Many body theory of magnetic susceptibility of bloch electrons

We present a theory of the total susceptibility (χ) of solids using a finite temperature Green's function formalism. The many body effects on orbital (χ_o), spin (χ_s) and spin-orbit (χ_s-o) contributions to χ are calculated. Our expression for χ_s is equivalent to the earlier results but with the g-factor replaced by the effective g-factor. An important aspect of our work is the analysis of many body effects on χ_s?o.     

To the theory of the longitudinal magnetic susceptibility of quasi-three-dimensional ferromagnetic dielectrics

The longitudinal magnetic susceptibility of quasi-three-dimensional easy-axis ferromagnets has been found. In this case, the ferromagnetic structure is considered as a fractal object with the dimension D = 3 − ɛ, where ɛ > 0. The approach is based on using a quasi-classical kinetic equation. The magnon dispersion strictly calculated using operations of the fractal differentiation is a strongly anisotropic (dependent on the parameter ɛ) function of angular variables, which leads to nontrivial frequency dependences of the magnetic susceptibility.     

Quantum tetrahedral mean field theory of the magnetic susceptibility for the pyrochlore lattice

A quantum mean field theory of the pyrochlore lattice is presented. The starting point is not the individual magnetic ions, as in the usual Curie-Weiss mean field theory, but a set of interacting corner-sharing tetrahedra. We check the consistency of the model against magnetic susceptibility data and find good agreement between the theoretical predictions and the experimental data. Implications of the model and future extensions are also discussed.     

On the theory of static magnetic susceptibility of NiF2

The magnetic susceptibility of the weak ferromagnet NiF₂ is calculated using self-consistent microscopic calculations. It is shown that a comparatively large longitudinal susceptibility can be due to the rotation of a spin subsystem under the action of a magnetic field. The expression for the piezomagnetic moment of NiF₂ is given.     

A study on the magnetic properties of two-phase particulate magnetic composites

The magnetic properties of two-phase particulate magnetic composites with a hard ferromagnetic component are studied theoretically and experimentally. The magnetic properties considered here are phase-distribution sensitive properties, including remanence, coercivity and the shape of hysteresis loop. These properties depend mainly on the properties of its constituents, volume fractions, phase distribution, packing fraction and orientation distribution for anisotropic particles. With fixed packing fraction and orientation distribution, the magnetic properties of the two phase mixture can be calculated in terms of its component properties, volume fractions and phase distribution. Here, the component properties include not only remanence B_r and coercivity H_c but also a variable m which is the rate of change of magnetic induction B with respect to field intensity H. For two-phase systems satisfying B - H relation of the type B = B_r + mH where m is a constant, the equations for calculating the magnetic properties B_r, H_c, etc., in terms of m are derived. The method for calculating m is also developed for the cases of parallel and series distributions. Bounds for m-values were also established. A modified Landauer's type equation is developed to calculate m-values in terms of the component properties of the mixture. Experiments were conducted to verify the theoretical calculations. Good agreements between the theoretical calculations and experimental results were obtained.     

Linear-temperature dependence of static magnetic susceptibility in LaFeAsO from dynamical mean-field theory

In this Letter we report the local density approximation with dynamical mean field theory results for magnetic properties of the parent superconductor LaFeAsO in the paramagnetic phase. Calculated uniform magnetic susceptibility shows linear dependence at intermediate temperatures in agreement with experimental data. Contributions to the temperature dependence of the uniform susceptibility are strongly orbitally dependent. For high temperatures (>1000 K) susceptibility first saturates and then decreases with temperature. Our results demonstrate that linear-temperature dependence of static magnetic susceptibility in pnictide superconductors can be reproduced without invoking antiferromagnetic fluctuations.     

Density expansion of the magnetic susceptibility

The pressure of a system may be expanded as a power series in the density, whose coefficients are the virial coefficients. In this paper, the magnetic susceptibility of a $\text{spin}\text{-}\text{1}\text{2}$ fermion system is also expanded in powers of density. This process explicitly separates the temperature and density dependence of the magnetic susceptibility. The coefficients of this series are shown to be related to certain virial coefficients. The first (previously established) and second corrections to Curie's law are explicitly expressed in terms of second and third virial coefficients. These corrections to Curie's law are small for temperatures above 4K, but become important below that temperature. The first correction has been previously measured. Given a set of second and third virial coefficients, the importance of the second correction can be calculated immediately at any density of interest.     

On the theory of magnetoresistance of binary composites in a weak magnetic field: Numerical experiment

A correction to the electric field strength linear in magnetic field H was found by computations for a two-dimensional disordered system. This correction was used to calculate and graphically tabulate two two-parameter functions present in the equation describing magnetoresistance in a wide range of parameter variations. This correction was also used to determine and tabulate the derivative of the function present in the equation for the effective Hall coefficient with respect to one of its arguments. The data obtained in this work combined with the earlier results of these authors allow the magnetoresistance of binary (composite) media to be completely described in the spirit of the similarity hypothesis.     

A fresh look into the neutron EDM and magnetic susceptibility

We reexamine the estimate of the neutron electric dipole moment (NEDM) from chiral and QCD spectral sum rules (QSSR) approaches. In the former, we evaluate the pion mass corrections which are about 5% of the leading Log results. However, the chiral estimate can be affected by the unknown value of the renormalizaton scale ν  . For QSSR, we analyze the effect of the nucleon interpolating currents on the existing predictions. We conclude that previous QSSR results are not obtained within the optimal choice of these operators, which lead to an overestimate of these results by about a factor 4. The weakest upper bound |θ|?2×¹⁰⁻⁹$|\theta |?2\times {10}^{\mathrm{-9}}$ for the strong CP  -violating angle is obtained from QSSR, while the strongest upper bound |θ|?1.3×¹⁰⁻¹⁰$|\theta |?1.3\times {10}^{\mathrm{-10}}$ comes from the chiral approach evaluated at the scale ν=MN$\nu =M_N$. We also re-estimate the proton magnetic susceptibility, which is an important input in the QSSR estimate of the NEDM.     

Temperature dependence of the magnetic susceptibility of magnetic dispersed nanosystems

The temperature dependence of the complex magnetic susceptibility of different magnetic nano-colloids and finely dispersed magnetite powder is studied. The results are explained with allowance for the magnetic moment relaxation in single-domain particles and phase transitions in a system of interacting dipoles.     

A different theory of anomalous magnetic moment

The invariance of Dirac's equation under rotation has been used to obtain the wave equation for a particle interacting with an electromagnetic field. The origin of the anomalous magnetic moment of a particle has been attributed to the existence of mass due to spin. These masses for a few representative particles have been calculated. In particular these calculations give a mass of 592.074 eV for a neutrino. An operator for the spin angular velocity has been constructed and the values of spin angular velocities for the particles have also been calculated.     

A theory of nonlinear AC response in coated composites

A method for determining effective dielectric responses of Kerr-like coated nonlinear composites under the alternating current (AC) electric field is proposed by using perturbation approach. As an example, we have investigated the composite with coated cylindrical inclusions randomly embedded in a host under an external sinusoidal field with finite frequency ω. The local field and potential of composites in general consists of components with all harmonic frequencies. The effective nonlinear AC responses at all harmonics are induced by the coated nonlinear composites because of the nonlinear constitutive relation. Moreover, we have derived the formulae of effective nonlinear AC responses at the fundamental frequency and the third harmonic in the dilute limit.     

On the theory of the magnetic,electric, and mixed susceptibility in magnetically and electrically ordered systems

On the basis of the unified macroscopic approach taking into account the initial structural state of the system and the parameters of external effects (frequency and orientation of probe and displacing fields), relations have been obtained for the frequency and orientation dependences of the magnetic, electric, and mixed susceptibility and magnetic capacity for magnets, ferroelectrics, and ferroelectromagnets, respectively.     

High-frequency behavior of magnetic composites

The paper reviews recent progress in the field of microwave magnetic properties of composites. The problem under discussion is developing composites with high microwave permeability that are needed in many applications. The theory of magnetic composites is briefly sketched with the attention paid to the laws governing the magnetic frequency dispersion in magnetic materials and basic mixing rules for composites. Recent experimental reports on the microwave performance of magnetic composites, as well as data on the agreement of the mixing rules with the measured permeability of composites that are available from the literature are discussed. From the data, a conclusion is made that the validity of a mixing rule is determined by the permeability contrast in the composite, i.e., the difference between permeability of inclusions and that of the host matrix. When the contrast is low, the Maxwell Garnet mixing rule is frequently valid. When the contrast is high, which is of the most interest for obtaining high microwave permeability of a composite, no conventionally accepted theory is capable of accurately predicting the permeability of the composites. Therefore, the mixing rules do not allow the microwave properties of magnetic composites to be predicted when the permeability of inclusions is high, that is the case of the most interest. Because of that, general limitations to the microwave performance of composites are of importance. In particular, an important relation constraining the microwave permeability of composites follows from Kittel's theory of ferromagnetic resonance and analytical properties of frequency dependence of permeability. Another constraint concerning the bandwidth of electromagnetic wave absorbers follows from the Kramers-Kronig relations for the reflection coefficient. The constraints are of importance in design and analysis of electromagnetic wave absorbers and other devices that employ the microwave magnetic properties of composites, such as magnetic substrates for microwave antennas, microwave inductors, etc.     

To the theory of conduction of binary composites

The relationship between the local electric field strength in a binary composite, as an example, with its macroscopic characteristics of a nonuniform medium (effective conductivity σ _e and the derivatives of σ _e with respect to the concentrations and conductivities of the individual components) is discussed. It was demonstrated that the determination of all these parameters in a numerical experiment makes it possible to obtain a detailed information on the properties of the effective conductivity σ _e , especially near the percolation threshold, a metal-dielectric phase transition.     

Concentration dependence of magnetic susceptibility for the system of dilute magnetic impurities

The concentration dependence of the magnetic susceptibility for the system of randomly distributed dilute magnetic impurities in a d-dimensional metal or on a metal surface is studied at zero-temperature under the mean-random-field approximation. It is shown that, if the amplitude of the RKKY-type interaction between the magnetic impurities behaves like $\text{1}\text{r}{}^{\mathrm{n}}$ at a large distance r, the susceptibility is proportional to $\text{c}{}^{\mathrm{<mtext>1?(</mtext><mtext>n</mtext><mtext>d</mtext><mtext>)</mtext>}}$, where c is the concentration of the magnetic impurities.     

On the theory of galvanomagnetic properties of composites

The conductivity of composites in the presence of a magnetic field H is considered. The galvanomagnetic characteristics for a weakly inhomogeneous medium are determined in explicit form in an approximation quadratic in the deviations of conductivity tensor $\hat \sigma $ (r) from its mean value 〈 $\hat \sigma $ 〉. The contribution to the effective conductivity tensor $\hat \sigma _e $ linear in concentration c of inclusions for a composite with a small value of c is expressed in terms of the dipole polarizability of an individual inclusion, which is defined in the transformed system in which it is surrounded by an isotropic matrix with a scalar conductivity. Transition to this system is performed using a symmetry transformation that does not change the dc equations. An approximate approach proposed for describing the galvanomagnetic properties of composites in the wide range of parameters appearing in the problem generalizes the standard theory of an effective medium to the case of anisotropic systems with inclusions of arbitrary shape in field H ≠ 0.