Local Moment Formation Research Articles

Density functional study of the electronic structure and magnetism of LaFeAsO alloyed with Zn

We report first-principles supercell investigations of ${\text{LaFe}}_{1\ensuremath{-}x}{\text{Zn}}_{x}\text{AsO}$. These are discussed in relation to existing experimental data on Zn-doped LaFeAsO. As expected, Zn occurs in a ${d}^{10}$ configuration in the alloy, similar to the pure Zn compound LaZnAsO. This is highly disruptive to the electronic structure of LaFeAsO near the Fermi energy, which is heavily derived from $\text{Fe}\text{ }d$ states. This favors localization and the formation of local moments on the Fe atoms near the Zn.

γ-Mn at the border between weak and strong correlations

We investigate the role of magnetic fluctuations in the spectral properties of paramagnetic gamma-Mn. Two methods are employed. The Local Density Approximation plus Dynamical Mean-Field Theory together with the numerically exact quantum Monte-Carlo solver is used as a reference for the spectral properties. Then the same scheme is used with the computationally less demanding perturbative spin-polarized fluctuation-exchange solver in combination with the Disordered Local Moment approach, and photoemission spectra are calculated within the one-step model. It is shown that the formation of local magnetic moments in gamma-Mn is very sensitive to the value of Hund's exchange parameter. Comparison with the experimental photoemission spectra demonstrates that gamma-Mn is a strongly correlated system, with the Hubbard band formation, which cannot be described by the perturbative approach. However, minor change of parameters would transform it into a weakly correlated system.

Shuffle dislocation induced magnetic moment in graphene

Graphene, a honeycomb arrangement of carbon atoms, is a promising material for nanoelectronics applications due to its unusual electronic properties. Recent experiments performed on suspended graphene indicate the existence of intrinsic defects on the samples. It is known that lattice defects such as vacancies or voids leaving unpaired atoms, lead to the formation of local magnetic moments (Vozmediano et al., 2005 [1]). The existence and ordering of these moments is largely determined by the bipartite character of the honeycomb lattice seen as two interpenetrating triangular sublattices. Dislocations made by pentagon–heptagon pairs or octagons with an unpaired atom have been studied recently and found to be stable in the graphene lattice (Carpio et al., 2008 [2]). These defects frustrate the sublattice structure and affect the magnetic properties of graphene. We study the magnetic properties of graphene in the presence of these defects. The system is described by a p z tight-binding model with electron–electron interactions modelled by a Hubbard term. Spin-polarized mean-field solutions are investigated within an unrestricted Hartree–Fock approximation.

Mott and Wigner-Mott transitions in doped correlated electron systems: Effects of superlattice potential and intersite correlation

We introduce the notion of superstructure Mottness to describe the Mott and Wigner-Mott transition in doped strongly correlated electron systems at commensurate filling fractions away from one electron per site. We show that superstructure Mottness emerges in an inhomogeneous electron system when the superstructure contains an odd number of electrons per supercell. We argue that superstructure Mottness exists even in the absence of translation symmetry breaking by a superlattice, provided that the extended or intersite Coulomb interaction is strong. In the latter case, superstructure Mottness offers a unifying framework for the Mott and Wigner physics and a nonperturbative, strong coupling description of the Wigner-Mott transition. We support our proposal by studying a minimal single-band ionic Hubbard $t$-$U$-$V$-$\Delta$ model with nearest neighbor Coulomb repulsion $V$ and a two-sublattice ionic potential $\Delta$. The model is mapped onto a Hubbard model with two effective ``orbitals'' representing the two sites within the supercell, the intra and interorbital Coulomb repulsion $U$ and $U^\prime \sim V$, and a crystal field splitting $\Delta$. Charge order on the original lattice corresponds to orbital order. Developing a cluster Gutzwiller approximation, we study the effects and the interplay between $V$ and $\Delta$ on the Mott and Wigner-Mott transitions at quarter-filling. We provide the mechanism by which the superlattice potential enhances the correlation effects and the tendency towards local moment formation, construct and elucidate the phase diagram in the unifying framework of superstructure Mottness.

Local magnetic moment formation at 119Sn Mössbauer impurity in [formula omitted] ( [formula omitted]) Laves phases compounds

The purpose of the present work is to theoretically study the local magnetic moment formation and the systematics of the magnetic hyperfine fields at a non-magnetic s–p Mössbauer 119Sn impurity diluted on R sites ( R = rare-earth metals ) of the cubic Laves phases intermetallic compounds R Fe 2 . One considers that the magnetic hyperfine field has two contributions (i) the contribution from R ions, calculated via an extended Daniel–Friedel [J. Phys. Chem. Solids 24 (1963) 1601] model and (ii) the contribution from the induced magnetic moments arising from the Fe neighboring sites. We have in this case a two-center Blandin–Campbell-like [Phys. Rev. Lett. 31 (1973) 51; J. Magn. Magn. Mater. 1 (1975) 1] problem, where a magnetic 3d-element located at a distance from the 119Sn impurity gives an extra magnetization to a polarized electron gas which is strongly charge perturbed at the 119Sn impurity site. We also include in the model, the nearest-neighbor perturbation due to the translational invariance breaking introduced by the impurity. Our self-consistent total magnetic hyperfine field calculations are in a very good agreement with recent experimental data.

Magnetic ordering in Fe/Co sandwiches on Cu(100)

We investigate magnetic correlations and local magnetic moments at finitetemperatures of some Fe and Co multilayers on Cu(100) substrates, such asComFenCom/Cu(100) andFemConFem/Cu(100). We use an ab initio mean-field theory of magnetic fluctuations for layered materials basedon the first-principles local spin-density functional theory implemented throughthe screened Korringa–Kohn–Rostoker method. We find that the presence ofFe layers in the neighbourhood of a Co layer always leads to a reduction in themagnetic moment of the Co atoms, whereas that of the Fe atoms is enhanced. Ofparticular interest is the lack of local moment formation on the single fcc-Co layersandwiched between two fcc-Fe layers. However, a Co layer completely immersedin a Cu environment remains ferromagnetic. The Curie temperature of theComFenCom/Cu(100) system oscillates as the Fe layer thickness is increased whereas that of theFemConFem/Cu(100) system increases almost monotonically with Co layer thickness.

Origin of ferromagnetism via hole doping in SnO 2: First-principles calculation

First-principles calculations are carried out in order to find the ferromagnetism dependence on the number of holes substituted for Sn sites. The results show that strong localization of defect states of the p bands of the oxygen atoms near the dopants favors high-spin states and local moment formation. These states appear to be ferromagnetically coupled with a rather long-range magnetic interaction, resulting in a half-metallic ferromagnetic ground state for the whole systems. Analysis of the total energies indicates that the induced well-confined ferromagnetism in the oxygen p orbitals due to hole doping is quite possible and easily controlled in these systems, which indicate a new way to develop a half-metallic ferromagnet in nonmagnetic d 0 oxides.

Localized magnetic states in Rashba dots

We study the formation of local moments in quantum dots arising in quasi-one-dimensional electron wires due to localized spin-orbit (Rashba) interaction. Using an Anderson-type model to describe the occurrence of the magnetic moments in these Rashba dots, we calculate the local magnetization within the mean-field approximation. We find that the magnetization becomes a nontrivial function of the Rashba coupling strength. We discuss both the equilibrium and nonequilibrium cases. Interestingly, we obtain a magnetic phase which is stable at large bias due to the Rashba interaction.

Local moments and magnetic order in the two-dimensional Anderson-Mott transition

We study the role of electronic correlation in a disordered two-dimensional model by using a variational wave function that can interpolate between Anderson and Mott insulators. Within this approach, the Anderson-Mott transition can be described both in the paramagnetic sector and in the magnetic sector. In the latter case, we find evidence for the formation of local magnetic moments that order before the Mott transition. The charge gap opening in the Mott insulator is accompanied by the vanishing of the ${\text{lim}}_{q\ensuremath{\rightarrow}0}\overline{⟨{n}_{q}⟩⟨{n}_{\ensuremath{-}q}⟩}$ (the bar denoting the impurity average). The role of a frustrating (second-neighbor) hopping is also discussed, with a particular emphasis to the formation of metastable spin-glass states.

Magnetism and chemical ordering in icosahedral Al-Pd-Mn quasicrystal

Even after nearly two decades of research efforts, the origin of the outstanding magnetic properties of icosahedral quasicrystals containing Mn remains unclear. Experiments have demonstrated that only a rather small fraction of the Mn atoms carry rather large magnetic moments, but it remains unclear whether these sites are intrinsic to the quasicrystalline structure or represent defects. We present ab initio density-functional calculations of the magnetic properties of a large 2/1 approximant to icosahedral $(i)$ Al-Pd-Mn, performed in the semilocal generalized gradient approximation. Structures for rational approximants to the quasicrystalline structure of bulk $i$-Al-Pd-Mn have been constructed using the cut-and-projection technique in six-dimensional (6D) hyperspace according to the Katz-Gratias-Boudard model. We studied magnetism in models of the 2/1 approximant with idealized coordinates obtained by projection from 6D hyperspace in models with coordinates relaxed using Hellmann-Feynman forces and in models simulating the structure of the quasicrystal at high temperature. In some idealized structural models a majority of the Mn atoms carry a large magnetic moment. Although this contradicts experiments demonstrating that only a very small fraction of the Mn atoms are magnetic, this provides the opportunity to investigate in detail the mechanisms leading to the formation of magnetic moments on the Mn atoms. We identify two major mechanisms: a loosely packed environment of the Mn atoms and a direct Mn-Pd interactions leading to a shift of the Mn $d$ band toward the Fermi level. The dominant contribution is the Mn-Pd interaction. A large magnetic moment on a Mn atom is formed at special sites where the Mn atom has two or three Pd nearest neighbors. These special sites are located at those regions of the quasiperiodic lattice where the pseudo-Mackay and Bergman clusters building the structure are linked along the threefold axes. At these sites, the building principles of the Mackay and Bergman clusters are in conflict: from the side of the Mackay cluster, occupation with an Al atom is required, whereas the symmetry of the Bergman cluster suggests occupation by a Pd atom. We have found that a small modification of the chemical decoration of these sites (corresponding to a slight modification of the internal shell structure of the occupation domains in the 6D hyperspace) can lead to a complete disappearance of magnetism in $i$-Al-Pd-Mn. Total energy calculations show that models with a chemical ordering disfavoring formation of magnetic moments on Mn atoms have the lowest energies. This re-establishes agreement with experiment. The ground state of $i$-Al-Pd-Mn is nonmagnetic; but at these specific sites, Al atoms can be replaced by Pd at low energetic cost and without strong violation of the building principles of the quasicrystalline structure, resulting in the formation of large local magnetic moments.

Evidence of local moment formation in Co-based Heusler alloys

In this work we use a combination of x-ray magnetic circular and linear dichroism (XMCD and XMLD) techniques to examine the formation of local moments in Heusler alloys of the composition Co{sub 2}MnX (where X=Si or Al). The existence of local moments in a half-metallic system is reliant upon the band gap in the minority-spin states. By utilizing the element-specific nature of x-ray techniques we are able to explore the origin of the minority-spin band gap in the partial density of states (PDOS), via the degree of localization of moments on Co and Mn atoms. We observe a crucial difference in the localization of the Co moment when comparing Co{sub 2}MnSi (CMS) and Co{sub 2}MnAl (CMA) films that is consistent with the predicted larger minority-spin gap in the Co PDOS for CMS. These results provide important evidence for the dominant role of the Co minority-spin states in realizing half-metallic ferromagnetism (HMF) in these Heusler alloys. They also demonstrate a direct method for measuring the degree of interfacial HMF in the raw materials without the need for fabricating spin-transport devices.

Tight-binding study of nonmagnetic-defect-induced magnetism in graphene

This review describes a study of nonmagnetic-defect-induced magnetism of graphene on the basis of a tight-binding model. A vacancy induces around itself a quasilocalized impurity state at the chemical potential and leads to formation of local magnetic moments. The connection between a quasilocalized state around a vacancy and the edge localized states near a zigzag edge is studied in detail. Magnetism associated with many vacancies and edge structures is also reviewed. Some new results of magnetism associated with many vacancies are presented.

Electronic structure and magnetic properties of dilute Cr alloys with transition-metal impurities

By using full-potential linearized augmented plane wave (FLAPW) method based on the density function theory (DFT), we have performed ab initio calculations of electronic structure and magnetic properties for dilute alloys of Cr with transition metals. Here, we present our results for the local magnetic properties of $3d$ (Sc\char21{}Ni) and $4d$ (Y\char21{}Pd) impurities in Cr, as well as changes in macroscopic properties such as total magnetization and specific heat of Cr alloys, arising from impurity-induced host perturbations. Results obtained for nonmagnetic local density of states is analyzed within Stoner model, and the condition for local moment formation is found to be satisfied only for Fe, Co, and Ni. For all other cases including Mn, the impurity moments are predicted to be induced by Cr host. Systematic trends for the impurity local moments and hyperfine fields in antiferromagnetic Cr host have been established. The trends show remarkable similarity with those observed in ferromagnetic Fe, Co, and Ni hosts. While the impurity moments for Sc, Ti, V, and Mn are found to be antiparallel with respect to the near neighbor Cr moment, self-consistent solutions for Fe, Co, and Ni show ferromagnetic coupling. For $4d$ impurities, except for Rh and Pd, the exchange interaction remain antiferromagnetic. The calculated changes of the total moment due to host perturbations show qualitative agreements with data from magnetization measurements. A detailed comparison is made for the impurity-induced changes in the total density of states at Fermi energy ${E}_{F}$ and experimentally measured electronic specific heat of Cr alloys. The results presented in this work provide comprehensive and improved understanding of electronic and magnetic properties of dilute Cr alloys. They are also useful for a common understanding of local magnetic properties of transition metal impurities in ferromagnetic and antiferromagnetic hosts.

Local Magnetic Moment Formation on Edges of Graphene

Local magnetic moment formation on edges of graphene is studied using the Hubbard model on a honeycomb lattice. The Coulomb interaction is treated with the mean field approximation. Realistic edges of graphene are composed of zigzag and armchair parts. It is found that, on a zigzag part of length n a larger than 3 a , where a is the lattice constant, local staggered moment whose magnitude is proportional to the length of the zigzag part, is developed, i.e., a zigzag part of length 3 a is sufficient for generation of local magnetism. This also means that one has to get rid of zigzag parts of n ≥3 to suppress local edge magnetism of graphene.

Failure of the mean-field approach in the out-of-equilibrium Anderson model

To explore the limitations of the mean-field approximation, frequently used in ab initio molecular electronics calculations, we study an out-of-equilibrium Anderson impurity model in a scattering formalism. We find regions in the parameter space where both magnetic and nonmagnetic solutions are stable. We also observe a hysteresis in the nonequilibrium magnetization and current as a function of the applied bias voltage. The mean-field method also predicts incorrectly local moment formation for large biases and a spin polarized current, and unphysical kinks appear in various physical quantities. The mean-field approximation thus fails in every region where it predicts local moment formation.

Defect-Induced Intrinsic Magnetism in Wide-Gap III Nitrides

Cation-vacancy induced intrinsic magnetism in GaN and BN is investigated by employing density-functional theory based electronic structure methods. The strong localization of defect states favors spontaneous spin polarization and local moment formation. A neutral cation vacancy in GaN or BN leads to the formation of a net moment of 3 muB with a spin-polarization energy of about 0.5 eV at the low density limit. The extended tails of defect wave functions, on the other hand, mediate surprisingly long-range magnetic interactions between the defect-induced moments. This duality of defect states suggests the existence of defect-induced or mediated collective magnetism in these otherwise nonmagnetic sp systems.

Formation and ordering of local magnetic moments in Fe–Al alloys

The density functional theory is used to study the local magnetic moments in Fe–Al alloys depending on concentration (from 29 to 44 at% Al) and the Fe nearest environment. We have found three different solutions for the system: a spin-spiral wave (SSW) which has a minimum energy and two collinear states, a ferromagnetic one and a state with both positive and negative Fe magnetic moments (the Fe atoms with many neighboring Al atoms around them have negative magnetic moments, while the other Fe atoms—positive). Both the SSW and the negative Fe moments agree with the experiments. Magnetization curves taken from the literature are analyzed. The assumption of percolation character of the size distribution of magnetic clusters describes well the experimental superparamagnetic behavior above 150 K.

Local magnetic moment formation at Sn119 Mössbauer impurity in RCo2 (R=Gd,Tb,Dy,Ho,Er) Laves phase compounds

In this work, we theoretically study the local magnetic moment formation and the systematics of the magnetic hyperfine fields at a Mösbauer Sn119 impurity diluted at the R site (R=Gd,Tb,Dy,Ho,Er) of the cubic Laves phase intermetallic compounds RCo2. One considers that the magnetic hyperfine fields have two contributions, (i) the contribution from R ions, calculated via an extended Daniel-Friedel [J. Phys. Chem. Solids 24, 1601 (1963)] model, and (ii) the contribution from the induced magnetic moments arising from the Co neighboring sites. Our calculated self-consistent total magnetic hyperfine fields are in a good agreement with recent experimental data.

Local magnetic moments and hyperfine fields at Ta impurities diluted in XFe2 (X = Y, Gd, Yb) Laves phases compounds

In this work, we study the systematics, at finite temperature, of the formation of local magnetic moments at a Ta impurity diluted in intermetallic Laves phases compounds XFe2 (X = Y, Gd, Yb). We use an extended two-coupled sublattice Hubbard Hamiltonian, to describe the Laves phases host. The d-d electronic interaction is treated via a functional integral approach in the quasi-static saddle point approximation. Temperature dependent pressure effects are included considering induced electron-phonon interaction which renormalizes the pure electron hybridization. The calculated magnetic hyperfine fields related to the obtained local magnetic moments, are in a quite good agreement with available experimental data.

Itinerant and localized magnetism on the triangular lattice: Sodium-rich phases ofNaxCoO2

We study the interplay between correlation, itinerant ferromagnetism, and local moment formation on the electron doped triangular lattice of sodium cobaltates ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$. We find that strong correlation renormalizes the Stoner criterion and stabilizes the paramagnetic state for $x<{x}_{c}\ensuremath{\simeq}0.67$. For $x>{x}_{c}$, ferromagnetic (FM) order emerges. The enhanced Na dopant potential fluctuations play a crucial role in the sodium-rich phases and lead to an inhomogeneous FM state, exhibiting nonmagnetic ${\mathrm{Co}}^{3+}$ patches, antiferromagnetic (AF) correlated regions, and FM clusters with AF domains. Hole doping the band insulator at $x=1$ leads to the formation of local moments near the Na vacancies and AF correlated magnetic clusters. We explain recent observations by neutron, $\ensuremath{\mu}\mathrm{SR}$, and NMR experiments on the evolution of the magnetic properties in the sodium-rich phases.