C. Infante,
A. P. Pyatakov,
S. Fusil,
E. Jacquet, [......],
D. Wang,
J-M. Le Breton,
M. Cazayous,
A. Sacuto,
J. Juraszek,
A. K. Zvezdin,
L. Bellaiche,
B. Dkhil,
A. Barthélémy,
M. Bibes
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ABSTRACT: Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Although this flurry of properties makes BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property—antiferromagnetism—has not been investigated extensively, especially in thin films. Here we bring insight into 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. Accordingly, we find that the magnonic response is entirely modified, with low-energy magnon modes being suppressed as strain increases. 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.
Nature Material 04/2013; advance online publication. · 32.84 Impact Factor
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D Sando,
A Agbelele,
D Rahmedov,
J Liu,
P Rovillain,
C Toulouse,
I C Infante,
A P Pyatakov,
S Fusil,
E Jacquet, [......],
D Wang,
J-M Le Breton,
M Cazayous,
A Sacuto,
J Juraszek,
A K Zvezdin,
L Bellaiche,
B Dkhil,
A Barthélémy,
M Bibes
[show abstract]
[hide abstract]
ABSTRACT: Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Although this flurry of properties makes BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property-antiferromagnetism-has not been investigated extensively, especially in thin films. Here we bring insight into 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. Accordingly, we find that the magnonic response is entirely modified, with low-energy magnon modes being suppressed as strain increases. 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.
Nature Material 04/2013; · 32.84 Impact Factor
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[show abstract]
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ABSTRACT: We measure the ferroelectric polarization of BiFeO_{3} films down to 3.6 nm using low energy electron and photoelectron emission microscopy. The measured polarization decays strongly below a critical thickness of 5-7 nm predicted by continuous medium theory whereas the tetragonal distortion does not change. We resolve this apparent contradiction using first-principles-based effective Hamiltonian calculations. In ultrathin films, the energetics of near open circuit electrical boundary conditions, i.e., an unscreened depolarizing field, drive the system through a phase transition from single out-of-plane polarization to nanoscale stripe domains. It gives rise to an average polarization close to zero as measured by the electron microscopy while maintaining the relatively large tetragonal distortion imposed by the nonzero polarization state of each individual domain.
Physical Review Letters 12/2012; 109(26):267601. · 7.37 Impact Factor
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[show abstract]
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ABSTRACT: THz-range dielectric spectroscopy and first-principle-based
effective-Hamiltonian molecular dynamics simulations were employed to elucidate
the dielectric response in the paraelectric phase of (Ba,Sr)TiO3 solid
solutions. Analysis of the resulting dielectric spectra suggests the existence
of a crossover between two different regimes: a higher-temperature regime
governed by the soft mode only versus a lower-temperature regime exhibiting a
coupled soft mode/central mode dynamics. Interestingly, a single
phenomenological coupling model can be used to adjust the THz dielectric
response in the entire range of the paraelectric phase (i.e., even at high
temperature). We conclude that the central peak is associated with thermally
activated processes, and that it cannot be discerned anymore in the dielectric
spectra when the rate of these thermally activated processes exceeds certain
characteristic frequency of the system.
11/2012;
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ABSTRACT: First-principles-based simulations are used to simulate the electrocaloric effect (ECE) in Ba(0.5)Sr(0.5)TiO(3) alloys. In analogy with experimental studies we simulate the effect directly and indirectly (via the use of Maxwell thermodynamics). Both direct and indirect simulations utilize the same atomistic framework that allows us to compare them in a systematic way and with an atomistic precision for the very first time. Such precise comparison allows us to provide a bridge between the atomistic and macroscopic descriptions of the ECE and identify the factors that may critically compromise or even destroy their equivalence. Our computational data reveal the intrinsic features of ECE in ferroelectrics with multiple ferroelectric transitions and confirm the potential of these materials to exhibit giant electrocaloric response. The coexistence of negative and positive ECE in one material as well as an unusual field-driven transition between them is predicted, explained at an atomistic level, and proposed as a potential way to enhance the electrocaloric efficiency.
Physical Review Letters 04/2012; 108(16):167604. · 7.37 Impact Factor
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C Daumont,
W Ren,
I C Infante, S Lisenkov,
J Allibe,
C Carrétéro,
S Fusil,
E Jacquet,
T Bouvet,
F Bouamrane,
S Prosandeev,
G Geneste,
B Dkhil,
L Bellaiche,
A Barthélémy,
M Bibes
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ABSTRACT: Epitaxial strain has recently emerged as a powerful means to engineer the properties of ferroelectric thin films, for instance to enhance the ferroelectric Curie temperature (T(C)) in BaTiO(3). However, in multiferroic BiFeO(3) thin films an unanticipated strain-driven decrease of T(C) was reported and ascribed to the peculiar competition between polar and antiferrodistortive instabilities. Here, we report a systematic characterization of the room-temperature ferroelectric and piezoelectric properties for strain levels ranging between -2.5% and +1%. We find that polarization and the piezoelectric coefficient increase by about 20% and 250%, respectively, in this strain range. These trends are well reproduced by first-principles-based techniques.
Journal of Physics Condensed Matter 03/2012; 24(16):162202. · 2.55 Impact Factor
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ABSTRACT: A first-principles-based effective Hamiltonian is used to investigate the thickness dependency of the size of straight-walled domains in ultrathin films made of the multiferroic BiFeO₃ (BFO) material. It is found that the Kittel law is followed, as in ferroelectric or ferromagnetic films. However, an original real-space decomposition of the different energetic terms of this effective Hamiltonian allows the discovery that the microscopic origins of such a law in BFO films dramatically differ from those in ferroelectric or ferromagnetic films. In particular, interactions between tilting of oxygen octahedra around the domain walls and magnetoelectric couplings near the surface (and away from the domain walls) play an important role in the observance of the Kittel law in the studied BFO films.
Physical Review Letters 10/2010; 105(14):147603. · 7.37 Impact Factor
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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,
B Dkhil
[show abstract]
[hide abstract]
ABSTRACT: We report the influence of epitaxial strain on the multiferroic phase transitions of BiFeO3 films. Using advanced characterization techniques and calculations we show that while the magnetic Néel temperature hardly varies, the ferroelectric Curie temperature TC decreases dramatically with strain. This is in contrast with the behavior of standard ferroelectrics where strain enhances the polar cation shifts and thus TC. We argue that this is caused by an interplay of polar and oxygen tilting instabilities and that strain can drive both transitions close together to yield increased magnetoelectric responses.
Physical Review Letters 07/2010; 105(5):057601. · 7.37 Impact Factor
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ABSTRACT: An atomistic scheme is used to study the dependence of nanodots and ultra-thin films made of the lead-free BiFeO<sub>3</sub> (BFO) multiferroic material on electrical boundary conditions. The ground-state patterns of the electrical dipoles, oxygen octahedral tiltings and magnetic dipoles are found to dramatically depend on such electrical boundary conditions, as well as on the dimensionality of the BFO nanostructure. These unusual dependencies are of large fundamental importance, and may open the door for novel devices with unprecedented performances.
High Performance Computing Modernization Program Users Group Conference (HPCMP-UGC), 2010 DoD; 07/2010
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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,
B Dkhil
Phys. Rev. Lett. 07/2010; 105(5):057601-.
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ABSTRACT: We report on the experimental and theoretical study of the lattice dynamics in Ba0.7Sr0.3TiO3 in 20-900 K temperature range. Complex dielectric response in the terahertz and far infrared range was evaluated from the time-domain THz transmission spectra and infrared reflectivity by means of damped harmonic oscillator models in classical and factorized form of the dielectric function. The anisotropic dielectric function in the ferroelectric phases was extracted using the Bruggeman model of the effective medium approximation. The fitted parameters of the temperature-dependent Slater-type vibrations (soft mode and central mode, CM) were compared with those obtained from ab initio simulations. The role of the lattice phonons and CM in the dielectric anomaly is discussed.
Phase Transitions 01/2010; 83(10-11):955. · 1.01 Impact Factor
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ABSTRACT: Ceramic Ba0.6Sr0.4TiO3 (BST-0.6) samples were studied in the broad spectral range of 10(6)-10(14) Hz by using several dielectric techniques in between 20 and 800 K. The dominant dielectric dispersion mechanism in the paraelectric phase was shown to be of strongly anharmonic soft-phonon origin. The whole soft-mode response in the vicinity of the ferroelectric transition was shown to consist of two coupled overdamped THz excitations, which show classical features of a coupled soft and central mode, known from many ferroelectric crystals with a dynamics near the displacive and order-disorder crossover. Similar behaviour has been recently revealed and theoretically simulated in pure BaTiO3 (see Ponomareva et al 2008 Phys. Rev. B 77 012102 and Hlinka et al 2008 Phys. Rev. Lett. 101 167402). Also for the BST system, this feature was confirmed by the theory based on molecular dynamics simulations with an effective first-principles Hamiltonian. In all the ferroelectric phases, additional relaxation dispersion appeared in the GHz range, assigned to ferroelectric domain-wall dynamics. The microwave losses were analysed from the point of view of applications. The paraelectric losses above 1 GHz are comparable with those in single crystals and appear to be of intrinsic multi-phonon origin. The ceramic BST system is therefore well suited for applications in the whole microwave range.
Journal of Physics Condensed Matter 11/2009; 21(47):474215. · 2.55 Impact Factor
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[show abstract]
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ABSTRACT: Ceramic Ba(0.6)Sr(0.4)TiO(3) (BST-0.6) samples were studied in the broad spectral range of 10(6)-10(14) Hz by using several dielectric techniques in between 20 and 800 K. The dominant dielectric dispersion mechanism in the paraelectric phase was shown to be of strongly anharmonic soft-phonon origin. The whole soft-mode response in the vicinity of the ferroelectric transition was shown to consist of two coupled overdamped THz excitations, which show classical features of a coupled soft and central mode, known from many ferroelectric crystals with a dynamics near the displacive and order-disorder crossover. Similar behaviour has been recently revealed and theoretically simulated in pure BaTiO(3) (see Ponomareva et al 2008 Phys. Rev. B 77 012102 and Hlinka et al 2008 Phys. Rev. Lett. 101 167402). Also for the BST system, this feature was confirmed by the theory based on molecular dynamics simulations with an effective first-principles Hamiltonian. In all the ferroelectric phases, additional relaxation dispersion appeared in the GHz range, assigned to ferroelectric domain-wall dynamics. The microwave losses were analysed from the point of view of applications. The paraelectric losses above 1 GHz are comparable with those in single crystals and appear to be of intrinsic multi-phonon origin. The ceramic BST system is therefore well suited for applications in the whole microwave range.
Journal of Physics Condensed Matter 11/2009; 21(47):474215. · 2.55 Impact Factor
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ABSTRACT: Properties of BiFeO_{3} under an electric field are simulated using an ab initio-based approach. Complex paths and anomalous phenomena occur, depending on the direction of the field. Examples of such phenomena are the rotations of the polarization and of the axis about which the oxygen octahedra tilt; isostructural transitions; disappearance and reappearance of the tilting of the oxygen octahedra; and reentrance into specific crystallographic classes.The magnetic order parameter is not always perpendicular to the polarization, especially when the tilting of the oxygen octahedra disappears. The governing "rule" is that the magnetic order parameter remains orthogonal to the axis about which the oxygen octahedra tilt.
Physical Review Letters 08/2009; 103(4):047204. · 7.37 Impact Factor
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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,
A Barthélémy
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ABSTRACT: In the search for multiferroic materials magnetic compounds with a strongly elongated unit-cell (large axial ratio c/a) have been scrutinized intensely. However, none was hitherto proven to have a switchable polarization, an essential feature of ferroelectrics. Here, we provide evidence for the epitaxial stabilization of a monoclinic phase of BiFeO3 with a giant axial ratio (c/a=1.23) that is both ferroelectric and magnetic at room temperature. Surprisingly, and in contrast with previous theoretical predictions, the polarization does not increase dramatically with c/a. We discuss our results in terms of the competition between polar and antiferrodistortive instabilities and give perspectives for engineering multiferroic phases.
Physical Review Letters 06/2009; 102(21):217603. · 7.37 Impact Factor
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ABSTRACT: Properties of BiFeO3 under high magnetic fields applied in the plane perpendicular to the polarization are investigated via an original first-principles-based effective Hamiltonian. Different phenomena are found, depending if the magnetic fields lie (a) along the initial direction of the antiferromagnetic vector, (b) perpendicular to it, or (c) in between these two latter directions. For instance, a spin-flop transition occurs for case (a), while a continuous transition occurs, for which both the antiferromagnetic vector and the field-induced magnetization rotate, for case (c). Such latter rotation leads to a controllable large enhancement of the magnetoelectric coefficient.
Phys. Rev. B. 01/2009; 79(1).
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ABSTRACT: An effective Hamiltonian scheme is developed to study finite-temperature properties of multiferroic BiFeO3. This approach reproduces very well (i) the symmetry of the ground state, (ii) the Néel and Curie temperatures, and (iii) the intrinsic magnetoelectric coefficients (that are very weak). This scheme also predicts (a) an overlooked phase above Tc approximately 1100 K that is associated with antiferrodistortive motions, as consistent with our additional x-ray diffractions, (b) improperlike dielectric features above Tc, and (c) that the ferroelectric transition is of first order with no group-subgroup relation between the paraelectric and polar phases.
Physical Review Letters 12/2007; 99(22):227602. · 7.37 Impact Factor
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Ingrid C.Infante, S Lisenkov,
B Dupé,
M. Bibes,
S. Fusil,
E. Jacquet,
G Geneste,
S. Petit,
A. Courtial,
J. Juraszek,
L Bellaiche,
A. Barthelemy,
B. Dkhil
[show abstract]
[hide abstract]
ABSTRACT: We report the influence of epitaxial strain on the multiferroic phase transitions of BiFeO3 films. Using advanced characterization techniques and calculations we show that while the magnetic Ne'el temperature hardly varies, the ferroelectric Curie temperature TC decreases dramatically with strain. This is in contrast with the behavior of standard ferroelectrics where strain enhances the polar cation shifts and thus TC. We argue that this is caused by an interplay of polar and oxygen tilting instabilities and that strain can drive both transitions close together to yield increased magnetoelectric responses.
Physical Review Letters.
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ABSTRACT: We report a temperature-dependent investigation of the multiferroic perovskite bismuth ferrite BiFeO3 (BFO) by using x-ray powder diffraction together with differential scanning calorimetry measurements. Our results provide evidence that the paraelectric phase above Tc=820 °C is not cubic but distorted and can be well refined in a monoclinic P21/m space group. An equivalent structure can be reconstructed based on the C2/m monoclinic space group and by assuming two types of bismuth sites. The marked change of the cell volume at Tc provides evidence for the first-order nature of the R3c-to-P21/m transition. The high-temperature P21/m phase is centrosymmetric and characterized by (i) strong oxygen octahedra tilting along the b axis; (ii) the occurrence of antiferroelectric displacements of the Fe cations; and (iii) an interesting lamellar structure characterized by two different types of BiO12 cages. The temperature-induced lamellar structure suggests a significant electronic rearrangement in terms of chemical bonding, which in turn might condition anisotropic electronic properties. The occurrence of a lamellar structure provides also an understanding of why BFO decomposes suddenly at higher temperatures. Finally, an anomaly in the evolution of the cell parameters at TN underlines the spin-lattice coupling in proximity of the magnetic transition.
Phys. Rev. B. 78(13).
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ABSTRACT: An ab initio scheme is developed, and first-principles calculations are performed, to investigate ferromagnetism in BiFeO3 (BFO) thick and ultrathin films. These systems all possess a weak magnetization that results from a spin canting (that is induced by the tilting of the oxygen octahedra) and that increases from 0 to ≃0.027μB as the temperature is decreased below the Neel temperature. Such findings contradict a suggestion that the coupling between magnetic dipoles and mismatch strain leads to the previously reported large values for the magnetization in BFO films. This spin canting is also found to be essential for the linear magnetoelectric effect to occur.
Phys. Rev. B. 81(14).
