Range Order Effects Research Articles

Short range order effects and the Falicov-Kimball model

The model of Falicov and Kimball is investigated incorporating short range order effects. This modifies both the electronic density of states and the configurational entropy, relative to the fully disordered case. The way this affects the conditions under which phase transitions occur is evaluated and discussed.

The microscopic structure of bulk amorphous red phosphorus: A Raman scattering investigation

The microscopic structure of bulk amorphous red P has been investigated by Raman scattering. It is concluded that the phosphorus tubes of pentagonal cross-section present in Hittorf's P and KP 15 are the structural units of the amorphous red P network. Intratube and intertube correlations are responsible for intermediate range order effects. The Raman data indicate that a continuous transition from the crystalline to the amorphous state takes place for P.

Magnetic properties of first- and second-stage MnCl 2 graphite intercalation compounds

The magnetic properties of graphite-MnCl 2 intercalation compounds (stages 1 and 2) were investigated from 3 to 300K by EPR and static susceptibility experiments. The behaviour of the magnetic susceptibility anisotropy, EPR linewidth and g-factor gives evidence for short range order effects increasing sharply below 50K. The data are analyzed in connection with the two-dimensionality of the compounds.

Electronic states in early-late transition metal glasses: Application to Cu 0.60Zr 0.40

The electronic properties of amorphous Cu 0.60Zr 0.40 are studied within the two component muffin-tin EMA framework. We argue that an understanding of the photoemission spectrum of this system requires that short range order effects be incorporated in both the construction of the atomic potentials and the calculation of the average spectrum. However, we show that the essential features of the density of states can be understood on the basis of simpler calculations in which the glass is modeled as either a substitutional alloy or a liquid with just a single pair distribution function.

Structural Investigation of the Metallic Glasses Mg85.5Cu14.5 and Mg70Zn30

Abstract The total structure factors as well as the total pair correlation functions for amorphous Mg85.5Cu14.5 (by neutron and X-ray diffraction) and for amorphous Mg70Zn30 (by X-ray diffraction) were determined. Both alloys show similar chemical short range order effects. From the total pair correlation function of the Mg85.5Cu14.5 glass, partial coordination numbers and atomic distances could be extracted. Comparison with the structure of crystalline Mg2Cu suggests that the short range order around the copper atoms is similar in the amorphous and the crystalline phase. The densities of both amorphous alloys were measured yielding negative excess volumina.

Density of states of vibrations in a disordered chain

We study the local density of vibrational states of a disordered binary chain using a renormalization approach suitable for the alloy problem. We include isotopic and spring constant disorder. Short range order effects in the alloy are taken into account in the pair approximation. We present some numerical results for the case of spring constant disorder and discuss their relevance for the interpretation of neutron scattering experiments in some metallic amorphous systems.

Short Range Order Effect in X-Ray Diffuse Scattering Due to Orientational Disorder

X-ray diffuse scattering due to orientational disorder is discussed in the disordered phase of a crystal having molecules with multiple orientations. The Bethe approximation is extended to multi-orientational system so that the short range correlation can be incorporated in the correlation function. The intensity of the diffuse scattering is written in terms of the probabilities of the pair configurations of the neighboring molecules.

Optical Birefringence in CsCuCl3: A Quasi One-Dimensional S=1/2 Ferromagnetic Heisenberg System

Short range order effects on the optical birefringence were investigated in an antiferromagnetically coupled linear-chain S =1/2 Heisenberg ferromagnet CsCuCl 3 in a temperature range between 6 K and 450 K. A nearly isomorphic diamagnet CsMgCl 3 was used to extract the magnetic part of birefringence in CsCuCl 3 . The magnetic birefringence was found to have a broad maximum at about 32 K. The effect of the interchain interaction on the birefringence was calculated in terms of a perturbation approach. By comparing the results with the observed temperature dependence of the birefringence, the ratio of inter- to intrachain exchange integrals was obtained as -0.07.

Segregation at stepped surfaces

A theoretical model of segregation at a (120) stepped surface with (110) monoatomic steps and terraces on the (110) plane of a body centered cubic alloy is presented. The model is based on pairwise interactions between nearest neighbours only and includes long and short range order effects. Results are presented for the concentration at different sites on the stepped surface as well as for the short range order parameters as functions of temperature.

Susceptibility and specific heat of insulating Magnéli phases VnO2n−1

The magnetic susceptibilities and the specific heats of single crystals of the vanadium Magnéli phases VnO2n−1(3?n?8) have been measured at low temperatures. Magnetic and thermodynamic properties associated with the antiferromagnetic ordering in the insulating state are strongly affected by spin pairing of vanadium ions. The Néel temperature TN is lowered rather smoothly with increasing n, which corresponds to larger spin pairing effects. A large short range order effect was observed in V3O5, which has no spin pairing. The susceptibility and the specific heat show the behavior of a low dimensional spin system. For V4O7 and V6O11 the magnetic entropy changes up to TN are less than 20% of the value R1n(2S+1) which would be expected if spin pairing were absent; this indicates considerable spin singlet pairing. On the other hand, V7O13 does not display a metal-insulator transition and remains metallic down to 1.2 K. Some remarkable features were observed for V8O15 in the susceptibility and the specific heat. These results suggest that the transition at low temperature (6.1 K) is not an ordinary antiferromagnetic one but another type of transition.

On the spin-wave theory of an amorphous asperomagnetic alloy

The spin wave theory of Bhattacharjee et al. (1978) in an asperomagnetic alloy is extended to include the short range order effects existing in amorphous systems.

Equivalence of the Krieger-James approximation and the constant-coupling approximation in magnetism

We have applied the Krieger-James approximation to a description of short range order effects in both ferro- and antiferromagnetism. In this method, a mechanical consistency condition is imposed on the probability distribution of a pair of near neighbours in a cluster of (Z+1) ions. The resulting theory is analytically equivalent to the familiar constant coupling approximation, thus ensuring thermodynamic equilibrium of the system.

Thermodynamic Properties of CoCl2·2NC5H5– A Quasi-One-Dimensional Ising System

Previously measured magnetic susceptibility and specific heat of powder samples CoCl 2 ·2NC 5 H 5 are compared with a theory for ferromagnetic Ising spin 1/2 chains weakly coupled by Ising antiferromagnetic interactions to their nearest neighbor chains. The weak interchain interaction is first treated within mean field theory; later short range order effects are included approximately via a reaction field formalism. Each chain, in its (approximate) effective field is treated exactly. Good agreement with experiment for both χ and C p is found with the following parameters: intrachain exchange J =10.8 K; interchain exchange J '=0.175 K; principal values of the g tensor, g // =7.0 and g ⊥ =5.0.

Ferromagnetic alloys: A Cluster-Bethe-lattice approach

We apply the Cluster-Bethe-lattice method to calculate the local density of one-magnon states for a binary substitutional ferromagnetic alloy. Short range order effects for the alloy are included. Simple clusters of one atom and one atom plus its nearest neighbors are studied. We show that, for the latter type of cluster, there are localized excitations the energies of which fall outside the continuum of excitations.

DETERMINATION OF INTERCHANGE ENERGIES FROM THERMODYNAMIC AND STRUCTURAL DATA

The numerical values of the interchange energies W(1), W(2) between nearest and aext-nearest neighbours in bcc and fcc solid solutions can be determined from energy measurements (e.g. enthalpy of formation) and from structural data (e.g. critical temperature of long range order). If the energy data do not refer to bcc (or fcc) standard states they must be appropriately converted. If critical temperatures are used short range order effects must be considered. For both fcc and bcc solid solutions W(1) and W(2) are found of the same order of magnitude. In the fcc alloys studied here W(2) is found of opposite sign of W(1). The consequences for the phase diagram are discussed.

Short range order effects on EPR frequencies in antiferromagnets

The short range order effects on EPR frequencies are studied for Heisenberg antiferromagnets with small anisotropy terms and inequivalent spins. The anisotropy terms lead to EPR line-shifts with the 3 cos 2 θ - 1 dependence, whereas the inequivalency in g-tensors always causes upward frequency shifts.

G-Shift in Low Dimensional Antiferromagnets at Low Temperatures

Anomalous g -shifts in low dimensional antiferromagnets, copper benzoate Cu(C 6 H 5 COO) 2 3H 2 O and copper tetra-amine sulfate Cu(NH 3 ) 4 SO 4 H 2 O (CTS) were observed mainly below 1 K. These compounds show positive g -shifts as temperature decreases. The results in copper benzoate are qualitatively explained by the theory recently developed by Nagata who introduced the one-dimensional short range order effect between inequivalent spins. The shifts in CTS are discussed from the two-dimensional point of view.

A Cluster-Bethe-Lattice approach to spin-waves in dilute ferromagnets

We study the spin-wave spectra of a dilute ferromagnet within the cluster-Bethe-lattice approximation. Short range order effects for the alloy are included. A study of finite size clusters connected at their edges to Bethe lattices of the same coordination number allows us to determine: (i) the stability condition for the magnetic system; (ii) the continuum spin-wave local density of states; and (iii) the existence of localized states below and above the continuum states.

Short Range Order Effect on the Magnetic Anisotropy in a Heisenberg Linear Chain Antiferromagnet CsMnCl32H2O

Torque measurements were done for CsMnCl 3 2H 2 O in order to investigate the short range order effect on the magnetic anisotropy in a Heisenberg linear chain antiferromagnet. Above 10 K, the anisotropy in the paramagnetic susceptibility has a uniaxial symmetry referred to the direction of chains ( a -axis) corresponding to the essential intrachain spin correlations and, below this temperature, it becomes orthorhombic because of the evolution of three-dimensional magnetic correlations. The magnitude and temperature dependence of the axial anisotropy observed above 10 K could be explained by the classical linear chain model including only a small dipolar term as an anisotropy term.

Critical behavior of the magnetic susceptibility of GdNi 2

We find the low field magnetic susceptibility of the ferromagnet, GdNi 2, is describable by the molecular field model to within approximately 8°K of the ordering temperature below which there are deviations which are consistent with short range order effects.