Intra-atomic Exchange Interaction Research Articles

Evidence of the Anomalous Fluctuating Magnetic State by Pressure-Driven 4f Valence Change in EuNiGe3.

In rare-earth compounds with valence fluctuation, the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration and the magnetic moment. Here, we provide direct experimental evidence for an induced magnetic polarization of the Eu3+ atomic shell with J = 0, due to intra-atomic exchange and spin-orbital coupling interactions with the Eu2+ atomic shell. By applying external pressure, a transition from antiferromagnetic to a fluctuating behavior in EuNiGe3 single crystals is probed. Magnetic polarization is observed for both valence states of Eu2+ and Eu3+ across the entire pressure range. The anomalous magnetism is discussed in terms of a homogeneous intermediate valence state where frustrated Dzyaloshinskii-Moriya couplings are enhanced by the onset of spin-orbital interaction and engender a chiral spin-liquid-like precursor.

Macrospin model of an assembly of magnetically coupled core-shell nanoparticles

Highly sophisticated synthesis methods and experimental techniques allow for precise measurements of magnetic properties of nanoparticles that can be reliably reproduced using theoretical models. Here, we investigate the magnetic properties of ferrite nanoparticles by using theoretical techniques based on Monte Carlo methods. We introduce three stages of sophistication in the macromagnetic model. First, by using tailor-made hamiltonians we study single nanoparticles. In a second stage, the internal structure of the nanoparticle is taken into consideration by defining an internal (core) and external (shell) region, respectively. In the last stage, an assembly of core/shell NPs are considered. All internal magnetic couplings such as inter and intra-atomic exchange interactions or magnetocrystalline anisotropies have been estimated. Moreover, the hysteresis loops of the aforementioned three cases have been calculated and compared with recent experimental measurements. In the case of the assembly of nanoparticles, the hysteresis loops together with the zero-field cooling and field cooling curves are shown to be in a very good agreement with the experimental data. The current model provides an important tool to understand the internal structure of the nanoparticles together with the complex internal spin interactions of the core-shell ferrite nanoparticles.

A local-orbital density functional formalism for a many-body atomic Hamiltonian: Hubbard–Hund’s coupling and DFT + U functional

In the conventional DFT + U approach, the mean field solution of the Hubbard Hamiltonian associated with the d or f (iσ) electrons of a transition metal atom is used to define the DFT + U potential acting on the iσ-electrons. In this work, we go beyond that mean field solution by analyzing the correlation energy and potential for a multi-level atom described by a Kanamori Hamiltonian connected to different channels representing the environment. As a first step, we analyze the many-body solution of our model, using a local-orbital density functional formalism that takes as independent variables the orbital occupancies, n iσ , of the atomic orbitals; accordingly, we present the corresponding density functional solution describing the correlation energy and potential as a function of n iσ . Then, we use this analysis to introduce a DFT + U potential extending previous proposals to materials with arbitrarily high correlation. In particular, we find that this potential mainly screens the conventional mean field potential contribution, and also yields new terms associated with the number of atomic electrons. Our results show that the atomic correlation effects enhance the role played by the intra-atomic exchange interaction and favor the formation of magnetic solutions.

Intra-atomic exchange energy quantified

An article in Physical Review X reports the first experimental measurement of the intra-atomic exchange interaction in rare-earth atoms.

Many-Body Physics of Single and Double Spin-Flip Excitations in NiO.

Understanding many-body physics of elementary excitations has advanced our control over material properties. Here, we study spin-flip excitations in NiO using Ni L_{3}-edge resonant inelastic x-ray scattering (RIXS) and present a strikingly different resonant energy behavior between single and double spin-flip excitations. Comparing our results with single-site full-multiplet ligand field theory calculations we find that the spectral weight of the double-magnon excitations originates primarily from the double spin-flip transition of the quadrupolar RIXS process within a single magnetic site. Quadrupolar spin-flip processes are among the least studied excitations, despite being important for multiferroic or spin-nematic materials due to their difficult detection. We identify intermediate state multiplets and intra-atomic core-valence exchange interactions as the key many-body factors determining the fate of such excitations. RIXS resonant energy dependence can act as a convincing proof of existence of nondipolar higher-ranked magnetic orders in systems for which, only theoretical predictions are available.

Spin effect on electronic, magnetic and optical properties of spinel CoFe2O4: A DFT study

This report demonstrates the structural, electronic, magnetic, elastic, and optical properties of spinel CoFe2O4 using generalized gradient approximation (GGA). Both the spin and non-spin polarized density functional theory (DFT) have been used to study the influence of spin interactions on electronic structures, spin magnetic moments, and optical properties. The calculated magnetic moments of CoFe2O4 from spin density of states are 6.98 μB per formula unit. The Fe and Co ions prefer high spin orientations owing to the cationic polarization because of crystal field strength and intra-atomic exchange interactions, which induces large spin magnetic moments. The high values of spin magnetic moments confirm strong spin orbit coupling due to strong electron-electron interactions and can be a promising for spintronic application. Moreover, the calculated high reflectivity of CoFe2O4 material (~100%) in the Infrared-Visible-Ultraviolet region up to ~30 eV, which suggesting that the CoFe2O4 can also be a good candidate for solar reflector.

Suppression of magnetism in Ba5AlIr2O11 : Interplay of Hund's coupling, molecular orbitals, and spin-orbit interaction

The electronic and magnetic properties of Ba$_5$AlIr$_2$O$_{11}$ containing Ir-Ir dimers are investigated using the GGA and GGA+SOC calculations. We found that strong suppression of the magnetic moment in this compound recently found in [J. Terzic {\it et al.}, Phys. Rev. B {\bf 91}, 235147 (2015)] is not due to charge-ordering, but is related to the joint effect of the spin-orbit interaction and strong covalency, resulting in the formation of metal-metal bonds. They conspire and act against the intra-atomic Hund's rule exchange interaction to reduce total magnetic moment of the dimer. We argue that the same mechanism could be relevant for other $4d$ and $5d$ dimerized transition metal compounds.

Temperature magnetism of disordered iron–aluminum alloys in a model of two intra-atomic interactions

A system of band electrons with the two constants of intra-atomic exchange interaction simulates the different local environment of magnetic atoms in alloys. Temperature behavior of the magnetic characteristics of disordered Fe-Al alloys is investigated on the basis of an improved theory of dynamic fluctuations in electron spin density. The nature of the alloys’ different properties and the unusual temperature dependence of the magnetization and magnetic susceptibility is discussed.

Spin-rotational-invariant theory of transition-metal magnetism at finite temperatures: Systematic study of a single-site model

A spin-rotational-invariant approach to the spin-fluctuation theory of itinerant-electron magnetism is proposed and evaluated in the framework of a $d$-band model Hamiltonian including intra-atomic exchange interactions $J$ and the coupling to a local magnetic field $B$. Using a vector-field Hubbard-Stratonovich transformation, we obtain a static approximation to the density matrix operator from which the equilibrium properties are directly derived. The method is applied to a single-site model taking Fe as a representative example. Exact and approximate analytical results are given for the local magnetic moments, their longitudinal and transversal components, the field-induced magnetizations, entropy, and heat capacity. Goals and limitations of various approximations are discussed as a function of $J,\phantom{\rule{0.16em}{0ex}}B$, and temperature. The quantum-mechanical origin of some important drawbacks found in previous vector-field static approaches is identified. The significant improvements achieved with the static density operator are demonstrated.

Magnetism of Fe-Al Alloys in Spin Density Fluctuation Theory

The electron spin density fluctuation theory is developed for disordered alloys. The model contains two types of magnetic atoms characterized by different intra-atomic exchange interactions because of their different nearest neighborhood. The microscopic approach allow us to describe the various temperature behaviors of the magnetization of the Fe-Al alloys (25 - 32 at.% Al) observed in experiments. Competition parallel and antiparallel orientations of subsystems Fe(1) and Fe(2) gives rise to a diverse and anomalous temperature behavior of magnetism in partially and strongly disordered alloys.

Spin Fluctuations in Disordered Metallic Ferrimagnetic Alloys

We propose a theory for the quantitative description of the thermodynamic, magnetic and electronic properties of disordered alloys of transition metals and normal, for example, the system Fe-Al, as obtained from melting, and in the form of metastable states produced by mechanical grinding. System of itinerantd-electrons with two constants of the intra-atomic exchange interaction is investigated by the fluctuating field approach. Fundamentally new feature of the system is the presence of several ferrimagnetic solutions, transitions between which give variety to the temperature dependence of the magnetic characteristics of disordered alloys. Results of calculation of the temperature dependences of the magnetic characteristics of iron aluminum alloys are considered as an application of the theory.

Electronic transitions in GdN band structure

Using the near-infrared (NIR) absorbance spectroscopy, electronic transitions and spin polarization of the GdN epitaxial film have been investigated; and the GdN epitaxial film was grown by a reactive rf sputtering technique. The GdN film exhibited three broad bands in the NIR frequency regimes; and those bands are attributable primarily to the minority and majority spin transitions at the X-point and an indirect transition along the Γ-X symmetric direction of GdN Brillouin zone. We experimentally observe a pronounced red-shift of the indirect band gap when cooling down below the Curie temperature which is ascribed to the orbital-dependent coulomb interactions of Gd-5dxy electrons, which tend to push-up the N-2p bands. On the other hand, we have evaluated the spin polarization of 0.17 (±0.005), which indicates that the GdN epitaxial film has almost 100% spin-polarized carriers. Furthermore, the experimental result of GdN electronic transitions are consistent with the previous reports and are thus well-reproduced. The Arrott plots evidenced that the Curie temperature of GdN film is 36 K and the large spin moment is explained by the nitrogen vacancies and the intra-atomic exchange interaction.

Route to high Néel temperatures in 4dand 5dtransition metal oxides

There are very few magnetic members among the 4$d$ and 5$d$ transition metal oxides. In the present work, we examine the recent observation of a high N\'eel temperature ${T}_{N}$ in the 4$d$ oxides SrTcO${}_{3}$ and CaTcO${}_{3}$. Considering a multiband Hubbard model, we find that ${T}_{N}$ is larger in the limit of a large bandwidth and vanishingly small intra-atomic exchange interaction strength, contrary to our conventional understanding of magnetism. This is traced to specific aspects of the ${d}^{3}$ configuration at the transition metal site and the study reveals additional examples with high ${T}_{N}$.

Thermal conductivity, electron transport, and magnetic properties of single-crystalCa3Co2O6

A recent neutron-diffraction study on the magnetic insulator ${\text{Ca}}_{3}{\text{Co}}_{2}{\text{O}}_{6}$ shows that spins in the $c$-axis chains of alternating Co1 octahedral and Co2 trigonal-prismatic face-sharing ${\text{CoO}}_{6/2}$ sites are ordered below ${T}_{N}$ with a long-wavelength modulation instead of a simple ferromagnetic chain as thought previously. Frustrated antiferromagnetic interchain interactions compete with ferromagnetic intrachain coupling to invalidate an Ising model despite a strong spin-orbit coupling $\ensuremath{\lambda}\mathbf{L}\ensuremath{\cdot}\mathbf{S}$ on the Co2 ions. We have carried out thermal conductivity, electron transport, and magnetic measurements on single-crystal ${\text{Ca}}_{3}{\text{Co}}_{2}{\text{O}}_{6}$ in order to characterize this complicated magnetic system. An anisotropic thermal-conductivity component superimposed at high temperatures on an isotropic phonon component is consistent with an exchange-interaction-mediated heat transfer. This observation is incompatible with an Ising model. The ferromagnetic intrachain interactions are argued to reflect a one-dimensional itinerant-electron band that is split by the $c$-axis translational symmetry and the on-site intra-atomic Hund exchange interaction with a localized $S=3/2$ spin on the Co2 ions.

First-principles investigation of symmetric and antisymmetric exchange interactions ofSrCu2(BO3)2

We report on a first-principles investigation of the electronic structure and of the magnetic properties of the quasi-two-dimensional Mott insulator SrCu$_{2}$(BO$_{3}$)$_{2}$. Based on the hopping integrals and Coulomb interactions calculated with LDA and LSDA+U, we provide a microscopic explanation of the symmetric Heisenberg and antisymmetric Dzyaloshinskii-Moriya exchange integrals of SrCu$_{2}$(BO$_{3}$)$_{2}$. The intra-atomic exchange interaction of oxygen is shown to strongly contribute to the intra-dimer isotropic exchange. The results are in good agreement with those derived from experimental data, both regarding the orientation of the Dzyaloshinskii-Moriya vectors and the magnitude of all exchange integrals. The microscopic analysis is confirmed by the results of Green function's and total energies difference methods.

Rotationally invariant ab initio evaluation of Coulomb and exchange parameters for DFT+U calculations

Conventional density functional theory (DFT) fails for strongly correlated electron systems due to large intra-atomic self-interaction errors. The DFT+U method provides a means of overcoming these errors through the use of a parametrized potential that employs an exact treatment of quantum mechanical exchange interactions. The parameters that enter into this potential correspond to the spherically averaged intra-atomic Coulomb (U) and exchange (J) interactions. Recently, we developed an ab initio approach for evaluating these parameters on the basis of unrestricted Hartree-Fock (UHF) theory, which has the advantage of being free of self-interaction errors and does not require experimental input [Mosey and Carter, Phys. Rev. B 76, 155123 (2007)]. In this work, we build on that method to develop a more robust and convenient ab initio approach for evaluating U and J. The new technique employs a relationship between U and J and the Coulomb and exchange integrals evaluated using the entire set of UHF molecular orbitals (MOs) for the system. Employing the entire set of UHF MOs renders the method rotationally invariant and eliminates the difficulty in selecting unambiguously the MOs that correspond to localized states. These aspects overcome two significant deficiencies of our earlier method. The new technique is used to evaluate U and J for Cr(2)O(3), FeO, and Fe(2)O(3). The resulting values of U-J are close to empirical estimates of this quantity for each of these materials and are also similar to results of constrained DFT calculations. DFT+U calculations using the ab initio parameters yield results that are in good agreement with experiment. As such, this method offers a means of performing accurate and fully predictive DFT+U calculations of strongly correlated electron materials.

Oxygen Vacancy Induced Ferromagnetism in V2O5-x

Ab initio calculations within density functional theory with generalized gradient approximation have been performed to study the effects of oxygen vacancies on the electronic structure and magnetism in undoped V 2 O 5- x (0 < x < 0.5). It is found that the introduction of oxygen vacancies would induce ferromagnetism in V 2 O 5- x with the magnetization being proportional to the O vacancy concentration x . The calculated electronic structure reveals that the valence electrons released by the introduction of oxygen vacancies would occupy mainly the neighboring V d x y -dominant band which then becomes spin-polarized due to intra-atomic exchange interaction, thereby giving rise to the half-metallic ferromagnetism.

Ab initioevaluation of Coulomb and exchange parameters forDFT+Ucalculations

Conventional density functional theory (DFT) fails for materials with strongly correlated electrons, such as late transition metal oxides. Large errors in the intra-atomic Coulomb and exchange interactions are the source of this failure. The $\mathrm{DFT}+\mathrm{U}$ method has provided a means, through empirical parameters, to correct these errors. Here, we present a systematic ab initio approach in evaluating the intra-atomic Coulomb and exchange terms, $U$ and $J$, respectively, in order to make the $\mathrm{DFT}+\mathrm{U}$ method a fully first-principles technique. The method is based on a relationship between these terms and the Coulomb and exchange integrals evaluated in the basis of unrestricted Hartree-Fock molecular orbitals that represent localized states of the extended system. We used this ab initio scheme to evaluate $U$ and $J$ for chromia $({\mathrm{Cr}}_{2}{\mathrm{O}}_{3})$. The resulting values are somewhat higher than those determined earlier either empirically or in constrained DFT calculations but have the advantage of originating from an ab initio theory containing exact exchange. Subsequent $\mathrm{DFT}+\mathrm{U}$ calculations on chromia using the ab initio derived $U$ and $J$ yield properties consistent with experiment, unlike conventional DFT. Overall, the technique developed and tested in this work holds promise in enabling accurate and fully predictive $\mathrm{DFT}+\mathrm{U}$ calculations of strongly correlated electron materials.

Fluctuations of the magnetization in thin films due to conduction electrons

A detailed analysis of damping and noise due to a $sd$ interaction in a thin ferromagnetic film sandwiched between two large normal metal layers is carried out. The magnetization is shown to obey, in general, a nonlocal equation of motion, which differs from the the Gilbert equation and is extended to the nonadiabatic regime. To the lowest order in the exchange interaction and in the limit where the Gilbert equation applies, we show that the damping term is enhanced due to interfacial effects, but it also shows oscillations as a function of the film thickness. The noise calculation is, however, carried out to all orders in the exchange-coupling constant. The ellipticity of the precession of the magnetization is taken into account. The damping is shown to have a Gilbert form only in the adiabatic limit, while the relaxation time becomes strongly dependent on the geometry of the thin film. It is also shown that the induced noise characteristic of $sd$ exchange is inherently colored in character and depends on the symmetry of the Hamiltonian of the magnetization in the film. We show that the $sd$ noise can be represented in terms of an external stochastic field, which is white only in the adiabatic regime. The temperature is also renormalized by the spin accumulation in the system. For large intra-atomic exchange interactions, the Gilbert-Brown equation is no longer valid.

Large-cluster-model calculation of O 1s XAS for Y 2−xCa xBaNiO 5

The electronic structure of a quasi-one-dimensional nickelate, Y 2−xCa xBaNiO 5, is discussed on the basis of numerically-exact diagonalization calculations of O 1s XAS using a four-site one-dimensional cluster model. In the undoped nickelate ( x = 0 ), the band dispersion along the chain direction is suppressed owing the intra-atomic Coulomb and exchange interactions between 3d holes. As a result, the width of the O 1s XAS peak that corresponds to the transition to the upper Hubbard band (UHB) is very narrow. Upon hole-doping, two new peaks appear about 2 eV below the UHB in accordance with the experiment. The dependence on the incident-photon polarization is also consistent with the experiment.