105 research outputs found
Energetics of the coupled electronic–structural transition in the rare-earth nickelates
No full textRare-earth nickelates exhibit a metal–insulator transition accompanied by a structural distortion that breaks the symmetry between formerly equivalent Ni sites. The quantitative theoretical description of this coupled electronic–structural instability is extremely challenging. Here, we address this issue by simultaneously taking into account both structural and electronic degrees of freedom using a charge self-consistent combination of density functional theory and dynamical mean-field theory, together with screened interaction parameters obtained from the constrained random phase approximation. Our total energy calculations show that the coupling to an electronic instability toward a charge disproportionated insulating state is crucial to stabilize the structural distortion, leading to a clear first order character of the coupled transition. The decreasing octahedral rotations across the series suppress this electronic instability and simultaneously increase the screening of the effective Coulomb interaction, thus weakening the correlation effects responsible for the metal–insulator transition. Our approach allows to obtain accurate values for the structural distortion and thus facilitates a comprehensive understanding, both qualitatively and quantitatively, of the complex interplay between structural properties and electronic correlation effects across the nickelate series
Combined first-principles and model Hamiltonian study of the perovskite series R MnO (R = La , Pr , Nd , Sm , Eu , and Gd)
No full textWe merge advanced ab initio schemes (standard density functional theory, hybrid functionals, and the GW approximation) with model Hamiltonian approaches (tight-binding and Heisenberg Hamiltonian) to study the evolution of the electronic, magnetic, and dielectric properties of the manganite family RMnO3 (R=La,Pr,Nd,Sm,Eu,and Gd). The link between first principles and tight binding is established by downfolding the physically relevant subset of 3d bands with eg character by means of maximally localized Wannier functions (MLWFs) using the VASP2WANNIER90 interface. The MLWFs are then used to construct a general tight-binding Hamiltonian written as a sum of the kinetic term, the Hund's rule coupling, the JT coupling, and the electron-electron interaction. The dispersion of the tight-binding (TB) eg bands at all levels are found to match closely the MLWFs. We provide a complete set of TB parameters which can serve as guidance for the interpretation of future studies based on many-body Hamiltonian approaches. In particular, we find that the Hund's rule coupling strength, the Jahn-Teller coupling strength, and the Hubbard interaction parameter U remain nearly constant for all the members of the RMnO3 series, whereas the nearest-neighbor hopping amplitudes show a monotonic attenuation as expected from the trend of the tolerance factor. Magnetic exchange interactions, computed by mapping a large set of hybrid functional total energies onto an Heisenberg Hamiltonian, clarify the origin of the A-type magnetic ordering observed in the early rare-earth manganite series as arising from a net negative out-of-plane interaction energy. The obtained exchange parameters are used to estimate the Néel temperature by means of Monte Carlo simulations. The resulting data capture well the monotonic decrease of the ordering temperature down the series from R=La to Gd, in agreement with experiments. This trend correlates well with the modulation of structural properties, in particular with the progressive reduction of the Mn-O-Mn bond angle which is associated with the quenching of the volume and the decrease of the tolerance factor due to the shrinkage of the ionic radii of R going from La to Gd
Strain-induced insulator-to-metal transition in d¹ perovskite systems within density functional theory plus dynamical mean field theory
No full textEffect of epitaxial strain on the spontaneous polarization of thin film ferroelectrics
No full textpeer-reviewedEpitaxial strain can substantially enhance the spontaneous polarizations and Curie temperatures of ferroelectric thin films compared to the corresponding bulk materials. In this Letter we use first principles calculations to calculate the effect of epitaxial strain on the spontaneous polarization of the ferroelectrics BaTiO3, PbTiO3, and LiNbO3, and the multiferroic material BiFeO3. We show that the epitaxial strain dependence of the polarization varies considerably for the different systems, and in some cases is, in fact, very small. We discuss possible reasons for this different behavior and show that the effect of epitaxial strain can easily be understood in terms of the piezoelectric and elastic constants of the unstrained materials. Our results provide a computational tool for the quantitative prediction of strain behavior in ferroelectric thin films
Strain-induced isosymmetric phase transition in BiFeO3
No full textWe calculate the effect of epitaxial strain on the structure and properties of multiferroic bismuth ferrite, BiFeO3, using first-principles density-functional theory. We investigate epitaxial strain corresponding to an (001)-oriented substrate and find that, while small strain causes only quantitative changes in behavior from the bulk material, compressive strains of greater than 4% induce an isosymmetric phase transition accompanied by a dramatic change in structure. In striking contrast to the bulk rhombohedral perovskite, the highly strained structure has a c/a ratio of similar to 1.3 and five-coordinated Fe atoms. We predict a rotation of polarization from [111] (bulk) to nearly [001], accompanied by an increase in magnitude of similar to 50%, and a suppression of the magnetic ordering temperature. Our calculations indicate critical strain values at which the two phases might be expected to coexist and shed light on recent experimental observation of a morphotropic phase boundary in strained BiFeO3
First principles study of the multiferroics BiFeO3, Bi2FeCrO6, and BiCrO3: Structure, polarization, and magnetic ordering temperature
No full textpeer-reviewedWe present results of an ab initio density-functional theory study of three bismuth-based multiferroics, BiFeO3, Bi2FeCrO6, and BiCrO3. We disuss differences in the crystal and electronic structure of the three systems and show that the application of the LDA+U method is essential to obtain realistic structural parameters for Bi2FeCrO6. We calculate the magnetic nearest-neighbor coupling constants for all three systems and show how Anderson\u27s theory of superexchange can be applied to explain the signs and relative magnitudes of these coupling constants. From the coupling constants we then obtain a mean-field approximation for the magnetic ordering temperatures. Guided by our comparison of these three systems, we discuss the possibilities for designing a multiferroic material with large magnetization above room temperature
Origin of ferroelectricity in the multiferroic barium fluorides BaM F4: A first principles study
No full textpeer-reviewedWe present a first-principles study of the series of multiferroic barium fluorides with the composition BaM F4, where M is Mn, Fe, Co, or Ni. We discuss trends in the structural, electronic, and magnetic properties, and we show that the ferroelectricity in these systems results from the freezing in of a single unstable polar phonon mode. In contrast to the case of the standard perovskite ferroelectrics, this structural distortion is not accompanied by charge transfer between cations and anions. Thus, the ferroelectric instability in the multiferroic barium fluorides arises solely due to size effects and the special geometrical constraints of the underlying crystal structure
Theory of x-ray magneto-optical effects and application to low-dimensional magnetic systems
No full textEs wurde ein Programm zur Berechnung magneto-optischer Effekte, insbesondere des magnetischen Röntgenzirkulardichroismus (XMCD), im Rahmen der Dichtefunktionaltheorie unter Verwendung der linearen "Muffin-Tin Orbital" Methode entwickelt. Dieses Programm wurde dann zur Untersuchung verschiedener niedrigdimensionaler magnetischer Systeme verwendet. Dabei wurde vor allem die Anwendbarkeit der sogenannten XMCD Summenregeln untersucht. Diese Summenregeln ermöglichen es, aus gemessenen XMCD Spektren die magnetischen Momente elementspezifisch und getrennt nach Spin- und Bahn-Anteil zu bestimmen. Während die Anwendbarkeit der Summenregeln für "bulk" Systeme sehr gut untersucht ist, ist bisher nicht klar, inwieweit sich diese Ergebnisse auch auf niedrigdimensionale Systeme übertragen lassen. Deshalb wurde in dieser Arbeit unter anderem eine systematische Untersuchung des Verhaltens der Spin- und Bahnmomente und der Anwendbarkeit der Summenregeln beim Übergang von dreidimensionalen "bulk" Systemen über zweidimensionale Monolagen bis hin zu eindimensionalen Ketten für die Übergangsmetalle Fe und Co durchgeführt. Es zeigte sich dabei, dass für Atome mit geringer lokaler Symmetrie der sogenannte magnetische Dipolterm, der in der Summenregel für das Spinmoment auftritt, nicht vernachlässigt werden kann. Diese Größe kann nur sehr schwer experimentell bestimmt werden und erschwert daher die Anwendung der Summenregeln erheblich. Außerdem wurde gezeigt, dass eine in der Vergangenheit vorgeschlagene Methode zur experimentellen Bestimmung dieses Terms für einige in dieser Arbeit untersuchte Systeme nicht anwendbar ist. Darüberhinaus wurde noch gezeigt, dass zur Beschreibung des orbitalen Magnetismus in niedrigdimensionalen Systemen die im Rahmen der Dichtefunktionaltheorie meistens benutzte Näherung, die lokale Dichte-Näherung, nicht ausreichend ist, und dass für eine realistische Beschreibung des orbitalen Magnetismus in diesen Systemen verbesserte Methoden verwendet werden müssen, wie z.B. die "orbital polarization" Methode oder die "LDA+U" Methode.A program has been developed for the calculation of magneto-optical effects, particularly x-ray magnetic circular dichroism (XMCD), based on the density functional theory by using the linear muffin-tin orbital method. This program then has been used for the investigation of several low-dimensional magnetic systems. Special attention has been paid to the applicability of the XMCD sum rules. These sum rules can be used for an element specific determination of both spin and orbital moments from the measured XMCD spectra. While the applicability of the sum rules is well established for bulk systems it is not clear if this is also the case for low-dimensional systems. Therefore, a systematic study of the spin and orbital moments and the applicability of the sum rules when going from three-dimensional bulk systems to two-dimensional monolayers and one-dimensional chains for the transition metals Fe and Co has been performed in this work. It turned out that for atoms with low local symmetry the so-called magnetic dipole term that appears in the spin sum rule is not negligible. Since it is very difficult to obtain this term experimentally, this is a serious difficulty for the application of the sum rules. It has also been shown that a recently proposed method for the experimental determination of this term is not applicable for some of the investigated systems. In addition, it has been shown that the most frequently used approximation in density functional theory, i.e. the local density approximation, is not sufficient for a realistic description of the orbital magnetism in these systems and that improved methods have to be used, e.g. the orbital polarization method or the LDA+U method
First principles studies of multiferroic materials
No full textpeer-reviewedMultiferroics, materials where spontaneous long-range magnetic and dipolar orders coexist, represent an attractive class of compounds, which combine rich and fascinating fundamental physics with a technologically appealing potential for applications in the general area of spintronics. Ab-initio calculations have significantly contributed to recent progress in this area, by elucidating different mechanisms for multiferroicity and providing essential information on various compounds where these effects are manifestly at play. In particular, here we present examples of density-functional theory investigations for two main classes of materials: a) proper multiferroics (where ferroelectricity is driven by hybridization or purely structural effects), with BiFeO3 as prototype material, and b) improper multiferroics (where ferroelectricity is driven by correlation effects and is strongly linked to electronic degrees of freedom such as spin, charge, or orbital ordering), with rare-earth manganites as prototypes. As for proper multiferroics, first-principles calculations are shown to provide an accurate qualitative and quantitative description of the physics in BiFeO3, ranging from the prediction of large ferroelectric polarization and weak ferromagnetism, over the effect of epitaxial strain, to the identification of possible scenarios for coupling between ferroelectric and magnetic order. For the class of improper multiferroics, ab-initio calculations have shown that, in those cases where spin-ordering breaks inversion symmetry (i.e. in antiferromagnetic E-type HoMnO3), the magnetically-induced ferroelectric polarization can be as large as a few _C/cm2. The presented examples point the way to several possible avenues for future research: On the technological side, first-principles simulations can contribute to a rational materials design, aimed at identifying spintronic materials that exhibit ferromagnetism and ferroelectricity at or above room-temperature. On the fundamental side, ab-initio approaches can be used to explore new mechanisms for ferroelectricity by exploiting electronic correlations that are at play in transition metal oxides, and by suggesting ways to maximize the strength of these effects as well as the corresponding ordering temperatures
Theory of induced magnetic moments and x-ray magnetic circular dichroism in Co-Pt multilayers
No full textpeer-reviewedFor Co-Pt multilayers, the magnetic moments of the Pt atoms and the x-ray magnetic circular dichroism (XMCD) spectra at the L2 and L3 edge of Pt are calculated-by the ab initio density-functional electron theory. The calculated magnetization profile for an ideal Co-Pt interface is in part different from the profile obtained by x-ray resonant magnetic reflectometry for the real interface. Some of the assumptions that are commonly adopted to determine the magnetic moments from the XMCD spectra via the sum rules are critically assessed for the Co-Pt system. It is shown that the orbital sum rule is strongly violated near the Co-Pt interface whereas the spin sum rule is approximately fulfilled provided the magnetic dipole term (Tz) is included in the analysis
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