Formation Of Magnetic Moments Research Articles

Temperature dependence on magnetic moment formation at a Ce impurity in Gd, Tb, and Dy: An intermediate valence model

Based on the influence of the Ce intermediate valence on magnetic properties in metallic systems, we theoretically study the local magnetic moment formation at a Ce impurity diluted in Gd, Tb, and Dy rare earth metals at finite temperatures. To investigate this point, our model considers the functional integral formalism, coherent potential approximation, and extracting relative charge from the Ce 4f-resonance and transferring it to the 5d Ce band at the temperature range from 0K to Curie critical temperature. As a result, our self-consistent calculation, focusing on temperature dependence, shows that the intermediate valence model explains the regimes of the local magnetic moments and magnetic hyperfine fields. Moreover, our results for the temperature dependence on magnetic hyperfine fields at the Ce site in Gd and Tb agree with experimental reports.

Higher-order multipoles in metaphotonic structures: reviewing symmetric approaches and constructing a comprehensive theoretical method

This research explores localized electric fields in two-dimensional (2D) photonic crystals, focusing on higher-order multipoles. Novel formulas are developed to accurately describe and analyze these complex electromagnetic modes. By considering various crystal element arrangements, we investigate the formation and interactions of electric, magnetic, and toroidal moments. We examine the effects of symmetry on the localized electromagnetic fields in 2D periodic structures and explain how bound states in the continuum (BICs) arise and behave in different symmetry classes. It is important to point out that every singularity point that occurs in multipolar series expansions depends on a BIC point, and it inspires us to study the characteristics of them. Furthermore, we analyze a variety of BICs and use perturbed and degeneracy breaking to investigate high-Q-factor quasi-BICs. The results provide a thorough understanding of how symmetry and multipolar interactions affect potential applications in cutting-edge metasurfaces and photonic devices (metaphotonic).

Electron Correlation Effects in Paramagnetic Cobalt

We study the influence of Coulomb correlations on spectral and magnetic properties of fcc cobalt using a combination of density functional theory and dynamical mean-field theory. The computed uniform and local magnetic susceptibilities obey the Curie–Weiss law, which, as we demonstrate, occurs due to the partial formation of local magnetic moments. We find that the lifetime of these moments in cobalt is significantly less than in bcc iron, suggesting a more itinerant magnetism in cobalt. In contrast to the bcc iron, the obtained electron self-energies exhibit a quasiparticle shape with the quasiparticle mass enhancement factor m*/m ~ 1.8, corresponding to moderately correlated metal. Finally, our calculations reveal that the static magnetic susceptibility of cobalt is dominated by ferromagnetic correlations, as evidenced by its momentum dependence.

Synchrotron radiation techniques and their application to actinide materials

The actinides comprise a 15-member group of metallic radioactive elements occupying the bottom row of the periodic table. Electron localization accompanied by the formation of large magnetic moments due to the strong Coulomb repulsion is balanced by hybridization with neighboring-atom electronic states. This hybridization promotes an opposite tendency toward itinerancy and the emergence of complex behavior. This review shows how x-ray synchrotron radiation techniques provide a variety of powerful tools to unravel the complexity of actinide materials. Experimental techniques, theoretical background, and applications to actinide materials are covered in detail.

Kondo screening of single magnetic impurity doped type-II Ising superconductors

We theoretically study the Kondo effect in type-II Ising superconductors with a single magnetic impurity. Type-II Ising superconductivity was found in two-dimensional centrosymmetric materials with multiple degenerate orbitals, such as in few-layer stanene [Falson et al., Science 367, 1454 (2020)] and ultrathin ${\mathrm{PdTe}}_{2}$ films [Liu et al., Nano Lett. 20, 5728 (2020)]. The type-II Ising spin-orbit coupling (SOC) in these materials generates out-of-plane effective Zeeman fields orienting opposite directions for opposing orbitals, which strongly protects the interorbital superconducting pairing states against in-plane magnetic fields. We show that the SOC-induced band splitting and the chemical potential significantly influence the formation of a localized magnetic moment, with particle-hole asymmetry in the phase boundary between magnetic and nonmagnetic states. The behaviors of spin-induced Yu-Shiba-Rusinov bound states and low-temperature magnetic susceptibility demonstrate that the SOC suppresses the Kondo screening of the magnetic moment, while the interorbital mixing weakens the SOC at finite momentum, then enhances the Kondo effect. The quantum phase transition between magnetic doublet and Kondo singlet ground states can be tuned through chemical potential in experiments.

Numerical studies on holographic paramagnetic-ferromagnetic phase transition in Gauss–Bonnet gravity

Based on the shooting method, we numerically investigate the properties of holographic paramagnetism-ferromagnetism phase transition in the presence of higher-order Gauss–Bonnet (GB) correction terms on the gravity side. On the matter field side, however, we consider the effects of Power-Maxwell (PM) nonlinear electrodynamics on the phase transition of this system. For this purpose, we introduce a massive 2-form coupled to PM field, and neglect the effects of 2-form fields and gauge field on the background geometry. We observe that increasing the strength of both the power parameter [Formula: see text] and GB coupling constant [Formula: see text] decreases the critical temperature of holographic model, and leads to the harder formation of magnetic moment in the black hole background. Interestingly, we find out that at low temperatures, the spontaneous magnetization and ferromagnetic phase transition happen in the absence of external magnetic field. In this case, the critical exponent for magnetic moment has the mean field value, [Formula: see text], regardless of the values of [Formula: see text] and [Formula: see text]. In the presence of external magnetic field, however, the magnetic susceptibility satisfies the Curie–Weiss law.

Transition from Pauli paramagnetism to Curie-Weiss behavior in vanadium

We study electron correlations and their impact on magnetic properties of bcc vanadium by a combination of density functional and dynamical mean-field theory. The calculated uniform magnetic susceptibility {in bcc structure} is of Pauli type at low temperatures, while it obeys the Curie-Weiss law at higher temperatures. Thus, we qualitatively reproduce the experimental temperature dependence of magnetic susceptibility without introducing the martensitic phase transition. Our results for local spin-spin correlation function and local susceptibility reveal that the Curie-Weiss behavior appears due to partial formation of local magnetic moments, which originate from $t_{2g}$ states and occur due to local spin correlations caused by Hund's rule coupling. At the same time, the fermionic quasiparticles remain well-defined, while the formation of local moments is accompanied by a deviation from the Fermi-liquid behavior. In particular, the self-energy of the $t_{2g}$ states shows the non-analytic frequency dependence, which is a characteristic of the spin-freezing behavior, while the quasiparticle damping changes approximately linearly with temperature in the intermediate temperature range $200$--$700$~K. By analyzing the momentum dependence of static magnetic susceptibility, we find incommensurate magnetic correlations, which may provide a mechanism for unconventional superconductivity at low temperatures.

Local magnetic moments due to loop currents in metals

We present Hartree-Fock calculations on a simple model to obtain the conditions of formation of local magnetic moments due to loop currents ${L}_{o}$ and spin-loop currents ${L}_{s}$ and compare them to the conditions of formation of local spin moments $M$ which were given long ago in a similar approximation by Anderson. A model with three degenerate orbitals sitting on an equilateral triangle, with on-site and nearest-neighbor repulsions $U$ and $V$, respectively, and intersite kinetic energy, hybridizing with conduction electrons with a parameter $\mathrm{\ensuremath{\Delta}}$ is investigated. ${L}_{o}$ and ${L}_{s}$ are promoted by large $V/\mathrm{\ensuremath{\Delta}}$ and their magnitude is relatively unaffected by $U/\mathrm{\ensuremath{\Delta}}$. Spin-magnetic moments $M$ promoted by large $U/\mathrm{\ensuremath{\Delta}}$ on the other hand are adversely affected by $V/\mathrm{\ensuremath{\Delta}}$. In this model, ${L}_{o}$ for $V$ multiplied by the number of neighbors is approximately the same as the $M$ promoted by $U$ in Anderson's local model for $M$. ${L}_{o}$ and ${L}_{s}$ are degenerate if exchange interactions and Hund's rule are neglected but ${L}_{o}$ is favored when they are included. Many of the qualitative results are visible in an expression for the Hartree-Fock ground state energy derived as a function of small ${L}_{o}, {L}_{s}$, and $M$. Numerical minimization of the Hartree-Fock energy is presented for larger values. We also briefly discuss the connection and differences of the interaction-generated orbital currents and spin currents discussed here and generalized to a lattice with the topological states in metals and semiconductors.

Effect of local magnetic moments on spectral properties and resistivity near interaction- and doping-induced Mott transitions

We study the effect of the formation and screening of local magnetic moments on the temperature- and interaction dependencies of spectral functions and resistivity in the vicinity of the metal-insulator transition. We use the dynamical mean-field theory for the strongly correlated Hubbard model and associate the peculiarities of the above mentioned properties with those found for the local charge $\chi_c$ and spin $\chi_s$ susceptibilities. We show that at half filling the maximum of resistivity at a certain temperature $T^*$ corresponds to the appearance of central quasiparticle peak of the spectral function and entering the metallic regime with well defined fermionic quasiparticles. At the same time, the temperature of the crossover to the regime with screening of local magnetic moments, determined by the minimum of double occupation, is smaller than the temperature scale $T^*$ and coincides at half filling with the boundary $T_{\beta=1}(U)$ corresponding to the exponent of resistivity $\beta\equiv d\ln \rho/d\ln T=1$. Away from half filling we find weak increase of the temperature of the beginning and completion of the screening (i.e. Kondo temperature) of local magnetic moments, while the unscreened local magnetic moments exist only up to few percents of doping. In the low temperature regime $T<T_{\beta=1}$ simultaneous presence of itinerant and localized degrees of freedom yields almost linear temperature dependence of scattering rate and resistivity.

The Nature of Ferromagnetism in a System of Self-Ordered α-FeSi2 Nanorods on a Si(111)-4° Vicinal Surface: Experiment and Theory.

In this study, the appearance of magnetic moments and ferromagnetism in nanostructures of non-magnetic materials based on silicon and transition metals (such as iron) was considered experimentally and theoretically. An analysis of the related literature shows that for a monolayer iron coating on a vicinal silicon surface with (111) orientation after solid-phase annealing at 450–550 °C, self-ordered two-dimensional islands of α-FeSi2 displaying superparamagnetic properties are formed. We studied the transition to ferromagnetic properties in a system of α-FeSi2 nanorods (NRs) in the temperature range of 2–300 K with an increase in the iron coverage to 5.22 monolayers. The structure of the NRs was verified along with distortions in their lattice parameters due to heteroepitaxial growth. The formation of single-domain grains in α-FeSi2 NRs with a cross-section of 6.6 × 30 nm2 was confirmed by low-temperature and field studies and FORC (first-order magnetization reversal curves) diagrams. A mechanism for maintaining ferromagnetic properties is proposed. Ab initio calculations in freestanding α-FeSi2 nanowires revealed the formation of magnetic moments for some surface Fe atoms only at specific facets. The difference in the averaged magnetic moments between theory and experiments can confirm the presence of possible contributions from defects on the surface of the NRs and in the bulk of the α-FeSi2 NR crystal lattice. The formed α-FeSi2 NRs with ferromagnetic properties up to 300 K are crucial for spintronic device development within planar silicon technology.

Refined guiding centre approximation for kinetic studies of Nitrogen seeding in medium density ITER L-mode scenarios with EMC3-EIRENE

• Improvements of kinetic ion transport module in guiding centre approximation in EMC3-EIRENE. • Validation of the kinetic ion transport module against magnetic moment conservation and formation of banana-orbits. • First stable simulation of Nitrogen seeding for ITER L-mode scenario with the improved kinetic ion transport module of EMC3-EIRENE.

Effect of Atomic Substitutions on the Electronic Structure of Pt1 – xNixMnSb Alloys (x = 0.0–1.0)

Using zone calculations in the FLAPW (the full-potential linearized augmented-plane-waves) model, the information on the energy, charge, and spin characteristics of Pt1-xNixMnSb alloys (x = 0.0÷1.0) is obtained. It is established that, with an increase in the concentration of nickel atoms in Pt1-xNixMnSb alloys, the interatomic space density of electrons decreases, covalent bonds weaken, and the cohesive energies of the alloys decrease. The dominant contributions to the formation of magnetic moments in Pt1-xNixMnSb alloys are made by 3d electrons of manganese atoms. In alloys with x ≥ 0.50, the complete polarization of Fermi electrons is registered, which converts these alloys to a half-metallic state.

Understanding the magnetic response of the Quantum Spin Liquid compound 1[formula omitted]-TaS[formula omitted

1T-tantalum disulphide (1T-TaS2) is a promising material to realize quantum spin liquid state. Here, we take a closer look on the temperature dependent magnetic susceptibility of 1T-TaS2 at low magnetic fields to understand the formation of local magnetic moment in this system and its response as a function of temperature and field. We found that the susceptibility increases with reducing temperature even in the nearly commensurate charge density wave (CDW) phase, which exhibits metallic conductivity and hence is expected to exhibit temperature independent Pauli paramagnetic susceptibility. It indicates that local moment starts forming in the nearly commensurate CDW phase itself, which is well above the temperature where evidence of Mott insulating state appears in resistivity measurement. In the commensurate CDW phase, susceptibility significantly deviates from the Curie–Weiss law. Below T = 10 K, temperature dependence of susceptibility indicates the formation of random singlet state in 1T-TaS2.

Resolving Atomic-Scale Defects in Conjugated Polymers On-Surfaces.

Atomic scale defects significantly affect the mechanical, electronic, and optical properties of π-conjugated polymers. Here, isolated atomic-scale defects are deliberately introduced into a prototypical anthracene-ethynylene π-conjugated polymer, and its local density of states is carefully examined on the atomic scale to show how individual defects modify the inherent electronic and magnetic properties of this one-dimensional systems. Scanning tunneling and atomic force microscopy experiments, supplemented with density functional theory calculations, reveal the existence of a sharp electronic resonance at the Fermi energy around certain defects, which is associated with the formation of a local magnetic moment accompanied by substantial mitigation of the mobility of charge carriers. While defects in traditionally synthesized polymers lead to arbitrary conformations, the presented results clearly reflect the preferential formation of low dimensional defects at specific polymer sites, which may introduce the possibility of engineering macroscopic defects in surface-synthesized conjugated polymers.

Ab initio approaches to high-entropy alloys: a comparison of CPA, SQS, and supercell methods

We present a comparative study of different modeling approaches to the electronic properties of the \(\text {Hf}_{0.05}\text {Nb}_{0.05}\text {Ta}_{0.8}\text {Ti}_{0.05}\text {Zr}_{0.05}\) high-entropy alloy. Common to our modeling is the methodology to compute the one-particle Green’s function in the framework of density functional theory. We demonstrate that the special quasi-random structures modeling and the supercell, i.e., the locally self-consistent multiple-scattering methods, provide very similar results for the ground state properties such as the spectral function (density of states) and the equilibrium lattice parameter. To reconcile the multiple-scattering single-site coherent potential approximation with the real space supercell methods, we included the effect of screening of the net charges of the alloy components. Based on the analysis of the total energy and spectral functions computed within the density functional theory, we found no signature for the long-range or local magnetic moments formation in the \(\text {Hf}_{0.05}\text {Nb}_{0.05}\text {Ta}_{0.8}\text {Ti}_{0.05}\text {Zr}_{0.05}\) high-entropy alloy; instead, we find possible superconductivity below \(\sim 9\)K.

Spin dynamics of itinerant electrons: Local magnetic moment formation and Berry phase

The state-of-the-art theoretical description of magnetic materials relies on solving effective Heisenberg spin problems or their generalizations to relativistic or multi-spin-interaction cases that explicitly assume the presence of local magnetic moments in the system. We start with a general interacting fermionic model that is often obtained in ab initio electronic structure calculations and show that the corresponding spin problem can be introduced even in the paramagnetic regime, which is characterized by a zero average value of the magnetization. Further, we derive a physical criterion for the formation of the local magnetic moment and confirm that the latter exists already at high temperatures well above the transition to the ordered magnetic state. The use of path-integral techniques allows us to disentangle spin and electronic degrees of freedom and to carefully separate rotational dynamics of the local magnetic moment from Higgs fluctuations of its absolute value. It also allows us to accurately derive the topological Berry phase and relate it to a physical bosonic variable that describes dynamics of the spin degrees of freedom. As the result, we demonstrate that the equation of motion in the case of a large magnetic moment takes a conventional Landau-Lifshitz form that explicitly accounts for the Gilbert damping due to itinerant nature of the original electronic model.

Local magnetic moment formation and Kondo screening in the half-filled single-band Hubbard model

We study the formation of local magnetic moments in the strongly correlated Hubbard model within dynamical mean-field theory and associate peculiarities of the temperature dependence of local charge ${\ensuremath{\chi}}_{c}$ and spin ${\ensuremath{\chi}}_{s}$ susceptibilities with different stages of local moment formation. The local maximum of the temperature dependence of the charge susceptibility ${\ensuremath{\chi}}_{c}$ is associated with the beginning of local magnetic moment formation, while the minimum of the susceptibility ${\ensuremath{\chi}}_{c}$ and double occupation, as well as the low-temperature boundary of the plateau of the effective local magnetic moment ${\ensuremath{\mu}}_{\mathrm{eff}}^{2}=T{\ensuremath{\chi}}_{s}$ temperature dependence are connected with the full formation of local moments. We also obtain the interaction dependence of the Kondo temperature ${T}_{K}$, which is compared to the fingerprint criterion of Chalupa et al. [Phys. Rev. Lett. 126, 056403 (2021)]. Near the Mott transition the two criteria coincide, while further away from the Mott transition the fingerprint criterion somewhat overestimates the Kondo temperature. The relation of the observed features to the behavior of eigenvectors/eigenvalues of fermionic frequency-resolved charge susceptibility and divergences of irreducible vertices is discussed.

Correlation strength, orbital-selective incoherence, and local moments formation in the magnetic MAX-phaseMn2GaC

We perform a theoretical study of the electronic structure and magnetic properties of the prototypical magnetic MAX-phase Mn$_2$GaC with the main focus given to the origin of magnetic interactions in this system. Using the density functional theory+dynamical mean-field theory (DFT+DMFT) method we explore the effects of electron-electron interactions and magnetic correlations on the electronic properties, magnetic state, and spectral weight coherence of paramagnetic and magnetically-ordered phases of Mn$_2$GaC. We also benchmark the DFT-based disordered local moment approach for this system by comparing the obtained electronic and magnetic properties with that of the DFT+DMFT method. Our results reveal a complex magnetic behavior characterized by a near degeneracy of the ferro- and antiferromagnetic configurations of Mn$_2$GaC, implying a high sensitivity of its magnetic state to fine details of the crystal structure and unit-cell volume, consistent with experimental observations. We observe robust local-moment behavior and orbital-selective incoherence of the spectral properties of Mn$_2$GaC, implying the importance of orbital-dependent localization of the Mn $3d$ states. We find that Mn$_2$GaC can be described in terms of local magnetic moments, which may be modeled by DFT with disordered local moments. However, the magnetic properties are dictated by the proximity to the regime of formation of local magnetic moments, in which the localization is in fact driven by the Hund's exchange interaction, and not the Coulomb interaction.

Environmental screening and ligand-field effects to magnetism in CrI3 monolayer

We study the microscopic origin of magnetism in suspended and dielectrically embedded CrI3 monolayer by down-folding minimal generalized Hubbard models from ab initio calculations using the constrained random phase approximation. These models are capable of describing the formation of localized magnetic moments in CrI3 and of reproducing electronic properties of direct ab initio calculations. Utilizing the magnet force theorem, we find a multi-orbital super-exchange mechanism as the origin of magnetism in CrI3 resulting from an interplay between ferro- and anti-ferromagnetic Cr-Cr d coupling channels, which is decisively affected by the ligand p orbitals. We show how environmental screening, such as resulting from encapsulation with hexagonal boron nitride, affects the Coulomb interaction in the film and how this controls its magnetic properties. Driven by a non-monotonic interplay between nearest and next-nearest neighbor exchange interactions we find the magnon dispersion and the Curie temperature to be non-trivially affected by the environmental screening.

Unraveling 5f correlated electronic states in paramagnetic PuSn3

Plutonium-based compounds establish an ideal platform for exploring the interplay between long-standing itinerant-localized 5f states and strongly correlated electronic states. In this paper, we exhaustively investigate the 5f correlated electronic states of PuSn3 dependence on temperature by means of a combination of the density functional theory and the embedded dynamical mean-field theory. It is found that the spectral weight of narrow 5f band grows significantly and remarkable quasiparticle multiplets appear around the Fermi level at low temperature. A striking c–f hybridization and prominent valence state fluctuations indicate the advent of coherence and itinerancy of 5f states. It is predicted that a 5f localized to itinerant crossover is induced by temperature accompanied by the change in Fermi surface topology. Therefore itinerant 5f states are inclined to take in active chemical bonding, suppressing the formation of local magnetic moment of Pu atoms, which partly elucidates the intrinsic feature of paramagnetic PuSn3. Furthermore, the 5f electronic correlations are orbital selective, which manifested themselves in differentiated band renormalizations and electron effective masses. Consequently, the convincing results remain crucial to our understanding of plutonium-based compounds and promote ongoing research.