Local Moment Formation Research Articles

Lattice distortion and magnetism of Fe impurity in Cu and Ag

First-principles electronic-structure calculations are performed for clusters representing an Fe impurity in the metal hosts Cu and Ag. The discrete-variational method is employed in the framework of local-spin-density theory. The host lattice distortions are taken into account in discussing the local magnetic moment formation and electronic properties. The obtained numerical results are compared with the previous theoretical and experimental studies.

Structural and Magnetic Studies of Amorphous and Icosahedral Al65Cr10Fe10Ge15 Synthesized through Mechanical Alloying

AbstractThe mechanical alloying is used to synthesize the amorphous powder of the nominal composition Al65Cr10Fe10Ge15. On the basis of DTA studies, the compound is heat treated at 530 K. The X‐ray diffraction of the heat treated phase can be indexed to icosahedral indices. The magnetic susceptibility measurement particularly exhibits some interesting features. A qualitative explanation based on the competition between on‐site Coulomb repulsion, which favours local moment formation and the s‐d interaction that leads to itinerancy, is attempted which is found to be well correlated with the structural disorder in the system.

Disorder induced local moment enhancement in compensated Si:P,B

The recently observed magnetic properties of the compensated Si:P,B and uncompensated Si:P are discussed in the light of a statistical mean field approach to the disordered Hubbard Hamiltonian. Besides giving an explanation for the observed enhancement of the local moment formation in the compensated materials, the model predicts the correct behaviour for the susceptibility as a function of the impurity density. In agreement with the experimental findings a larger persistence of the magnetic properties at high density is expected for larger compensation values.

Disorder-induced fluctuations in the magnetic properties of an Anderson-Hubbard model.

We present a microscopic description of the inhomogeneous magnetic response of a disordered, interacting system, with local susceptibilities determined via a random-phase-approximation-type approach about stable, inhomogeneous mean-field ground states. A careful treatment of the role of disorder is vital in describing: the phase boundary to local moment formation; site-differential inhomogeneity in the distribution of local susceptibilities in a local moment regime, and its marked disorder-induced enhancement; and, on the length scales probed, large disorder-induced fluctuations in the total magnetic susceptibility.

Magnetic 4d systems studied by implanted ions

By application of the perturbed γ-ray distribution method following heavy-ion reactions and recoil implantation techniques, we have found an experimental way of producing and investigating magnetic 4d states in metals. Strong 4d magnetism has been found for 4d ions in alkali metal hosts and in Pd hosts. In alkali metals, 4d ions reflect the phenomena of well-defined ionic ground states, orbital magnetism, mixed valence, and crystal field splittings smaller than theLS coupling. Magnetic 4d states in alkali metals cannot be described by one-electron approaches based on Anderson-type models, but requires an analysis in terms of many electron ionic configurations exhibiting basic features common to the physics of stable and unstable f stales in metals. In contrast, the local moment formation of 4d and 3d ions in Pd is governed by inter-atomic interactions of the magnetic d states with host d-band electrons, giving rise to spin magnetic behavior of the 4d impurity and to strong spin polarizations of the 4d electrons of the Pd host. Thus, the magnetism and electronic structure of 4d ions in metals exhibit qualitative differences in alkali metal hosts compared to Pd. The existence of magnetic 4d systems strongly depends on the 4d ion species and the host matrix, and on spin fluctuation rates or the corresponding Kondo temperatures. The results can be directly compared to theoretical work and also to the magnetic behavior of 3d ions in sp metal hosts and in hosts with d-band electrons.

Electronic band structure and magnetic effects in ternary Zr 2(Ni 1−xM x) 1 glassy alloys

The electronic and magnetic properties of the amorphous Zr 2(Ni 1−xM x) 1 alloys (M = Ti, V, Cr, Mn, Fe, Co, Ni and Cu) have investigated in the temperature range from 1.5K to 300K. As for the other Zr-3d glassy alloys (3d = Fe, Co, Cu or Ni) the temperature dependence of the electrical resistivity(T ≥ 20K) could be described in terms of the incipient electron localization. The new features are the pronounced variations of the magnetic and electron-transport properties with M. The origin of these variations are the systematic changes in the electronic band structure of the alloys on going from M = Cu towards M = Ti and the tendency to the formation of localized magnetic moments for M around the middle of 3d-series (M = V, Cr, Mn and Fe). The magnetic interactions strongly suppress the effects of the incipient localization.

Strong correlations and disorder in d=

The behavior of strongly correlated electrons in disordered systems is investigated using a functional integral formulation of the problem. In the mean-field approximation, which becomes exact in the limit of large spatial dimensionality, the problem reduces to the solution of an ensemble of self-consistently determined Anderson impurity models. The methods are applied to different classes of disorder, and the possible phases of the system are analyzed. We present results for the behavior of the thermodynamic and transport properties near the metal-insulator transitions for each case considered. When strong hopping disorder is present, disorder-induced local-moment formation is found, leading to qualitative modifications of metallic phases even away from the transition. Finally, we indicate how our approach can be systematically extended beyond the mean-field limit, where the presence of spatial fluctuations makes it possible to address the problem of Anderson localization in strongly correlated electronic systems.

Local Moment Formation at Cu(2) Sites around Co Atom in YBa2(Cu1-xCox)3O7+yStudied by Cu-NQR

Cu-NQR spectra in YBa 2 (Cu 1- x Co x ) 3 O 7+ y were investigated systematically from x =0 to 0.18. In the normal state, T =100 K, the integrated intensities of the spectra for both Cu(1) and Cu(2) sites decrease rapidly with increasing x , which indicates that the large region around Co atom extending to 4∼5th n.n. in Cu(1)-chain plane and 3rd∼4th n.n. in Cu(2)O 2 plane are wiped out from observation. From temperature dependence of the spectra, the magnetic ordered state associated with Co moments is confirmed to coexist with the superconducting state. The magnetic transition temperature, T M , is estimated from temperature dependence of the resonance intensities. At 1.5 K, the abnormally broad spectra ranging from 6 to 130 MHz appear, indicating the local moment formation at Cu(2) sites in the region wiped out at 100 K. Some discussions are made on the origins of the destruction of the superconductivity from the experimental results.

Density of states and magnetic susceptibilities on the octagonal tiling

We study electronic properties as a function of the six types of local environments found in the octagonal tiling. The density of states has six characteristic forms, although the detailed structure differs from site to site since no two sites are equivalent in a quasiperiodic tiling. We present the site-dependent magnetic susceptibility of electrons on this tiling, which also has six characteristic dependences. We show the existence of a non-local spin susceptibility, which decays with the square of the distance between sites and is the quasiperiodic version of Ruderman-Kittel oscillations. These results are obtained for a tight-binding Hamiltonian with pure hopping.Finally, we investigate the formation of local magnetic moments when electron-electron interactions are included.

Iron on substitutional and interstitial lattice sites in alkali metals and isomer shift systematics for interstitial iron in elemental metals.

In-beam M\ossbauer spectroscopy is applied to study implanted Fe atoms in the alkali metals Li, Na, and K. From the M\ossbauer parameters we infer that the Fe implants take up substitutional as well as interstitial sites. The interstitial Fe experiences a strongly increased s-electron density which is shown to hold quite generally in elemental metals and can be explained by the resulting lattice pressure. It is concluded that recently reported local moment formation in the alkali metals has contributions from both substitutional and interstitial Fe.

Interplay between disorder and electron interactions in a d=3 site-disordered Anderson-Hubbard model: A numerical mean-field study.

We consider a numerical mean-field study, at the unrestricted Hartree-Fock level, of a Gaussian site-disordered Anderson-Hubbard model on a simple cubic lattice. The phase diagram at half filling is obtained, including magnetic and metallic-insulating phases, and all relevant phase boundaries are found to occur in a relatively weak-coupling regime. Variation with filling fraction y is also considered, with particular reference to the y-differential disorder-induced enhancement of electron interactions that lead to site-differential local-moment formation

Hubbard models with random hopping in d=

A class of Hubbard models with random hopping is examined as models of strongly correlated electrons in disordered systems. In the limit of large spatial dimensionality the problem is reduced to solving an ensemble of self-consistently determined Anderson impurity models. Even at intermediate correlation, disorder-induced local moment formation occurs, leading to a diverging specific heat coefficient inside the metallic phase. For stronger correlation, the system undergoes a metal-insulator transition where the dc conductivity abruptly drops to zero, displaying minimum metallic conductivity.

The formation of localized moments in dilute alloys: a critical behaviour

The formation of localized moments of impurities in simple-metal hosts is investigated in the case of an Mn solute atom in jellia with continuously varying density. The mean-field critical behaviour of the transition from a spin-polarized to a non-spin-polarized state of the impurity, which is deduced from self-consistent local-spin-density (LSD) functional calculations, is analysed assuming a Landau-type expansion of the energy. This analysis is illustrated and supported by constrained LSD functional calculations.

Interplay between disorder and electron interactions: mean-field phase diagram of an Anderson-Hubbard model

To obtain a broad understanding of the combined effects of disorder and electron interactions, the phase diagram of a half-filled Gaussian site-disordered Anderson-Hubbard model is assessed numerically on a simple cubic lattice, at the unrestricted Hartree-Fock mean field level. Metallic/insulating and magnetic phases are considered. Paramagnetic, disordered antiferromagnetic and spin glass magnetic phases are found. Particular attention is given to obtaining a microscopic picture of the interplay between disorder and interaction-induced local moment formation, which underlies the metal-(gapless) insulator transition.

Statistical mean-field approach to a disordered Hubbard model

The presence of disorder, reflecting a range of local environments may lead to local moment formation on an inhomogeneous scale; the essential element of the theory is a self-consistent description of local charges and magnetic moments on sites of different site energies, in , arising from the occurrence of site disorder. The authors consider self-consistently determined local and total pseudoparticle spectra, in -dependent site occupation probabilities, and a measure of the Fermi-level charge distribution over the sites. Particular attention is given to the evolution of the interplay between disorder and interactions as the band filling fraction, y, is increased from y approximately=0 through to the half-filled limit, y=1.

In-beam M�ssbauer spectroscopy on single iron atoms in alkali metals

In-beam Mossbauer spectroscopy is applied to study implanted Fe atoms in the alkali metals Li, Na and K. From the Mossbauer parameters we infer that the Fe implants take up substitutional as well as interstitial sites. The strongly increased electron density at the interstitial position is qualitatively explained by the pressure resulting from the small interstitial volume. It is concluded that recently reported local moment formation in these systems cannot be due to substitutional Fe only.

Local spin moments of transition-metal impurities in monovalent simple-metal hosts.

The local magnetic properties of 3d and 4d substitutional impurities in noble and alkali-metal hosts are investigated by means of self-consistent local-spin-density-functional calculations using the jellium model as well as the first-principles Korringa-Kohn-Rostoker--Green's-function method. The results for the impurity moment from both calculations are in good agreement and compare well with the available experimental data. Our calculations give a consistent picture for the formation of local impurity spin moments in free-electron-like hosts.

Resonant-photoemission investigation of the Heusler alloys Ni2MnSb and NiMnSb.

Photoemission was used to investigate the electronic density of states (DOS) in ${\mathrm{Ni}}_{2}$MnSb and NiMnSb. Comparisons with calculated band structures reveal reasonable agreement, but there are indications that the calculations overemphasize Ni 3d contributions at some binding energies. Resonant effects at the Mn 3p core threshold were employed to obtain information on the Mn 3d partial densities of states and these indicate that Mn 3d character is spread throughout the valence bands, in agreement with theory. These effects are strongest in the bottom of the valence band around a binding energy of 3.1 eV and produce structure which agrees well with the theoretical Mn 3d DOS, but are weak for other portions of the calculated Mn DOS. These results are discussed in the context of models for the formation of localized moments in these materials.

Local spin-density theory of itinerant magnetism in crystalline and amorphous transition metal alloys

The authors present self-consistent spin-polarized electronic structure calculations for realistic models of amorphous transition metal alloys. The atomic structure is prepared by a simulated molecular-dynamic quench, based on interatomic forces calculated using hybridized nearly-free-electron tight-binding-bond theory. The electronic structure is calculated in the local spin-density approximation, using a linear muffin-tin orbital (LMTO) supercell approach. Detailed results for crystalline and amorphous alloys of Ni, Co and Fe with Zr are presented. NixZr1-x alloys are predicted to be paramagnetic for x<or=0.85, both in the crystalline and in the amorphous state. In CoxZr1-x the onset of magnetic ordering occurs at x approximately=0.67 for crystalline and at x approximately=0.50 for amorphous alloys. The Co-rich alloys are predicted to be strong ferrimagnets. The formation of the negative Zr moments is related to a covalent coupling of Co- and Zr-d states, which is strongest for the Co minority-spin states. The enhancement of magnetism in the glassy alloys is related to an increase of the density of states at the Fermi level induced by structural disorder. In both crystalline and amorphous FexZr1-x alloys, the onset of ferrimagnetic ordering occurs at x approximately=0.33. In contrast to the Co-based alloys, FexZr1-x alloys are weak magnets. For x>or=0.75 the competition between ferromagnetic and antiferromagnetic exchange interactions leads to the formation of negative moments on isolated Fe sites. The number of negative Fe moments increases strongly for x>or=0.90, leading to a decrease of the average moment in the Fe-rich limit. The authors show that the formation of negative local Fe moments is related to a large number of contracted Fe-Fe pairs. The predictions of local spin-density theory are found to be in excellent agreement with experiment. For all crystalline and amorphous alloys the local exchange splitting of the 3d states is shown to be correlated linearly with the local magnetic moment, with a slope of approximately=1 eV mu B-1. This relation holds for all types of magnetic order and also for the crystalline and amorphous pure metals.

Crystal structure and magnetic properties of U-Os intermetallic compounds

We have investigated the structural and physical properties of the uranium/osmium intermetallics U 3Os, U 2Os and UOs 2. The U-rich compounds crystallize in structures of low symmetry with small interatomic distances. Therefore they show band-like magnetic behavior. In contrast cubic UOs 2, crystallizing in the MgCu 2 - (Laves phase-)structure with a inter-uranium distance just below the “Hill limit” for local moment formation, orders antiferromagnetically below T N = 41.2 K. We relate these properties to the respective crystal structures and compare with other actinide/transition metal compounds.