Isothermal Susceptibility Research Articles

Proxy-climate and geomagnetic palaeointensity records extending back to Ca. 75,000 BP derived from sediments cored from Lago Grande di Monticchio, Southern Italy

As part of a multi-disciplinary study of maar lake sediments, magnetic properties of some 3000 samples from two long cores extracted from the bottom sediments of Lago Grande di Monticchio located in southern Italy have been measured and the results connected with lithological, palynological and geochemical data obtained from the same cores. A time-scale, based on microlaminations, radiocarbon and Ar Ar age determinations (see companion papers in this special issue of Quaternary Science Reviews) dates the base of the longer 51 m core at about 76 ka before the present. The strengths of anhysteretic remanence, saturation isothermal remanence and susceptibility are highly sensitive to the contrast between stadial and interstadial conditions, showing almost flip-flop transitions at Oxygen Isotope Stage 5a/4 and Stage 2/1 boundaries as previously observed for Lac du Bouchet, another maar lake site located in the Haute Loire Province of France. On the other hand, these laboratory installed magnetizations vary smoothly through sediment deposited during stadials. A prolonged period of heavy (modelled) precipitation starting at ∼25 ka BP coincides with a marked decrease in measured magnetic intensities possibly associated with the consequent increase in lake level and lake water chemistry. The natural remanent magnetization is dominated by climatic and environmental influences: nevertheless a weak signal attributed to variations in the palaeointensity of the geomagnetic field has been isolated, and it shows good broad-scale agreement with that previously reported for Lac du Bouchet.

Landau Theory of a System with Two Bilinearly Coupled Order Parameter sin External Field: Exact Mean Field Solution, Critical Properties and Isothermal Susceptibility

Abstract We present simple description of a system with two bilinearly coupled order parameters in external field based on an exact mean-field solution of the Landau Hamiltonian. It reproduces the qualitative form of the "field-temperature" phase diagram given by a molecular-field model and by more sophisticated theories and experiments on metamagnets. The solution gives the same critical exponents as the molecular-field theory, but it is not restricted to the magnetic systems only and it is easier to handle, since it formulates the results in explicit analytical form. The susceptibility in this model does not diverge at the second order transition line (far from a higher order critical point separating the second and first order transition lines), but jumps down from the lower temperature wing to the higher temperature one. The jump amplitude is proportional to the square of the field in small fields and diverges in large fields close to the higher order critical point.

Introduction to the Theory of the Integer Quantum Hall Effect

Part 1 Introduction - Basic Facts: The Integer Quantum Hall Effect Classical Dynamics Quantizing Magnetic Fields The Eigenvalue Problem The Landau Model Models of Confinement Bloch Representation Disorder Broadening. Part 2 Quantum Hall Effect for Pedestrians - High Field Model: Confined Cylinder Model Spectral Conditions for the Quantum Hall Effect Systems with Contacts Robustness of the Quantum Hall Effect. Part 3 Linear Response Isothermal Susceptibilities: Dynamic Susceptibilities Conductivity Tensor Spectral Decomposition Conductivitie in Spectral Decomposition Hall Conductivity in Terms of the Center Coordinates - Multi-Probe Systems Conductance and Geometry Problems in Magnetotransport Calculations. Part 3 Phenomenology of Global Conductivities - Quantum Langevin Equation: Mori Theory Localization Criteria Hall Plateaus and Mobility Edges Finite Temperatures. Part 4 Localization in High Landau Bands - Quantum Corrections to the Conductivity: Quantum Wires Weak Localization Regime in the Quantum Hall Effect Localization in the Tails of High Landau Bands. Part 5 Averaging Green Functions - Gaussian Path Integrals: Supersymmetry Method Replica Tyick The Instanton Free Energy. Part 6 Localization in the Lowest Landau Band Density of States: Improved Perturbation Methods Inverse Participation Number Instanton Method - Density of States Localization in Band Tails.

Nonlinear AC susceptibility and critical current of (Y 1− xPr x)Ba 2Cu 3O 7−δ ceramics

The intergranular critical current density J cJ( T) of (Y 1− x Pr x )Ba 2Cu 3O 7−σ ceramic samples was determined at zero DC magnetic field by temperature-dependent AC susceptibility measurements χ( T) with different field amplitudes h AC as well as by isothermal low-field AC susceptibility measurements χ( h AC) (140 mG ≤ h AC ≤ 18.4 G). In the “low” temperature regime, the temperature dependence of j cJ was deduced from the initial slope of isothermal χ′( h AC) measurements. The shift of the intergrain peak in χ″( T) to lower temperatures with increasing field amplitude h AC was used to determine the temperature dependence of j cJ in the “high” temperature regime which could be fitted to a power-law behaviour j cJ (T) ∝ (1 − T T cJ ) β with β ≈ 1 . This temperature dependence of the intergranular critical current density j cJ indicates that the superconducting grains are connected by insulating interfaces (network of SIS junctions). j cJ was found to decrease monotonically with increasing Pr content x.

Static Crossover of Critical Behavior in Polymer Blend Solutions: Scaling Analysis of the Ginzburg Number

By means of light scattering, the static critical behavior was investigated for symmetrical polymer blend solutions of polystyrene/poly(methyl methacrylate)/deuterated benzene and polystyrene/poly(2-chlorostyrene)/deuterated benzene with different polymeric indices N and polymer concentrations c. The temperature dependence of the isothermal susceptibility S T and the correlation length ξ t near the stability limit showed the static crossover from the mean-field to the three-dimensional Ising behavior in all solutions. The Ginzburg number Gi BK was evaluated with the aid of the crossover function of Belyakov and Kiselev. As c increases from the semidilute region to the bulk limit, the scaled Ginzburg number gi BK = Gi BK N decreased, with the power α of gi BK ∞ c α being -1.25 to -2 as expected by the theory based on the blob concept. The values of gi BK (bare) evaluated by the bare interactions were smaller than experimental values.

Collective excitations in the anharmonic crystal

We consider a lattice model of an anharmonic crystal and compute the isothermal ( χ T ) and adiabatics ( χ S ) susceptibilities for the lattice displacement below T c. A detailed analysis is given in terms of the dimension of the lattice and a decay parameter of the harmonic force. It turns out that there is a phase-space region for which 0 < χ T / χ S < 1, indicating breaking of ergodicity, and a region for which χ T is finite, indicating the presence of a plasmon frequency due to the anharmonicity.

Dynamical properties of critical and paramagnetic spin fluctuations in anisotropic Heisenberg magnets

Decay rates for long-wavelength spin fluctuations in the standard x-y model, and a second model described by the sum of the full Heisenberg exchange interaction and a single-site anisotropy, are calculated on the basis of a coupled mode theory in which the isothermal susceptibilities are consistent with corresponding spherical models (in consequence, quantum fluctuations are absent). Results for the decay rates are provided for two- and three-dimensional lattices and ferromagnetic and antiferromagnetic exchange interactions. Their dependence on the wave vector, q, and the inverse correlation lengths are derived from integral equations that admit homogeneous functions as solutions. The decay rates may thus be expressed as qd2/R1(q/ kappa 0) and q(d+2/2)R2(q/ kappa ,q/ kappa 0) where R1 and R2 are, respectively, scaling functions for the x-y and ferromagnetic single-site anisotropy models, d is the spatial dimension, and kappa (KO) is the out of plane (in plane) inverse correlation length. Analytical and numerical values of the scaling functions are presented for the limits of main physical interest.

Intergranular properties of YBCO and BSCCO ceramic superconductors at low fields

Isothermal low-field AC susceptibility measurements have been used to analyze the intergranular critical current density J c( T) on sintered, non-oriented YBa 2Cu 3O 7−δ and Bi 2Sr 2CaCu 2O 8+δ ceramic samples at zero field. Below the critical temperature, potential variations, J c( T) ≈ (1− t j) m with t j = T/ T j, have been found, T j being the onset of grain's coherence, but with different exponents, supporting that different mechanisms limit the intergranular J c values. Moreover, the effect of texture has been also considered on Bi 2Sr 2CaCu 2O 8+δ ceramics grown by the laser floating zone method, which have stronger intergranular junctions. Their high-temperature behaviour is limited by intrinsic effects, while at low temperatures the quality of the junctions is the limiting factor. The temperature dependence of the χ′( h 0) extrapolation at zero filed has also been correlated with the evolution of the intergranular penetration depth, λ J( T).

AC susceptibility in the spin-Peierls compound CuGeO 3 under high magnetic fields

The AC susceptibility in the cuprate compound CuGeO 3, in which the spin-Peierls (SP) transition occurs around 14K in the absence of magnetic field, was measured in the magnetic field region up to 20 T to establish the magnetic phase diagram of CuGeO 3. It was found that the isofield AC susceptibility shows a “knee” or peak at the phase boundary between the SP and uniform (U) phases and that between another magnetic (M) and U phases. The SP-M phase transition was clearly observed in the isothermal AC susceptibility. The determined phase diagram is consistent with the theoretical one after Cross and those of three organic SP materials.

Mean field theory of magnetic susceptibilities of metallic spin glasses

A mean field approach which emphasizes spin-spin pair correlations is used to calculate the d.c. isothermal magnetic susceptibility χ of metallic spin glasses. Finite-temperature effects and intrinsic sd scattering of electrons by magnetic ions play important roles in establishing the structure of χ. The result is a parameter-free expression which reveals a cusp in χ at the freezing temperature T g . The sharpness of the cusp is shown to increase with an increase in either the concentration of magnetic ions or the magnitude S of spins in the metallic spin glass.

The spectrum of longitudinal spin fluctuations in a ferromagnet including dipolar and Zeeman energies

The spectrum of longitudinal spin fluctuations, observed in inelastic neutron scattering experiments, is provided for a Heisenberg model of an ordered ferromagnet including dipolar forces and a magnetic field. A numerical method is used to obtain results, from an expression derived with linear spin-wave theory, for wavevectors (k) throughout the Brillouin zone as a function of frequency ( omega ) and at various temperatures. Particular attention is given to realistic models of EuO and EuS. Results show that dipolar forces diminish the intensity in the longitudinal response function S(k omega ) by about a factor of 2, in accord with previous results for the isothermal susceptibility. The evolution of S(k, omega ) with k displays several pronounced features, and for EuS there is a strong dependence on the direction of k for k near the zone boundary.

Weak coupling analysis of the optical conductivity of the two dimensional Hubbard model

We analyze the dynamical conductivity at zero momentum transfer of the two dimensional Hubbard model applying a projection technique approach in Liouville space. In the limit of small correlation strength we present a perturbative evaluation of the relevant current selfenergy and the isothermal current susceptibility using a novel real space summation technique. A dicussion of our results for the zero frequency Drude weight, the finite frequency relaxation rate and the incoherent spectrum of the optical conductivity at various filling densities and for zero temperature will be given.

Weak ferromagnetism and magnetic interactions in La2NiO4

Experimental evidence of weak ferromagnetic behaviour in the low-temperature tetragonal phase of stoichiometric LA2NiO4 is given from both isothermal magnetization and magnetic susceptibility studies. A clear anomaly is observed in the magnetic susceptibility around the first-order structural phase transition at Tc1 approximately=80 K. The symmetric and anti-symmetric exchange interactions in La2NiO4 are evaluated within the scope of the mean-field approximation and compared to those reported in La2CuO4. In spite of the strong similarities in the overall magnetic behaviour of La2NiO4 and LA2CuO4 oxides, severe quantitative differences exist in the magnetic interactions, which leads to a strong reduction in the two-dimensional character in La2NiO4. It is suggested that this different magnetic behaviour may be related to the absence of superconducting behaviour in the Ni oxides.

Scaling of the field effect on the magnetic relaxation about the Curie point of EuS.

Above ${\mathit{T}}_{\mathit{c}}$ and also in the homogeneous ferromagnetic phase below ${\mathit{T}}_{\mathit{C}}$, susceptibility measurements between 1.6 MHz and 1.5 GHz in parallel static fields up to 0.5 kOe reveal Lorentzian shapes in \ensuremath{\omega}. In the dipolar critical region on both sides of ${\mathit{T}}_{\mathit{C}}$, the field and temperature effects on the resulting kinetic coefficient of the magnetization scale with the isothermal susceptibility. We discuss this feature phenomenologically based on a recent mode-coupling result valid in the limit H=0 and T&gt;${\mathit{T}}_{\mathit{C}}$.

Paleomagnetism of Late Mesozoic granitic intrusions in Hong Kong: Implications for Upper Cretaceous reference pole of south China

Two Late Jurassic‐early Cretaceous granite plutons in Hong Kong were sampled at 16 sites for paleomagnetic determination. The two granites show differences in magnetic properties and mineralogy but similar natural remanent magnetization (NRM) directions. Multiple magnetizations and dual polarity have been detected in the samples. Isothermal remanent magnetism, magnetic susceptibility, and coercivity measurements reveal multidomain titanomagnetite as the principal magnetic carrier. A rating of paleomagnetic results from individual sites was based on the number of specimens, efficacy of the demagnetization procedures, and within‐site variance of NRM directions. The pole position from 12 high quality sample sites is 171.9°E, 78.2°N, A95=10.6°. The dual polarity and cooling ages of the granites suggest a Late Cretaceous acquisition of the remanent magnetism. Based on four different studies, the Upper Cretaceous reference pole of South China is located at 194.3° longitude, 73.5°N latitude, A95=10.4°. Paleomagnetic results from North China, South China, and Eurasia show no large‐scale Cenozoic relative movements among the three blocks. But the small separation between the paleomagnetic poles of South China and Eurasia probably implies a minor relative movement since the Late Cretaceous.

The study of the free radical DPPH in pulsed magnetic fields

Isothermal differential susceptibility measurements in pulsed magnetic fields up to 25 T in the spin system of orthorhombic DPPH were performed at liquid helium temperatures. The measurements in the level-crossing region can be well described by a four-spin cluster model.

Susceptibility divergence of the spin 1 Ising model on the Cayley tree

Abstract The temperature T2(1), below which the zero-field isothermal susceptibility diverges for the spin 1 Ising model on the M-generation Cayley tree, in the limit as M → ∞, is located. The general approach follows closely that of Falk for the spin 1 2 problem. Pruning and retraction identities are used to find upper and lower bounds on the bond-length dependence of the general two-spin correlation function and thence on the susceptibility. A conjecture is advanced regarding the divergence of the higher-order susceptibilities.

Spin-lattice relaxation in paramagnetic CdMnTe

Paramagnetic Relaxation has been used to determine the interaction time between Mn 2+ spins and phonons in Cd 1− x Mn x Te for x ≤ 0.02 in the temperature range 2–30 K. We measure the frequency dependance of the differential susceptibility, as a function of a DC magnetic field applied parallel to the modulated field (∼1 gauss, 100 Hz ≤ v ≤ 1 MHz), in a Faraday experiment. As shown by Casimir and Du Pré, the basic physics is contained into the relation between isothermal and adiabatic magnetic susceptibility. The interaction time τ SL is found to decrease roughly as T −2 and decreases also when x increases. We discuss these results in the context of other experiments, in the field of Semimagnetic Semiconductors, concerned with localized spins relaxation.

Quenching of ferrimagneticlike ordering in SrCr8Fe4O19 hexagonal ferrite

We report here measurements of high field (H≤150 kOe) isothermal magnetization, dc and ac susceptibility, and Mössbauer spectroscopy of the exchange frustrated insulator SrCr8Fe4O19. The isothermal magnetization and Mössbauer spectroscopy results are consistent with a ferrimagneticlike transition at TC≊250 K. Low field dc and ac susceptibility measurements reveal a reentrant spin-glass transition at low temperature but they fail to show any anomaly at the nominal Curie temperature TC. This unusual feature is interpreted as indicative of a transition where long range ferrimagnetic ordering has been replaced by a correlated spin-glass phase with finite spin correlation length.

Comparison between the magnetic susceptibilities of Pd 1− xAg x and single phase PdH n

The rapid decrease of the isothermal magnetic susceptibility χ of single phase PdH n with n ( = atomic ratio H/Pd) above the critical temperature T c = 564 differs from that of the magnetically similar single phase system Pd 1− x Ag x ( x: silver mole fraction) at the same temperature and at equal valence electron concentrations ( n = x), i.e. χ (PdH n )−χ(Pd 1− x Ag x ) <0. By use of a semiphenomenological susceptibility ansatz related to the nonideal solution behaviour of H (Ag) in Pd the susceptibility difference is interpreted as an electronic excess effect. The analysis of the steep descent of the magnetic susceptibility also applies to the Pd-rich side of PdH n in the subcritical temperature region (so-called α-phase) and can be supported by 105Pd Knight shift data at 348 K.