Spin Magnetic Susceptibility Research Articles

Coexistence of antiferromagnetic ordering and high- Tc superconductivity in electron-doped superconductor Nd 2− xCe xCuO 4

Muon spin relaxation and magnetic susceptibility measurements were performed for the electron-doped superconductor Nd 2− x Ce x CuO 4 (NCCO) with a particular attention to the vicinity of the boundary in phase diagram between the antiferromagnetic (AF) and the superconducting phases. The obtained phase diagram clearly exhibits a competitive change in the volume fraction between the two phases around the phase boundary; a dramatic decrease of the AF phase and a sudden appearance of bulk superconductivity upon doping beyond x∼0.13. However, the spatial coexistence of both phases is also suggested around the boundary. In contrast to the hole-doped system, the onset of T c is approximately independent on the doping in the underdoped region of NCCO.

Superconducting 2D system with lifted spin degeneracy: mixed singlet-triplet state.

Motivated by recent experimental findings, we have developed a theory of the superconducting state for 2D metals without inversion symmetry modeling the geometry of a surface superconducting layer in a field-effect transistor or near the boundary doped by adsorbed ions. In such systems the twofold spin degeneracy is lifted by spin-orbit interaction, and singlet and triplet pairings are mixed in the wave function of the Cooper pairs. As a result, spin magnetic susceptibility becomes anisotropic and Knight shift retains finite and rather high value at T = 0.

Role of Hydrogen Bonds in Benzimidazole-Based Organic Magnetic Materials: Crystal Scaffolding or Exchange Linkers?

The highly stable nitroxide radicals 5(6)-chloro-2-(N-tert-butyl-N-aminoxyl)benzimidazole (4) and 5,6-dimethyl-2-(N-tert-butyl-N-aminoxyl)benzimidazole (5) were synthesized and characterized by electron spin resonance spectroscopy, crystallography, and magnetic susceptibility. Both crystallize as dimeric pairs with N−H hydrogen bond donors and N−O acceptors. This behavior contrasts to that of analogous 2-(N-tert-butyl-N-aminoxyl)benzimidazole (3), which forms hydrogen-bonded chains (Ferrer, J. R.; Lahti, P. M.; George, C.; Antorrena, G.; Palacio, F. Chem. Mater. 1999, 11, 2205−2210). Susceptibility analysis shows dimeric antiferromagnetic exchange coupling with J/k = −22 K for 4 and −24 K for 5. Density functional theory (DFT) computations support a N−O to N−O throughspace antiferromagnetic exchange mechanism as the major contributor to the overall magnetic behavior of 4 and 5. A spin−polarization exchange coupling mechanism involving a cyclic exchange path using the −N−H···O−N hydrogen bonds in the dimer...

Phase diagram of theXXZchain with next-nearest-neighbor interactions

We calculate the quantum phase diagram of an XXZ chain with nearest-neighbor (NN) ${J}_{1}$ and next-NN exchange ${J}_{2}$ with anisotropies ${\ensuremath{\Delta}}_{1}$ and ${\ensuremath{\Delta}}_{2},$ respectively. In particular we consider the case ${\ensuremath{\Delta}}_{1}=\ensuremath{-}{\ensuremath{\Delta}}_{2},$ to interpolate between the XX chain $({\ensuremath{\Delta}}_{i}=0)$ and the isotropic model with ferromagnetic phase ${J}_{2}.$ For ${\ensuremath{\Delta}}_{1}<\ensuremath{-}1,$ a ferromagnetic phase and two antiferromagnetic phases exist. For $|{\ensuremath{\Delta}}_{i}|<1,$ the boundary between the dimer and spin fluid phases is determined by the method of crossing of excitation spectra. For large ${J}_{2}{/J}_{1},$ this method seems to indicate the existence of a second spin fluid critical phase. However, an analysis of the spin stiffness and magnetic susceptibility for ${\ensuremath{\Delta}}_{1}={\ensuremath{\Delta}}_{2}=1$ suggests that a small gap is present.

On the theory of ferromagnetism of hexagonal diborides

The electronic structure and the magnetic properties of the MnB2 and CrB2 compounds with hexagonal AlB2-type lattices were studied. The problem was treated in terms of the generalized Hubbard model with an infinite electron-electron repulsion energy in the same atom. Equations for spin magnetic susceptibility were derived and used to determine the conditions of ferromagnetic instability and construct the phase diagram of the existence of ferromagnetic ordering.

ESR and magnetic susceptibility studies of polyaniline and its blend with poly (methyl methacrylate)

We present an analysis of electron spin resonance, magnetic susceptibility, and dc conductivity measurements on polyaniline and polyanilinc-PMMA blend to understand the reason for enhanced N(E F ) in the case of the blend. A number of experiments performed under different conditions can be understood using a model in which metallic particles are surrounded by disordered regions or shells. Our results are in agreement with an earlier investigation that found different structural arrangement in the blend compared with polyaniline.

Observation of the Pressure-Induced Weak Ferromagnetic Moment in the Layered Antiferromagnetic Compound (C2H5NH3)2CuCl4by Magnetic Susceptibility and Neutron Scattering Measurements

In the two-dimensional Heisenberg antiferromagnet (C 2 H 5 NH 3 ) 2 CuCl 4 , the intralayer exchange interaction is ferromagnetic, whereas the interlayer exchange interaction, which is less than 10 -4 of the intralayer one, is antiferromagnetic, and the spins collinearly lie in each layer. To investigate the effect of pressure on its spin structure, magnetic susceptibility and neutron scattering measurements under pressures up to 2.5 GPa were performed. The magnetic susceptibility measurements revealed the appearance of a pressure-induced weak ferromagnetic moment along the c -axis around 0.6 GPa. The neutron scattering measurements clarified the noncollinear spin structure which causes the weak ferromagnetic moment mentioned above. That is, the ferromagnetically coupled spins in each layer cant from the layer, which brings about the weak ferromagnetic moment along the c -axis. The ferromagnetic coupling in each layer and the antiferromagnetic coupling among the nearest-neighboring layers are maintained a...

Incommensurate modulation of spin magnetic susceptibility in single band Hubbard model

By means of a cumulant expansion method, we investigate the incommensurate phase in the low doping region of the two dimensional single-band Hubbard model. We calculate the spin magnetic susceptibility as a function of doping and analyze the effect of a Van Hove singularity on a commensurate-incommensurate transition. A comparison with the experimental results of LSCO is discussed.

Phase transitions and exact ground-state properties of theone-dimensional Hubbard model in a magnetic field

The exact phase diagrams of the one-dimensional Hubbard model, bothattractive and repulsive, in the presence of an arbitrary magneticfield h for various electron concentrations n and on-siteinteraction strengths U<0 or U>0 are calculated andinvestigated. The exact ground-state properties, namely, theground-state energy, the average spin (magnetization), theconcentration of the doubly occupied sites, the kinetic energy, thechemical potential, the spin (magnetic) susceptibility and thecharge compressibility, are calculated and examined over a wide rangeof interaction strengths U for various h and n. It is foundthat the spin susceptibility at half-filling is non-analytic andchanges discontinuously as U→0. The exact theory shows theabsence of a charge energy gap in the U⩽0 region for all n andprovides, for the chemical potential, the rigorous upper and lowerbounds for half-filled and empty bands respectively. Theanalytical results derived for the weak-coupling limit and asymptoticexpansions for strong coupling are in full agreement with thenumerical calculations.

Structure and Properties of a New Electride, Rb+(cryptand[2.2.2])e.

This is the sixth electride whose crystal structure has been determined and the fourth to show polymorphism. Crystals of the title electride prepared from mixed solvents have a structure similar to that of Li+(cryptand[2.1.1])e-. Electrons occupy cavities that are connected by "ladder-like" channels. The static and spin magnetic susceptibilities of polycrystalline samples that contain this polymorph (called phase α) show Heisenberg 1D antiferromagnetic behavior with -J/kB = 30 K. Similar to other electrides with "localized" electrons, this electride is a poor conductor (σ < 10- 4 ohm-1cm-1). Thin films prepared by high vacuum co-deposition of Rb metal and cryptand[2.2.2] have optical spectra and near-metallic electrical conductivity nearly identical with those of K+(cryptand[2.2.2])e-. These properties would not be expected if the film structure were the same as that obtained for crystals. Rather, they suggest that the films consist of microcrystals whose structure is similar to that of K+(cryptand[2.2.2])e-. Polycrystalline samples prepared by slow evaporation of methylamine from stoichiometric solutions at -78 °C (called phase β) have properties similar to those of K+(cryptand[2.2.2])e-. The conductivity of samples that contain phase β is more than an order of magnitude larger than those with phase α. Magnetic and spin susceptibilities show that phase β samples have much larger electron-electron interactions. As with K+(cryptand[2.2.2])e-, the magnetic susceptibility of phase β is compatible with alternating linear chain Heisenberg antiferromagnetism, with -J/kB ≈ 300 K and -J'/kB ≈ 240 K. Thin vapor co-deposited films show abrupt changes in the conductivity and optical spectrum at -12 °C that suggest a transition that may be conversion of phase β to phase α.

Spin magnetic susceptibility in the two-layer Hubbard model

The two-layer Hubbard model is studied by using the composite operator method. Magnetic properties of the model in the normal state are analyzed by studying the uniform static spin magnetic susceptibility as a function of doping and temperature. The relevance of the model to describe the physical behavior of two-layered cuprate high- T c superconductors is discussed.

Spin dynamics in perovskites, pyrochlores, and layered manganites

High temperature electron spin resonance (ESR) and magnetic susceptibility (χ) are analyzed for manganites related with colossal magnetoresistance (CMR). The properties of compounds with different crystalline structures: three-dimensional (3D) perovskites, pyrochlore, and La1.2Sr1.8Mn2O7, a two-dimensional layer, are compared. In the paramagnetic regime, and outside the critical regions associated with phase transitions, the temperature dependence of the ESR linewidth presents a universal behavior dominated by the variations of χ(T), ΔHpp(T)=[C/Tχ(T)]ΔHpp(∞). The high temperature limit of the linewidth, ΔHpp(∞), is related to the parameters of the Hamiltonian describing the interactions of the spin system. The role played by magnetic anisotropy, isotropic superexchange, and double exchange is revealed and discussed in the analysis of the experimental data. In CMR and non-CMR pyrochlores, ΔHpp(∞)∝ωp2/J where J is proportional to the Curie–Weiss temperature, including the hybridization mechanism producing CMR. Instead, ΔHpp(∞) of CMR perovskites seems not to be affected by the double-exchange interaction. In contrast with the 3D perovskites, the ESR linewidth and resonance field of La1.2Sr1.8Mn2O7, a bilayer compound, although isotropic at high temperatures, becomes anisotropic for Tc=125 K&amp;lt;T&amp;lt;Tp≈450 K.

Exact magnetic phase diagram of the one-dimensional Hubbard model

The exact solution of the one-dimensional Hubbard model, both attractive and repulsive, is investigated numerically over a wide range of interaction strength U/ t (−∞< U/ t<∞), electron concentration n (0⩽ n⩽1) and magnetic field h. The magnetic phase diagram and the ground-state properties, including the total energy, the average spin (magnetization) and the spin (magnetic) susceptibility are studied under the variation of U/ t, h/ t and n. The critical behavior near the onset of magnetization and magnetic saturation is analyzed. Our results are in full agreement with the analytical expansions in strong and weak interaction limits and previous numerical studies for h=0.

Spin and orbital magnetic susceptibility of the alloy system Ag xPt 1− x

We present results of a first application of linear response formalism to calculate the spin and orbital magnetic susceptibility of a disordered alloy system using the Korringa–Kohn–Rostoker-Coherent Potential Approximation (KKR-CPA). To account for all relativistic effects in a proper way, the formalism was used in its fully relativistic version (RKKR-CPA). Results obtained for the alloy system Ag x Pt 1− x are presented and compared with experimental data.

Spin−Spin Correlation Function and Magnetic Susceptibility of Quantum Ferrimagnetic Spin Chains as Models for Organic Molecule-Based Ferrimagnetics

Spin−spin correlation function and temperature dependence of magnetic susceptibility were calculated for the finite-size Hamiltonian of an S = 1 and S = 1/2 antiferromagnetic Heisenberg chain. The ...

EXACT GROUND-STATE PROPERTIES OF ONE-DIMENSIONAL HUBBARD MODEL IN THE PRESENCE OF MAGNETIC FIELD

The exact phase diagram and the ground-state properties of the one-dimensional Hubbard model with arbitrary on-site interaction of electrons are calculated over a wide range of magnetic field and electron concentrations by means of the Bethe-ansatz formalism. The ground-state properties, including the total energy, the average spin (magnetization) and spin (magnetic) susceptibility are investigated for both signs of the interaction strength U/t. The critical behavior near the onset of magnetization and magnetic saturation are also analyzed. At the onset of magnetization and near the magnetic saturation the spin susceptibility χ diverges at all U/t for half-filling case n=1, whereas for n≠1 it is always finite. The reverse susceptibility χ-1(U) exhibits anomalous hump, which increases with h or n, and shows discontinuity as U/t→±0 at infinitesimal h→0. The analytical results for the ground-state properties in strong and weak interaction limits are in full agreement with our numerical calculations.

Electron spin resonance and magnetic susceptibility suggest superconductivity in Na doped WO samples

In WO3 doped with Na (WO3:Na) an Electron Spin Resonance (ESR) signal with unresolved fine and/or hyperfine structure is detected and used as a probe for the state of the carriers. Using the saturation method we determined the spin-lattice relaxation rate 1/ T 1 of these paramagnetic centers. Upon cooling below 100 K, 1/ T 1 decreases markedly, as known to occur in NMR when a gap opens in the superconducting state. At low temperatures, 1/ T 1 quantitatively follows BCS behavior with a gap Δ= 160 K or2Δ/kTc=3.5. The magnetic susceptibility exhibits a strong difference between magnetic-field cooled and zero-field cooled regimes below Tc which also supports a so far unknown superconducting phase resulting from Na doping.

Doping and temperature dependence of the spin susceptibility in the p-d model

The spin magnetic susceptibility of the p-d model is calculated by means of a perturbation theory in the hybridization term V through a generalized cumulant expansion (GCE). The analysis is approached from the paramagnetic metallic phase. The results qualitatively reproduce some unusual magnetic properties in the normal state of the hole-doped cuprates, supporting the scenario of a Van Hove singularity near the Fermi level.

Relation of spin susceptibility and a.c. conductivity in nearly antiferromagnetic Fermi liquid

Coupling of the charge carriers through the magnetic spin susceptibility which is strongly peaked at ({pi}, {pi}) in the two dimensional CuO{sub 2} Brillouin zone leads to strong momentum anisotropies and energy dependencies in the inelastic optical scattering rates. The a.c. conductivity is isotropic and can be used to extract an effective, isotropic spin-charge carrier spectral density which reproduces almost exactly the optical scattering rates. The technique is then applied to data in superconducting, optimally doped YBCO and used to measure the charge carrier coupling to the 41 me V peak observed in spin polarized neutron scattering.

Antiferromagnetic domains inYBa2Cu3O6+xprobed byGd3+ESR

The anisotropy of the static homogeneous magnetic spin susceptibility of the antiferromagnetic CuO2 bilayers and the crystal-field parameters are measured in Gd-doped YBa 2Cu3O61x ~small x! single crystals using Gd 31 ESR at 9, 75, 150, and 225 GHz. We show that the easy magnetization direction is along @100# and that there is a magnetostriction leading to an orthorhombic lattice distortion. We observe an antiferromagnetic domain structure corresponding to the two equivalent orthorhombic distortions of the tetragonal lattice which depends on magnetic fields of the order of 1 T. The domain structure is unchanged between 10 and 150 K and is independent of thermal and magnetic history. We discuss two models: ~i! charged domain walls, ~ii! magnetization pinned to a small number of defective oxygen-rich Cu~1! layers. @S0163-1829~99!01702-6#