Showing papers on "Paramagnetism published in 2007"
15 Dec 2007
TL;DR: In this article, the authors present a detailed overview of the development of spin wave dynamics and its application in a variety of applications in the field of computer science and engineering, such as magnetism, physics, and digital logic.
Abstract: VOLUME 1: Fundamentals and Theory Part 1: Electron Theory of Magnetism Density Functional Theory of Magnetism Hubbard Model Dynamical Mean Field Theory of Itinerant Electron Magnetism Quantum Monte Carlo Methods Part 2: Strongly Correlated Electronic Systems Heavy Fermions: electrons at the edge of magnetism The Kondo Effect Orbital physics in transition metal oxides: Magnetism and optics Part 3:Theory of Magnetic Spectroscopy and Scattering Magnetic Spectroscopy X-ray and Neutron Scattering by Magnetic Materials Part 4:Spin Dynamics and Relaxation Spin Waves History and A Summary of Recent Developments Dissipative Magnetization dynamics close to the adiabatic regime Part 5:Phase Transitions and Finite Temperature Magnetism Experiment and Analysis Electron Theory of Finite Temperature Magnetism Theory of Magnetic Phase Transitions Disordered and Frustrated Spin Systems Quantum Phase Transitions Part 6: Theory of Magneocrystalline Anisotropy and Magnetoelasticity Theory of Magnetocrystalline Anisotropy and Magnetoelasticity in transition metal systems Theory of Magnetocrystalline Anisotropy and Magnetoelasticity for 4f and 5f Metals Magnetostriction and Magnetoelasticity Theory: a Modern View Part 7: Theory of Transport and Exchange Phenomena in Layer Systems Exchange Coupling in Magnetic Multilayers Enhanced Magnetoresistance Berry phase in magnetism and the anomalous Hall effect Theory of Spin-Dependent Tunneling Part 8: Magnetism of Low Dimensions Magnetism of Low-dimensional Metallic Structures Magnetism of Low-Dimensional Systems: Theory Part 9: Molecular Magnets: Phenomenology and Theory Molecular Magnets: Phenomenology and Theory Part 10: Magnetism and Superconductivity Interplay of Superconductivity and Magnetism Magnetic Superconductors VOLUME 2: Micromagnetism Part 1: Fundamentals of Micromagnetism and Discrete Computational Models General Micromagnetic Theory Numerical Micromagnetics : Finite Difference Methods Numerical Methods in Micromagnetics (FEM) Magnetization dynamics including thermal fluctuations: basic phenomenology, fast remagnetization processes and transitions over high energy barriers Nonlinear Magnetization Dynamics in Nanomagnets Classical Spin Models Part 2: Micromagnetics Applications: Distribution of Equilibrium Configurations, Phase Diagrams and Hysteretic Properties- Small Objects Magnetization Configurations and reversal in small magnetic elements Magnetic Properties of Systems of Low Dimensions Part 3: Micromagnetics Applications: Distribution of Equilibrium Configurations, Phase Diagrams and Hysteretic Properties- Wall in Nanowires Domain Wall Propagation in Magnetic Wires Current Induced Domain-Wall Motion in Magnetic Nanowires The Motion of Domain Walls in Nano-Circuits and its Application to Digital Logic Part 4: Micromagnetics Applications: Distribution of Equilibrium Configurations, Phase Diagrams and Hysteretic Properties- Microstructure and Magnetization Processes Guided Spin Waves Micromagnetism-Microstructure Relations. Micromagnetism of the Hysteresis Loop Modelling of Non-linear Behaviour and Hysteresis in Magnetic Materials Part 5: Magnetization dynamics, solitons, Modes and Thermal Excitations Magnetization Dynamics: Thermal Driven Noise in Magnetoresistive Sensors Modes, Theory and Experiment Nonlinear Multi-dimensional Spin Wave Excitations in Magnetic Films Part 6: Micromagnetics of Spin angular transfer Theory of Spin-Transfer Torque Microwave Generation in Magnetic Multilayers and Nanostructures VOLUME 3: Novel Techniques for Characterizing and Preparing Samples Part 1: X-Ray and Neutron Diffraction Techniques Spin Structures and Spin Wave Excitations Domain States determined by Neutron Refraction and Scattering Polarized neutron reflectivity and scattering of magnetic nanostructures and spintronic materials Part 2: Synchrotron Radiation Techniques, Circular Dichroism of Hard & Soft X-Rays Synchrotron radiation techniques based on X-ray magnetic circular dichroism Part 3: Time and Space Resolved Magnetization Dynamics Ultrafast Magnetodynamics with Lateral Resolution: A View by Photoemission Microscopy Part 4: Electron Microscopy and Electron Holography Lorentz Microscopy of Thin Film Systems Electron Holography Of Ferromagnetic Materials Spin-Polarized Low Energy Electron Diffraction Spin-polarized Low Energy Electron Microscopy (SPLEEM) Scanning Electron Microscopy with Polarisation Analysis Part 5: Magneto-optical Techniques Investigation of Domains and Dynamics of Domain Walls by the Magneto-optical Kerr-effect Magnetization-induced second harmonic generation technique Investigation of Spin Waves and Spin Dynamics by Optical Techniques Time-resolved Kerr-effect and spin dynamics in itinerant ferromagnets Part 6: Spin Polarized Electron Spectroscopies Investigation of Ultrathin Ferromagnetic Films by Magnetic Resonance Spin-Polarized Photoelectron Spectroscopy as a probe of Magnetic Systems High-energy surface spin-waves studied by Spin-polarized Electron Energy Loss Spectroscopy Part 7: Nano Magnetism- Application and Charaterisation Scanning Probe Techniques: MFM and SP-STM Alternative Patterning Techniques : Magnetic Interactions in Nanomagnet Arrays Chemical Synthesis of Monodisperse Magnetic Nanoparticles Nanoimprint Technology for Patterned Magnetic Nanostructures Part 8: Growth Techniques Growth of Magnetic Materials using Molecular Beam Epitaxy Epitaxial Heusler alloys on III-V semiconductors Crystal Growth of magnetic materials VOLUME 4: Novel Materials Part 1: Soft Magnetic Materials Amorphous Alloys Soft Magnetic Materials - Nanocrystalline Alloys Soft Magnetic Bulk Glassy and Bulk Nanocrystalline Alloys Advanced Soft Magnetic Materials for Power Applications Part 2 : Hard Magnetic Materials Rare earth intermetallics for permanent magnet applications Rare-earth (RE) Transition-Metal (T M) Magnets Rare earth nanocrystalline and nanostructured magnets Current Status of Magnetic Industry in China & its Future Part 3: Ferro- and ferrimagnetic oxides and alloys Ferrimagnetic Insulators Crystallography and Chemistry of Perovskites Chalcogenides and Pnictides Dilute Magnetic Oxides and Nitrides Heusler alloys Half Metals Part 4: Ferro- and ferrimagnetic particles Superparamagnetic Particles Novel Nanoparticulate Magnetic Materials and Structures Part 5: Micro- and Nanowires Advanced Magnetic Microwires Template-based Synthesis and Characterization of High-Density Ferromagnetic Nanowire Arrays Magnetic Carbon Part 6: Magnetic Thin Films Magnetic Ultra-hyphen thin Films Magnetic Thin Films Hard Magnetic Films Part 7: Magnetic Materials with outstanding properties Magneto-optical materials Magnetocaloric Materials Magnetostrictive Materials and Magnetic Shape Memory Materials Ferroelectricity in Incommensurate Magnets Magnetism and Quantum Critically in Heavy-Fermion Compounds: Interplay with Superconductivity Molecular nanomagnets Part 8: Biomagnetic Materials Spintronic Biochips For Biomolecular Recognition Application of Magnetic Particles in Medicine and Biology VOLUME 5: Spintronics and Magnetoelectronics Part 1: Metal Spintronics Magnetic Tunnel Junctions including Applications Spin angular momentum transfer in magnetoresistive nano-junctions Spin-transfer in high magnetic fields and single magnetic layer nanopillars Microwave Excitations in Spin Momentum Transfer Devices Theory of Spin-Polarized Current and Spin-Transfer Torque in Magnetic Multilayers Part 2: Exotic Materials High Temperature Superconductivity- Magnetic Mechanisms Ferromagnetic Manganite Films Magnetic Polarons Kondo Effect in Mesoscopic Quantum Dots Ferromagnetic Semiconductors Diluted ferromagnetic semiconductors - theoretical aspects Part 3: Semiconductor spintronics Spin Engineering in Quantum Well Structures Hot Electron Spintronics Spin-dependent transport of carriers in semiconductors Spintronic devices/spin relaxation Theory of Spin Hall Effects in Semiconductors Manipulation of Spins and Coherence in Semiconductors Quantum computing with spins in solids Part 4: Quantum computation The Magnetic Resonance Force Microscope Part 5: Magnetoresistance Tunneling Magnetoresistance in Semiconductors Spin-dependent Tunneling: Role of Evanescent and Resonant States Unusual magnetoresistance including extraordinary and Ballistic
2,246 citations
15 Oct 2007
TL;DR: In this article, electron paramagnetic resonance (EPR) spectra were used to study the properties of N-atom impurities in TiO2 under irradiation with visible light and showed that the Nb centers are responsible for visible light absorption with promotion of electrons from the localized N-impurity states to the conduction band or to electron scavengers.
Abstract: Nitrogen doped titanium dioxide is attracting a continuously increasing attention because of its potential as material for environmental photocatalysis In this paper we review experimental and theoretical work done on this system in our groups in recent years The analysis is largely based on electron paramagnetic resonance (EPR) spectra and on their interpretation based on high-level ab initio calculations N-doped anatase TiO2 contains thermally stable single N-atom impurities either as charged diamagnetic Nb- centers or as neutral paramagnetic Nb centers (b stays for bulk) The N-atoms can occupy both interstitial or substitutional positions in the solid, with some evidence for a preference for interstitial sites All types of Nb centers give rise to localized states in the band-gap of the oxide, thus accounting for the related reduction of absorption band edge The relative abundance of these species depends on the oxidation state of the solid In fact, upon reduction, oxygen vacancies form and transfer electrons from Ti3+ ions to the Nb with formation of Ti4+ and Nb- EPR spectra measured under irradiation show that the Nb centers are responsible for visible light absorption with promotion of electrons from the localized N-impurity states to the conduction band or to electron scavengers like O2 adsorbed on the surface These results provide an unambiguous characterization of the electronic states associated with N-impurities in TiO2 and a realistic picture of the processes occurring in the solid under irradiation with visible light
939 citations
1 Dec 2007
TL;DR: In this paper, the magnetic ordering in nanosized (100 and 1500 nm) nickel oxide NiO powders, prepared by the plasma synthesis method, was studied using Raman scattering spectroscopy in a wide range of temperatures from 10 to 300 K.
Abstract: Magnetic ordering in nanosized (100 and 1500 nm) nickel oxide NiO powders, prepared by the plasma synthesis method, was studied using Raman scattering spectroscopy in a wide range of temperatures from 10 to 300 K. It was observed that the intensity of two- magnon band decreases rapidly for smaller crystallites size. This effect is attributed to a decrease of antiferromagnetic spin correlations and leads to the antiferromagnetic-to- paramagnetic phase transition .
446 citations
TL;DR: The volume fraction of the superconducting phase estimated from the diamagnetic susceptibility reached approximately 40 vol % at 1.8 K, substantiating that LaNiOP is a bulk superconductor.
Abstract: A layered oxyphosphide, LaNiOP, was synthesized by solid-state reactions. This crystal was confirmed to have a layered structure composed of an alternating stack of (La3+O2-)+ and (Ni2+P3-)-. We found that the resulting LaNiOP shows a superconducting transition at ∼3 K. This material exhibited metallic conduction and Pauli paramagnetism in the temperature range of 4−300 K. The resistivity sharply dropped to zero and the magnetic susceptibility became negative at <4 K, indicating that a superconducting transition occurs. The volume fraction of the superconducting phase estimated from the diamagnetic susceptibility reached ∼40 vol % at 1.8 K, substantiating that LaNiOP is a bulk superconductor.
316 citations
Posted Content•
TL;DR: In this article, the enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature has been reported.
Abstract: We report here enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field upto 300K and a finite thermoremanent magnetization upto 340K for the annealed sample. We propose that magnetic moments can form at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers, or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagnetic like behaviour below 10K.
279 citations
TL;DR: In this article, the magnetic properties of antiferromagnetic nanoparticles are reviewed and the authors discuss how the magnetic dynamics can be studied by use of magnetization measurements, Mossbauer spectroscopy and neutron scattering.
Abstract: The magnetic properties of nanoparticles of antiferromagnetic materials are reviewed. The magnetic structure is often similar to the bulk structure, but there are several examples of size-dependent magnetic structures. Owing to the small magnetic moments of antiferromagnetic nanoparticles, the commonly used analysis of magnetization curves above the superparamagnetic blocking temperature may give erroneous results, because the distribution in magnetic moments and the magnetic anisotropy are not taken into account. We discuss how the magnetic dynamics can be studied by use of magnetization measurements, Mossbauer spectroscopy and neutron scattering. Below the blocking temperature, the magnetic dynamics in nanoparticles is dominated by thermal excitations of the uniform mode. In antiferromagnetic nanoparticles, the frequency of this mode is much higher than in ferromagnetic and ferrimagnetic nanoparticles, but it depends crucially on the size of the uncompensated moment. Excitation of the uniform mode results in a so-called thermoinduced moment, because the two sublattices are not strictly antiparallel when this mode is excited. The magnetic dipole interaction between antiferromagnetic nanoparticles is usually negligible, and therefore such particles present a unique possibility to study exchange interactions between magnetic particles. The interactions can have a significant influence on both the magnetic dynamics and the magnetic structure. Nanoparticles can be attached with a common crystallographic orientation such that both the crystallographic and the magnetic order continue across the interfaces.
249 citations
TL;DR: In this article, the coprecipitation technique was used to synthesize polycrystalline Co-doped SnO2 diluted magnetic semiconductors with Co concentrations of 0, 0.5, and 2.0%.
Abstract: We have used the coprecipitation technique to synthesize polycrystalline Co-doped SnO2 diluted magnetic semiconductors with Co concentrations of 0, 0.5, and 2.0%. X-ray diffraction patterns showed for all samples the expected SnO2 tetragonal structure with no additional peaks corresponding to parasitic phases. Transmission electron microscopy (TEM) did not indicate the presence of magnetic parasitic phases and confirmed that Co ions are uniformly distributed inside the samples. Optical absorption measurements showed an energy band gap which decreases when increasing the Co concentration. Raman spectroscopy shows an intensity loss of classical cassiterite SnO2 vibration lines, which is an indication of significant structural modifications and disorder of the SnO2 lattice. Magnetization measurements revealed a mixture of paramagnetic and antiferromagnetic behavior for Co-doped SnO2 with no sign of ferromagnetism.
232 citations
TL;DR: A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.
Abstract: We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet $\mathrm{RbFe}({\mathrm{MoO}}_{4}{)}_{2}$. Ferroelectricity is observed only when the magnetic structure has chirality and breaks inversion symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.
211 citations
TL;DR: The use of bidentate acetate bridging ligands to link [Fe3(m3-O)(m-OAc)6] + cations together to form a new compound with the desired magnetically frustrated star lattice is reported.
Abstract: The preparation of new geometrically spin-frustrated magnetic materials that approximate theoretical models is a challenge. Although theMermin–Wagner theorem indicates that long-range magnetic order can exist in two dimensions at zero Kelvin, order can be destroyed either by quantum fluctuations or geometric frustration even at this temperature. Theoretical studies indicate that the ground state of a spin-1/2 Heisenberg antiferromagnet is most likely to be semiclassically ordered. However, the interplay of geometric frustration and quantum fluctuations has been found to give rise to a paramagnetic ground state without semi-classical long-range order in two types of lattice. The first of these lattices is the famous Kagom+ lattice (T8) and the second is the so-called “star” lattice (T9; Scheme 1), which may serve as a new example of a quantum paramagnet. 5] The triangles are corner-sharing in the Kagom+ lattice whereas they are separated by a bridge in the star lattice, which means that their next-nearest-neighbor exchange interactions are different. 5] The magnetic J exchange pathways in the Kagom+ lattice are all equivalent, whereas the intra-triangular JT pathway in the star lattice is weaker than the inter-triangular JD pathway. In contrast to the rapid development of Kagom+-type antiferromagetic lattices 7] and related, geometrically spin-frustrated lattices, there appears to date to be no report of a compound with a genuine star lattice. Triangular clusters with superexchange pathways, such as the widely employedM3(m3-O) clusters, whereMmay be Fe , Fe, Co, Ni, Cu, V, or Cr, can be used to generate frustrated lattices, including the desired magnetically frustrated star lattice. This star lattice can be described in vertex notation as 3.12 (see Scheme S1 in the Supporting Information), a lattice that is a uniform, three-connected twodimensional net with large voids. Three-connected node subunits that prefer to bond in a planar fashion, such as the basic cationic iron(III) carboxylate cluster [Fe3(m3-O)(mO2CR)6L3] , where L may be water, methanol, or pyridine, must be used to avoid three-dimensional connections. These carboxylate clusters are good potential building blocks because they are easily prepared, prefer planar bonding, and the R groups and L ligands can easily be varied. The cationic [Fe3(m3-O)(m-O2CR)6L3] + moiety has previously served as a sixor three-connected node (see Scheme S2 in the Supporting Information) to form either threeor zero-dimensional porous frameworks depending upon the nature of the carboxylate, which may be either fully or partially substituted by dicarboxylates; the L ligands are usually retained as terminal ligands. Although no example is known to date, it should be possible to substitute the L ligands located in the triangular [Fe3(m3-O)(m-O2CR)6L3] + cation plane with other bridging bidentate ligands that are better at both mediating antiferromagnetic interactions and producing a two-dimensional star lattice. Herein, we report the use of bidentate acetate bridging ligands to link [Fe3(m3-O)(mOAc)6] + cations together to form [Fe3(m3-O)(m-OAc)6(H2O)3][Fe3(m3-O)(m-OAc)7.5]2·7H2O (1), a new compound with the desired star lattice. Single-crystal X-ray diffraction studies of 1 at 293 and 90 K revealed that isolated [Fe3(m3-O)(m-OAc)6(H2O)3] + cations (Figure 1) occupy the dodecagonal channels formed by the stacking of acetate-bridged [Fe3(m3-O)(m-OAc)7.5] 1/2 anionic layers; the dihedral angle between the triangular [Fe3(m3-O)(m-OAc)6(H2O)3] + cations and the [Fe3(m3-O)(mScheme 1. A comparison of the Kagom (T8, left) and star (T9, right) lattices with indication of the magnetic J exchange pathways.
208 citations
TL;DR: In this article, the basic theory and experimental geometry for the off-axis mode of electron holography are presented, and representative studies over a wide range of materials are then described.
Abstract: Electron holography provides a unique phase-imaging approach for characterizing nanoscale electrostatic and magnetic fields. From the relative phase shifts of the electron wave that has passed through the sample, quantitative field measurements that can be related back to specific features of the object can be made. The basic theory and experimental geometry for the off-axis mode of electron holography are first presented. Representative studies over a wide range of materials are then described. Applications involving electrostatic fields include p-n junctions and dopant profiles, piezoelectric fields and ferroelectrics, and charged defects and boundaries. Applications involving magnetic materials include hard magnets, thin films, and nanostructures, both man-made and naturally occurring. Finally, prospects for future developments and applications are briefly discussed.
196 citations
TL;DR: In this article, the magnetic and transport properties of 5% Co-doped and undoped ZnO thin films were investigated by pulsed laser deposition and they showed paramagnetic and ferromagnetic behavior as well as high magnetoresistance and a small anomalous Hall effect.
Abstract: We have investigated magnetic and transport properties of 5% Co-doped and undoped ZnO thin films deposited on $r$ plane ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrates by pulsed laser deposition. The Co doped films showed paramagnetic and ferromagnetic behavior as well as a high magnetoresistance and a small anomalous Hall effect. In a range of $0\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ at low temperatures we observed a double sign change of the magnetoresistance. For undoped ZnO films, prepared by the same conditions, only a negative MR was observed, but surprisingly also a very small anomalous Hall effect. We explain our results by applying a semiempirical fit consisting of a positive and a negative contribution to the magnetoresistance. Using x-ray magnetic circular dichroism we investigated element specific magnetic moments in Co-doped laser ablated ZnO films. As the Co atoms show a paramagnetic behavior, we attribute the ferromagnetism to a spontaneously spin impurity band induced by oxygen vacancies and defects due to the transition metal doping and/or interface stress to the substrate.
TL;DR: Experimental studies support the theoretical calculations with regard to the basic electronic structure and the formation of local magnetic moments and it remains to be seen if these materials are magnetically ordered and, if so, below what temperature.
Abstract: We describe a possible pathway to new magnetic materials with no conventional magnetic elements present. The substitution of nitrogen for oxygen in simple nonmagnetic oxides leads to holes in N 2p states which form local magnetic moments. Because of the very large Hund's rule coupling of Nitrogen and O 2p electrons and the rather extended spatial extent of the wave functions these materials are predicted to be ferromagnetic metals or small band gap insulators. Experimental studies support the theoretical calculations with regard to the basic electronic structure and the formation of local magnetic moments. It remains to be seen if these materials are magnetically ordered and, if so, below what temperature.
TL;DR: In this paper, the structural and magnetic properties of polycrystalline CrxZn1−xO powders are compared with as-deposited, annealed, and powdered samples.
Abstract: Polycrystalline CrxZn1−xO films and powders are prepared by cosputtering and sol-gel method, respectively. While room-temperature ferromagnetism is found in as-deposited films, the powders exhibit paramagnetism. Comparison of the structural and magnetic properties of the as-deposited, annealed, and powdered samples indicates that the interstitial zinc, together with Cr doping, plays an important role in the ferromagnetic origin of Cr:ZnO. The ferromagnetism in films can be described by bound magnetic polaron models with respect to defect-bound carriers.
TL;DR: In this article, a theoretical study of the magnetism induced in transition metal dioxides ZrO2 and TiO2 by substitution of the cation by a vacancy or an impurity from the groups 1A or 2A of the periodic table, where the impurity is either K or Ca.
Abstract: We perform a theoretical study of the magnetism induced in transition metal dioxides ZrO2 and TiO2 by substitution of the cation by a vacancy or an impurity from the groups 1A or 2A of the periodic table, where the impurity is either K or Ca. In the present study both supercell and embedded cluster methods are used. It is demonstrated that the vacancy and the K-impurity leads to a robust induced magnetic moment on the surrounding O-atoms for both the cubic ZrO2 and rutile TiO2 host crystals. On the other hand it is shown that Ca-impurity leads to a non magnetic state. The native O-vacancy does not induce a magnetic moment in the host dioxide crystal.
TL;DR: It is found that in Cu(4)(OD)(6)Cl(2), where distorted kagome planes are weakly coupled, a dispersionless excitation mode appears in the magnetic excitation spectrum below approximately 20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence-bond solid that breaks translational symmetry.
Abstract: A three-dimensional system of interacting spins typically develops static long-range order when it is cooled. If the spins are quantum (S=1/2), however, novel quantum paramagnetic states may appear. The most highly sought state among them is the resonating-valence-bond state, in which every pair of neighbouring quantum spins forms an entangled spin singlet (valence bonds) and these singlets are quantum mechanically resonating among themselves. Here we provide an experimental indication for such quantum paramagnetic states existing in frustrated antiferromagnets, Zn(x)Cu(4-x)(OD)(6)Cl(2), where the S=1/2 magnetic Cu2+ moments form layers of a two-dimensional kagome lattice. We find that in Cu(4)(OD)(6)Cl(2), where distorted kagome planes are weakly coupled, a dispersionless excitation mode appears in the magnetic excitation spectrum below approximately 20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence-bond solid, that breaks translational symmetry. Doping with non-magnetic Zn2+ ions reduces the distortion of the kagome lattice, and weakens the interplane coupling but also dilutes the magnetic occupancy of the kagome lattice. The valence-bond-solid state is suppressed, and for ZnCu(3)(OD)(6)Cl(2), where the kagome planes are undistorted and 90% occupied by the Cu2+ ions, the low-energy spin fluctuations become featureless.
TL;DR: In this article, the lattice and electronic and magnetic transport properties of the antiperovskite structure of Mn3Zn1−xGexN compounds were investigated, and it was shown that the partial substitution of Ge for Zn induces a lattice contraction near the magnetic transition temperature.
Abstract: The lattice and electronic and magnetic transport properties of the antiperovskite structure Mn3Zn1−xGexN compounds were investigated. For Mn3ZnN, there is a magnetic transition from antiferromagnetic to paramagnetic near 185K. Correspondingly, the resistivity shows an abrupt drop, but any sudden change of lattice parameters is not found. However, it is interesting that the partial substitution of Ge for Zn induces a lattice contraction near the magnetic transition temperature, where a drop of the resistivity remain, and the transition temperature point increases and the temperature range is broadened with increasing doped Ge contents. The thermodynamics properties were also investigated.
TL;DR: In this article, an autocombustion method was used to synthesize Mn doped ZnO nanowires, which were characterized by X-ray diffraction and transmission electron microscopy.
TL;DR: X-ray crystallographic analyses revealed that 1·2Et2O underwent a single-crystal-to-single-Crystal transformation with the release of crystal solvent molecules, and a reversible conversion between paramagnetism and spin-crossover induced by solvent molecules was observed.
Abstract: An iron(II) complex with ferrocenyl groups, [Fe(dppFc)2](BF4)2·2Et2O (1·2Et2O, dppFc = 1-ferrocenyl-2-{(2,6-bis(pyrazolyl)pyridyl}ethylene), was prepared. X-ray crystallographic analyses revealed that 1·2Et2O underwent a single-crystal-to-single-crystal transformation with the release of crystal solvent molecules. In magnetic susceptibility measurements, a reversible conversion between paramagnetism and spin-crossover induced by solvent molecules was observed.
TL;DR: In this article, Ni-doped ZnO (Zn1-xNixO, in which 0 £ x £ 0.05) diluted magnetic semiconductors nanoparticles are prepared by an ultrasonic assisted sol-gel process.
Abstract: In this paper, Ni-doped ZnO (Zn1-xNixO, in which 0 £ x £ 0.05) diluted magnetic semiconductors nanoparticles are prepared by an ultrasonic assisted sol-gel process. Transmission electron microscopy shows sphere- like nanoparticles with an average size of about 25 nm. From the analysis of X-ray diffraction, the Ni-doped ZnO nanoparticles are identified to be a wurtzite structure, but impurity phases are observed when the Ni content x reaches 0.05. Sample structures are further studied by Raman spectra, from which a broad and strong Raman band in the range of 500-600 cm -1 is observed in Zn1-xNixO. With the increment of x, wurtzite structures degrade gradually. The magnetic properties are measured using superconducting quantum interference device at room temperature; the Zn1-xNix O( x £ 0.02) nanoparti- cles show ferromagnetism. However, for the sample of Zn0.95Ni0.05O, paramagnetism is observed, which may be ascribed to ferromagnetic-antiferromagnetic competition.
Book•
1 Jan 2007
TL;DR: In this paper, the authors present a framework for the definition of MCD terms and the interpretation of the MCD spectra, as well as the threefold symmetry groups of SO3 O and O D4 Chains.
Abstract: Preface. 1. Introduction. 2. Polarized Light. 3. Theoretical Framework: Definition of MCD Terms. 4. Measurement of MCD Spectra. 5. The Interpretation of MCD Spectra. 6. Case Studies I. Diamagnetic Systems: A and B terms. 7. Case Studies II. Paramagnetic Systems: C Terms. 8. Magnetic Vibrational Circular Dichroism (MVCD) and X-Ray magnetic Circular Dichroism (XMCD). 9. Magnetic Linear Dichroism Spectroscopy. Appendix A. tables for the Symmetry Group O and Td. Appendix B. Tables for the Fourfold Symmetry Group D4. Appendix C. Tables for the Threefold Symmetry Group D3. Appendix D. 3jm Factors for Single-Valued Irreps of the SO3 O and O D4 Chains. Reviews and References. Index.
TL;DR: In this paper, the magnetocaloric effect and refrigerant capacity of Ho30Y26Al24Co20, Dy50Gd7Al23Co20 and Er50Al 24Co20Y6 bulk metallic glasses were studied.
Abstract: The authors study the magnetocaloric effect and refrigerant capacity of Ho30Y26Al24Co20, Dy50Gd7Al23Co20, and Er50Al24Co20Y6 bulk metallic glasses. Their magnetic entropy changes associated with spin glass to paramagnetic transition are larger than those of Gd, Gd5Si2Ge1.9Fe0.1, and many other intermetallic compounds reported in the same temperature range. The good refrigerant efficiency combined with their high electrical resistivity, high thermal stability, outstanding mechanical properties, and tunable nature makes these glassy materials be attractive candidates for magnetic refrigerants in helium and hydrogen liquefaction temperature range of 2–50K.
TL;DR: The XPS data collected from Zn(1-x)Co(x)O samples showed a gradual increase in the oxygen concentration, changing the oxygen-deficient undoped ZnO to an excess oxygen state for x = 0.12, indicating that such high Co concentrations and appropriate oxygen stoichiometry may be needed to achieve adequate ferromagnetic exchange coupling between the incorporated Co(2+) ions.
Abstract: We report the results of a detailed investigation of sol?gel-synthesized nanoscale Zn1?xCoxO powders processed at 350??C with 0?x?0.12 to understand how the structural, morphological, optical and magnetic properties of ZnO are modified by Co doping, in addition to searching for the theoretically predicted ferromagnetism. With x increasing to 0.03, both lattice parameters a and c of the hexagonal ZnO decreased, suggesting substitutional doping of Co at the tetrahedral Zn2+ sites. For x>0.03, these trends reversed and the lattice showed a gradual expansion as x approached 0.12, probably due to additional interstitial incorporation of Co. Raman spectroscopy measurements showed a rapid change in the ZnO peak positions for x>0.03, suggesting significant disorder and changes in the ZnO structure, in support of additional interstitial Co doping possibility. Combined x-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectroscopy, photoluminescence spectroscopy and diffuse reflectance spectroscopy showed clear evidence for tetrahedrally coordinated high-spin Co2+ ions occupying the lattice sites of ZnO host system, which became saturated for x>0.03. Magnetic measurements showed a paramagnetic behaviour in Zn1?xCoxO with increasing antiferromagnetic interactions as x increased to 0.10. Surprisingly, a weak ferromagnetic behaviour was observed for the sample with x = 0.12 with a characteristic hysteresis loop showing a coercivity Hc~350?Oe, 25% remanence Mr, a low saturation magnetization Ms~0.04?emu?g?1 and with a Curie temperature Tc~540?K. The XPS data collected from Zn1?xCoxO samples showed a gradual increase in the oxygen concentration, changing the oxygen-deficient undoped ZnO to an excess oxygen state for x = 0.12. This indicates that such high Co concentrations and appropriate oxygen stoichiometry may be needed to achieve adequate ferromagnetic exchange coupling between the incorporated Co2+ ions.
Abstract: A three-dimensional system of interacting spins typically develops static long-range order when it is cooled. If the spins are quantum (S = 1/2), however, novel quantum paramagnetic states may appear. The most highly sought state among them is the resonating valence bond (RVB) state in which every pair of neighboring quantum spins form entangled spin singlets (valence bonds) and the singlets are quantum mechanically resonating amongst all the possible highly degenerate pairing states. Here we provide experimental evidence for such quantum paramagnetic states existing in frustrated antiferromagnets, ZnxCu4-x(OD)6Cl2, where the S = 1/2 magnetic Cu2+ moments form layers of a two-dimensional kagome lattice. We find that in Cu4(OD)6Cl2, where distorted kagome planes are weakly coupled to each other, a dispersionless excitation mode appears in the magnetic excitation spectrum below ~ 20 K, whose characteristics resemble those of quantum spin singlets in a solid state, known as a valence bond solid (VBS), that breaks translational symmetry. Doping nonmagnetic Zn2+ ions reduces the distortion of the kagome lattice, and weakens the interplane coupling but also dilutes the magnetic occupancy of the kagome lattice. The VBS state is suppressed and for ZnCu3(OD)6Cl2 where the kagome planes are undistorted and 90% occupied by the Cu2+ ions, the low energy spin fluctuations in the spin liquid phase become featureless.
TL;DR: In this article, a phenomenological theory of easy-axis antiferromagnets displaying spin flops is developed and the components of the magnetic susceptibility tensor are calculated for homogeneous and multidomain states in the vicinity of the spin flop.
Abstract: The classical spin flop is the field-driven first-order reorientation transition in easy-axis antiferromagnets. A comprehensive phenomenological theory of easy-axis antiferromagnets displaying spin flops is developed. It is shown how the hierarchy of magnetic coupling strengths in these antiferromagnets causes a strongly pronounced two-scale character in their magnetic phase structure. In contrast to the major part of the magnetic phase diagram, these antiferromagnets near the spin-flop region are described by an effective model akin to uniaxial ferromagnets. For a consistent theoretical description both higher-order anisotropy contributions and dipolar stray fields have to be taken into account near the spin flop. In particular, thermodynamically stable multidomain states exist in the spin-flop region, owing to the phase coexistence at this first-order transition. For this region, equilibrium spin configurations and parameters of the multidomain states are derived as functions of the external magnetic field. The components of the magnetic susceptibility tensor are calculated for homogeneous and multidomain states in the vicinity of the spin flop. The appreciable anomalies in these measurable quantities provide an efficient method to investigate magnetic states and to determine materials parameters in bulk and confined antiferromagnets, as well as in nanoscale synthetic antiferromagnets. The method is demonstrated for experimental data on the magnetic properties near the spin-flop region in the orthorhombic layered antiferromagnet ${({\mathrm{C}}_{2}{\mathrm{H}}_{5}\mathrm{N}{\mathrm{H}}_{3})}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$.
TL;DR: The cluster rotation and the visualization of viscoelastic modes are independent techniques to probe the rheological properties of the cluster and agreement is found when determining the 2D cluster viscosity eta(c) approximately 10(-11) N s/m.
Abstract: Paramagnetic particles in a liquid above a solid dynamically self-assemble into two-dimensional (2D) viscoelastic clusters in a processing magnetic field if the precession angle exceeds the magic angle. Hexagonal clusters rotate with a frequency proportional to the precession frequency of the magnetic field. The rotation is explained by viscoelastic shear waves excited in the clusters that can be visualized slightly above the magic angle. The cluster rotation and the visualization of viscoelastic modes are independent techniques to probe the rheological properties of the cluster. We find agreement between both techniques when determining the 2D cluster viscosity eta(c) approximately 10(-11) N s/m.
TL;DR: In this paper, the measurement of proton nuclear magnetic spin-lattice relaxation as a function of magnetic field strength (and hence nuclear Larmor frequency) can provide reliable information on the microstructure (specific surface area and pore size distribution) throughout the progressive hydration of cement-based materials.
Journal Article•
TL;DR: The ordering of a spin-1 condensate when quenched from its paramagnetic phase to its ferromagnetic phase by reducing the magnetic field is discussed, and the creation of vortices through growth of the magnetization fluctuations is discussed.
Abstract: We discuss the ordering of a spin-1 condensate when quenched from its paramagnetic phase to its ferromagnetic phase by reducing the magnetic field. We first elucidate the nature of the equilibrium quantum phase transition. Quenching rapidly through this transition reveals XY ordering either at a specific wave vector, or the "light-cone" correlations familiar from relativistic theories, depending on the end point of the quench. For a quench proceeding at a finite rate the ordering scale is governed by the Kibble-Zurek mechanism. The creation of vortices through growth of the magnetization fluctuations is also discussed. The long-time dynamics again depends on the end point, conserving the order parameter in a zero field, but not at a finite field, with differing exponents for the coarsening of magnetic order. The results are discussed in the light of a recent experiment by Sadler et al.
TL;DR: In this paper, the X-band, Q-band and far infrared electron paramagnetic resonance (EPR) spectra have been measured between 4 and 600K and compared with magnetic susceptibility and magnetization data.
Abstract: To check on the nature of the weak magnetic order in polycrystalline magnetoelectric Pb(Fe1∕2Nb1∕2)O3 the X-band, Q-band, and far infrared electron paramagnetic resonance (EPR) spectra have been measured between 4 and 600K and compared with magnetic susceptibility and magnetization data. The asymmetric line shapes can be simulated at higher temperature by thermally fluctuating superparamagnetic nanoclusters. The pronounced temperature dependence of the position of the spectra demonstrates the presence of an internal magnetic field which is small but nonzero even at room temperature, i.e., far above the antiferromagnetic transition. The electronic spin-spin exchange has been found to be in the terahertz range. The magnetization data reveal a weak ferromagnetism even above 300K and a break in the temperature dependence of susceptibility at the paramagnetic to ferromagnetic transition.
TL;DR: In this article, the phonon Hall effect in the paramagnetic dielectric garnet Tb3Ga5O12 has been investigated and it has been found that the coefficient of the PHE is positive and is equal to (3.5 ± 2) × 10−5T−1 in a magnetic field of 3 T at a temperature of 5.13 K.
Abstract: The phonon Hall effect in the paramagnetic dielectric garnet Tb3Ga5O12 has been investigated. It has been found that the coefficient of the phonon Hall effect is positive and is equal to (3.5 ± 2) × 10−5T−1 in a magnetic field of 3 T at a temperature of 5.13 K. The results are experimental evidence of the phonon Hall effect in the paramagnetic dielectric found by C. Strohm, G. L. J. A. Rikken, and P. Wyder, Phys. Rev. Lett. 95, 155901 (2005).