F-center induced Raman scattering in RbCl

Experimental Raman spectra of RbCl containing F-centers are reported. Lines of A_1u, E_u and F_2u symmetry were observed. Calculations of the first-order Raman spectra are presented; they have been based on the assumption that the electron-phonon interaction is linear with displacements of F-center nearest neighbours. A shell-model fitted to neutron data was used to evaluate the Green-function matrix elements and the projected densities of states. The best agreement between theory and experiment was found to occur when using a zero-force-constant change for the F_2u spectrum and a 0·35 fractional decrease in the longitudinal force-constant A₁₂ for the A_3u and E_u spectra.     

Spin-glass behavior as a function of concentration and local order by NMR in nearly random NiMn systems

Unidirectional anisotropy fields (H_A) in disordered Ni_{100 ? x}Mn_x (25 ? x ? 32) systems after field cooling are found by NMR (spin echo) to be very sensitive to the Mn concentration and to local order. The H_A increases by roughly a factor of 10 when the concentration increases 5%. The Mn NMR spectra show that all Mn have several first nearest Mn neighbours. For a given concentration, in the above range, the H_A are very sensitive to the annealing-quenching temperatures (T_AQ) and vanish when the sample is cooled down from 900°C in 2 h. The NiMn system can be tuned by heat treatments from spin-glass to a ferromagnet. The Mn NMR spectra with decreasing T_AQ show a clear metallurgical evolution tending to states where a Mn atom sees fewer first nearest Mn neighbours. We find evidence that the anisotropy has its roots in atomic randomness.     

Critical temperature for a system with Heisenberg-like nearest neighbour and Ising-like next nearest neighbour interactions

A ferromagnetic system with $\text{S =}\text{1}\text{2}$, where the interactions between nearest neighbours are isotropic (having Heisenberg character) and those between the next nearest neighbours anisotropic (Ising-like), is investigated. Using the effective hamiltonian approach we find the change in the critical temperature due to the inclusion of next nearest neighbour interactions of different character. The change is greatest in two dimensions where the critical temperature is shifted up from its zero value for a system with the Ising-like interactions switched off. We also calculate the T_c for a system with both nearest and next nearest neighbours interactions of the Ising-type. The results for the two models are compared.     

Evidence for fcc-like short range order in Fe-Ni-B metallic glasses

Soft X-ray appearance potential spectra have been measured for iron and nickel in (Fe_0.5Ni_0.5)_100?xB_x metallic glasses with x = 16, 18, 20, 22, 24, 26. For x = 16 both iron and nickel spectra exhibit structural features characteristic for the electronic structure of fcc transition metals. It is concluded that in the amorphous state Ni and Fe are arranged in fcc-like geometry beyond nearest neighbours.     

Mossbauer studies of Np hydrides

The Mossbauer spectra of ²³⁷Np in NpH_2+x(x = 0.1, 0.3 and 0.5) consist of several absorption lines which are attributed to Np inequivalent sites corresponding to different numbers of octahedral hydrogen neighbours. The relative intensities of the various lines do not correspond to a random distribution of the hydrogen ions. A model which takes into account first nearest neighbour interactions among octahedral hydrogen ions can reproduce the experimental spectra.     

Structure magnetique du sulfure de fer FeS de type blende

Zinc blende type FeS, which is cubic at room temperature and becomes orthorhombic, pseudo-tetragonal ($\text{c}\text{a}\text{< 1}$) at the magnetic transition, has been studied by neutron diffraction. The magnetic space group is F_c 2 2' 2' : ferromagnetic (001) planes are coupled antiferromagnetically, and the moments lie along the a axis, in agreement with previous Mössbauer measurements. The exchange integral between nearest neighbours in the ab plane is thus weaker than those between nearest neighbours in the ac and bc planes.     

The magnetic and hyperfine properties of iron in silicon carbide

The magnetic and hyperfine properties of iron impurities in 3C- and 6H- silicon-carbide are calculated using the abinitio method of full-potential linear-augmented-plane-waves. The iron atoms are introduced at substitutional carbon, Fe _C , and silicon, Fe _Si , sites as well as at the tetrahedral interstitial sites with four nearest neighbours carbon atoms, Fe _I (C), and four nearest neighbours silicon atoms, Fe _I (Si). The effect of introducing vacancies at the neighbours of these sites is also studied. Fe atoms with complete neighbors substituted at Si or C sites are found to be nonmagnetic, while Fe atoms at interstitial sites are magnetic. Introduction of a vacancy at a neighboring site reverse the picture.     

Cems study of the carbon distribution in austenite

CEMS resolution allows a careful study of Fe−C pure austenite spectra with high carbon content. Three Fe environments are detected which are ascribed to Fe atoms with one carbon first nearest neighbour and zero carbon second nearest neighbour and two environments with no carbon first nearest neighbour but zero carbon second nearest neighbour and one to four carbon second nearest neighbours respectively. This confirms the repulsive interaction between carbon interstitials and the tendency towards Fe₈C ordering is suggested.     

M-Zentrenbildung in röntgenbestrahltem KCl

The equilibrium betweenF- andM-center concentration in KCl after X-irradiation at temperatures between 77 °K and 298 °K has been studied. At low temperatures (<200 °K) onlyF-centers which are formed statistically as nearest neighbours give aM-center. At higher temperaturesF-centers within a certain sphere of interaction with a radiusr formM-centers. The temperature dependance of this radiusr has been evaluated. The equilibrium constantK depends on the temperature in the form K=K₀exp(-Q/kT). The activation energy was determined to be Q=(0.5±0.1) eV.     

Magnetic properties of MnxMg1-xS solid solutions

The static magnetic properties of the polycrystalline solid solutions Mn_xMg_1-xS(0<x?1) are reported. The experimental data reveal the appearance of a critical concentration x_c=0.13±0.01 for long-range magnetic ordering, which is in good agreement with the result of a series expansion of the mean cluster size for a face-centered cubic lattice with exchange interactions up to 12 nearest neighbours and 6 next-nearest neighbours. At concentrations far below x_c the existence of long-range exchange interactions is demonstrated.     

Die magnetischen Eigenschaften der Europium-Chalkogenide unter hydrostatischem Druck

The magnetic ordering temperatures and the magnetization curves of the europium chalcogenides have been measured in compressed helium. Under the application of hydrostatic pressures up to 4 kbar the Curie temperatures increase, the Néel temperatures remain constant and the ferrimagnetic transition temperature of EuSe decreases. Magnetization curves of Eu(Se, S) mixed crystals are similar to those of EuSe under pressure. The results are analysed within the molecular field approximation with exchange interactionsJ ₁ andJ ₂ between nearest and next nearest neighbours, respectively.J ₁ increases with pressure in EuO and EuS but is not changed in EuSe.J ₂ decreases under pressure in EuSe but remains constant in EuTe. The results are compared with different theoretical models for the exchange interactions in these compounds.     

Magnetic structure and phase diagram of the frustrated spinel Cd1−xZnxCr2Se4

In attempt to characterise the magnetic ordering in the whole composition range of the Cd_1−xZn_xCr₂Se₄ system, various magnetic measurements were performed on both crystalline and polycrystalline samples with 0?x?1. The magnetic properties of the system are typical of a ferromagnet below x=0.4 and of a complex antiferromagnet one above x=0.6. In this work the intermediate region was carefully studied. The variations of both M(T) and χ_ac at low fields suggest that transitions from ferromagnetic to Gabay–Toulouse ferromagnetic-spin-glass mixed phase at low temperature occur in the range 0.41?x?0.58. The high-temperature susceptibility measurements show that for the whole concentration range the system obeys Curie–Weiss laws. The results can be explained by the coexistence of competing interactions (ferromagnetic between nearest neighbours and antiferromagnetic between higher order neighbours) and disorder due to the random substitution between zinc and cadmium ions in the tetrahedral sites of the spinel lattice. An experimental magnetic phase diagram of the system is established.     

Magnetic ordering in the system Co2-xNix TiSn studied by 119Sn-Mössbauer spectroscopy and magnetization measurements

Magnetization and Mössbauer with ¹¹⁹Sn measurements have been performed in the system Co_2-xNi_xTiSn. Transferred field distribution at the Sn site is very well consistent with the assumption, that only the nearest magnetic neighbours contribute to the field in contradiction with the Blandin-Campbell model, where the long range spin polarization of the conduction band occurs.     

Local structure of Ni2Si

The local structure around the Ni atom in Ni₂Si has been studied by measuring and analyzing the Extended Energy Loss Fine Structure above the Ni M_2,3 edge. The radial distribution function we found is characterized by the efficiency of the only nearest neighbours Si atoms. We also obtained the phase shift of the Ni-Si pair and the backscattering amplitude.     

A scalar model for MHD turbulence

A model for homogeneous MHD turbulence is proposed. Nonlinear interactions acts between nearest neighbours in a discretized wavenumbers space and conservation properties (total energy and v-b correlation) are verified. The model can be truncated at will. With three modes, a bifurcation analysis is given. In the turbulent case (dissipation and kinetic forcing are present) one obtains time fluctuations at all scales and time-averaged power law spectra, the small scales exhibit intermittency effects. Typical MHD phenomena such as the dynamo effect or the increase of v-b correlation in the decaying cases are also observed.     

Short range order in amorphous FeGe alloys

The short range order in Fe_xGe_1-x amorphous alloys is analysed by use of electron diffraction diagrams. It is shown that the number of nearest neighbours increases from a value of 4 when x equals zero to a value of 9 for x ≥ 0.47. It is suggested that hybridization between the sp states of germanium and the d states of iron plays an important role in stabilizing the high coordination amorphous phase found in the alloys with high content of iron.     

Neutron diffraction determination of the magnetic structure of Dy PO4

Dy PO₄ is antiferromagnetically ordered below 3.39 K. The spin-structure is described by an antiparallel arrangement of nearest neighbours along the crystallographic c-axis. The magnetic moment at 1.8 K is (9.0±0.3) μ_β. These results confirm that Dy PO₄ may be considered as an ideal three-dimensional Ising system.     

Magnetic properties of intermetallic ruthenium compounds with the CsCl structure

Magnetization measurements on six ruthenium compounds with the CsCl structure (B2-compounds) are reported. The magnetic properties can be described in terms of a temperature independent Pauli susceptibility and a contribution of localized moments of about 4μ_B which are very likely connected with magnetic centers consisting of one ruthenium atom which substitutes for an X-atom in an otherwise ordered compound RuX and its eight nearest neighbours. The results show that a weak antiferromagnetic coupling exists between these moments of which the concentration is about 0.003 per ruthenium atom.     

二元固溶体的配分函数

这篇论文是作者之一的某一篇论文的继续。论文分四节。在第一节中我们讨论的对象是应用二元混合气体的买厄理论至一个在AB型晶体上的二元固溶体而获得的不可约集团积分B(v₁,v₂)。我们严格地证明了它们是某一些晶体常数z₁,z₂,…的线性函数。这些常数的定义为1/((1/2)N)∑λ_abλ_a′b′…,式中N代表晶点阵的点的总数,λ_ab为邻矩阵,定义为λ_ab=1 如果ab为近邻, =0,如果ab不是近邻, a,a′,…互不相等,b,b′,…也互不相等,而乘积λ_ab…为全部下标的一个不可约乘积。证明共用了三个定理,同时也援用了一个所谓基本不可约集团的定义。在第二节中,我们应用了第一节的讨论对B(2,3)作了计算。在第三节中,我们应用第一节的结果,简化了求固溶体自由能的Kirkwood方法,同时也利用了简化的方法求得了自由能对(kT)^-1展开的幂级数中(kT)^-7的系数。比(kT)^-7更早的项的系数早已在作者之一的一篇论文中求得。在第四节中,我们利用第一节的结果,证明了严格理论在忽略z₂,z₃,…时即成了Bethe理论的第一级近似。如果将自由能表为z₁,z₂,…的函数而称它们的系数为f₁,f₂,…,那末我们指出:只消研究在某些赝点阵上的固溶体的配分函数,便可以求出f₁,f₂,…,同时我们也指出:在这些赝点阵上的固溶体的配分函数是可以(而也容易)严格地求出的,因此以上的理论供给了我们一个较好的求固液体的自由能的方法。     

Uniaxial stress and polarisation studies of F2 centre luminescence in MgO

Uniaxial stress and polarised luminescence measurements of additively coloured MgO show that the 375 nm luminescence band is associated with pairs of F centres in nearest neighbour sites along 〈110〉 crystal directions (i.e. F₂ centres).