Showing papers on "Magnetization published in 1993"
TL;DR: The samples show a drop in the resistivity at the magnetic transition, and the existence of magnetic polarons seems to dominate the electric transport in this region.
Abstract: At room temperature a large magnetoresistance, \ensuremath{\Delta}R/R(H=0), of 60% has been observed in thin magnetic films of perovskitelike La-Ba-Mn-O. The films were grown epitaxially on ${\mathrm{SrTiO}}_{3}$ substrates by off-axis laser deposition. In the as-deposited state, the Curie temperature and the saturation magnetization were considerably lower compared to bulk samples, but were increased by a subsequent heat treatment. The samples show a drop in the resistivity at the magnetic transition, and the existence of magnetic polarons seems to dominate the electric transport in this region.
3,424 citations
TL;DR: In this article, it was shown that the magnetization of the Mn12 cluster is highly anisotropic and the magnetisation relaxation time becomes very long below a temperature of 4 K, giving rise to pronounced hysteresis.
Abstract: MAGNETIC materials of mesoscopic dimensions (a few to many thousands of atoms) may exhibit novel and useful properties such as giant magnetostriction, magnetoresistivity and magnetocaloric effects1–4. Such materials also allow one to study the transition from molecular to bulk-like magnetic behaviour. One approach for preparing mesoscopic magnetic materials is to fragment bulk ferromagnets; a more controllable method is to take a 'bottom-up' approach, using chemistry to grow well defined clusters of metal ions5,6. Lis7 has described a twelve-ion manganese cluster in which eight of the Mn ions are in the +3 oxidation state (spin S=2) and four are in the +4 state (S=3/2). These ions are magnetically coupled to give an S=10 ground state8, giving rise to unusual magnetic relaxation properties8,9. Here we report that the magnetization of the Mn12 cluster is highly anisotropic and that the magnetization relaxation time becomes very long below a temperature of 4 K, giving rise to pronounced hysteresis. This behaviour is not, however, strictly analogous to that of a bulk ferromagnet, in which magnetization hysteresis results from the motion of domain walls. In principle, a bistable magnetic unit of this sort could act as a data storage device.
3,327 citations
TL;DR: Arrays of ferromagnetic nickel and cobalt nanowires have been fabricated by electrochemical deposition of the metals into templates with nanometer-sized pores prepared by nuclear track etching, displaying distinctive characteristics because of their one-dimensional microstructure.
Abstract: Arrays of ferromagnetic nickel and cobalt nanowires have been fabricated by electrochemical deposition of the metals into templates with nanometer-sized pores prepared by nuclear track etching. These systems display distinctive characteristics because of their one-dimensional microstructure. The preferred magnetization direction is perpendicular to the film plane. Enhanced coercivities as high as 680 oersteds and remnant magnetization up to 90 percent have also been observed.
1,146 citations
TL;DR: In this article, the authors used circularly polarized soft x-rays with an imaging photoelectron microscope to record images of magnetic domains at a spatial resolution of 1 micrometer.
Abstract: Circularly polarized soft x-rays have been used with an imaging photoelectron microscope to record images of magnetic domains at a spatial resolution of 1 micrometer. The magnetic contrast, which can be remarkably large, arises from the fact that the x-ray absorption cross section at inner-shell absorption edges of aligned magnetic atoms depends on the relative orientation of the photon spin and the local magnetization direction. The technique is element-specific, and, because of the long mean free paths of the x-rays and secondary electrons, it can record images of buried magnetic layers.
512 citations
TL;DR: In this paper, a combination of room temperature coercivity analysis and low temperature remanence measurements provided a characteristic magnetic signature for intact chains of single domain (SD) particles of magnetite from magnetotactic and dissimilatory iron-reducing bacteria.
430 citations
TL;DR: In this paper, the existence of a ferromagnetic transition in a purely organic, non-ionic material with a Curie temperature of 1.48 ± 0.02 K was reported.
Abstract: IN one of the pioneering studies of magnetism at the microscopic level, Heisenberg1 concluded that ferromagnetism could not exist in compounds consisting only of light elements. A few well defined2, purely organic materials (consisting only of the elements carbon, hydrogen, oxygen and nitrogen) have subsequently been found3–5 to exhibit evidence of ferromagnetic interactions at low tem-peratures, with a small number of these showing a transition to a true ferromagnetic state. For example, the p-nitrophenyl nitronyl nitroxide radical has a Curie (transition) temperature of 0.60 K (ref. 6), and a possible ferromagnetic transition has been identified7,8 in a C60-based ionic complex. More recently, it was reported9 that the magnetization and magnetic susceptibility behaviour of a crystalline nitroxide biradical, N,N′-dioxy-l,3,5,7-tetramethyl-2,6-diazaadamantane, are indicative of ferromagnetic interactions. Here we report the existence of a ferromagnetic transition in this system, with a Curie temperature of 1.48 ± 0.02 K. As yet, this is the highest transition temperature found for a purely organic, non-ionic material.
423 citations
TL;DR: Magnetic moments μ(N) of iron clusters in a molecular beam, with temperatures ranging from 100 to 1000 K, are investigated from their Stern-Gerlach deflections and it is found that at a temperature of 120 K, μ is 3μ B per atom, decreasing to about the bulk value near N=500.
Abstract: Magnetic moments μ(N) of iron clusters in a molecular beam, with temperatures ranging from 100 to 1000 K, are investigated from their Stern-Gerlach deflections. We find that at a temperature of 120 K, μ (25≤N≤130) is 3μ B per atom, decreasing to about the bulk value (2.2μ B per atom) near N=500. For all sizes, μ decreases with increasing temperature, and is approximately constant above a temperature T C (N). For example, T C (130) is about 700 K, and T C (550) is about 550 K (T C bulk=1043 K). Limitations of the superparamagnetic model due to rotational erects are discussed
386 citations
Book•
01 Jan 1993
TL;DR: In this paper, the main components of magnetic fluids are described and compared to the physical properties of the magnetic fluids, such as equilibrium particle concentration in a ferrocolloid magnetization of a magnetic fluid, density of magnetic fluid viscosity of the fluid, and the noninteracting particles model for actual fluids.
Abstract: Part 1 Manufacture of magnetic fluids composition of magnetic fluids manufacture of magnetic fluids with different ferrophases manufacture of different-base magnetic fluids. Part 2 Physical properties of magnetic fluids: equilibrium particle concentration in a ferrocolloid magnetization of magnetic fluids density of magnetic fluids viscosity of magnetic fluids validity of the non-interacting particles model for actual fluids heat conduction and heat capacity electrical characteristics of magnetic fluids acoustic characteristics optical properties forecast of magnetic fluid properties physical properties of the main components of magnetic fluids. Part 3 Dynamics of magnetic fluids in gravitational, thermal and magnetic fields: governing equations hydrostatics phenomena in non-isothermal magnetic fluids interfacial phenomena specific features in the dynamics of non-equilibrium magnetization magnetic fluids non-equilibrium magnetization problems and their statement hydrodynamic effects of non-equilibrium magnetization. Part 4 Magnetic fluids in chemical engineering: seals of shafts and rods magnetofluid lubricants supports, bearings, dampers and shock-absorbers. Part 5 Magnetic fluids as applied to technological processes: magnetohydrostatic separation of ores drag reduction and flow separation control heat transfer enhancement magnetic fluids in chemical technological processes. Part 6 Magnetic fluid-based devices and arrangements: sensors attenuating mechanisms printers acoustic radiators.
385 citations
TL;DR: In this article, it was shown that hysteresis in the magnetostriction k is coupled to the hystresis of the magnetization M due to the dependence of the magnetic properties on the magnetisation.
Abstract: It is demonstrated that hysteresis in the magnetostriction k is coupled to hysteresis in the magnetization M because of the dependence of the magnetostriction on the magnetization. At the same time, when stress is present, the magnetization is in turn coupled to the behavior of the part of the magnetostriction associated with domain moment rotation. An expression for the magnetostriction is formulated, and numerical modeling results for magnetostriction hysteresis are compared to experimental results. Although some features of the magnetostriction in iron and steel still need additional explanation, the main features of the magnetostriction are accounted for. These include liftoff (failure of the magnetostriction to return to its value in the demagnetized state as the hysteresis loop is cycled) and a magnetostriction increase after flux density B reaches its maximum and starts to decrease. A macromagnetic, multidomain formulation that yields zero magnetostriction in the demagnetized specimen is used. >
371 citations
TL;DR: Nanocrystalline structures offer a new opportunity for tailoring soft magnetic materials The most prominent example are devitrified glassy FeCuNbSiB alloys which reveal a homogeneous ultrafine grain structure of bcc-FeSi with grain sizes of typically 10-15 nm and random texture.
Abstract: Nanocrystalline structures offer a new opportunity for tailoring soft magnetic materials The most prominent example are devitrified glassy FeCuNbSiB alloys which reveal a homogeneous ultrafine grain structure of bcc-FeSi with grain sizes of typically 10-15 nm and random texture Owing to the small grain size the local magneto-crystalline anisotropy is randomly averaged out by exchange interaction so that there is only a small anisotropy net-effect on the magnetization process Moreover the structural phases present lead to low or vanishing saturation magnetostriction which minimizes magneto-elastic anisotropies Both the suppressed magnetocrystalline anisotropy and the low magnetostriction provide the basis for the superior soft magnetic properties comparable to those of permalloys or near zero-magnetostrictive Co-base amorphous alloys but at a higher saturation induction Like in other soft magnetic material the hysteresis loop can be tailored by uniaxial anisotropies induced by magnetic field annealing
301 citations
TL;DR: In this article, it was shown that the amplitude of the analytic signal is dependent on magneti-sation strength and the direction of geological strike with respect to the magnetisation vector, this dependency is easier to deal with in the interpretation of analytic signal amplitude than in the original total field data or pole-reduced magnetic field.
Abstract: The interpretation of magnetic field data at low magnetic latitudes is difficult because the vector nature of the magnetic field increases the complexity of anomalies from magnetic rocks. The most obvious approach to this problem is to reduce the data to the magnetic pole (RTP), where the presumably vertical magnetisation vector will simplify observed anomalies. However, RTP requires special treatment of north-south features in data observed in low magnetic latitudes due to high amplitude corrections of such features. Furthermore, RTP requires the assumption of induced magnetisation with the result that anomalies from remanently and anisotropically magnetised bodies can be severely disturbed. The amplitude of the 3-D analytic signal of the total magnetic field produces maxima over magnetic contacts regardless of the direction of magnetisation. The absence of magnetisation direction in the shape of analytic signal anomalies is a particularly attractive characteristic for the interpretation of magnetic field data near the magnetic equator. Although the amplitude of the analytic signal is dependent on magnetisation strength and the direction of geological strike with respect to the magnetisation vector, this dependency is easier to deal with in the interpretation of analytic signal amplitude than in the original total field data or pole-reduced magnetic field. It is also straightforward to determine the depth to sources from the distance between inflection points of analytic signal anomalies.
TL;DR: In this article, the field-induced adiabatic magnetic entropy change of a series of iron-substituted gadolinium garnets (GGIG) Gd3Ga5−xFexO12 was investigated.
Abstract: The working refrigerant material in the majority of magnetic refrigerators has been Gd3Ga5O12 (GGG) which has an upper temperature limit near 15 K. In this paper we report on the field‐induced adiabatic magnetic entropy change, ΔSm(H,T), of a series of iron‐substituted gadolinium garnets (GGIG) Gd3Ga5−xFexO12 which have the potential to increase the working temperature range or to reduce the field requirements of cryogenic magnetic refrigeration. Depending on Fe concentration, x, the entropy change of these materials at applied fields of 0.9 and 5.0 T is much greater than that of GGG, especially at temperatures above 15 K. At low Fe concentrations, the results are consistent with formation of magnetically ordered clusters of spins at low temperatures. Room temperature electron paramagnetic resonance measurements show that Fe3+ ions mediate exchange interactions which are responsible for clustering at low temperatures.
TL;DR: The present theory can explain qualitatively these anomalous magnetic properties observed in Co compounds Co(S,Se,Se${)}_{2}$, ${\mathrm{YCo}}_{2}, ${\ mathrm{LuCo}$, and others.
Abstract: An itinerant-electron metamagnetism is discussed at finite temperature, by taking into account the effect of spin fluctuations on the Landau-Ginzburg theory. It is shown that the paramagnetic susceptibility always shows a maximum in its temperature dependence when the metamagnetic transition from the paramagnetic to the ferromagnetic state is induced by the external magnetic field at low temperature. This metamagnetic transition, associated with a hysteresis in the magnetization curve, is shown to disappear at high temperature. Moreover, the first-order transition in the temperature dependence of the spontaneous magnetization is shown to occur under a certain condition among the Landau coefficients. Three characteristic temperatures, at which the susceptibility reaches a maximum, the field-induced metamagnetic transition disappears, and the temperature-induced first-order transition of the magnetization occurs, are discussed. The present theory can explain qualitatively these anomalous magnetic properties observed in Co compounds Co(S,Se${)}_{2}$, ${\mathrm{YCo}}_{2}$, ${\mathrm{LuCo}}_{2}$, and others.
TL;DR: For Fe-Cr alloys it is found that the disordered-moment state has a higher energy than the ferromagnetic state in the entire region of Fe concentrations, and the theory fails to explain the spin-glass state observed around Fe 0.14 Cr 0.86.
Abstract: Local moment disorder, defined as a random arrangement of two distinct magnetic states of the same atomic species in a metallic system, is discussed in the framework of the Korringa-Kohn-Rostoker coherent-potential approximation combined with the local-density-functional method and applied to Fe-Cr, Ni-Fe, and Ni-Mn alloys. For Fe-Cr alloys it is found that the disordered-moment state has a higher energy than the ferromagnetic state in the entire region of Fe concentrations. Thus the theory fails to explain the spin-glass state observed around ${\mathrm{Fe}}_{0.14}$${\mathrm{Cr}}_{0.86}$. The theory, on the other hand, can explain the transition of Ni-Fe alloys from ferromagnetism to paramagnetism around the Invar region; the transition, however, is of first order, in contrast to experimental indications. The volume contraction due to the reversal of the magnetic-moment alignment from parallel to antiparallel with respect to the bulk magnetization is also discussed in connection with the Invar anomalies. For Ni-Mn alloys the calculation shows that, when the Mn concentration is larger than 15 at. %, magnetic states with local moments parallel and antiparallel to the average magnetization coexist even in the ferromagnetic region. The results are quite consistent with NMR experiments, which clearly show the existence of the antiparallel Mn local moments in addition to the parallel ones.
TL;DR: In this article, the authors have demonstrated conclusively that self potentials develop owing to fluid flow and that both resistivity and magnetization change when rocks are stressed, however, field experiments have had much less success.
Abstract: Despite over 2 decades of international and national monitoring of electrical signals with the hope of detecting precursors to earthquakes, the scientific community is no closer to understanding why precursors are observed only in some cases. Laboratory measurements have demonstrated conclusively that self potentials develop owing to fluid flow and that both resistivity and magnetization change when rocks are stressed. However, field experiments have had much less success. Many purported observations of low-frequency electrical precursors are much larger than expectations based on laboratory results. In some cases, no precursors occurred prior to earthquakes, or precursory signals were reported with no corresponding coseismic signals. Nonetheless, the field experiments are in approximate agreement with laboratory measurements. Maximum resistivity changes of a few percent have been observed prior to some earthquakes in China, but the mechanism causing those changes is still unknown. Anomalous electric and magnetic fields associated with fluid flow prior to earthquakes may have been observed. Finally, piezomagnetic signals associated with stress release in earthquakes have been documented in measurements of magnetic fields.
TL;DR: In this paper, the current density and local magnetic field are calculated analytically for a strip of a type-II superconductor in perpendicular magnetic field Ha for constant critical current density.
Abstract: The current density and local magnetic field are calculated analytically for a strip of a type-II superconductor in perpendicular magnetic field Ha for constant critical current density. The penetrating flux front has vertical slope and the initial penetration depth, magnetization change and hysteretic losses are ~ Ha2, ~ Ha3 and ~ Ha4, respectively. The analytical results differ from the widely used Bean model and explain numerous experiments in a natural way without the assumption of a surface barrier.
TL;DR: In this paper, the magnetic properties of Br.H 2 O, a molecule consisting of eight iron(III) ions bridged by oxo and hydroxogroups, are reported, and the magnetic susceptibility and magnetization indicate that levels of high spin multiplicity are populated at 4.2 K.
Abstract: The magnetic properties of {[(tacn) 6 Fe 8 (μ 3 -O) 2 (μ 2 -OH) 12 ]Br 7 (H 2 O)}Br.H 2 O, Feg, tacn=1,4,7-triazacyclononane, a moleculecomprising eight iron(III) ions bridged by oxo and hydroxogroups, are reported. The magnetic susceptibility, both dc and ac, and the magnetization indicate that at low-temperature spin levels with 8
TL;DR: In this article, the authors discussed the domain structure in ultrathin magnetic films, where the easy direction of magnetization is perpendicular to the film, and calculated the dependence of the width D of domains on film thickness L within the continuum approximation.
TL;DR: In this article, the magnetic properties of carbon nanotubes (CN's) are calculated in the effective mass theory, and the magnetic moment in a magnetic flux parallel to the tube axis oscillates as a function of the flux with period determined by the flux quantum and phase depending on whether CN is metallic or semiconducting.
Abstract: Magnetic properties of carbon nanotubes (CN's) are calculated in the effective-mass theory. The magnetic moment in a magnetic flux parallel to the tube axis oscillates as a function of the flux with period determined by the flux quantum and phase depending on whether CN is metallic or semiconducting. For a magnetic field perpendicular to the axis, CN's exhibit a large diamagnetism independent of metallic or semiconducting. The magnetic moment induced by a perpendicular field is about three order of magnitude as large as that by a parallel field.
TL;DR: In this paper, the spin configurations found in multilayer systems determine macroscopic properties such as the static susceptibility and magnetization, and can lead to anomalous field and temperature behavior.
Abstract: The authors discuss some of the fundamental properties unique to magnetic multilayers. Complex spin configurations are examined for many different systems and are shown to arise from a simple competition between exchange and Zeeman energies. The spin configurations found in multilayer systems determine macroscopic properties such as the static susceptibility and magnetization, and can lead to anomalous field and temperature behaviour. The authors also discuss the dynamic behavior of magnetic multilayers. Emphasis is placed on spin waves in magnetic multilayers with canted spin configurations and the softening of modes at magnetic phase transitions. Furthermore they show that spin wave excitations provide a powerful method for studying exchange interactions and spin configurations. Finally, the phenomenon of giant magnetoresistance in magnetic multilayers where the resistivity of the metallic structure can be changed by over 60% at room temperature, is discussed. Simple theoretical approaches are used to understand and predict the properties of the multilayer systems and comparisons between theory and experiment are stressed.
TL;DR: In this paper, the authors successfully grown ferromagnetic MnGa ultrathin films on GaAs substrates by molecular beam epitaxy using high energy electron diffraction and cross-sectional transmission electron microscopy.
Abstract: We have successfully grown ferromagnetic MnGa ultrathin films on GaAs substrates by molecular beam epitaxy. Reflection high energy electron diffraction and cross‐sectional transmission electron microscopy show that monocrystalline MnGa films are grown with the c axis of the tetragonal unit cell normal to the (001) GaAs substrates. Both magnetization measurements by vibrating sample magnetometer and extraordinary Hall effect (EHE) measurements indicate perpendicular magnetization, with the remnant magnetization of 225 emu/cm3 and EHE resistivity in the range of 0.5–4 μΩ cm at room temperature. The material possesses properties ideal for certain nonvolatile magnetic memory coupled with underlying III‐V circuitry.
TL;DR: In this article, the optical absorption edge is red shifted with respect to that of an epitaxially grown single-crystal film of γ-Fe2O3, attributed to lattice strain in the small particles.
Abstract: γ-Fe2O3 nanocrystals with a mean radius of 4.2 nm have been synthesized in a polymer matrix by an ion exchange and precipitation reaction. Magnetization and susceptibility data from experiment and computer simulations indicate that the system is superparamagnetic. The optical absorption edge is red shifted with respect to that of an epitaxially-grown single-crystal film of γ-Fe2O3. The red shift is attributed to lattice strain in the small particles.
TL;DR: A new type of magnetic linear dichroism in angle-resolved, spin-integrated photoemission is observed: for p-polarized light under oblique incidence the Fe 3p core level peak position and line shape change when the sample magnetization is reversed.
Abstract: We have observed a new type of magnetic linear dichroism in angle-resolved, spin-integrated photoemission: For p-polarized light under oblique incidence the Fe 3p core level peak position and line shape change when the sample magnetization is reversed. Spin-resolved measurements show that the erect is due to spin-orbit interaction in the presence of exchange interaction. The erect can be used for chemically specific diagnostics of magnetic structures
TL;DR: The coercive field of a magnetically soft prismatic grain of irregular cross-section embedded in a hard magnetic matrix has been calculated as a function of the size of the soft magnetic grain this paper.
TL;DR: In this paper, a method is proposed to study the possible contribution of remanent magnetization to a particular anomaly, by comparing two functions that are calculated directly from the observations: (1) the amplitude of the analytic signal, and (2) the horizontal gradient of pseudogravity.
Abstract: Remanent magnetization can have a significant influence on the shape of magnetic anomalies in areas that are generally characterized by induced magnetization. Since modeling of magnetic anomalies is nonunique, additional constraints on the direction of magnetization are useful. A method is proposed here to study the possible contribution of remanent magnetization to a particular anomaly, by comparing two functions that are calculated directly from the observations: (1) the amplitude of the analytic signal, and (2) the horizontal gradient of pseudogravity. From the amplitude and relative position of maxima in these derived quantities, we infer the deviation of the magnetization direction from that of the ambient field. The approach is applied to the magnetic anomaly in the center of the Manicouagan impact structure (Canada). Our results, based only on the magnetic anomaly observations, are in close agreement with constraints on the direction of remanent magnetization from rock samples.
TL;DR: In this article, the authors measured the enthalpy of transformation to equilibrium using differential scanning calorimetry and determined the magnetic moments and Curie temperatures for the metastable solid solutions of Fe and Cu alloys.
Abstract: Metastable solid solutions of Fe and Cu, which are immiscible in equilibrium, have been formed using high-energy ball milling of elemental powder mixtures. Single-phase face-centered-cubic (fcc) solid solution was obtained for 0
TL;DR: In this paper, it was shown that if magnetization is scaling with a single scaling exponent, then the observed f−3 power spectra of aeromagnetic data implies a three-dimensional f−4 susceptibility distribution for length scales of 1 to 1000 km.
Abstract: Many geophysical variables exhibit fractal or at least scaling behaviour, notably in having power spectra proportional to some power of frequency. Power spectra of magnetic susceptibility logs from boreholes are proportional to spatial frequency f−2 for wavelengths from 0.1 to 100 m. If the power spectrum of the susceptibility or magnetization of the crust is known then the power spectrum of the magnetic field at the surface can be calculated. We show that if magnetization is scaling with a single scaling exponent then the observed f−3 power spectra of aeromagnetic data implies a three-dimensional f−4 susceptibility distribution for length scales of 1 to 1000 km. One way to reconcile the two results is to introduce anisotropy in the form of a f−4 variation for the horizontal direction and f−2 for the vertical.
TL;DR: This paper presents a combination of speculative approaches, some related to earlier work and some apparently novel, which show great promise in providing a new class of MRI machines that would be considerably less expensive, over and above that of low cost.
Abstract: This paper presents a combination of speculative approaches, some related to earlier work and some apparently novel, which show great promise in providing a new class of MRI machines that would be considerably less expensive. This class would have advantages and disadvantages as compared to existing MRI, over and above that of low cost. The disadvantages include the apparent inability to perform classic spectroscopy, and limited flexibility in the area of selective excitation. The advantages include a fundamental immunity to inhomogeneity and susceptibility problems, the ability to create a wide class of machines that are designed for specific anatomy-related applications, the ability to design open machines for physician access, and improved capability for high speed imaging. Generic to all of the methods presented are a pulsed polarizing field and an oscillatory read-out bias field. The pulsed field initially polarizes the magnetic moments. Since it is not on during the readout operation it has negligible homogeneity requirements since changes in the field amplitude will merely shade the image intensity. During readout a relatively low bias field is used. To enable the use of a relatively inhomogeneous bias field, an oscillatory field is used that has a zero average value. This prevents any long-term buildup of phase errors due to a frequency error associated with inhomogeneity. Thus the average bias frequency will be determined solely by the frequency rather than the amplitude of the bias field. Three methods are described, all including the above features. The first two involve imaging in the laboratory frame, while the third involves imaging in the rotating frame. The second approach requires no RF excitation and the third approach uses RF bias and gradient signals. Some approaches to slice selection are described.
TL;DR: A detailed quantitative study of the physical properties of the infinite-dimensional Hubbard model at half filling using an exact mapping onto a single-impurity model supplemented by a self-consistency condition is presented.
Abstract: A detailed quantitative study of the physical properties of the infinite-dimensional Hubbard model at half filling is presented The method makes use of an exact mapping onto a single-impurity model supplemented by a self-consistency condition This coupled problem is solved numerically Results for thermodynamic quantities (specific heat, entropy, ), one-particle spectral properties, and magnetic properties (response to a uniform magnetic field) are presented and discussed The nature of the Mott-Hubbard metal-insulator transition found in this model is investigated A numerical solution of the mean-field equations inside the antiferromagnetic phase is also reported
TL;DR: In this paper, a triple resonance magic angle spinning (MAS) NMR experiment is reported that can be used to probe carbon-nitrogen interatomic distances in polycrystalline and amorphous solids, without the need for 15N isotopic labeling.
Abstract: A novel triple resonance magic angle spinning (MAS) NMR experiment is reported that can be used to probe carbon–nitrogen interatomic distances in polycrystalline and amorphous solids, without the need for 15N isotopic labeling. Theory is presented for an S=1 nucleus under conditions of MAS and a spin‐locking radio frequency (r.f.) field. An off‐resonance r.f. field is only effective in periods during the rotor cycle where it is of the same order of magnitude as the first order quadrupolar splitting Q. This occurs when Q changes sign (the zero crossing) and passages between the S=1 Zeeman levels result. Numerical calculations are used to determine the conditions for adiabatic passages. An adiabatic passage results in the greatest change in the density matrix, inverting the Zeeman magnetization and creating quadrupolar order; faster passages, caused by faster MAS or a larger quadrupole coupling constant (e2qQ/h), result in the loss of some Zeeman magnetization to non‐spin‐locked coherences. Applying an off‐resonance 14N spin‐locking field, during the evolution period of a 13C spin‐echo experiment, alters the evolution of the dipolar coupled spin and leads to a loss of 13C intensity at the echo. Calculations of the dephasing of 13C magnetization, caused by adiabatic 14N passages, are in good agreement with experimental results obtained at slow spinning speeds for a sample of glycine, and an estimate of the dipolar coupling between the nitrogen and directly bonded carbon can be made. Faster 14N passages result in less 13C dephasing. Despite the large value for e2qQ/h expected for the amide 14N nucleus in the polymer polyamide‐6, significant dephasing is still observed for carbon atoms that are more than 3.7 A away from the nitrogen in the polymer chain. Methods for calculating the 13C dephasing under conditions of fast 14N passages are considered.