arXiv subjects

Claude Ederer

Publications and source records attributed to Claude Ederer.

85 records · Page 5Linked to original sources

arXiv2008

Towards a microscopic theory of toroidal moments in bulk periodic crystals

We present a theoretical analysis of magnetic toroidal moments in periodic systems, in the limit in which the toroidal moments are caused by a time and space reversal symmetry breaking arrangement of localized magnetic dipole moments. We summarize the basic definitions for finite systems and address the question of how to generalize these definitions to the bulk periodic case. We define the toroidization as the toroidal moment per unit cell volume, and we show that periodic boundary conditions lead to a multivaluedness of the toroidization, which suggests that only differences in toroidization are meaningful observable quantities. Our analysis bears strong analogy to the modern theory of electric polarization in bulk periodic systems, but we also point out some important differences between the two cases. We then discuss the instructive example of a one-dimensional chain of magnetic moments, and we show how to properly calculate changes of the toroidization for this system. Finally, we evaluate and discuss the toroidization (in the local dipole limit) of four important example materials: BaNiF_4, LiCoPO_4, GaFeO_3, and BiFeO_3.

cond-mat.str-el↗

arXiv2007

Magnetic coupling in CoCr_2O_4 and MnCr_2O_4: an LSDA+U study

We present a first principles LSDA+U study of the magnetic coupling constants in the spinel magnets CoCr_2O_4 and MnCr_2O_4. Our calculated coupling constants highlight the possible importance of AA interactions in spinel systems with magnetic ions on both A and B sites. Furthermore, we show that a careful analysis of the dependence of the magnetic coupling constants on the LSDA+U parameters provides valuable insights in the underlying coupling mechanisms, and allows to obtain a quantitative estimate of the magnetic coupling constants. We discuss in detail the capabilities and possible pitfalls of the LSDA+U method in determining magnetic coupling constants in complex transition metal oxides.

cond-mat.mtrl-sci↗

arXiv2006

Origin of ferroelectricity in the multiferroic barium fluorides BaMF4

We present a first principles study of the series of multiferroic barium fluorides with the composition BaMF4, 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.

cond-mat.mtrl-sci↗

arXiv2006

BaNiF4: an electric field-switchable weak antiferromagnet

We show that in the antiferromagnetic ferroelectric BaNiF4 the Dzyaloshinskii-Moriya interaction leads to a small canting of the magnetic moments, away from the easy axis, resulting in a noncollinear magnetic structure. The canting corresponds to an additional "weak" antiferromagnetic order parameter whose orientation is determined by the polar structural distortion and can be reversed by switching the ferroelectric polarization with an electric field. Our results point the way to a more general coupling mechanism between structural distortions and magnetic order parameters in magnetoelectric multiferroics which can be exploited in the design of electric field-switchable magnets.

cond-mat.mtrl-sci↗

arXiv2005

Recent progress in first-principles studies of magnetoelectric multiferroics

Materials that combine magnetic and ferroelectric properties have generated increasing interest over the last few years, due to both their diverse properties and their potential utility in new types of magnetoelectric device applications. In this review we discuss recent progress in the study of such magnetoelectric multiferroics which has been achieved using computational first-principles methods based on density functional theory. In particular, we show how first-principles methods have been successfully used to explain various properties of multiferroic materials and to predict novel effects and new systems that exhibit multiferroic properties.

cond-mat.mtrl-sci↗

arXiv2005

First principles study of the multiferroics BiFeO$_{3}$, Bi$_{2}$FeCrO$_{6}$, and BiCrO$_{3}$: Structure, polarization, and magnetic ordering temperature

We present results of an {\it ab initio} density functional theory study of three bismuth-based multiferroics, BiFeO$_{3}$, Bi$_{2}$FeCrO$_{6}$, and BiCrO$_{3}$. We disuss differences in the crystal and electronic structure of the three systems, and we show that the application of the LDA+$U$ method is essential to obtain realistic structural parameters for Bi$_{2}$FeCrO$_{6}$. We calculate the magnetic nearest neighbor coupling constants for all three systems and show how Anderson's 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.

cond-mat.mtrl-sci↗

arXiv2005

Effect of epitaxial strain on the spontaneous polarization of thin film ferroelectrics

We investigate the variation of the spontaneous ferroelectric polarization with epitaxial strain for BaTiO$_3$, PbTiO$_3$, LiNbO$_3$, and BiFeO$_3$ using first principles calculations. We find that while the strain dependence of the polarization is very strong in the simple perovskite systems BaTiO$_3$ and PbTiO$_3$ it is only weak in LiNbO$_3$ and BiFeO$_3$. We show that this different behavior can be understood purely in terms of the piezoelectric and elastic constants of the unstrained bulk material, and we discuss several factors that determine the strain behavior of a certain material.

cond-mat.mtrl-sci↗

arXiv2005

Influence of strain and oxygen vacancies on the magnetoelectric properties of multiferroic bismuth ferrite

The dependencies on strain and oxygen vacancies of the ferroelectric polarization and the weak ferromagnetic magnetization in the multiferroic material bismuth ferrite, BiFeO_3, are investigated using first principles density functional theory calculations. The electric polarization is found to be rather independent of strain, in striking contrast to most conventional perovskite ferroelectrics. It is also not significantly affected by oxygen vacancies, or by the combined presence of strain and oxygen vacancies. The magnetization is also unaffected by strain, however the incorporation of oxygen vacancies can alter the magnetization slightly, and also leads to the formation of Fe^{2+}. These results are discussed in light of recent experiments on epitaxial films of BiFeO_3 which reported a strong thickness dependence of both magnetization and polarization.

cond-mat.mtrl-sci↗

arXiv2004

Weak ferromagnetism and magnetoelectric coupling in bismuth ferrite

We analyze the coupling between the ferroelectric and magnetic order parameters in the magnetoelectric multiferroic BiFeO_3 using density functional theory within the local spin density approximation and the LSDA+U method. We show that weak ferromagnetism of the Dzyaloshinskii-Moriya type occurs in this material, and we analyze the coupling between the resulting magnetization and the structural distortions. We explore the possibility of electric-field-induced magnetization reversal and show that, although it is unlikely to be realized in BiFeO_3, it is not in general impossible. Finally we outline the conditions that must be fulfilled to achieve switching of the magnetization using an electric field.

cond-mat.mtrl-sci↗

arXiv2004

An experimental and computational investigation of structure and magnetism in pyrite Co$_{1-x}$Fe$_x$S$_2$: Chemical bonding and half-metallicity

Bulk samples of the pyrite chalcogenide solid solutions Co$_{1-x}$Fe$_x$S$_2$ 0 <= x <= 0.5, have been prepared and their crystal structures and magnetic properties studied by X-ray diffraction and SQUID magnetization measurements. Across the solution series, the distance between sulfur atoms in the persulfide (S$_2^{2-}$) unit remains nearly constant. First principles electronic structure calculations using experimental crystal structures as inputs point to the importance of this constant S-S distance, in helping antibonding S-S levels pin the Fermi energy. In contrast hypothetical rock-salt CoS is not a good half metal, despite being nearly isostructural and isoelectronic. We use our understanding of the Co$_{1-x}$Fe$_x$S$_2$ system to make some prescriptions for new ferromagnetic half-metals.

cond-mat.mtrl-sci↗

arXiv2003

Anisotropy of the orbital methods and the magnetic dipole term $T_z$ in ${\rm CrO_2}$: An {\it ab-initio} studt

A systematic study is performed by the {\it ab-initio} density functional theory of the anisotropy of the orbital moments $ $ and the magnetic dipole term $ $ in bulk ${\rm CrO_2}$. Two different band-structure techniques are used (FLAPW and LMTO-ASA), and the electronic correlations are treated by the local-spin-density approximation (LSDA), the LSDA+ orbital polarization method, and the LSDA+$U$ method. The calculated anisotropies of $ $ and $ $ are very large compared to Fe, Ni and Co but still a factor of 5 and 2 smaller than the anisotropies obtained from a recently suggested analysis of the X-ray magnetic circular dichroism spectra for a thick layer of ${\rm CrO_2}$.

cond-mat.mtrl-sci↗

arXiv2003

Magnetism in systems with various dimensionality: A comparison between Fe and Co

A systematic ab initio study is performed for the spin and orbital moments and for the validity of the sum rules for x-ray magnetic circular dichroism for Fe systems with various dimensionality (bulk, Pt-supported monolayers and monatomic wires, free-standing monolayers and monatomic wires). Qualitatively, the results are similar to those for the respective Co systems, with the main difference that for the monatomic Fe wires the term in the spin sum rule is much larger than for the Co wires. The spin and orbital moments induced in the Pt substrate are also discussed.

cond-mat.mtrl-sci↗

arXiv2002

From the bulk to monatomic wires: An ab-initio study of magnetism in Co systems with various dimensionality

A systematic ab-initio study within the framework of the local-spin-density approximation including spin-orbit coupling and an orbital-polarization term is performed for the spin and orbital moments and for the X-ray magnetic circular dichroism (XMCD) spectra in hcp Co, in a Pt supported and a free standing Co monolayer, and in a Pt supported and a free standing monatomic Co wire. When including the orbital-polarization term, the orbital moments increase drastically when going to lower dimensionality, and there is an increasing asymmetry between the L_2 and L_3 XMCD signal. It is shown that spin and orbital moments can be obtained with good accuracy from the XMCD spectra via the sum rules. The term of the spin sum rule is surprisingly small for the wires, and the reason for this is discussed.

cond-mat.mtrl-sci↗