Structure, stability and magnetism of cobalt doped (ZnO)n clusters

Clusters of magnetic impurities are believed to play an important role in retaining ferromagnetism in diluted magnetic semiconductors (DMS), the origin of which has been a long debated issue. Controlling the dopant homogeneity in magnetic semiconductors is therefore a critical issue for the fabrication of high performance DMS. The current paper presents a first principle study on the stability and magnetic properties of Co doped (ZnO)n (n = 12 and 15) clusters using density functional theory. The results show that cobalt ions in these clusters tend to increase their stabilities by maximizing their co-ordination numbers to oxygen. This will likely to be the case for (ZnO)n clusters with n other than 12 and 15 in order for Co to reside in a stable local crystal field. Expansive (shrinkage) stress is introduced when cobalt resides in exohedral substitutional (endohedral interstitial) sites; such strain can be offset by the cluster deformation. Bidoped cluster is found to be unstable due to the increase of system strain energy. All the doped clusters were found to preserve 3 microg of magnetic moments from Co in the overall clusters, but with part of the local moments on cobalt re-distributed onto neighboring oxygen atoms. Current findings may provide a better understanding on the structural chemistry of magnetic dopants in nanocrystallined DMS materials.     

Anomalous magnetic susceptibility and inhomogeneous magnetic state in lanthanum strontium manganites

Anomalous magnetic susceptibility χ has been observed in mono-and polycrystalline (ceramic) samples of lanthanum strontium manganites. The oscillations of χ observed for single crystal samples in the vicinity of the Curie temperature (and in the paramagnetic region) are explained by the existence of magnetic clusters. The appearance of susceptibility oscillations in ceramic samples is attributed to the formation of magnetic clusters, which may occur both in grains (at the interface between ferro-and antiferromagnetic phases) and at the grain boundaries.     

Magnetic Sensor Principle for Susceptibility Imaging of Para- and Diamagnetic Materials

In ferromagnetic materials, magnetic domain walls interact with microstructure over similar mechanisms as dislocations do. Under the absence of stress as an additional influence, the magnetic properties of these materials are often correlated with mechanical-technological characteristics such as hardness and strength. This fundamental observation is the basis of micro-magnetic materials characterization. It is not yet well known, but quite legitimate to assume that such a correlation also exists between magnetic and mechanical properties of para- and diamagnetic materials, since the interactions on the electron spin level depend on the molecular structure and determine the magnetic behavior. Similar effects are known to exist in the electrical domain. As an example, the electrical conductivity of nonmagnetic materials such as aluminum or nickel based alloys, which can be assessed non-destructively using eddy current impedance measurements, is affected by stress and dislocation density. Much less is known about the correlation between the mechanical properties and the magnetic susceptibility of non-ferromagnetic materials such as graphite, aluminum and plastics. Today, there is no commercialized non-destructive method which uses the concept of magnetic susceptibility measurements for the characterization of dia- and paramagnetic materials. This article proposes a force-based magnetic sensor principle for susceptibility imaging of such materials.     

Magnetic Properties of Iron-Containing Particles in a Quartz Matrix after Mechanochemical Processing

Quartz powders prepared by mechanochemical processing in a planetary mill in the presence of FeCl₃ or monohydric or dihydric alcohols were found to exhibit magnetic properties. The magnetic susceptibility of milled quartz depends on the milling time and the amount of additions. The material was shown to undergo aging: its susceptibility and electron paramagnetic resonance spectrum vary during storage. The magnetization of quartz is interpreted in terms of Fe-containing clusters present in the amorphized surface layer.     

A Study of the Paramagnetic to Ferromagnetic Transition in the Optimally Doped CMR Compound La0.65Ca0.35MnO3 and Its Dependence on Oxygen Mass

We present a study of the paramagnetic to ferromagnetic transition in the CMR compound La_0.65Ca_0.35MnO₃ and its dependence on magnetic field and oxygen mass. The transition is characterized by two temperatures, the thermodynamic transition temperature at T _c, obtained from specific heat and thermal expansion data, and the resistive transition obtained from the resistivity maximum. The resistive transition occurs well within the paramagnetic range. The magnetic susceptibility in the paramagnetic range is isotope dependent up to 400 K. The magnitude of the Curie-Weiss constant indicates the presence of small clusters of about 4–5 unit cells. The resistive transition occurs when the percolation limit for these clusters is reached.     

A Study of the Paramagnetic to Ferromagnetic Transition in the Optimally Doped CMR Compound La0.65Ca0.35MnO3 and Its Dependence on Oxygen Mass

We present a study of the paramagnetic to ferromagnetic transition in the CMR compound La_0.65Ca_0.35MnO₃ and its dependence on magnetic field and oxygen mass. The transition is characterized by two temperatures, the thermodynamic transition temperature at T _c, obtained from specific heat and thermal expansion data, and the resistive transition obtained from the resistivity maximum. The resistive transition occurs well within the paramagnetic range. The magnetic susceptibility in the paramagnetic range is isotope dependent up to 400 K. The magnitude of the Curie-Weiss constant indicates the presence of small clusters of about 4–5 unit cells. The resistive transition occurs when the percolation limit for these clusters is reached.     

Oxidation of magnesia-supported Pd-clusters leads to the ultimate limit of epitaxy with a catalytic function

Oxide-supported transition-metal clusters and nanoparticles have attracted significant attention owing to their important role as components of model catalysts, sensors, solar cells and magnetic recording devices. For small clusters, functionality and structure are closely interrelated. However, knowledge of the structure of the bare cluster is insufficient as the interaction with the chemical environment might cause drastic structural changes. Here we show by ab initio simulations based on the density functional theory that the reaction with molecular oxygen transforms small, non-crystalline, magnesia-supported Pd-clusters to crystalline Pd(x)O(y) nano-oxide clusters that are in epitaxy with the underlying support. Restructuring of the Pd backbone is controlled by the electrostatic interaction with magnesia leading to a strong reduction of the O2 dissociation barrier. The supported Pd(x)O(y) clusters are likely to serve as Mars-van Krevelen oxygen reservoirs in catalytic oxidation reactions as observed for PdO overlayers and demonstrated here for the oxidation of CO molecules.     

Numerical simulation of chainlike cluster movement of feeble magnetic particles by induced magnetic dipole moment under high magnetic fields

In this paper, the motion of a chainlike cluster of feeble magnetic particles induced by high magnetic field is discussed on the basis of the results of numerical simulations. The simulations were performed on glass particles with a diameter of 0.8 mm; and the viscosity, applied magnetic field and magnetic properties of the surrounding medium were changed. In addition to the magnetic field and the difference in magnetic susceptibility between the particles and the surrounding medium, the obtained results indicate that the viscosity is an essential factor for the formation of the chainlike alignment of feeble magnetic particles. We also carried out simulations using glass particles with a smaller diameter of 0.1 mm. Chainlike clusters were produced similar to those of ferromagnetic particles formed in a ferromagnetic fluid.     

The Nucleation of a Nanoparticle of Silicon Dioxide in a Closed Domain. Computer Experiment

The method of molecular dynamics is used to study SiO₂ clusters located in cavities of spherical, cylindrical, and cubic shape. The effect of the elastically stressed state of the cell boundaries on the physical properties of nanoparticles contained in them is investigated. Given the same pattern of application of external forces, a nanoparticle in a spherical cell experiences a stronger compression than in clusters surrounded by envelopes of other geometries. The clusters assume the shape of the cavity in which they were contained. The effect of compressive forces results in the breakage of Si–O bonds and in the subsequent oxygen enrichment of the cluster surface. The behavior of nanoparticles after the elimination of the stimulation by the cavity boundaries is treated. It is only for a cluster of spherical geometry that a significant number of Si–O bonds is restored. For other clusters, the evaporation of oxygen atoms is observed.     

Electronic and magnetic properties of dilute transition clusters in noble (or transition) metals

Using a realistic band structure for the host (noble or transition metal), we present a detailed study of the electronic and magnetic properties of transitional impurity clusters without interactions between the clusters. For the calculation of the one-electron properties, the Hartree-Fock environment effects are self-consistently taken into account by Friedel's rule. The impurity potentials and densities of states are very sensitive to the impurity-impurity interactions inside the clusters. For the calculation of the magnetic properties, the electron-electron interactions are taken into account in the random phase approximation, which allows one to obtain simple expressions for the specific heat and the low-temperature-dependent resistivity. These properties are expressed in terms of three characteristic temperatures T _f(1), T _f(2) ⁺ , and T _f(2) ^– corresponding to three different modes of spin fluctuations for the considered model. We present numerical results from the study of the local magnetic susceptibility, which depends on local environments via several combined effects. Our conclusions on nearly magnetic copper-based alloys are in good agreement with the experimental data. In particular, our calculations confirm that a magnetic moment appears on a nickel atom when it is surrounded by approximately eight nearest-neighboring Ni atoms.This publication is a part of a thesis to be presented at the University Louis Pasteur.     

Numerical simulation of chainlike cluster movement of feeble magnetic particles by induced magnetic dipole moment under high magnetic fields

Abstract

In this paper, the motion of a chainlike cluster of feeble magnetic particles induced by high magnetic field is discussed on the basis of the results of numerical simulations. The simulations were performed on glass particles with a diameter of 0.8 mm; and the viscosity, applied magnetic field and magnetic properties of the surrounding medium were changed. In addition to the magnetic field and the difference in magnetic susceptibility between the particles and the surrounding medium, the obtained results indicate that the viscosity is an essential factor for the formation of the chainlike alignment of feeble magnetic particles. We also carried out simulations using glass particles with a smaller diameter of 0.1 mm. Chainlike clusters were produced similar to those of ferromagnetic particles formed in a ferromagnetic fluid.     

Structure and Magnetic Properties of Polymer Microspheres Filled with Magnetite Nanoparticles

The structure and magnetic properties of collagen microspheres filled with magnetite nanoparticles are studied. The average interparticle separation in the polymer matrix and the size of magnetite nanoparticles before and after the introduction of the nanoparticles into the matrix are determined using electron microscopy. The magnetization curve of the microspheres has a superparamagnetic character. The magnetite nanoparticles undergo no aggregation during the synthesis of microspheres and are evenly distributed over the matrix. The magnetic susceptibility data for magnetic polymer microspheres of different diameters suggest that, at small diameters (<300 m), all of the nanoparticles, aligned in chains, contribute to magnetization; at large diameters, some of the chains give way to clusters, the chains are shorter, and, accordingly, the susceptibility is lower.     

Magnetic Moments due to Orbital Currents in an Electron-Lattice Model of Cuprate Superconductors

It has been shown in an earlier paper that loop currents and magnetic moments can be generated in an electron-lattice model of superconducting cuprates if a predominant mode of vibration of the oxygen clusters is an unusual nonlinear anti-ferro pattern of 2-D vibrations called a Q ₂ mode. This makes the electron-lattice mechanism a possible candidate to explain the experimental evidence of ubiquitous strong electron-lattice interaction involving oxygen clusters and also a possible candidate to explain experimental evidence of unusual magnetic moments in the CuO ₂ planes, if confirmed. In this paper, we report a detailed numerical study of exact 2-D modes involving the Jacobian elliptic functions. The magnetic fields generated, energy of modes, frequency, and elliptic modulus are studied as the parameters of an anharmonic molecular crystal model are varied. With a suitable choice of parameters, the orders of magnitude of magnetic field and energy are consistent with the plausible estimates made in an earlier paper. The broken π/2 rotational symmetry in each CuO ₂ unit cell due to the generation of small magnetic fields makes our model a candidate to explain intra-unit-cell electronic nematicity measured recently.     

Effect of quenching on the magnetic behaviour of some Ni-rich NiAl alloys

The effects of quenching from 1443 K on the magnetic properties of NiAl alloys in the composition range 32.0 to 40.0 at% Al have been studied. Changes were observed in the magnetic properties of all the alloys and, in particular, large changes were observed for the 35 at% Al alloy when heated from an as-quenched state. These magnetic changes were also accompanied by a sample weight loss. The magnetic susceptibility and weight loss effects have been attributed to the absorption of hydrogen when the alloys were quenched into water from a high temperature and to its subsequent evolution when the samples were reheated under vacuum. The changes in the field dependence are thought to have a different origin and have been attributed to the diffusion of Ni antistructure atoms to form Ni clusters large enough to exhibit ferromagnetism. The composition dependence of the susceptibility at room temperature and the large changes seen in the susceptibility of the 35 at% Al alloy on heating both suggest that the Fermi energy of this composition lies on or close to a subsidiary peak in the electronic density of states curve. This observation has been correlated with the theoretical density of states curve for NiAl.     

Measuring Magnetoresistance in a 2D Intergranular Magnetic-Semiconducting Material

A new 2D intergranular semiconducting-magnetic material obtained by depositing magnetic clusters of permalloy in a doped Si surface, is presented. The material, exhibiting ohmic contacts, has an ample range of properties and versatility by simply controlling deposition time. We can have clusters separated by relatively large, intermediate distances or a continuous film. This permits to study the magnetic and electrical properties of the clusters mediated by currents through the n-doped Si. We observe anisotropic and giant magnetoresistances of and between clusters. Also, by preventing oxidation of the Si surface, we suggest the possibility of formation of magnetic Si up to 205 K. The material reported here is technologically promising since it is grown directly on Si and its magnetoresistance of up to 1.5% is obtained at low fields and RT. The spin transfer occurs for long distances since the current flows coherently from cluster to cluster via the Si matrix. The method developed may be also adequate to study superconductivity properties of isolated or weakly percolative clusters by coupling through Josephson currents. The system is diluted when clusters are separated on average by 50 nm. This could help to better understand dilute magnetic semiconductor materials in general. We also put forward a method to distinguish univocally giant from anisotropic magnetoresistance by depositing a thin film of gold and comparing magnetic properties by spin transfer through Si or metal.     

Dynamics of the Magnetic Flux Trapped in Fractal Clusters of a Normal Phase in Percolative Superconductors

The effect of the fractal clusters of a normal phase, which act as pinning centers, on the dynamics of magnetic flux in percolative type-II superconductor is considered. The main features of these clusters are studied in detail: the cluster statistics is analyzed; the fractal dimension of their boundary is estimated; the distribution of critical currents is obtained, and its peculiarities are explored. It is found that there is the range of fractal dimension where this distribution has anomalous statistical properties, specifically, its dispersion becomes infinite. It is examined how the finite resolution capacity of the cluster geometric size measurement affects the estimated value of fractal dimension. The effect of fractal properties of the normal phase clusters on the electric field arisen from magnetic flux motion is investigated for the cluster area distribution of different kinds. The voltage-current characteristics of fractal superconducting structures in the resistive state are obtained for an arbitrary fractal dimension. It is revealed that the fractality of the boundaries of the normal phase clusters intensifies the magnetic flux trapping and thereby raises the critical current of a superconductor.     

Electronic structure and optical property of semiconductor nanocrystallites

Semiconductor nanostructures show many special physical properties associated with quantum confinement effects, and have many applications in the opto-electronic and microelectronic fields. However, it is difficult to calculate their electronic states by the ordinary plane wave or linear combination of atomic orbital methods. In this paper, we review some of our works in this field, including semiconductor clusters, self-assembled quantum dots, and diluted magnetic semiconductor quantum dots. In semiconductor clusters we introduce energy bands and effective-mass Hamiltonian of wurtzite structure semiconductors, electronic structures and optical properties of spherical clusters, ellipsoidal clusters, and nanowires. In self-assembled quantum dots we introduce electronic structures and transport properties of quantum rings and quantum dots, and resonant tunneling of 3-dimensional quantum dots. In diluted magnetic semiconductor quantum dots we introduce magnetic-optical properties, and magnetic field tuning of the effective g factor in a diluted magnetic semiconductor quantum dot.     

Inorganic molecular wires: Physical and functional properties of transition metal chalco-halide polymers

The rapid development of nanotechnology has lead to demands on new one-dimensional materials with new functional properties. Carbon nanotubes have received most attention, followed by nanowires of very different kinds. Most recently inorganic molecular wires - particularly molybdenum halide or chalcogenide cluster polymers - have emerged as a new type of one-dimensional materials with remarkable molecular-scale functionality. These transition metal chalco-halide molecular wires are unique in terms of structure and molecular properties, setting them apart from the plethora of nanowires and nanotubes discovered in recent years.Their one-dimensional polymer structure gives rise to some very unusual physical properties. Anionic bridges which bind Mo clusters together into one-dimensional chains are extraordinarily strong, yet highly deformable, giving rise to exceptionally high Young’s moduli and nonlinear mechanical properties respectively. The very weak interaction between individual polymer chains within crystalline bundles leads to observation of extreme one-dimensional electronic and magnetic character on one hand, and also to easy dispersion in common polar solvents and ultralow shear moduli on the other. The sulfur atoms within the structure facilitate diverse functionalization chemistry to thiol-containing molecules, such as proteins. The connectivity of the molecular wire ends to gold nanoparticles and surfaces with covalent bonds and good electronic coupling enable self-assembled molecular-scale connections to be made between individual molecules.Experimental and theoretical progress in the field has been extremely rapid since 2004 and in this review we try and summarize the most important structural, mechanical, electronic and magnetic properties of molybdenum chalco-halide molecular wires. We also discuss their functional properties which make these new materials of great interest for a wide variety of diverse applications including conducting composites, nonlinear optics, field emission, tribology, spin-modulators, molecular and gas sensing and potentially revolutionary applications in molecular electronics.     

Curie temperature of METGLAS magnetic alloys measured by different techniques

A series of experiments was performed to determine the Curie temperature Tcof six METGLAS® magnetic alloys using three different techniques: thermogravimetry with magnetic field, differential scanning calorimetry (DSC), and magnetic susceptibility. The purpose of the work was to assess the sensitivity of the three methods, to establish the relationships between magnetic properties and enthalpy in the vicinity of Tc, and finally, to determine physically valid criteria for Tcwhen using the susceptibility method. The full matrix of data is given for the following METGLAS alloys: 2605S-2, 2605SC, 2605S-3A, 2705M, 2826MB, and 2605CO. Good accord between the temperature-dependence of magnetic moment, susceptibility, and enthalpy of the first five alloys is shown. A feature which can be identified with Tcis defined for each of the measurement methods. Interesting behavior was observed in the case of the METGLAS alloy 2605CO which has so small an enthalpy of the magnetic transition that DSC cannot be used. Both magnetic methods used, thermogravimetry with magnetic field and susceptibility, reveal a steplike temperature-dependence of magnetic moment and susceptibility. A strong dependence of Tcon the heating rate during measurement was observed, indicating superposition of the magnetic disordering and relaxation processes in the amorphous state. Extrapolation of the data shows that the virtual Tcfor METGLAS 2605CO in the completely amorphous state should be as high as 547°C (820 K).     

Spin-glass and cluster ferromagnetism in RuSr2Y1.5Ce0.5Cu2O10 magneto-superconductor synthesized by HPHT

We report structural, DC magnetization, linear and non-linear AC susceptibility for phase pure RuSr₂Y_1.5Ce_0.5Cu₂O₁₀ (YRu-1222) magneto-superconductor. The studied compound is synthesized through HPHT (high pressure high temperature: 6 GPa, 1400° C) technique at NIMS-Japan. As synthesized YRu-1222, is crystallized in single phase tetragonal structure with space group I4/mmm. DC magnetic susceptibility showed that studied YRu-1222 is a magneto-superconductor with Ru spins magnetic ordering at around 120 K and superconductivity in the Cu-O₂ planes below ～40 K. The frequency and field dependent AC susceptibility measurements confirmed the spin-glass (SG) behavior with homogenous/non-homogenous ferromagnetic clusters for the studied system. The frequency variation followed the Vogel–Fulcher law with homogeneous/non-homogenous magnetically interacting ferromagnetic clusters embedded in spin-glass (SG) matrix. Detailed linear and non-linear AC susceptibility measurements are carried out to probe the exact magnetic nature of the studied magneto-superconductor YRu-1222.