Formation Of Magnetic Moments Research Articles

Magnetic properties of Co clusters deposited on Pt(111)

Using the fully relativistic version of the Korringa–Kohn–Rostoker method for electronic structure calculations within local spin density functional theory, the magnetic and spectroscopic properties of Co clusters deposited on Pt(111) have been investigated. Of central interest was the role of spin–orbit coupling, since it influences the spontaneous formation and orientation of magnetic moments and gives rise among other things to the occurrence of orbital magnetic moments, magnetic anisotropy energy and magnetic circular dichroism in X-ray absorption. The results have been complemented by calculations of the exchange coupling parameters Jij used within Monte Carlo simulations on the basis of the extended classical Heisenberg Hamiltonian. This allowed us to simulate the magnetic properties at finite temperatures, which are of central importance for applications.

Influence of magnetic moment formation on the conductance of coupled quantumwires

In this paper, we develop a model for the resonant interaction between a pair of coupledquantum wires, under conditions where self-consistent effects lead to the formation of alocal magnetic moment in one of the wires. Our analysis is motivated by the experimentalresults of Morimoto et al (2003 Appl. Phys. Lett. 82 3952), who showed that the conductanceof one of the quantum wires exhibits a resonant peak at low temperatures, whenever theother wire is swept into the regime where local-moment formation is expected. In order toaccount for these observations, we develop a theoretical model for the inter-wireinteraction that calculated the transmission properties of one (the fixed) wirewhen the device potential is modified by the presence of an extra scattering term,arising from the presence of the local moment in the swept wire. To determinethe transmission coefficients in this system, we derive equations describing thedynamics of electrons in the swept and fixed wires of the coupled-wire geometry. Ouranalysis clearly shows that the observation of a resonant peak in the conductanceof the fixed wire is correlated to the appearance of additional structure (near0.75 × 2e2/h or0.25 × 2e2/h) in the conductance of the swept wire, in agreement with the experimental results ofMorimoto et al.

Density-functional theory calculation of electronic and magnetic properties of hexagonalV∕Ruthin films and superlattices

Stimulated by recent experimental evidence for the formation of ultrathin close-packed films of vanadium (V) and also indication of ferromagnetic order in epitaxial $\mathrm{V}∕\mathrm{Ru}$ multilayers, we have performed first-principles electronic structure and total energy calculations to study the stability and magnetism of the metastable hcp and fcc structures of V and also the effects of the surface and interface on these properties. The systems include ultrathin films of a few monolayers of V grown on hcp (0001) ruthenium (Ru) substrates and $\mathrm{V}∕\mathrm{Ru}$ superlattices as well as bulk bcc, fcc, and hcp structures. Our investigations are based on density-functional theory with local density approximation plus generalized gradient corrections. Both the frozen-core projector augmented-wave approach and the all-electron full-potential linearized augmented-plane-wave method are utilized. First, our calculations show that all three bulk structures are nonmagnetic at their respective minimal energy lattice constants. However, all the three structures transit first to a low-spin ferromagnetic phase and then to a high-spin phase as the lattices expand. Second, we find that thin films on hcp (0001) Ru with one monolayer of V in either hcp or fcc stacking sequence have a magnetic moment of the order of $1{\ensuremath{\mu}}_{B}$. A thin film of three monolayers of V in fcc stacking sequence is weakly ferromagnetic, though the other thin films with more than one monolayer of V are essentially nonmagnetic. A free-standing film of three V monolayers in hcp stacking sequence is also ferromagnetic with sizable magnetic moments. We also find that the ${\mathrm{V}}_{2}(\mathrm{hcp})∕{\mathrm{Ru}}_{6}(\mathrm{hcp})$ and ${\mathrm{V}}_{3}(\mathrm{hcp})∕{\mathrm{Ru}}_{5}(\mathrm{hcp})$ superlattices exhibit ferromagnetism with a small total magnetic moment of a few tenths of ${\ensuremath{\mu}}_{B}$. Lattice relaxation has the trend of decreasing the magnitude of the magnetic moments. All the other ${\mathrm{V}}_{n}∕{\mathrm{Ru}}_{m}(\mathrm{hcp})$ superlattices ($n=1,4,5$ and $m=5,6$ as well as $n=3$ with fcc or bcc stacking sequence) are essentially nonmagnetic. Finally, our calculations show that the stacking sequence has significant effects on the formation of stable atomic magnetic moments of V and is also important for the energetic stability of these systems.

Magnetic moment formation in quantum point contacts

We study the formation of local magnetic moments in quantum point contacts. Using a Hubbard-like model to describe point contacts formed in a two dimensional system, we calculate the magnetic moment using the unrestricted Hartree approximation. We analyze different type of potentials to define the point contact, for a simple square potential we calculate a phase diagram in the parameter space (Coulomb repulsion - gate voltage). We also present an analytical calculation of the susceptibility to give explicit conditions for the occurrence of a local moment, we present a simple scaling argument to analyze how the stability of the magnetic moment depends on the point contact dimensions.

Electronic structure and magnetism of Fe3–xMnxAl alloys

AbstractThe electronic structure of Fe3–xMnxAl and the effect of local environment on formation of local magnetic moments are examined using self‐consistent spin‐polarised TB‐LMTO method. The electronic properties of the Fe3–xMnxAl compound have been calculated for the supercell structure with 32 atoms and in the range 0 ≤ x ≤ 0.5. The values and orientations of magnetic moments of the iron and the manganium atoms are presented. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure and magnetic moment formation in Mn–Ga–As alloys

We present calculations of the electronic and magnetic structure of the cubic phases ofMn–Ga–As. We find that for high coordination numbers, as for a bcc and fcc environment,the Ga/As atoms form broad metallic-like sp bands which interact with Mn. For most ofthese systems antiferromagnetic structures are the most stable configurations. For AsMn inthe ZnS structure we find a half metallicity both in the ferromagnetic and the [001]anti ferromagnetic modification. Calculations of Mn–GaAs supercells allow us toformulate a possible mechanism to enhance the magnetic ordering temperature. SinceTc is found to depend strongly on the number of excess electrons in the system, alloying withconstituents with fewer electrons than As could help to stabilize the FM state and thusmight increase the magnetic ordering temperature.

Real structure and magnetic properties of UN thin films

Uranium nitride thin films were prepared by reactive sputter deposition at temperatures between −200 and +400 °C. These conditions provided a large variety of microstructure. At low temperatures, nanocrystalline layers (mean crystallite size 17 nm) grew with a strong preferred orientation {1 1 1} perpendicular to the substrate, with a large compressive residual stress and with a high density of structure defects (i.e. with large microstrain). At elevated temperatures, the crystallites had only a moderate preferred orientation; the residual stress was partly relaxed and the density of structure defects lower. For the first time, for an actinide material, we have investigated how the microstructure influences the formation of magnetic moments and magnetic ordering. Bulk UN is an itinerant antiferromagnet. The increasing disorder suppresses the long-range antiferromagnetism and induces a ferromagnetic component in exchange interactions, leading to a cluster glass type of ordering at low temperatures. In highly disturbed thin films (low temperature deposition), UN exhibits a weak Pauli paramagnetism.

Magnetism of cubic Mn–Ga–As compounds: an ab initio study

We present calculations of the electronic and magnetic structure of the cubic phases ofMn–Ga–As and provide a systematic analysis of the magnetic moment formation of Mnand the stability of its various ordered magnetic configurations. We find that for highcoordination number like for a bcc and fcc environment, the Ga/As atoms form broadmetallic-like sp bands which interact with Mn. For most of these systems antiferromagneticstructures are the most stable configurations. For AsMn in the ZnS structure wefind a half-metallicity both in the ferromagnetic and the [001] antiferromagneticmodification. Calculations for Mn–GaAs supercells allow us to formulate a possiblemechanism to enhance the magnetic ordering temperature. Since the stability of theferromagnetic phase is found to depend strongly on the number of excess electrons in thesystem, alloying with constituents with fewer electrons than As could help tostabilize the FM state and thus might increase the magnetic ordering temperature.

Density-functional calculations of alpha, beta, gamma, delta, delta', and epsilon plutonium.

Total energies for the six known polymorphs of plutonium metal have been calculated within spin and orbital polarized density-functional theory as a function of lattice constant. Theoretical equilibrium volumes and bulk moduli correspond well with experimental data and the calculated total energies are consistent with the known phase diagram of Pu. It is shown that a preference for the formation of magnetic moments, increasing through the alpha-->beta-->gamma phases, explains their position in the ambient pressure phase diagram and their anomalous variation of atomic density. A simple model is presented that establishes a relationship between atomic density, crystal symmetry, and magnetic moments which is universally valid for all Pu phases.

Neutron reflectometry studies of exchange springs in DyFe2/YFe2 superlattices

Spin-polarised neutron reflectometry has been used to investigate a magnetic exchange spring, formed in a (75 Å DyFe2/150 Å YFe2)×18 multilayer thin film. The existence of an exchange spring is found to enhance the level of neutron spin flipping during reflection, a strong indicator of the formation of transverse magnetic moments within the YFe2 layers.

Formation of magnetic moments in the cuprate superconductor Hg 0.8Cu 0.2Ba 2Ca 2Cu 3O 8+ δ below Tc seen by NQR

We report pure zero field nuclear magnetic resonance (NQR) measurements on the optimally doped three layer high- T c-compounds HgBaCaCuO and HgBaCaCuO(F) with T c=134 K. Above T c two Cu NQR line pairs are observed in the spectra corresponding to the two inequivalent Cu lattice sites. Below T c the Cu NQR spectra show additional lines leading to the extreme broadened Cu NQR spectra at 4.2 K well known for the HgBaCaCuO compounds. The spin–lattice relaxation curves follow a triple exponential function with coefficients depend onto the saturation time (number of saturation pulses), whereas the spin–spin relaxation curve is described by a single exponential function. From the spin–lattice relaxation we deduced a complete removal of the Kramers degeneracy of the Cu quadrupole levels indicating that the additional lines are due to a Zeemann splitting of the 63/65Cu lines due to the spontaneous formation of magnetic moments within the CuO layers. Below 140 K, the spectra are well fitted by a number of 6 63/65Cu line pairs. From the number of the Cu lines, the position of the lines relative to each other and the complete removal of the Kramers degeneracy we deduced an orientation of the magnetic moments parallel to the c-axis with magnitudes of the order of 1000 G. We also discuss the possible microscopic origin of the observed internal magnetic fields.

Magnetic ordering in doped coupled frustrated quantum spin- [formula omitted] chains with four-spin exchange

The role of various magnetic inter-chain couplings has been investigated recently by numerical methods in doped frustrated quantum spin chains. A non-magnetic dopant introduced in a gapped spin chain releases a free spin- 1 2 soliton. The formation of a local magnetic moment has been analyzed in term of soliton confinement. A four-spin coupling which might originate from cyclic exchange is shown to produce such a confinement. Dopants on different chains experience an effective space-extended non-frustrating pairwise spin interaction. This effective interaction between impurity spins is long ranged and therefore is expected to play a crucial role in the mechanism of antiferromagnetic long-range ordering observed in spin-Peierls compounds such as CuGeO 3 doped with non-magnetic impurities.

Dependence of the superconducting transition temperature of organic molecular crystals on intrinsically nonmagnetic disorder: A signature of either unconventional superconductivity or the atypical formation of magnetic moments

We give a theoretical analysis of published experimental studies of the effects of impurities and disorder on the superconducting transition temperature ${T}_{c}$ of the organic molecular crystals $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}X$ (where $X=\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_{2}]\mathrm{Br}$ and $\mathrm{Cu}(\mathrm{NCS}{)}_{2}$ and BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene) and $\ensuremath{\beta}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}X$ (for ${X=I}_{3}$ and ${\mathrm{IBr}}_{2}).$ The Abrikosov-Gorkov (AG) formula describes the suppression of ${T}_{c}$ both by magnetic impurities in singlet superconductors, including s-wave superconductors and by nonmagnetic impurities in a non-$s$-wave superconductor. We show that various sources of disorder (alloying anions, fast electron irradiation, disorder accidentally produced during fabrication, and cooling rate induced disorder) lead to the suppression of ${T}_{c}$ as described by the AG formula. This is confirmed by the excellent fit to the data, the fact that these materials are in the clean limit and the excellent agreement between the value of the interlayer hopping integral ${t}_{\ensuremath{\perp}}$ calculated from this fit and the value of ${t}_{\ensuremath{\perp}}$ found from angular-dependent magnetoresistance and quantum oscillation experiments. There are only two scenarios consistent with the current state of experimental knowledge. If the disorder induced by all of the four methods considered in this paper is, as seems most likely, nonmagnetic then the pairing state cannot be s wave. We show that published measurements of the cooling rate dependence of the magnetization are inconsistent with paramagnetic impurities. Triplet pairing is ruled out by NMR and upper critical field experiments. Thus if the disorder is nonmagnetic then this implies that $l>~2,$ in which case Occam's razor suggests that d-wave pairing is realized in both $\ensuremath{\beta}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}X$ and $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}X.$ However, particularly given the proximity of these materials to an antiferromagnetic Mott transition, it is possible that the disorder leads to the formation of local magnetic moments via some atypical mechanism. Thus we conclude that either $\ensuremath{\beta}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}X$ and $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}X$ are d-wave superconductors or else they display an atypical mechanism for the formation of localized moments, possibly related to the competition between the antiferromagnetic and superconducting grounds states. We suggest systematic experiments to differentiate between these two scenarios.

Neutron reflectometry studies of exchange springs in DyFe2/YFe2 superlattices

Spin-polarised neutron reflectometry has been used to investigate a magnetic exchange spring, formed in a (75 A DyFe2/150 A YFe2)×18 multilayer thin film. The existence of an exchange spring is found to enhance the level of neutron spin flipping during reflection, a strong indicator of the formation of transverse magnetic moments within the YFe2 layers.

Magnetic moment formation at a dilute Ta impurity in RCo 2 intermetallic compounds

In this work we study the formation of the local magnetic moments and the related magnetic hyperfine fields at a Ta impurity diluted on the R site of the RCo 2 intermetallic hosts (R= rare-earth elements). In order to calculate the local magnetic moment at a Ta impurity diluted in these intermetallics, we use a two band model, including the nearest-neighbor perturbation due to the translational invariance breaking introduced by the impurity. The calculated total hyperfine fields are in good agreement with the available experimental data.

Doped coupled frustrated spin-1/2 chains with four-spin exchange.

The role of various magnetic interchain couplings is investigated by numerical methods in doped frustrated quantum spin chains. A nonmagnetic dopant introduced in a gapped spin chain releases a free spin-1/2 soliton. The formation of a local magnetic moment is analyzed in terms of soliton confinement. A four-spin coupling which might originate from cyclic exchange is shown to produce such a confinement. Dopants on different chains experience an effective space-extended nonfrustrating pairwise spin interaction.

Zeeman splitting of the 63,65Cu NQR line in optimally doped Hg1Ba2Ca2Cu3O8+δ below Tc

We report pure zero field nuclear magnetic resonance (NQR) measurements on the optimally doped three layer compound Hg1Ba2Ca2Cu3O8+δ (Hg-1223) with Tc=134 K. Above Tc two Cu line pairs are observed in the spectrum corresponding to the two inequivalent Cu lattice sites. Below Tc the Cu NQR spectrum shows a splitting of these two line pairs into six line pairs. From a multiexponential spin lattice relaxation with a dependency on the initial conditions, we find a formation of magnetic moments in the CuO layers as the origin of the line splitting. Furthermore, the magnetic moments are oriented parallel to the symmetry axis of the electrical field gradient tensor with magnitudes of the order of 1000 G.

Hyperfine interactions in Sc1−x YxFe2 cubic Laves alloys

Effects of hybridization of 3d bands of iron with 3d bands of scandium and 4d bands of yttrium in Sc1−xYxFe2 cubic Laves alloys (0≤x≤1) are studied by the nuclear magnetic resonance method. The concentration dependences of the lattice parameters a, saturation magnetization σ, and hyperfine fields at the 57Fe, 45Sc, and 89Y nuclei—as well as the 27Al impurity nuclei, whose atoms substitute iron atoms in the lattices of these alloys—are measured. The “local” and “induced” contributions to hyperfine fields at the 57Fe nuclei are separated and the magnetic moments at iron atoms are estimated. It is found that the hybridization effect leads to the formation of magnetic moments at Sc and Y atoms (whose direction is opposite to the direction of the magnetic moment at iron atoms) and is responsible for the ferrimagnetic structure in Sc1−xYxFe2 alloys.

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.

Magnetic susceptibility of (Zr 80Fe 20) 1− xH x metallic glasses

The magnetisation of hydrogen-doped (Zr 80Fe 20) 1− x H x metallic glasses has been measured in the temperature range 1.7–100 K for various dopant concentrations. The system is paramagnetic with magnetic susceptibilities that are only weakly temperature dependent down to about 25 K and hydrogen concentration x<0.03. For a larger hydrogen concentration, x=0.1, the magnetic susceptibility becomes strongly temperature dependent and shows the Curie–Weiss behaviour. It has been also found that the magnetic susceptibility at 100 K, χ(100 K), decreases for hydrogen concentration x=0.03, while for x=0.1 it increases again. This behaviour is primarily due to the increased contribution of the Fe 3d-electronic states to the density of states at the Fermi level and to the formation of spin fluctuations and magnetic moments on the Fe sites at hydrogen concentrations x>0.03. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the Curie–Weiss term and the quantum corrections to the susceptibility.