Three-point interpolation approximation for the macroscopic properties of isotropic two-component materials

A simple unified interpolation scheme, which involves certain three-point correlation information about the microgeometry of an isotropic two-component material, is suggested to approximate the macroscopic conductivity and elastic properties of the composite. The approximation satisfies Hashin–Shtrikman bounds as well as the three-point bounds on the effective properties, and reduces to the well-established self-consistent and matrix-mixture (Maxwell) approximations for the respective microgeometries. Applications to some high-contrast property composites illustrate the results.     

Nd-Ce-Fe-B纳米复合薄膜的磁性及交换耦合作用

采用磁控溅射技术制备了具有永磁特征的Nd-Ce-Fe-B多层纳米复合薄膜,并对其进行了退火处理.通过改变退火温度,研究其对薄膜磁性能和晶体结构的影响.结果表明,随着退火温度的提高薄膜磁性能逐渐增大,但当温度达到695℃以上时,薄膜的磁性能急剧下降.当退火温度为675℃时,薄膜的矫顽力Hci=10.1 kOe(1Oe=79.5775 A/m),垂直于薄膜表面方向的剩余磁化强度4πM_(r⊥)=5.91 kG(1 G=10~3/(4π)A/m).薄膜的X射线衍射结果表明,磁性薄膜具有较好的c轴取向.通过对薄膜磁化反转过程的研究,发现随着外加磁场的增大,M_(rev)的极小值向M_(irr)减小的方向移动,这与畴壁弯曲模型类似,表明在薄膜中存在较强烈的局部钉扎作用,而剩余磁化强度曲线表明这种钉扎作用在薄膜矫顽力机制中并不占支配作用.此外,薄膜的Henkel曲线结果表明在薄膜中存在较强的交换耦合作用,在经过685℃退火的薄膜中磁相互作用更加显著.     

Magnetic exchange driven magnetoelastic properties in GdCu2

The magnetoelastic properties of GdCu₂ have been investigated by thermal expansion and magnetostriction measurements. GdCu₂ orders antiferromagnetically with a noncollinear magnetic structure. The anisotropic magnetostriction is of similar magnitude as in other RCu₂ compounds and can be explained by a contribution of the bilinear exchange interaction to the magnetoelastic energy. For several compounds this contribution is as important as the single ion magnetoelastic exchange. The pressure dependence of the Néel temperature of GdCu₂ is found to be in agreement with the data of thermal expansion.     

High orders isotropic exchange in symmetrical clusters

The group-theoretical classification of exchange multiplets is suggested. The exchange levels are characterized by full spin S and irreducible representations of SU(2s+1), R(2s+1) or Sp(2s+1) groups. In the cases s≤$\text{3}\text{2}$ the exchange Hamiltonian can be expressed through Casimir's operators of these groups. It makes possible to find the expression for energy of symmetrical systems in analitical form. It is shown that Schrödinger exchange operator H(s), which is the generalization of the Dirac exchange Hamiltonian, is the Casimir's operator of the corresponding unitary U(2s+1) group.     

Nonlocal density approximation to exchange and correlation: Ground state properties of solid copper and vanadium

Fermi surface parameters and x-ray scattering form factors of Cu and V are calculated using an approximation for the exchange-correlation potential, which is nonlocal in the electronic density. For Cu the results are very close to recent experimental data, especially the neck radius of the Fermi surface is reproduced almost exactly. For V, where previous calculations in LDA so far gave much too large Fermi surface ellipses, non-local effects in the long range part of the xc-potential decrease the size of these ellipses so drastically, that they now become too small. The residual discrepancy may be attributed to a local approximation of the short range part of the xc-potential in the present calculation.     

Markovian approximation in foreign exchange markets

In this paper, using the exit-time statistic, we study the structure of the price variations for the high-frequency data set of the bid–ask Deutschemark/US dollar exchange rate quotes registered by the inter-bank Reuters network over the period October 1, 1992 to September 30, 1993. Having rejected random-walk models for the returns, we propose a Markovian model which reproduce the available information of the financial series. Besides the usual correlation analysis we have verified the validity of this model by means of other tools all inspired by information theory. These techniques are not only severe tests of the approximation but also evidence of some aspects of the data series which have a clear financial relevance.     

纳米结构CoxFe3-xO4多孔微球的磁性及交换偏置效应研究

用乙二醇为溶剂,用三氯化铁、二氯化钴和醋酸铵为起始反应试剂,通过溶剂热反应首次合成了纳米结构Co_xFe_3-xO₄多孔微球.用X射线衍射仪(XRD)、透射电子显微镜(TEM)和扫描电子显微镜(SEM)表征样品的结构和形貌,结果表明,所制备的单分散Co_xFe_3-xO₄多孔微球为立方多晶结构,其直径约300 nm,是由约30 关键词： xFe_3-xO₄')" href="#">Co_xFe_3-xO₄ 多孔微球 磁性 交换偏置效应     

Magnetic properties and composition range of non-stoichiometric m-type hexagonal ferrites prepared by ion exchange

Single crystals of Ba, Sr M-type hexagonal ferrites were prepared by ion exchange in Ba, Sr containing molten salts from single crystals of β″ -ferrites. A fast diffusion of the divalent Ba²⁺, Sr²⁺ is observed leading to a non-stoichiometric M-type ferrite with chemical formula: Ba_1+xFe_10.5Co_0.25O_17+x (0 ≤ x ≤ 0.25). x depending on the exchange reaction time. Saturation magnetization ranges from 19 to 64 emu/g depending on exchange conditions. The Curie temperature is (470 ± 5)°C. An easy axis direction (M_s c) has been determined in all cases. The observed anisotropy is considerably lower than that of M ferrite. The calculated anisotropy constants are, in 10⁶ erg/cm³, K₁ = 0.8 and K₂ = 1.0 at room temperature.     

Magnetic properties of soft layer/FePt--MgO exchange coupled composite perpendicular recording media

The magnetic properties of exchange coupled composite （ECC） media that are composed of perpendicular magnetic recording media FePt MgO and two kinds of soft layers have been studied by using an x-ray diffractometer, a polar Kerr magneto-optical system （PMOKE） and a vibrating sample magnetometer （VSM）. The results show that ECC media can reduce the coercivities of perpendicular magnetic recording media FePt-MgO. The ECC media with granular-type soft layers have weaker exchange couplings between magnetic grains and the magnetization process, for ECC media of this kind mainly follow the Stoner Wohlfarth model.     

Optical absorption spectra of erbium iron garnet (ErIG)

Absorption spectra of the transition⁴ I _15/2→⁴ I _9/2 have been measured in erbium iron garnet (ErIG). From these spectra the exchange tensors for four Kramers dubletts can be obtained. A tentative analysis of these data in terms of a phenomenological exchange Hamiltonian will be presented.     

Magnetic exchange energy of two-dimensional electrons

The exchange energy of a two-dimensional electron gas was evaluated exactly in the presence of a magnetic field when only the lowest Landau level is occupied fractionally.     

Magnetic and electric birefringence in the isotropic phase of nematic liquid crystals

A theory is developed of the anomalous magnetic and electric birefringence in the isotropic phase of nematic liquid crystals taking into account orientational correlations between neighbouring molecules. Use is made of a modification of Bethe’s method due to Krieger and James, and the properties of the system are derived in terms of a single parameter, viz., the two-particle interaction constant. The expressions for the magnetic and electric birefringence are similar in form to those given by the phenomenological model of de Gennes. Theoretical curves forp-azoxyanisole reproduce the trends in the observed data. A calculation of the nematic-isotropic transition point confirms that this treatment is an improvement over the mean field approximation in describing pre-transition phenomena in the isotropic phase.     

Electroinduced anisotropy of acoustical properties of isotropic media

The possibility of observing a new acoustical effect related to the variation in the potential of the intermolecular interaction in an external electric field is estimated. It is shown that the effect can be used for coherent amplification of sound.     

Magnetic anisotropy and exchange interactions in Li-Al ferrites

The mechanism of exchange interactions in Li-Al ferrites is examined along with the nature of the magnetic crystallographic anisotropy. Calculation from a modified statistical model and another from Wolf's model may be used with observed curves for the temperature dependence of the magnetization and the first magnetic crystalline anisotropy constant to obtain values for the exchange integrals and the spin-Hamiltonian parameters SHP for Fe³⁺. It is shown that the SHP are substantially influenced by the exchange interactions, and the concentration relationships cannot be explained within the framework of the ionic crystalline-field theory.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 54–57, November, 1982.     

The use of OPW's in the Lindgren approximation of conduction-core exchange and the lattice dynamical properties of rubidium

This paper reports a first principles nonlocal pseudopotential calculation for the lattice dynamical properties of rubidium metal. In these calculations, an orthogonalized plane wave (OPW) has been used for the first time to represent the conduction state in the Lindgren approximation of conduction-core exchange. A comparison is made of phonon spectra and elastic shear constants using both plane wave (PW) and OPW conduction states. It is found that the OPW results are in excellent agreement with experiment and consistently better than the PW results for both phonon spectra and elastic constant. Similar results are found for other simple alkali metals.     

Flow properties of the optically isotropic tetrahedratic phase

We discuss the coupling between flow and other hydrodynamic variables that can occur in the optically isotropic tetrahedratic phase (T_d) characterized by a third rank tensor order parameter T_ijk. We point out that an applied electric field or an applied temperature gradient will lead to flow. Reciprocally we predict that, for example, a shear flow applied to a tetrahedratic phase leads to an induced electric field and a temperature gradient. Similarities to recent experimental observations in the vicinity of the isotropic-B7 phase transitions in materials formed by banana-shaped molecules are discussed. Received 31 July 2001     

Magnetic properties of the spin S = 1/2 Heisenberg chain with hexamer modulation of exchange

We consider the spin-1/2 Heisenberg chain with alternating spin exchange in the presence of additional modulation of exchange on odd bonds with period 3. We study the ground state magnetic phase diagram of this hexamer spin chain in the limit of very strong antiferromagnetic (AF) exchange on odd bonds using the numerical Lanczos method and bosonization approach. In the limit of strong magnetic field commensurate with the dominating AF exchange, the model is mapped onto an effective XXZ Heisenberg chain in the presence of uniform and spatially modulated fields, which is studied using the standard continuum-limit bosonization approach. In the absence of additional hexamer modulation, the model undergoes a quantum phase transition from a gapped phase into the only one gapless Lüttinger liquid (LL) phase by increasing the magnetic field. In the presence of hexamer modulation, two new gapped phases are identified in the ground state at magnetization equal to [Formula: see text] and [Formula: see text] of the saturation value. These phases reveal themselves also in the magnetization curve as plateaus at corresponding values of magnetization. As a result, the magnetic phase diagram of the hexamer chain shows seven different quantum phases, four gapped and three gapless, and the system is characterized by six critical fields which mark quantum phase transitions between the ordered gapped and the LL gapless phases.