Appearance Of Magnetic Moments Research Articles

多体問題に対する微分形式による数値厳密解

The Many-body problem is very important issue in physics. Usually perturbation calculations based on the Green's function have been used to analyze such a problem, however, infinite orders of perturbation calculations are required to obtain exact solutions, thus resulting in the analytical and numerical difficulties. To overhaul such difficulties, we propose the new calculation method based on the differential forms, which are able to evaluate exact solutions if Hamiltonian is composed of Fermi particles. Since our proposed method is based on time evolution equations, comparisons with the calculations derived from Feynman Kernel is possible, with showing complete agreement. Furthermore we applied this method to the simplest Anderson Hamiltonian to investigate the appearance of magnetic moment. The calculation results show that magnetic moment easily disappears with small Coulomb repulsive energy, possibly implying the spin fluctuations.

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.

Magnetic States of the Zigzag Edge of a Graphene Nanoribbon

Using a simple structural model and the multicenter Anderson Hamiltonian, Green’s functions are obtained for the atoms of the zigzag edge of an epitaxial graphene nanoribbon. The electronic structure of the free nanoribbon is discussed in detail. Specifically, expressions for the band spectrum and density of states are found and estimates of the occupation numbers and magnetic moments are given. For a nanoribbon strongly bonded to a metal substrate, the criteria for the appearance of magnetic moments are determined. As it is shown for both free and epitaxial nanoribbons, the probability of the appearance of magnetic moments and their magnitude for zigzag edge atoms that have two nearest neighbors is higher than for atoms with three nearest neighbors.

О магнитных состояниях зигзагообразной кромки графеновой наноленты

Abstract Using a simple structural model and the multicenter Anderson Hamiltonian, Green’s functions are obtained for the atoms of the zigzag edge of an epitaxial graphene nanoribbon. The electronic structure of the free nanoribbon is discussed in detail. Specifically, expressions for the band spectrum and density of states are found and estimates of the occupation numbers and magnetic moments are given. For a nanoribbon strongly bonded to a metal substrate, the criteria for the appearance of magnetic moments are determined. As it is shown for both free and epitaxial nanoribbons, the probability of the appearance of magnetic moments and their magnitude for zigzag edge atoms that have two nearest neighbors is higher than for atoms with three nearest neighbors.

First-Principles Modeling of Atomic Structure and Chemical and Optical Properties of β-C3N4

In our work, we report the results of first-principles modeling of optical and chemical properties of β-C3N4 in bulk (pristine and defected), surface, and nanoclusters. We demonstrate significant sensitivity of adsorption spectra of β-C3N4 to any kinds of disorder in atomic structure. Formation and passivation of the surface provides similar changes in optical properties. The value of the indirect bandgap depends on the chemical structure of the surface. The surface of the bulk crystal and nanocluster is chemically active and unavoidably passivated at ambient conditions. Partial oxidation of the surface of β-C3N4 provides decreasing of the bandgap. Functionalization of the active sites on the surface by monovalent species (hydrogen and fluorine) leads to vanishing of the bandgap in the case of (001) surface and changes the value of the bandgap in the case of nanoclusters. Results of our calculations also demonstrate the appearance of magnetic moments in hydrogenated and fluorinated (001) surface of β-C3N4.

Electronic Structure and Magnetic Properties of CuFeS2.

Chalcopyrite (CuFeS2) is an antiferromagnetic semiconductor with unusual magnetic and electrical properties, which are still not clearly understood. Neutron diffraction experiments reveal a phase transition at ∼50 K that has been attributed to an unexpected appearance of magnetic moments on Cu ions, having a paramagnetic arrangement down to 50 K and then ordering to an antiferromagnetic state at lower temperatures. In this study we use DFT-based computational methods to investigate the electronic structure and magnetic properties of CuFeS2 in order to obtain a reliable source of information for the interpretation of the observed magnetic behavior, and in particular to shed some light on the magnetic behavior of copper atoms in this compound. We have calculated the electronic structure of the ground and low-energy magnetically excited states and deduced a set of exchange coupling constants that are used afterward in classical Monte Carlo simulations to obtain magnetic susceptibility data, which compare successfully with our experimental results above ∼170 K. From our results it can be inferred that copper atoms remain in a diamagnetic state in this temperature range, although spin delocalization from neighboring iron atoms results in a non-negligible spin density on the copper atoms at high temperatures.

Magnetic properties of AII–BIV–C2V chalcopyrite semiconductors doped with 3d‐elements

The ab initio calculations of electronic and magnetic properties of chalcopyrites (II = Be, Zn, Cd; IV = Si, Ge; V = P, As) doped with 3d metals are presented within the scope of interest for semiconductor spintronics. Different concentrations of V, Cr, Mn, Fe, and Co atoms replacing II–II, II–IV, and IV–IV group sites are checked and the most stable configurations for each case are determined. The appearance of magnetic moments and the ferromagnetic state upon doping is analyzed. It is revealed that for all materials incorporation of Cr or Mn atoms leads to the appearance of magnetic moments, but only substitution of II group atoms yields a stable configuration with a ferromagnetic state. High spin polarization is obtained for Zn–IV–P2 and Zn–IV–As2 compounds whose lattice parameters are close to that of silicon and germanium (or GaAs), respectively. Moreover, the substitution of Be by Cr atoms in Be–IV–As2 semiconductors (having a lattice parameter close to silicon) transforms them into half‐metals stable in the ferromagnetic state. Recommendations upon the further applicability of investigated materials for spintronics are proposed.

Synthesis, structural and magnetic properties of self-organized single-crystalline nanobricks of chalcopyrite CuFeS2

A thermal pyrolysis method has been developed to synthesize the tetragonal phase of chalcopyrite magnetic semiconductor CuFeS2 nanoparticles. All nanoparticles have the same anisotropic brick-like shape, and the “bricks” are self-organized in a certain orientation, creating well-ordered nanocomposites. High-resolution transmission electron microscopy and electron diffraction data show that every nanobrick is a single crystal with a layered atomic structure and characteristic dimensions of about 5nm×20nm in plane. Magnetic measurements support the antiferromagnetic spin structure and reveal the appearance of a small ferromagnetic component below 60K. Magnetic anomalies observed in the zero-field cooled magnetization curves at low temperatures may be related to an appearance of magnetic moment at the Cu ion site. The Mössbauer spectra show that only about 50% of Fe atoms are in the magnetically ordered α-phase of chalcopyrite. The remaining Fe is non-magnetic and may be located either in the γ-phase of chalcopyrite or in isocubanite.

Specific Heat of theS=1/2 Two-Dimensional Shastry–Sutherland Antiferromagnet SrCu2(BO3)2in High Magnetic Fields

We characterize the field-induced magnetic phases of SrCu 2 (BO 3 ) 2 , a frustrated spin-1/2 Heisenberg antiferromagnet in the two-dimensional Shastry–Sutherland lattice, using specific heat in magnetic fields up to 33 T. We find that the spin gap persists above the expected critical field H c = Δ/gµ B of 21 T despite the appearance of magnetic moment in the ground state. At the magnetization plateau at 1/8 of the saturation, the triplets that carry the magnetization of the ground state are observed to form a two-dimensional gas of large mass. A gas consisting of the same number of heavy particles continues to completely dominate the specific heat in the field region above the plateau, although the magnetization increases with increasing field. Ordering is observed immediately below the temperature region of the gas phase.

Magnetism induced by single carbon vacancies in a three-dimensional graphitic network

We present an ab initio density functional theory study of the magnetic moments that arise in graphite by creating single carbon vacancies in a three-dimensional (3D) graphite network using full potential, all electron, spin polarized electronic structure calculations. In previous reports, the appearance of magnetic moments was explained in a two-dimensional graphene sheet just through the existence of the vacancies itself [Carbon-Based Magnetism, edited by F. Palacio and T. Makarova (Elsevier, Amsterdam, 2005); D. C. Mattis, Phys. Rev. B 71, 144424 (2005); Y. Kobayashi et al., ibid. 73, 125415 (2006); R. Yoshikawa Oeiras et al., ibid. (to be published); P. O. Lehtinen et al., Phys. Rev. Lett. 93, 187202 (2004)]. The dependence of the arising magnetic moment on the nature and geometry of the vacancies for different supercells is reported. We found that the highest value of magnetic moment is obtained for a $3\ifmmode\times\else\texttimes\fi{}3\ifmmode\times\else\texttimes\fi{}1$ supercell and that the highly diluted $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}1$ supercell shows no magnetic ordering. The results obtained in this paper are indicative of the importance of interlayer interactions present in a 3D stacking. We conclude that this should not be underestimated when vacancy-based studies on magnetism in graphitic systems are carried out.

Magnetic ordering in spin-chainEu2BaNiO5 from 151Eu Mössbauer measurements

We present 151Eu Mössbauermeasurements on Eu2BaNiO5, where the Ni2+ formwell separated S = 1 antiferromagnetic chains. The appearance of magnetic moments in thiscompound evidences a ‘crossed bootstrap’ polarization since neither theEu3+ northe Ni2+ will carry magnetic moments unless polarized by an exchangefield coming from the other sublattice. We confirm that theEu3+ ion carries a magnetic moment below 30 K implying thatNi2+ ordered moments also appear below this temperature. We measure the thermal dependence of the sizeof the Eu3+ moment and of the polarizing field produced by theNi2+. Both show distributions which we link to the role of defects in the nickel chains. Themeasurements also provide the thermal dependence of the relative size of theNi2+ moments. We compare the rare earth and nickel properties ofEu2BaNiO5 with those of isomorphous compounds.

Small magnetic moment and superconductivity in URu 2Si 2

One of the most puzzling properties observed in URu 2Si 2 has been the large jump of specific heat around 17 K for small magnetic moments. This specific heat jump is attributed to the quadrupolar order rise around 17 K instead of the dipolar order. Neutron experiments showed about 0.03 μ B at low temperatures whereas no evidences were found in NMR experiments for static magnetic order. These experiments suggest that Bragg peaks observed in the neutron experiments are not really due to static magnetic order but due to quasi-static ones. However, new NMR experiments clearly show the internal field below 17 K, corresponding to the appearance of magnetic moments of 0.2 μ B. The intensity of the signal is small, suggesting that a small portion of the sample is magnetic while the other portions are non-magnetic. We present a scenario, consistent with these experiments, based on the assumption that the sample is a mixture of dipolar and quadrupolar phases, and also taking account of quasi-static magnetic fluctuations. We also discuss the pressure effects in the sample.

Magnetic properties of the Gd 3 Ni single crystal

The magnetization measurements of the Gd3Ni single crystal are presented. Below the ordering temperature, TN=100 K, the field dependence of the magnetization, measured along three basic symmetry axes, exhibit one or two magnetic transitions associated with ferromagnetic and antiferromagnetic components of the complex magnetic structure. The anomalous behavior in paramagnetic range of temperatures is, probably, related with field induced magnetic ordering and appearance of magnetic moments of nickel atoms.

Conditions on the occurrence of magnetic moments in quasicrystals and related phases

We analyze the criterion for the appearance of magnetic moments in quasicrystals approximants and liquids Al–Mn. In a Hartree–Fock scheme, it is shown that the Stoner criterion (Und(EF)≥1) does not apply to these systems due to the presence of the pseudogap in the density of states. We give a generalized criterion that can take into account the particular density of state. The main result is that the appearance of a magnetic moment on a Mn atom depends strongly on its position in the crystalline structure. Indeed, some particular positions of the Mn atoms lead to magnetic moments whereas other positions are more stable when the Mn is non-magnetic. This criterion is used to analyze the electronic structure of two approximants (1/1-Al–Pd–Mn–Si and T–Al–Pd–Mn) calculated by the self-consistent linear muffin tin orbitals (LMTO)-ASA method.

Appearance of Magnetic Moments in 3d Transition Metals in bcc and fcc lattices.

Spin-unrestricted band calculation for 3d transition metals (atomic number Z=24-28) in bcc and fcc lattices was performed with various lattice parameter. It is shown that magnetic moments generally appear or disappear in all 3d metals investigated when the atomic volume is increased or decreased for bcc and fcc structures.Comparison between the total energy obtained by spin-restricted and spin-unrestricted calculation indicate that there is a region in which non-magnetic state is stable, in spite of the appearance of a ferromagnetic self-consistent solution.

Surface enhancement of palladium

The critical concentration required for the appearance of magnetic moments on nickel atoms deposited on a palladium substrate and covered with a thin palladium overlayer is calculated in the Ginzburg-Landau model. The effect of the palladium surface is characterised in this model by a surface extrapolation length lambda . The extrapolation length lambda is determined by comparing the calculated and observed critical concentrations for a Pd-Ni-Pd sandwich with 1% of Ni atoms covered by a palladium overlayer of four atomic layers. The negative value of lambda obtained by this method implies that the exchange enhancement of the palladium surface is stronger than in the bulk.

An analytic study of the friedel criterion in disordered magnetic states in transition metals in the presence ofS-D hybridisation, using the cluster bethe lattice approach

In this paper we present a new theory for including both short range order and long range order simultaneously in the well known cluster Bethe lattice method for binary alloys. We have used this theory for obtaining the Friedel criterion for the appearance of magnetic moments in disordered states using the single band Hubbard model. This is followed by a study of this criterion in a two-bands-d hybridised Hubbard model, which is considered as a simulation of real transition metals. A new technique for solving this problem in the Bethe lattice network is presented, which yields an analytic solution for the critical correlation strength in the presence of hybridisation and short range order. It is found that in all cases hybridisation tends to diminish the tendency for magnetisation, which is in accord with physical expectations.

Magnetic moments in metallic sandwiches

The Ginzburg-Landau equation, previously used for bulk Pd-Ni alloys, is applied to discuss the appearance of magnetic moments at T=0 in a Pd-Ni-Pd sandwich. Using parameters deduced from the analysis of the bulk alloys the authors predict that the critical concentration of Ni atoms is 14.1% for a (100) monolayer and 11.1% for a (111) monolayer.

Appearance of magnetic moment in vanadium metal fine particles

The magnetic susceptibility of vanadium metal fine particles (90 to 300 Å) which were prepared by the gas evaporation method was measured in the temperature range between 4.2 and 300 K. The magnetic susceptibility of these fine particles depends upon the diameter of the particle and it consists of two parts, one was the Pauli type and the other was the Curie-Weiss type overlapping the temperature independent one. The fact that the Curie constant estimated from the C-W part of the susceptibility is proportional to the reciprocal diameter of fine particle suggests that magnetic moment originates from surface of vanadium fine particles. Compared with vanadium fine particles, the magnetic properties of other metal fine particles were measured.

Appearance of Magnetic Moments in Hyperfine Particles of Vanadium Metal

Hyperfine particles (100 to 1000 A) of vanadium were prepared by the evaporation method and the magnetic susceptibility of these fine particles was measured in the temperature range between 1.6 to 300 K. The magnetic susceptibility of these fine particles was found to consist of two parts, one was the Pauli type which was also observed in the bulk state of the metal and the other was the Curie-Weiss type overlapping the temperature independent one. The C-W component of the susceptibility increases with the decrease of particle size as the inverse of average diameter, suggesting that the magnetic moment which revealed the Curie-Weiss type susceptibility originates from the surface region of the particle. Localization of 3d-electron at the surface of particle is a probable origin of the magnetic moment.