Local Moment Formation Research Articles

Evidence for local moment formation around a positive muon in graphite

The local-electronic structure of the positive muon has been investigated in the semimetal graphite using muon spin rotation/relaxation. Both the muon Knight shift and spin-relaxation rate in highly oriented pyrolytic graphite are anomalously large compared to simple metals and both have an unusual temperature dependence. These results indicate that a local moment forms around the muon due to the low carrier density. In contrast, measurements on metallic ${\mathrm{LiC}}_{6}$ reveal a smaller muon Knight shift, which is opposite in sign (negative) and almost temperature independent. We suggest this is due to the core polarization of a ${\mathrm{Mu}}^{\ensuremath{-}}{\mathrm{Li}}^{+}$ complex.

Electronic structure and magnetism of Fe 3− xCr xSi alloys

The study of the electronic structure and magnetic properties of the Fe 3− x Cr x Si compound is motivated by the Mössbauer and neutron spectroscopy measurements showing unusual behaviour of Cr in such alloys. The site preferences of Cr in Fe 3Si and the effect of local environment on formation of local magnetic moments are examined using self-consistent spin-polarised TB-LMTO method. The results of calculations essentially confirm the values and orientations of magnetic moments of iron and chromium atoms. However, they do not explain an experimental observation of almost equal occupation of A, B and C sites by chromium.

Local-moment formation in the periodic Anderson model with superconducting correlations

We study local-moment formation in the presence of superconducting correlations among the f electrons in the periodic Anderson model. Local moments form if the Coulomb interaction $U>{U}_{\mathrm{cr}}.$ We find that ${U}_{\mathrm{cr}}$ is considerably stronger in the presence of superconducting correlations than in the nonsuperconducting system. Our study is done for various values of the f-level energy and electronic density. The smallest critical ${U}_{\mathrm{cr}}$ values occur for the case where the number of f electrons per site is equal to 1. In the presence of d-wave superconducting correlations we find that local-moment formation presents a quantum phase transition as function of pressure. This quantum phase transition separates a region where local moments and d-wave superconductivity coexist from another region characterized by a superconducting ground state with no local moments. We discuss the possible relevance of these results to experimental studies of the competition between magnetic order and superconductivity in ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}.$

Signatures of spin and charge energy scales in the local moment and specific heat of the half-filled two-dimensional Hubbard model

Local moment formation driven by the on--site repulsion $U$ is one of the most fundamental features in the Hubbard model. At the simplest level, the temperature dependence of the local moment is expected to have a single structure at $T \sim U$, reflecting the suppression of the double occupancy. In this paper we show new low temperature Quantum Monte Carlo data which emphasize that the local moment also has a signature at a lower energy scale which previously had been thought to characterize only the temperatures below which moments on {\it different} sites begin to correlate locally. We discuss implications of these results for the structure of the specific heat, and connections to quasiparticle resonance and pseudogap formation in the density of states.

Local magnetic moments and hyperfine magnetic fields in disordered metal-metalloid alloys

The local magnetic moments and hyperfine magnetic fields (HFF's) in the ordered alloys ${\mathrm{Fe}}_{15}\mathrm{Sn}$ and ${\mathrm{Fe}}_{15}\mathrm{Si}$ are calculated with the first-principles full-potential linear augmented plane wave method. The results are compared with the experimental data on Fe--M (M=Si, Sn) disordered alloys at small metalloid concentration. The relaxation of the lattice around the impurity and its influence on the quantities under consideration are studied. The mechanism of the local magnetic moment formation is described. It is proved that the main distinction between these alloys is connected with the different lattice parameters. Three contributions to the HFF are discussed: the contributions of the core and valence electron polarization to the Fermi-contact part, and the contribution from the orbital magnetic moment.

Local magnetic moments and hyperfine magnetic fields in Fe-M (M = Si, Sn) alloys at small metalloid concentrations

The main tendencies in the formation of local magnetic moments and hyperfine magnetic fields at Fe nuclei in Fe-Sn and Fe-Si alloys at low metalloid concentrations are analyzed on the basis of “first-principles” calculations. The results of calculations are compared with experimental data. The main differences between these alloys were proved to be due to the differences in their lattice parameters. It is shown that a significant contribution to the formation of the hyperfine field comes from the orbital magnetic moment and the Ruderman-Kittel-Kasuya-Yosida polarization, which depend on the impurity concentration and the distance to an impurity atom in the crystal lattice.

Magnetic behavior of interstitial Fe impurities in divalent Ca, Sr, and Yb hosts

We have used a first-principles real-space approach to investigate the electronic structure and the magnetic behavior of interstitial Fe impurities in divalent Ca, Sr, and Yb hosts. The dependence of the local moment as a function of lattice relaxation around the impurity is obtained and contrasted with that of interstitial Fe in trivalent and tetravalent Zr, Y, Ti, and Sc hosts. The trends obtained for local moment formation at the impurity site are in agreement with experimental time-differential perturbed g-ray angular distribution technique observations.

Specific heat anomaly and adiabatic hysteresis in disordered electron systems in a magnetic field

: We consider the thermodynamic behavior of a disordered interacting electron system in two dimensions. We show that the corrections to the thermodynamic potential in the weakly localized regime give rise to a non monotonic behavior of the specific heat both in temperature and magnetic field. From this effect we predict the appearance of adiabatic hysteresis in the magnetoconductance. Our results can be interpreted as precursor effect of formation of local moments in disordered electron systems. We also comment on the relevance of our analysis in three dimensional systems.

Magnetic phases of FexV1−x S and their electronic structure

A study has been made of the FexV1−x S solid solutions with 0 0.1, x-ray diffraction analysis discloses a V5S8 superstructure. Samples with x>0.1 are magnetically ordered at room temperature. The concentration dependences of resistivity and magnetization exhibit sharp peaks for x=0.1 and x=0.2, respectively. The main features of the structure and electronic properties have been qualitatively explained in terms of the three-band exciton-insulator model, and the maxima in resistivity and magnetization are assigned to the formation of localized magnetic moments with S=1, which become delocalized with increasing x.

Local moment formation in Zn-doped cuprates

We suggest that when zinc is substituted for copper in the copper oxide planes of high $T_{c}$ superconductors, it does not necessarily have a valency of 2+. Rather, the valency of a zinc impurity should be determined by its surrounding medium. In order to study this hypothesis, we examine the effect of static impurities inducing diagonal disorder within a one band Hubbard model coupled to a localised state. We use this model to discuss the physics of zinc doping in the cuprates. Specifically, we discuss the formation of local moments near impurity sites and the modification of the transverse spin susceptibility in the antiferromagnetic state.

Theoretical study of hyperfine fields at impurity nuclei in GdX (X=Zn,Cd) compounds: A two-center model

We theoretically discuss the local moment formation and the hyperfine field behavior of nonmagnetic (s−p and noble) impurities diluted in ferromagnetic rare-earth compounds GdX (X=Zn, Cd). It is experimentally observed in these systems that all impurities enter substitutionally at the “nonmagnetic” X lattice site, creating a strong local charge perturbation. In our simplified model, one calculates the local magnetic moment in the perturbed conduction band at the origin (a charge perturbed X site). The polarization of the conduction band arises from the Gd 4f moments and the conduction electron polarization produced by the host magnetic moment at the origin is treated within the Born approximation. The general trend of the experimental hyperfine field data is well reproduced by our self-consistent calculations.

Effect of Level Statistics on Local Magnetism in Nanoscale Metallic Grains

Effect of level statistics on local electronic states and local magnetism in nanoscale metallic grains with transition-metal impurity in the ballistic regime is studied. It is shown that the mean occupation of local electron and the local magnetic moment in nanoscale metallic grains with odd conduction-electrons are larger than those with even conduction-electrons. The effect of even-odd parity on the condition for the occurrence of local magnetic moment is also discussed, it is found that the critical value, ρ d (0) U c , for the formation of local moment in nanoscale metallic grains is much smaller than that in bulks. The dependences of the local spin susceptibility on size and the Coulomb interaction are obtained. These results show that the level statistics plays an important role for the local magnetism, it distinguishes the properties of nanoscale metallic grains from those of clusters and bulks.

Magnetic properties of strongly disordered electronic systems

We present a unified, global perspective on the magnetic properties of strongly disordered electronic systems, with special emphasis on the case where the ground state is metallic. We review the arguments for the instability of the disordered Fermi liquid state towards the formation of local magnetic moments, and argue that their singular low temperature thermodynamics are the ``quantum Griffiths'' precursors of the quantum phase transition to a metallic spin glass; the local moment formation is therefore not directly related to the metal-insulator transition. We also review the the mean-field theory of the disordered Fermi liquid to metallic spin glass transition and describe the separate regime of ``non-Fermi liquid'' behavior at higher temperatures near the quantum critical point. The relationship to experimental results on doped semiconductors and heavy-fermion compounds is noted.

Foreword: Nuclear methods in magnetism

Usually, the occurrence of magnetism on isolated, substitutional 3d, 4d and 5d impurity ions in metals is restricted to certain 3d ions (mainly Cr, Mn, Fe, Co, Ni) in alloying metallic systems. The application of the perturbed γ-ray distribution method following heavy ion reactions and recoil implantation has offered an experimental technique for producing and investigating new magnetic systems. Of special importance are nonalloying systems, which can exhibit extreme variations of e.g. density of states and atomic volume in the probe–host combinations produced by recoil implantation. Recent developments in this field include the following: Magnetism and the Kondo effect observed for 43Sc ions in alkali metal hosts are found to be consistent with a nearly localised, ionic 3d1 single-electron configuration, and parallel the behaviour observed in certain Ce systems. More generally, essential features of the magnetism of 3d and 4d ions in sp metal hosts are similar to those of 4f systems. Recent experimental and theoretical studies of 54Fe in d-band metal hosts are of key importance for an understanding of the basic features of local moment formation on substitutional Fe ions in transition metal hosts in general. In many nonalloying 54Fe probe-host combinations, (at least) two different magnetic responses have been detected. These components correspond to substitutional and interstitial sites of the implanted probes, as has been verified by in-beam Mössbauer spectroscopy of 57Fe in a series of host metals. This provides new insight into lattice site occupation as a function of host properties and allows directed investigations of the magnetic behaviour of Fe (and Mo) ions on interstitial lattice sites. Depending on the host metal, interstitial Fe is found to be nonmagnetic, e.g. in Zr, or magnetic, e.g. in Yb. Surprisingly, even the 4d ion Mo can be magnetic on interstitial sites. The experimental results for the substitutional as well as the interstitial sites can be compared to extensive theoretical work within the framework of local spin density calculations.

Magnetic Behaviour of Implanted Transition Metal Probes at Different Lattice Sites in Metals

Usually, the occurrence of magnetism on isolated, substitutional 3d, 4d and 5d impurity ions in metals is restricted to certain 3d ions (mainly Cr, Mn, Fe, Co, Ni) in alloying metallic systems. The application of the perturbed γ-ray distribution method following heavy ion reactions and recoil implantation has offered an experimental technique for producing and investigating new magnetic systems. Of special importance are nonalloying systems, which can exhibit extreme variations of e.g. density of states and atomic volume in the probe–host combinations produced by recoil implantation. Recent developments in this field include the following: Magnetism and the Kondo effect observed for 43Sc ions in alkali metal hosts are found to be consistent with a nearly localised, ionic 3d1 single-electron configuration, and parallel the behaviour observed in certain Ce systems. More generally, essential features of the magnetism of 3d and 4d ions in sp metal hosts are similar to those of 4f systems. Recent experimental and theoretical studies of 54Fe in d-band metal hosts are of key importance for an understanding of the basic features of local moment formation on substitutional Fe ions in transition metal hosts in general. In many nonalloying 54Fe probe-host combinations, (at least) two different magnetic responses have been detected. These components correspond to substitutional and interstitial sites of the implanted probes, as has been verified by in-beam Mössbauer spectroscopy of 57Fe in a series of host metals. This provides new insight into lattice site occupation as a function of host properties and allows directed investigations of the magnetic behaviour of Fe (and Mo) ions on interstitial lattice sites. Depending on the host metal, interstitial Fe is found to be nonmagnetic, e.g. in Zr, or magnetic, e.g. in Yb. Surprisingly, even the 4d ion Mo can be magnetic on interstitial sites. The experimental results for the substitutional as well as the interstitial sites can be compared to extensive theoretical work within the framework of local spin density calculations.

Zinc valency and local moments in impurity doped cuprates

We suggest that when zinc is substituted for copper in the high T c superconductors, it does not necessarily have a valency of 2 +. Rather, the valency of a zinc impurity should be determined by its surrounding medium. To elaborate on this, we examine the effect of static impurities inducing diagonal disorder within a one band Hubbard model coupled to a localised state. We discuss the formation of local moments on and near impurity sites in the antiferromagnetic phase.

Strong antiferromagnetic spin fluctuations and the quantum spin-liquid state in geometrically frustrated -Mn, and the transition to a spin-glass state caused by non-magnetic impurity

The magnetic properties of -Mn and the effect of the substitution of non-magnetic Al were systematically investigated by measuring thermodynamic and transport properties, and the NMR and polarized neutron scattering for alloys with -Mn is paramagnetic down to the lowest temperature (1.4 K) but with strong spin fluctuations. Substitution of Al atoms which preferentially occupy one of two crystallographic sites (site II) results in the formation of local moments on Mn atoms, and makes the ground state into a spin-glass-like state. NMR measurements revealed almost independent magnetic behaviour of the two different sites, and static magnetic ordering of Mn at site II for , although the possibility of very weak magnetism of the site-I atoms cannot be excluded. The temperature dependence of the spin - lattice relaxation rate indicates antiferromagnetic correlations of paramagnetic spin fluctuations, and its saturation at higher temperatures for and a critical slowing down of the fluctuations at low temperatures for . Polarized neutron scattering experiments showed directly strong spin fluctuations with antiferromagnetic correlations for both -Mn and . The characteristic energy of fluctuations for -Mn is fairly large even at 7 K , implying a quantum origin of the fluctuations, while the energy spectrum of becomes very sharp at low temperatures, within a resolution-limited energy width, indicating damping of spin fluctuations into a spin-glass-like state. These results can be interpreted in terms of the transition from a spin liquid to a spin glass caused by the substitution of Al that gives rise to the release of the antiferromagnetic frustration of the characteristic crystal lattice. Considerable similarity of the characteristic features to those of the highly frustrated Laves phase compounds is argued. The possible frustration on site II in the -Mn structure, a three-dimensional network of corner-sharing regular triangles, which is similar to the two-dimensional Kagomé lattice, is pointed out.

Local moment formation at interstitial impurity sites in metals

We have calculated the electronic structure and related properties around an interstitial impurity in several metallic hosts. This was done using the real-space linear muffin tin orbital scheme (RS-LMTO-ASA), a first-principles, self-consistent approach implemented directly in real space. We show that interstitial Fe does not develop a local moment in trivalent and tetravalent Sc, Y, Ti and Zr hosts. In divalent Ba, Ca, Sr and Yb we find that the appearance of local moment is extremely dependent on nearest neighbor relaxation, while in alkali metals such as K, we expect the interstitial Fe impurity to be magnetic. We show that these trends can be qualitatively understood using simple ideas based on the Wolff model and the Stoner criterium. We also consider Fe impurities in Gd, a trivalent magnetic host. We find an unusually large induced local moment at the interstitial Fe site and discuss the origin of this effect. Finally, we compare our results with TDPAD and in beam Mossbauer experiments in these systems.

Magnetic Behavior of Fe Impurities in Tc and Re, and Its Relevance to the General Problem of the Magnetism of Fe in d-Band Metal Hosts.

We have observed implanted Fe ions to be nonmagnetic in the group VIIb hosts Tc and Re, by applying the in-beam perturbed angular {gamma}-ray distribution method. This result is in agreement with local spin density calculations for Fe in Tc using both supercell and single impurity approaches. The results are of key importance for a reasonable and consistent understanding of the basic, general features of local magnetic moment formation on Fe ions in transition metal hosts. {copyright} {ital 1996 The American Physical Society.}

Local properties at Fe and Co impurities in Nb1-xMox alloys.

We have investigated the influence of the local environment on the magnetic behavior of substitutional Fe or Co impurities in ${\mathrm{Nb}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mo}}_{\mathit{x}}$ alloys. We have found that the behavior of Fe and Co impurities is very similar in the systems considered here. We have studied the physical mechanisms of the local moment formation at the Fe site in those alloys and discussed what has been suggested in the literature by a different theoretical approach. The isomer shift and hyperfine field at the impurity site have been calculated and compared with experimental data available in the literature. The self-consistent calculations were performed using the real-space linear muffin-tin orbital scheme. \textcopyright{} 1996 The American Physical Society.