M. BAADJI Nadjib

Density functional theory (DFT) and nonequilibrium Green’s function (NEGF) calculations were employed to elucidate the single-molecule conductance properties of a series of polynuclear alkynylplatinum-based organometallic wires, AcS[[(p-tol3P)2PtC≡C–C≡C–C≡C–C≡C]mPt(Pp-tol3)2SAc (m = 1–3), featuring platinum centers incorporated within polyyne chains. This investigation aimed to determine the influence of the embedded platinum centers on conductance compared to all-carbon polyynes and organometallic wires with terminal metal entities. While the structural and electronic properties of the individual molecular units are minimally affected by the chain extension, DFT-NEGF calculations on the corresponding Au-wire-Au junctions reveal an exponential decay in conductance with increasing molecular length, consistent with a tunneling-dominated transport mechanism. However, for longer wires (m = 3), the markedly low conductance and partial localization of frontier orbitals indicate that hopping transport could contribute to the overall transport mechanism. These findings provide valuable insights into the factors governing charge transport, including the interplay of length, platinum incorporation, and orbital localization in polynuclear alkynylplatinum-based organometallic wires, which may have implications for the future design of molecular wires.

The increasing demand for efficient and low-cost multifunctional materials in the fields of spintronics, solar cell technology, and thermoelectric applications is a big challenge. The exceptional electronic and magnetic properties of Heusler alloys have made them promising candidates, allowing for fine-tuning for specific applications This study investigates the elastic, structural, electronic, magnetic, optical, and thermoelectric properties of the FeMnSCAl quaternary Heuser alloy using density functional theory with the full-potential linearized augmented plane wave method via Wien2K software and BoltzTraP for thermoelectric analysis. Results confirm that FeMnScAl is thermodynamically and mechanically stable, asevide need byits negative formation energy and elastic constants meeting the stability criteria, with an optimized lattice parameter of 6.116 A. Density of statemand stasis reveal its half-metallic charace cal ultimoment of 3.00 uB follows the Slater-Pauling rule and stands for ferromagnetism. FeMnScAl also possesses special opticall properties, such as a narrow band gap of 0.677 eV, a high refractive index (2.63 < n(u) < 3.77), low reflectivity in the visible region, strong UV absorption, and superluminal effects. This provides

Electronic transport in Magnetic and molecular junctions

Intense UV emission is observed in Ce3+-doped GdBO3 nano-phosphor, successfully prepared by simple sol gel method. The synthetized nano-powder sample is characterized by diffractometry (XRD) and room temperature steady photoluminescence. The XRD analysis shows that the crystallite size is 55 nm and crystallizes in the vaterite structure. The room temperature excitation and emission observed bands are assigned to 4f ↔ 5d inter-configurational transitions and investigated. First principal Density Functional theory DFT + U calculations, implemented in VASP code was used to performed the study of the structural and electronic structure of GdBO3:Ce3+ nanomaterial system in both vaterite P63/mmc and calcite R-3C. The U parameter is varied to give best 4f Ce position in GdBO3 bandgap. Also, it was found that the R-3C is the ground state crystalline structure and that the observed P63/mmc is higher in energy by about 0.1 eV/atom. By comparison between experimental and calculations results energy Ce3+ energy levels scheme relative to electronic band structure was constructed.

We report on the transport properties of junctions based on the polar metal LiOsO3[1], using a combination of density functional theory and the non-equilibrium Green’s function [2,3]. We show that the ferroelectric like-transition from the high-temperature centro-symmetric crystalline structure (space group no 167) to a low-temperature non-centro-symmetric crystalline structure (space group no 161) [4] is accompanied by a change of the electric resistivity. The electronic and magnetic properties are also investigated. Furthermore, the interplay between the electronic correlation and the spin-orbit coupling and their effect on the electronic properties are studied and we show that the LiOsO3 is rather a weakly correlated material. The reduced conductance of LiOsO3 is attributed to its crystalline structure.

The all-electron full-potential linearized augmented plane-wave method is used to investigate the structural, electronic, and magnetic properties of the cubic C1 FeMnGa. We show that FeMnGa is a spin-gapless semiconductor, and by changing the lattice constant, we obtain a nearly compensated half-metallic ferrimagnet. This makes it suitable for spintronic devices. Then, a combined non-equilibrium Green’s function and density functional theory are used to show that both symmetric and asymmetric FeMnGa/Pd/FeMnGa junctions present infinite giant magneto-resistance, with higher conductance obtained for the symmetric junction with Fe–Pd interfaces.

Une étude théorique, basée sur la théorie de la fonctionnelle de la densité (DFT) pour la structure électronique combinée avec le formalisme de la fonction de Green hors équilibre (NEGF) pour le transport électronique, a été utilisée pour déterminer les propriétés électroniques et magnétiques de l’antiferromagnétique L10 MnPt et les jonctions à base de MnPt, notamment Au/MnPt/S/Fe/Au, où S est la couche séparatrice, soit métallique (Au) ou isolante (MgO). On a pu montrer que le magnétisme de l’antiferromagnétique MnPt est corrélé à sa structure cristalline. Donc le MnPt peut être utilisé dans un dispositif piezo-spintronique. En plus on a étudié le transport électronique dans les jonctions à base de l’AFM MnPt et on a montré pour les jonctions Au/MnPt/Au/Fe/Au et Au/MnPt/MgO/Fe/Au présentent une GAMR de 30 %, et une TAMR de 20 % respectivement et par conséquent l’existence deux états logiques distincts et stables 0 et 1 correspondant respectivement aux deux configurations parallèle et perpendiculaire. On a montré aussi que dans la jonction antiferromagnétique avec un isolant comme séparateur (MgO), on peut passer de l’état logique 1 à l’état logique 0 par un stimulus externe, et cela est du au couplage d’échange inter couches non colinéaire, qui présente une barrière largement inférieure à celle de la jonction où le séparateur est métallique (Au). Ce dernier couplage combiné avec l’anisotropie magnétique représentent les ingrédients suffisants pour déterminer la dynamique de spin.

Ferroelectric metal is a concept, initially proposed by Blount and Anderson [1], and identified recently, by Shi et al. [2]. Experimentally, The metallic bulk LiOsO 3 undergo a ferroelectric-like phase transition from a high-temperature centro-symmetric structure R-3C (non-polarized paraelectric state) to non-centro-symmetric phase R3C (a low-temperature “FE” polarized state). This distinctive characteristic offers a unique opportunity to investigate the interaction between free electrons and electric dipoles. However, the bad conductivity and enhancements from the electron contribution to the specific heat and the paramagnetic susceptibility indicate a strong electron correlation in LiOsO 3 [3] whereas it expected to be small in 5d transition metal. In addition, the strong spin-orbit coupling in 5d may explain the puzzling experimental fact is its paramagnetic state down to very low temperature with negligible magnetic moment[4]. Consequently, the electron correlation, spin-orbit coupling and crystal field splitting should be all treated of the same footing, as they are all on the scale of an electron volt. Here we show, using density functional theory that in presence of SOC and within the right crystalline structure we could explain most of the observed properties of the polar metal LiOsO 3 without resort to the electron correlations and that free electron model can not be used to describe the physical properties of the ferroelectric metal LiOsO 3 .

References :
[1] P. W. Anderson,& E. I. Blount, “Symmetry considerations on martensitic transformations: Ferroelectric metals?”, Phys. Rev. Lett. 14, 217219 (1965).
[2] Y. Shi, et al. “A ferroelectric-like structural transition in a metal”, Nature Materials 12, 1024 (2013)
[3] J.-S. Zhou, et al. “Strongly correlated electrons in the ferroelectric metal LiOsO3”, Phys. Rev. B 104, 115130 (2021).
[4] Yu Zhang et al. “Possible Origin of the Absence of Magnetic Order in LiOsO3: Spin–Orbit Coupling Controlled Ground State”, physica status solidi (RRL), 12 1800396 (2018)

In this work, we have developed a series of materials AFe1−xMnxO3 (A= La0.67Ca0.13Ba0.2) with x = 0, 0.03 and 0.06 by the self-combustion method using glycine and annealed at 1173 K. Analysis of X-ray diffractograms shows that these compounds crystallize in the same cubic structure with the same space group Pm̅m. Morphology and EDS analysis confirm the presence of all the elements and attests to the purity of these compounds. Electrical measurements show that at low temperatures, these compounds obey Jonscher's modified law, whereas above 420 K they obey Jonscher's classical law. The adjustment of the Nyquist diagrams leads to modeling these samples at low temperatures by two electric circuits connected in series (Rg//CPEg) + (Rgb//CPEgb) reflecting the contributions of the grains and the grain boundaries. Dielectric analysis indicates high permittivity and low loss suggesting that these compounds are good candidates for use in the electronic systems industry and supercapacitor applications.

We report the synthesis, structure, and magnetic investigation of a novel one-dimensional copper (II) coordination polymer: formula [Cu(μ1,1single bondN3)(μ1,3single bondOAc)(μ-DMSO)]∞ (1) (OAc=acetate). The connection between neighbouring copper ions arises through a triple bridge involving EO type azide, syn–syn carboxylate and μ-DMSO molecule in apical positions forming an elongated octahedral coordination of copper ions. Magnetic susceptibility measurements show that complex (1) exhibits ferromagnetic intrachain interactions Jchain = +66 cm−1 whereas a metamagnetic behaviour occurs below 2.1 K related with antiferromagnetic inter-chain interactions with a critical field of 600 Oe at 1.8 K. DFT calculations were caried out to rationale the in-chain ferromagnetic interactions between copper ions. We found that the ferromagnetic coupling between copper ions is less sensitive to variation of bond length and/or Cu-N-Cu angle that may be caused by external stimulus such as temperature or pressure.

We report on first-principles spin-polarised quantum transport calculations (from NEGF + DFT) in MgO-spaced magnetic tunnel junctions (MTJs) based on two different Mn-based Heusler ferrimagnetic metals, namely Mn3Al and Mn3Ga in their tetragonal DO22 phase. The former is a fully compensated half-metallic ferrimagnet, while the latter is a low-moment high-spin-polarisation ferrimagnet, both with a small lattice mismatch from MgO. In identical symmetric and asymmetric interface reconstructions across a 3-monolayer thick MgO barrier for both ferrimagets, the linear response (low-voltage) spin-transfer torque (STT) and tunneling magneto-resistance (TMR) effects are evaluated. A larger staggered in-plane STT is found in the Mn3Ga case, while the STT in Mn3Al vanishes quickly away from the interface (similarly to STT in ferromagnetic MTJs). The roles are reversed for the TMR, which is practically 100% in the half-metallic Mn3Al-based MTJs (using the conservative definition) as opposed to 60% in the Mn3Ga case. The weak dependence on the exact interface reconstruction would suggest Mn3Ga–Mn3Al solid solutions as a possible route towards optimal trade-off of STT and TMR in the low-bias, low-temperature transport regime.

The all-electron full-potential linearized augmented plane-wave method is used to show that the variation of tetragonality of the L1 0 MnPt antiferromagnet switches the magnetization from an easy axis to an easy plane. The optimum lattice parameters are obtained and the changing from inplane to out-of plane is explained by a competition between the strong spin orbit coupling and weak magnetization of Pt with the large magnetization and weak spin–orbit coupling of Mn. Finally we present the X-ray magnetic linear dichroism used to investigate the magnetic anisotropy of the antiferromagnet MnPt.

The mechanical properties of transition metal carbides Ti x M1–x C (0 ≤ x ≤ 1) alloys, with TM = Nb, V and Zr, were carried out using a first-principles pseudopotential calculation. The Ti substitution by a transition metal TM is assumed to completely random so we can use the virtual crystal approximation (VCA) for the disordered alloys. These calculation shows that the mechanical properties are largely affected by the alloying due to the interplay between ionic and covalent bonding of TM-C. The TiNbC and TiVC show an increase of the hardness with increasing x, while for TiZrC a decrease is observed. The variation of elastic constants is correlated to the electronic structure and the nature of bonding for TMC.

By considering the intrinsic anisotropy, present in almost all magnetic systems, as a perturbation to the usual Zeeman term, we show that the spin-spin dipolar interaction also known as zero-field splitting (ZFS) leads to an extra geometrical phase in addition to the conventional Berry's phase. Furthermore, we suggest some ways to observe the energy shift in electron paramagnetic resonance spectra due to Berry's phase and how we can separate it from the conventional Zeeman Berry's phase.

We have used the full potential linearized augmented plane waves (FP-LAPW) method to investigate the magnetic properties of Zn0.50Mn0.50S/Rh (001) system. Spin-orbit coupling and different spin-polarized states for the electronic structure are considering in LSDA potential. The L23-edges X-ray absorption (XAS) and X-ray magnetic dichroism (XMD) are calculated locally at Mn–Mn sites. The results show no circular dichroism spectra (leading to zero in sum for a spin up and spin down conspicuously of the independent Mn atoms), which is a behavior specific to an antiferromagnetic material. The system shows a perpendicular magnetic anisotropy of (664 ± 25) µeV. Hence, the magnetic moments are arranged antiparallel and reach the value of ±3.72 µB especially between nearest neighbor Mn atoms. This can be explaining by a large number of d-electrons available to be spin-polarized in the low-dimensional …

In the emerging fields of spintronics, spin-orbitronics and antiferromagnetic
spintronics, both degrees of liberty of electron, charge and spin, are used and their
current can be manipulated and even interchanged. A proper definition of charge and
spin currents are described and some calculations for the charge current are
performed in magnetic junctions with a ferromagnetic leads (spintronics) and
antiferromagnetic (AFM) junctions with a nonmagnetic leads. We show that even in
the absence of spin orbit coupling, AFM junction can lead to a detectable anisotropic
magnetoresistance due to the structural anisotropy. As a charge current is
accompanied by a heat current, where we also investigate the figure of merit ZT in
such junctions.

Simple sol-gel method was used to synthesize nanoscale yttrium phosphate-vanadates with general formula YP1−xVxO4: 1 at. % Tb3+ (x = 0–1.0). The nanopowder samples were characterized by X-ray diffraction (XRD), infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and room temperature steady and time resolved photoluminescence spectroscopy. The structural analysis reveals that the volume, the crystallite size of the nanophosphors increases with increasing vanadium content and their morphologies, changes from spherical, for low vanadium content, to rods for high content. Blue and green PL emissions of Tb3+and (VO4)3− were observed in YP1−xVxO4 and investigated in details. In addition, the role of Inter-Valence Charge transfer IVCT band in the quenching process of Tb3+ emission in YP1−xVxO4 is highlighted.

The strained body centered tetragonal (bct) Mn ultrathin film from lattice parameter a=2.89 Å to lattice value of 2.73 Å induces anti-ferromagnetic behavior between Mn layers. The magnetic easy axis of Mn film was demonstrated theoretically to switch from the in-plane to out-of-plane by magneto-optical Kerr effect investigation. By including spin–orbit coupling in full potential linearized augmented plane waves and linearized muffin-tin orbitals methods, manganese ultrathin film displays different magnetic behaviors and the spin-reorientation transition is shown to be correlated to these structural changes. The calculated magnetic moment of manganese planes are enhanced and reach a value of ~4.02 μB. The polar magneto-optical Kerr effect is calculated for a photon energy range extended to 15 eV. It shows a pronounced peak in visible light.

Mn 3 Ga bulk material and thin films deposited on several different substrates have been investigated using x-ray and neutron diffraction, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and electronic structure calculations using density-functional theory with the aim of determining the atomic site occupancy, magnetic moments, and magnetic structure of this tetragonal D0 22-structure compound. The Mn 3 Ga has close to ideal site occupancy, with Ga on 2 a sites and Mn on 2 b and 4 d sites. The magnetic structure is basically ferrimagnetic, with the larger Mn moment of about 3 μ B on the 2 b site, which is coordinated by 8 Mn 4 d and 4 Ga, and the smaller one on the 4 d site, which is coordinated by 4 Mn 2 b, 4 Ga, and 4 Mn 4 d. The Mn d-band occupancy is close to 5 on both sites, and the orbital moments are small,< 0.2 μ B. The material nevertheless exhibits substantial uniaxial anisotropy

Scanning tunneling microscopy (STM) can be used to detect inelastic spin transitions in magnetic nanostructures comprising only a handful of atoms. Here we demonstrate theoretically that STM can uniquely identify the electrostatic spin-crossover effect, whereby the exchange interaction between two magnetic centers in a magnetic molecule changes sign as a function of an external electric field. The fingerprint of such effect is a large drop in the differential conductance as the bias increases. Crucially in the case of a magnetic dimer the spin-crossover transition inverts the order between the ground state and the first excited state, but does not change their symmetry. This means that at both sides of the conductance drop associated to the spin-crossover transition there are two inelastic transitions between the same states. The corresponding conductance steps split identically in a magnetic field and provide a unique ...

We report on a density functional theory study demonstrating the coexistence of weak ferromagnetism and antiferroelectricity in boron-deficient MgB6. A boron vacancy produces an almost one dimensional extended molecular orbital, which is responsible for the magnetic moment formation. Then, long-range magnetic order can emerge from the overlap of such orbitals above percolation threshold. Although there is a finite density of states at the Fermi level, the localized nature of the charge density causes an inefficient electron screening. We find that the Mg2+ ions can displace from the center of their cubic cage, thus generating electrical dipoles. In the ground state these order in an antiferroelectric configuration. If proved experimentally, this will be the first material without d or f electrons displaying the coexistence of magnetic and electric order.

The electronic origin of a large resistance change in nanoscale junctions incorporating spin-crossover molecules is demonstrated theoretically by using a combination of density functional theory and the nonequilibrium Green’s function method for quantum transport. At the spin-crossover phase transition, there is a drastic change in the electronic gap between the frontier molecular orbitals. As a consequence, when the molecule is incorporated in a two-terminal device, the current increases by up to 4 orders of magnitude in response to the spin change. This is equivalent to a magnetoresistance effect in excess of 3000%. Since the typical phase transition critical temperature for spin-crossover compounds can be extended to well above room temperature, spin-crossover molecules appear as the ideal candidate for implementing spin devices at the molecular level.

An important development in recent synthesis strategies is the formation of electronically coupled one and two-dimensional organic systems for potential applications in nanoscale molecule-based devices. Here, we assemble one-dimensional spin chains by covalently linking basic molecular building blocks on a Au(111) surface. Their structural properties are studied by scanning tunneling microscopy and the Kondo effect of the basic molecular blocks inside the chains is probed by scanning tunneling spectroscopy. Tunneling spectroscopic images reveal the existence of separate Kondo regions within the chains while density functional theory calculations unveil antiferromagnetic coupling between the spin centers.

The conductance profiles of magnetic transition-metal atoms, such as Fe, Co, and Mn, deposited on surfaces and probed by a scanning tunneling microscope (STM), provide detailed information on the magnetic excitations of such nanomagnets. In general, the profiles are symmetric with respect to the applied bias. However, a set of recent experiments has shown evidence for inherent asymmetries when either a normal or a spin-polarized STM tip is used. In order to explain such asymmetries, here we expand our previously developed perturbative approach to electron-spin scattering to the spin-polarized case and to the inclusion of out of equilibrium spin populations. In the case of a magnetic STM tip, we demonstrate that the asymmetries are driven by the nonequilibrium occupation of the various atomic spin levels, an effect that is reminiscent of electron spin transfer. In contrast, when the tip is not spin polarized, such a nonequilibrium population cannot be built up. In this circumstance, we propose that the asymmetry simply originates from the transition metal ion density of states and from the uneven electronic coupling between the scattering magnetic atoms and both the tip and the substrate. These effects are included in the formalism as a nonvanishing real component to the spin-scattering self-energy.

Oriented thin films of Mn3Ge with the tetragonal D022 structure, grown on strontium titanate substrates, exhibit a low magnetization Ms = 73 kA m−1 combined with high uniaxial anisotropy Ku = 0.91 MJ m−3 at 300 K, making them potentially useful for thermally stable sub-10 nm spin torque memory elements. The highly ordered epitaxial Mn3Ge (001) films show 46% diffusive spin polarization at the Fermi level, measured by point contact Andreev reflection. Density functional calculations show that the compounds are ferrimagnetic, with anisotropic spin polarization at the Fermi level.

We present a theoretical study of the spin transport properties of monoatomic magnetic chains with a focus on the spectroscopical features of the I-V curve associated with spin-flip processes. Our calculations are based on the s-d model for magnetism with the electron transport treated at the level of the nonequilibrium Green’s function formalism. Inelastic spin-flip scattering processes are introduced perturbatively via the first Born approximation, and an expression for the associated self-energy is derived. The computational method is then applied to describe the I-V characteristics and its derivatives of one-dimensional chains of Mn atoms, and the results are then compared to available experimental data. We find a qualitative and quantitative agreement between the calculated and the experimental conductance spectra. Significantly we are able to describe the relative intensities of the spin excitation features in the I-V curve, by means of a careful analysis of the spin transition selection rules associated with the atomic chains.

Recent experimental advances in scanning tunneling microscopy make the measurement of the conductance spectra of isolated and magnetically coupled atoms on nonmagnetic substrates possible. Notably, these spectra are characterized by a competition between the Kondo effect and spin-flip inelastic electron tunneling. In particular they include Kondo resonances and a logarithmic enhancement of the conductance at voltages corresponding to magnetic excitations, two features that cannot be captured by second-order perturbation theory in the electron-spin coupling. We have now derived a third-order analytic expression for the electron-spin self-energy, which can be readily used in combination with the nonequilibrium Green’s function scheme for electron transport at finite bias. We demonstrate that our method is capable of semiquantitative description of the competition between Kondo resonances and spin-flip inelastic electron tunneling at a computational cost significantly lower than that of other approaches. The examples of Co and Fe on CuN are discussed in detail.

Spin scattering at the interface formed between metallic Fe and Cu-phthalocyanine molecules is investigated by spin-polarized scanning tunneling spectroscopy and spin-resolved photoemission. The results are interpreted using first-principles electronic structure theory. The combination of experimental and theoretical techniques allows us to shed light on the role of hybrid interface states for the spin scattering. We show that Cu-phthalocyanine acts, via hybrid interface states, as a local spin filter up to room temperature both below and above the Fermi energy, E F. At the same time, the molecule behaves as a featureless scattering barrier in a region of about 1 eV around E F. Similar properties are found for both single molecules and self-assembled molecular layers, so that the acquired microscopic knowledge can be transferred to operating devices.

This contribution concerning the effect of spin–orbit coupling on the magnetic properties of materials is divided into two sections. In the first section we review the method based on the density functional theory (DFT) within the local density approximation (LDA) used to compute the electronic structure, the magnetic anisotropy, the x-ray absorption spectra, and the x-ray magnetic circular dichroism. We give the major approximations used to derive the Kohn–Sham equations with or without the Hubbard interaction for correlated orbitals. We give also a brief introduction to the generalized gradient approximation (GGA). We then provide a solution of the latter equations using the full-potential linear augmented plane wave (FLAPW) basis set and discuss the so-called LDA+U method, where the Hubbard U is included for localized orbitals. We show how the relativistic effects, such as the spin–orbit coupling, can be introduced into band structure calculations and show their effect on magnetism, i.e., magnetic anisotropy energy (MAE), magnetooptical properties, and x-ray magnetic circular dichroism (XMCD). Then we show a brief derivation of the force theorem for the calculation of the magnetic anisotropy as well as a description of its application to the MAE calculations and show the details of the calculation of the XMCD matrix elements in the electric dipole approximation. The second section of this contribution includes some applications of the method to the computation of the electronic, magnetic, and spectroscopic properties of spintronics materials. In particular, we investigate the electronic structure and x-ray magnetic circular dichroism (XMCD) of Sr2FeMoO6 (SFMO for short) and other useful ferromagnetic half-metals with 100% spin polarization, materials useful for spin injection. In particular, we show that the spin–orbit coupling reduces the spin polarization, while the intra-site electronic correlations tend to increase it. For example, SFMO is found to be a half-metallic ferrimagnet with a gap in the spin-up channel. The calculated spin magnetic moments on iron and Mo sites confirm the ferromagnetic ordering and settle the controversy existing between the earlier experimental works. The orbital magnetism at the Fe and Mo sites agrees quite well with the recent experimental XMCD measurements. The computed L2,3 XMCD at the Fe and the Mo sites compares fairly well with the experiment. The XMCD sum rule computed spin and orbital magnetic moments are in good agreement with the values obtained from the direct self-consistent calculations. In the last application, we focus on the GGA+U treatment of the electronic and magnetic structure of Gd and Gd-related compounds, such as GdN and GdFe2. We compare the calculated density of states to the experimental photoemission and inverse photoemission spectra (XPS and BIS) and determine the Fermi surface with and without the Hubbard U and spin–orbit coupling. The GGA+U is found to be the most appropriate for treating the 4f Gd electrons. We have investigated the bulk properties and calculated the XMCD spectra at the L2,3 edges at the Gd site of GdN. The agreement of the calculated spectra with experiment is the indication of the relevance of the XMCD formalism within the one-electron picture. The results also show that the ground-state electronic structure of GdN is that of a half-metal. Finally our computational method is used to determine the magnetic anisotropy aspect of Gd and its compounds GdN and GdFe2. Using force theorem, we have calculated the MAE of Gd, GdN, and GdFe2 for different directions of the magnetization. Indeed, owing to the nil spin–orbit interaction of the 4f half-filled shell, the force theorem is expected to be efficient for Gd and Gd compounds’ MAE calculations. This theorem allows a considerable computational effort gain since the spin–orbit coupling could be calculated only for one self-consistent iteration. Once again, the GGA+U method is found to be the most adequate approach for the force theorem calculations of the Gd MAE. The GGA and GGA-core model treatments of the 4f states have led to a wrong MAE. It turns out that the electronic properties and the magnetic properties of 4f systems are tightly related, and the 4f electrons play a crucial role in the computed magnetic anisotropy. Although the Gd MAE is found to be similar to that of a typical 3d transition metal like hcp Co, the GdN and GdFe2 cubic crystal MAEs are found to be different from that of a pure 3d cubic material like fcc Ni.

This contribution concerning the effect of spin–orbit coupling on the magnetic properties of materials is divided into two chapters. In the first chapter we review the method based on the density functional theory (DFT) within the local density approximation (LDA) used to compute the electronic structure, the magnetic anisotropy, the x-ray absorption spectra, and the x-ray magnetic circular dichroism. We give the major approximations used to derive the Kohn–Sham equations with or without the Hubbard interaction for correlated orbitals. We give also a brief introduction to the generalized gradient approximation (GGA). We then provide a solution of the latter equations using the full-potential linear augmented plane wave (FLAPW) basis set and discuss the so-called LDA+U method, where the Hubbard U is included for localized orbitals. We show how the relativistic effects, such as the spin–orbit coupling, can be introduced into band structure calculations and show their effect on magnetism, i.e., magnetic anisotropy energy (MAE), magneto-optical properties, and x-ray magnetic circular dichroism (XMCD). Then we show a brief derivation of the force theorem for the calculation of the magnetic anisotropy as well as a description of its application to the MAE calculations and show the details of the calculation of the XMCD matrix elements in the electric–dipole approximation. The second chapter of this contribution includes some applications of the method to the computation of the electronic, magnetism, and spectroscopic properties of spintronics materials. In particular , we investigate the electronic structure and x-ray magnetic circular dichroism (XMCD) of Sr2FeMoO6 (SFMO for short) and other useful ferromagnetic half-metals with 100% spin polarization, materials useful for spin injection. In particular, we show that the spin–orbit coupling reduces the spin polarization while the intra-site electronic correlations tend to increase it. For example, SFMO is found to be a half-metallic ferrimagnet with a gap in the spin-up channel. The calculated spin magnetic moments on Fe and Mo sites confirm the ferromagnetic ordering and settle the controversy existing between the earlier experimental works. The orbital magnetism at the Fe and Mo sites agrees quite well with the recent experimental XMCD measurements. The computed L2,3 XMCD at the Fe and the Mo sites compares fairly well with experiment. The XMCD sum rule computed spin and orbital magnetic moments are in good agreement with the values obtained from the direct self-consistent calculations. In the last application, we focus on the GGA+U treatment of the electronic and magnetic structure of Gd and Gd-related compounds, such as GdN and GdFe2. We compare the calculated density of states to the experimental photoemission and inverse photoemission spectra (XPS and BIS) and determine the Fermi surface with and without the Hubbard U and spin–orbit coupling. The GGA+U is found to be the most appropriate for treating the 4f Gd electrons. We have investigated the bulk properties and calculated the XMCD spectra at the L2,3 edges at the Gd site of GdN. The agreement of the calculated spectra with experiment is the indication of the relevance of the XMCD formalism within the one-electron picture. The results also show that the ground-state electronic structure of GdN is that of a half-metal. Finally our computational method is used to determine the magnetic anisotropy aspect of the Gd and its compounds GdN and GdFe2. Using the force theorem, we have calculated the MAE of Gd, GdN, and GdFe2 for different directions of the magnetization. Indeed, owing to the nil spin–orbit interaction of the 4f half-filled shell, the force theorem is expected to be efficient for Gd and Gd compounds MAE calculations. This theorem allows a considerable computational effort gain since the spin–orbit coupling could be calculated only for one self-consistent iteration. Once again, the GGA+U method is found to be the most adequate approach for the force theorem calculations of the Gd MAE. The GGA and GGA-core model treatments of the 4f states have led to a wrong MAE. It turns out that the electronic properties and the magnetic properties of 4f systems are tightly related, and the 4f electrons play a crucial role in the computed magnetic anisotropy. Although the Gd MAE is found to be similar to that of a typical 3d transition metal like hcp Co, the GdN and GdFe2 cubic crystals MAEs are found to be different from that of a pure 3d cubic material like fcc Ni.

The electronic structure, magnetic properties, and magnetic anisotropy energy (MAE) of the insulating antiferromagnetic bulk CoO and CoO strained by silver or MnO substrates have been studied by means of three different methods: (1) a simple model based on the crystal-field approach, (2) the full-potential linear augmented plane-waves method within the generalized gradient approximation (GGA), and (3) the so-called GGA+U method. In all methods the spin-orbit coupling is included. In the third method, the Hubbard interaction U is used to treat the strong electron-electron interaction due to the cobalt localized open d shell. The results of the various calculations for bulk CoO and those of CoO strained by silver or MnO substrates are discussed and compared with available experimental data and other calculations. The GGA predicts incorrectly that bulk CoO is a metallic oxide and its spin magnetic moment is oriented along the inclination angle θ of 66° and the azimuth angle ϕ of 45° and the hard axis along the c direction. The orbital magnetic moment is also much smaller compared to experiment. On the contrary, within the GGA+U bulk CoO becomes an insulator as expected and the spin magnetic moment is oriented along the tetragonal c axis; it rotates in plane when CoO is strained by a silver substrate, and out of plane when strained by an MnO substrate. Within the GGA+U an unquenched orbital magnetic moment on the order of 1μB is obtained in both bulk CoO and CoO strained by MnO substrate but is much smaller (0.45μB) when CoO is strained by silver. All these results together with the values of MAE are in good agreement with experiment and our crystal-field analysis.

Scanning tunneling microscopy (STM) is today the most powerful and versatile tool available for imaging and manipulating single molecules on surfaces. Here, we explore its ultimate limit by demonstrating the possibility of controlling sequential di-dehydrogenation of single Co-Salen molecules sublimated on Cu. In particular, we are able to explore the final products of the H2 dissociation as well as the intermediate state, in which only one H atom is separated from the molecule. This is achieved by low-temperature STM with the dissociation induced by either point spectroscopy or in the standard constant-current mode. Crucial for the interpretation of the data is our ability to perform state-of-the-art density-functional theory simulations of both topographic and spectroscopical STM images. This work demonstrates that STM combined with theory can give access to the atomic details of a chemical reaction even when the reaction products are completely unknown.

In a non-polar geometry for non-cubic ferromagnetic materials, it is shown that the X-ray absorption dichroism spectrum is composed of a non-magnetic contribution (natural dichroism) emanating from the reduced crystal symmetry and a magnetic contribution from spin-orbit coupling and spin polarization. Their computation at the L 2, 3 edges of the half-metal CrO 2 chromium site, illustrate the good agreement with experiment. This is found only when both dichroic contributions are taken into account. Moreover, X-ray magnetic dichroism sum rules can be applied directly to extract the spin and orbital magnetic moments only after removing the natural dichroism from the full spectrum.

The magnetic configuration of a nanostructure can be altered by an external magnetic field, by spin-transfer torque or by its magnetoelastic response. Here, we explore an alternative route, namely the possibility of switching the sign of the exchange coupling between two magnetic centres by means of an electric potential. This general effect, which we name electrostatic spin crossover, occurs in insulating molecules with super-exchange magnetic interaction and inversion symmetry breaking. As an example we present the case of a family of di-cobaltocene-based molecules. The critical fields for switching, calculated from first principles, are of the order of 1 V nm−1 and can be achieved in two-terminal devices. More crucially, such critical fields can be engineered with an appropriate choice of substituents to add to the basic di-cobaltocene unit. This suggests that an easy chemical strategy for achieving the synthesis of suitable molecules is possible.

We present rocksalt MgN as a d 0 magnet with potential for spintronics applications. Our density-functional theory calculations demonstrate that rocksalt MgN is at the verge of half-metallicity, with an electronic structure robust against the choice of exchange and correlation functional. Furthermore the calculated heat of formation describes the compound as metastable and suggests that it can be fabricated by tuning the relative Mg and N abundance during growth. Intriguingly the equilibrium lattice constant is close to that of MgO so that MgN is likely to form as an inclusion during the fabrication of N-doped MgO. If MgN can be made, the MgO/MgN system may become a materials platform for magnetic tunnel junctions not incorporating any transition metals.

The electronic and magnetic properties of gadolinium are studied by means of the full-potential linear augmented plane-wave method including the Hubbard interaction for describing a 4f electron-electron interaction in the mean-field approximation. The manner in which the 4f localized orbitals are approximated seems to be of great importance in elucidating the mechanisms responsible for the strong magnetic moment of gadolinium and its ordered magnetic structure as well as the relative energy positions of the occupied and empty 4f states. Through various calculations, including the spin-orbit coupling in a second-variation scheme, within the local-spin-density approximation (LSDA), the generalized-gradient approximation (GGA), the LDA+U, and the GGA+U, it is clearly shown that the LDA+U or the GGA+U are the most adequate methods for describing the electronic and magnetic structures as well as the x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) of the the strongly correlated 4f electrons of gadolinium. In particular, the calculated L2,3 and M4,5 XAS and XMCD spectra are found in good agreement with the experimental ones except that the shoulder above the principal peak of the M4,5 spectra is not present in the calculation. The spin magnetic moments of the 5d states obtained from the XMCD spectra using the XMCD sum rules compare favorably with the self-consistent band-structure results only when the dipole magnetic term is included in the calculation.

The electronic structure and magnetic properties of Fe/GaAs/Fe (001) tunnel junction have been studied using a first-principles Green's function technique, based on the tight-binding linear muffin-tin orbital method in its atomic spheres approximation, in conjunction with the coherent potential approximation to describe the disorder effects such as the interdiffusion at the interface. The results show that at the Fe/GaAs interface there is a charge transfer from iron to the semiconductor region and an enhancement of Fe magnetic moment. The magnetic properties are found to be sensitive to the interface terminations and influenced by the interdiffusion. The spin dependent transport properties are also discussed.