First-principles property assessment of hybrid formate perovskites.

Hybrid organic-inorganic formate perovskites, AB(HCOO)3, are a large family of compounds that exhibit a variety of phase transitions and diverse properties, such as (anti)ferroelectricity, ferroelasticity, (anti)ferromagnetism, and multiferroism. While many properties of these materials have already been characterized, we are not aware of any study that focuses on the comprehensive property assessment of a large number of formate perovskites. A comparison of the properties of materials within the family is challenging due to systematic errors attributed to different techniques or the lack of data. For example, complete piezoelectric, dielectric, and elastic tensors are not available. In this work, we utilize first-principles density functional theory based simulations to overcome these challenges and to report structural, mechanical, dielectric, piezoelectric, and ferroelectric properties of 29 formate perovskites. We find that these materials exhibit elastic stiffness in the range 0.5-127.0 GPa; highly anisotropic linear compressibility, including zero and even negative values; dielectric constants in the range 0.1-102.1; highly anisotropic piezoelectric response with the longitudinal values in the range 1.18-21.12 pC/N; and spontaneous polarizations in the range 0.2-7.8 µC/cm2. Furthermore, we propose and computationally characterize a few formate perovskites that have not been reported yet.

Unusual Properties of Hydrogen-Bonded Ferroelectrics: The Case of Cobalt Formate.

Hybrid organic-inorganic perovskites is a class of materials with diverse chemically tunable properties and outstanding potential for multifunctionality. We use first-principles simulations to predict room temperature ferroelectricity in a representative of the formate family, [NH_{2}NH_{3}][Co(HCOO)_{3}]. The ferroelectricity arises as a "by-product" of structural transition driven by the stabilization of the hydrogen bond. As a consequence the coupling with the electric field is relatively weak giving origin to large intrinsic coercive fields and making material immune to the depolarizing fields known for its detrimental role in nanoscale ferroelectrics. Insensitivity to the electric field and the intrinsic dynamics of the order-disorder transition in such material leads to the supercoercivity defined as significant increase in the coercive field with frequency. Room temperature polarization measurements provide further support for the predictions.

Emergence of ferroelectricity in antiferroelectric nanostructures.

First-principles-based finite-temperature simulations are used to predict the emergence of ferroelectricity in antiferroelectric nanostructures made of PbZrO3. The phenomenon is expected to occur in antiferroelectric nanodots, nanowires, and thin films with good surface charge compensation and can be explained by the recently proposed surface effect. Our computations provide a microscopic insight into the equilibrium phases, phase competition, and electrical properties of PbZrO3 nanostructures. The dependence of these properties on the electrical boundary conditions and nanostructure size is investigated.

Publisher's Note: Critical Thickness for Antiferroelectricity in PbZrO_{3} [Phys. Rev. Lett. 115, 097601 (2015)].

This corrects the article DOI: 10.1103/PhysRevLett.115.097601.

Publisher's Note: Thermally Mediated Mechanism to Enhance Magnetoelectric Coupling in Multiferroics [Phys. Rev. Lett. 114, 177205 (2015)].

This corrects the article DOI: 10.1103/PhysRevLett.114.177205.

Scaling law for electrocaloric temperature change in antiferroelectrics.

A combination of theoretical and first-principles computational methods, along with experimental evidence from the literature, were used to predict the existence of a scaling law for the electrocaloric temperature change in antiferroelectric materials. We show that the temperature change scales quadratically with electric field, allowing a simple transformation to collapse the set of ΔT(E) onto a single curve. This offers a unique method that can be used to predict electrocaloric behavior beyond the limits of present measurement ranges or in regions where data are not yet available.

Critical Thickness for Antiferroelectricity in PbZrO3.

Ferroelectrics and antiferroelectrics appear to have just the opposite behavior upon scaling down. Below a critical thickness of just a few nanometers the ferroelectric phase breaks into nanodomains that mimic electric properties of antiferroelectrics very closely. On the other hand, antiferroelectric thin films were found to transition from the antiferroelectric behavior to a ferroelectric one under certain growth conditions. At present, the origin of such a transition is controversial. Here, we use accurate first-principles-based finite-temperature simulations to predict the existence of a critical thickness for antiferroelectricity in the most celebrated antiferroelectric, PbZrO3. The origin of this effect is traced to the intrinsic surface contribution that has been previously overlooked. The existence of a critical thickness below which the antiferroelectric phase is replaced with a ferroelectric one not only complements the discovery of a critical thickness for ferroelectricity, but also suggests that ferroelectricity and antiferroelectricity are just two opposite manifestations of the same phenomenon: the material's tendency to develop a long-range order. Nanoscaling offers the opportunity to manipulate this order.

Thermally mediated mechanism to enhance magnetoelectric coupling in multiferroics.

The main roadblock on the way to practical realization of magnetoelectric devices is the lack of multiferroics with strong magnetoelectric coupling. We propose an unusual route to dramatically enhance this coupling through a thermally mediated mechanism. Such a thermally mediated magnetoelectric effect is quantified by an isentropic rather than isothermal magnetoelectric response and is computed here from first principles. A robust enhancement of the magnetoelectric coupling is predicted for both naturally occurring and heterostructured materials.

Control of ferroelectricity and magnetism in multi-ferroic BiFeO3 by epitaxial strain.

Recently, strain engineering has been shown to be a powerful and flexible means of tailoring the properties of ABO3 perovskite thin films. The effect of epitaxial strain on the structure of the perovskite unit cell can induce a host of interesting effects, these arising from either polar cation shifts or rotation of the oxygen octahedra, or both. In the multi-ferroic perovskite bismuth ferrite (BiFeO3-BFO), both degrees of freedom exist, and thus a complex behaviour may be expected as one plays with epitaxial strain. In this paper, we review our results on the role of strain on the ferroic transition temperatures and ferroic order parameters. We find that, while the Néel temperature is almost unchanged by strain, the ferroelectric Curie temperature strongly decreases as strain increases in both the tensile and compressive ranges. Also unexpected is the very weak influence of strain on the ferroelectric polarization value. Using effective Hamiltonian calculations, we show that these peculiar behaviours arise from the competition between antiferrodistortive and polar instabilities. Finally, we present results on the magnetic order: while the cycloidal spin modulation present in the bulk survives in weakly strained films, it is destroyed at large strain and replaced by pseudo-collinear antiferromagnetic ordering. We discuss the origin of this effect and give perspectives for devices based on strain-engineered BiFeO3.

Crafting the magnonic and spintronic response of BiFeO3 films by epitaxial strain.

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.

Bridging the macroscopic and atomistic descriptions of the electrocaloric effect.

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.

Strain dependence of polarization and piezoelectric response in epitaxial BiFeO3 thin films.

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.

Thickness-dependent polarization of strained BiFeO3 films with constant tetragonality.

We measure the ferroelectric polarization of BiFeO3 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.

Bridging multiferroic phase transitions by epitaxial strain in BiFeO3.

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.

Kittel law in BiFeO3 ultrathin films: a first-principles-based study.

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.

Electric-field-induced paths in multiferroic BiFeO3 from atomistic simulations.

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.

Evidence for room-temperature multiferroicity in a compound with a giant axial ratio.

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.

Broad-band dielectric spectroscopy and ferroelectric soft-mode response in the Ba(0.6)Sr(0.4)TiO(3) solid solution.

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.

Finite-temperature properties of multiferroic BiFeO3.

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.