Magnetic phase diagram of GdAg1−xZnx

The magnetic phase diagram of GdAg_1?xZn_x, an intermetallic solid solution of an antiferromagnet (GdAg: T_N = 136 K) and a ferromagnet (GdZn: T_C = 269 K), has been elaborated from magnetization measurements. The antiferromagnetic phase boundary T_N(x) first passes a broad maximum meeting the ferromagnetic phase boundary T_C(x) at x₁ = 0.575 and T₁ = 72 K, where four phases coexist. On approaching (x₁, T₁) along T_N(x) the magnetization phenomena vanish. At x₁ the phase transition still has ferromagnetic appearance but proceeds into a state without spontaneous magnetic moment. Two different ferromagnetic phases (F₁, F₂) and one ferrimagnetic phase (F₃) occur in the composition intervals 0.69 < x₁ <1, and 0.61 <x₂ < 0.69 and 0.51 < x₃ < 0.61. All phase transitions seem to be of second order except th e F₁?F₂ one at x = 0.69 which is of first order. This phase line meets the paramagnetic to ferromagnetic phase boundary T_C(x) in a multicritical point with the coordinates x_m = 0.69, T_m = 123 K.Six further mixed magnetic phases, M_{1, 2, ..., 6}, are observed between x = 0.33 and 0.6 below the antiferromagnetic branch and exhibit irreversible thermodynamic properties, such as hysteresis, below about 40 K.Assuming local magnetic interactions only between nearest and next-nearest Gd neighbours, the T_N(x) and T_C(x) phase boundaries can be described fairly well by a simple model calculation using different exchange parameters for a few relevant distributions of Ag and Zn atoms.     

Concentrated Kondo systems

This review considers the experimental and theoretical studies of concentrated Kondo systems (CKS), Kondo lattices, substitutional solid solutions and their transition from Kondo impurity to Kondo lattice, and ‘intermediate valence compounds’ which are, in fact, high T _K CKS (T _K is the Kondo temperature). The anomalous low temperature properties of CKS are related to the formation of the narrow (～k _B T _K) high-amplitude Abrikosov-Suhl resonance E _R in the vicinity of the Fermi level E _F. This resonance is situated exactly at E _F in low T _K CKS with T _K < Δ_CF and near E _F in high T _K CKS with T _K > Δ_CF (Δ_CF is the crystal field splitting). In low T _K ‘j=1/2’ CKS the condition E _R=E _F leads to an increase of the density of states at E _F, which is large enough to induce heavy fermion superconductivity in CeCu₂Si₂, UBe₁₃. We demonstrate that the transition from low T _K (E _R=E _F) to high T _K CKS (E _R≠E _F) might be what was formerly considered as a ‘Kondo-lattice-intermediate valence state’ transition. It appears that in many cases the essentially non-integer valence state of the rare-earth elements in metallic compounds is thermodynamically unstable with respect to a transition to an almost integer valence state, because it realizes the maximum gain in free energy from the Kondo condensation.     

Intermolecular vibrations in pure and doped C60

We discuss the results of inelastic neutron time-of-flight spectroscopy on powder samples of C₆₀, K₃C₆₀ and Rb₃C₆₀. A well structured density of states is obtained from the raw data where the details can be attributed to translational acoustic/optic and librational modes. The intermolecular excitation spectra for all three compounds can be well understood within the frame-work of lattice dynamics of a molecular crystal. We show that a particular sample preparation is necessary to avoid impurity contributions in particular within the gap region between inter- and intramolecular vibrations. There are indications for a weak anomalous temperature dependence of low energy phonons in K₃C₆₀ atT _c .     

Critical behaviour of the magnetic susceptibility of two nearly bidimensional Heisenberg ferromagnets

The parallel magnetic susceptibilities of ferromagnetic K₂ Cu F₄ and (CH₃ NH₃)₂ Cu Cl₄ have been measured near the critical temperature. In both ferromagnets, the thermal dependence cannot be described by the simple power law ?^-γ with $\text{? =}\text{T}\text{T}{}_{\mathrm{<mtext>C</mtext>}}\text{? 1}$ over a large range of temperatures. Due to crossover with dimensionality, the γ value increases from 1 for K₂ Cu F₄ and 1.23 for (CH₃ NH₃)₂ Cu Cl₄ in the immediate vicinity of T_c, to a maximum value of about 2.     

Low-temperature specific heat of icosahedral and amorphous Pd−U−Si alloys

Icosahedral (I) Pd_0.588U_0.206Si_0.206 can be obtained from melt-spun amorphous (A) ribbons by annealing. The specific heatC (measured betweenT=0.1 K and 20 K) shows very similar behavior for both phases. The main features ofC are as follows. (i) The vibrational heat capacityC _ph dominatesC at highT.C _ph is almost identical in both phases, in agreement with recent inelastic neutron scattering data. (ii) Shallow maxima in (C–C _ph)/T vs.T are found at 5.4 and 4.3 K forI andA phases, respectively, associated with magnetic order. These maxima are suppressed by 20% in an applied magnetic field of 6 T. (iii) A large quasi-linear contribution is observed with a low-T coefficient =165 mJ/mole U K² for theI phase and =120 mJ/mole U K² for theA phase. In the low-T region,C is hardly affected by a field of 6 T. This hints at the formation of a narrow 5f band with a comparable density of states for bothI andA phases.     

Metal-insulator transition in a radiation-disordered La<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> manganite

The temperature dependences of the electrical resistivity ρ(T) and the ac magnetic susceptibility χ(T, H = 0) are thoroughly investigated for a perovskite-like lanthanum manganite, namely, La_0.85Sr_0.15MnO₃, which is preliminarily exposed to neutron irradiation with a fluence _F = 2 × 10¹⁹ cm^?2 and then annealed at different temperatures ranging from 200 to 1000°C. The results of the electrical resistance measurements demonstrate that neutron irradiation of the samples leads to the disappearance of the low-temperature insulating phase. As the annealing temperature increases, the insulating phase is not restored and the manganite undergoes a transformation into a metallic phase. Analysis of the magnetic properties shows that, under irradiation, the ferromagnet-paramagnet phase transition temperature T_C decreases and the magnetic susceptibility is reduced significantly. With an increase in the annealing temperature, the phase transition temperature T_C and magnetic susceptibility χ(_{T, H} = 0) increase and gradually approach values close to those for an unirradiated sample. This striking difference in the behavior of the electrical and magnetic properties of the radiation-disordered La_0.85Sr_0.15MnO₃ manganite is explained qualitatively.     

Inelastic neutron scattering study of the specific features of the phase transitions in (NH4)2WO2F4

Oxyfluoride (NH₄)₂WO₂F₄ has been studied by the inelastic neutron scattering method over a wide temperature range 10–300 K at two initial neutron energies of 15 and 60 meV. The role of tetrahedral ammonium groups in the mechanism of sequential phase transitions at T ₁ = 201 K and T ₂ = 160 K has been discussed.     

A measurement of the QCD colour factors and a limit on the light gluino

Using data collected from 1992 to 1995 with the ALEPH detector at LEP, a measurement of the colour factor ratios C _A/C _F and T _F /C _F and the strong coupling constant α _s = C _F α _s(M_Z)/(2π) has been performed by fitting theoretical predictions simultaneously to the measured differential two-jet rate and angular distributions in four-jet events. The result is found to be in excellent agreement with QCD, {fx4-1} Fixing C _A/C _F and T _F/C _F to the QCD values permits a determination of α _s(M_Z) and η _f, the number of active flavours. With this measurement the existence of a gluino with mass below 6.3 GeV/c² is excluded at 95% confidence level.     

Random-field induced memory effects in inhomogeneously diluted antiferromagnets K2NixZn1−xF4

Using neutron diffraction a random-field generated memory is observed in K₂Ni_xZn_1−xF₄ with x = 0.96, 0.85, and 0.75. The intensities and profiles of magnetic Bragg reflections are found to follow unique trajectories determined by switching the external magnetic field on and off while cooling. The effect, associated with a inhomogeneous spread in T_N on a microscopic scale, is a direct manifestation of freezing in of multi-domain states at the ordering temperature.     

F-Zentren und Kolloide in der amorphen,hexagonalen und kubischen Phase von LiJ

By simultaneous evaporation of LiI and Li onto a cooled substrate F centers can be produced in the hexagonal (78 K<T _K <200 K) and amorphous (T _K <78 K) phase of one and the same salt. In both modifications there exist two types of centers F and F^*. The F^* center differs from the cubic F center (T _d -symmetry) by a nearby Frenkel defect. In hexagonal films the normal F band peaks at 2.58 eV, whereas the transitions of the F^* center appear at 2.92 and 2.58 eV too. Polarized irradiation at 20 K causes a dichroic behaviour of the F^* centers. Both types of centers can be transformed into one another photochemically. In the amorphous phase all transitions are shifted to lower energies by about 0.1 eV. After the phase change amorphous→hexagonal the absorption bands shift back by the same amount of energy. AboveT _K =230 K the excess metal forms colloids. The absorption bands are due to colloidal centers embedded in the crystalline material (2.25 eV) and films adsorbed to the crystallites (3.1 eV), respectively. By annealing a particle growth can be observed. After electrolytic colouration cubic single crystals of LiI exhibit an absorption band peaking at 2.36 eV. However, it is not yet sure, if this band is allowed to be ascribed to F centers.     

Recovery of copper alloys after neutron irradiation at low temperatures

The effects of fast neutron irradiation near 4.2°K on dilute Cu, Cu₃Au, α-CuZn, β-CuZn and CuNi alloys were studied by electrical resistivity measurements. For Cu₃Au, the recovery stage at ～ 100°K becomes more distinct with increasing long-range order, and is attributed to migration of interstitials. The ratio of the number of replacements to the number of displacements (C _R /C _F ) is estimated to be about 50 for neutron irradiation, in contrast to the previously reported value of ～2 for electron irradiation. For α-CuZn, a large recovery stage at ～150°K is found and attributed to annihilation of interstitids whose migration produces ordering. For β-CuZn, migration of interstitials also produces ordering. It is also suggested that in β-CuZn, the ratio C _R /C _F is larger for neutron irradiation than for electron irradiation as in the case of Cu₃Au. The results on CuNi alloys are presented without explanation.     

High-pressure effects on the crystal and magnetic structure of the Nd0.78Ba0.22CoO3 cobaltite

The crystal and magnetic structure of the Nd_0.78Ba_0.22CoO₃ cobaltite is studied by neutron diffraction at high pressures up to 4.2 GPa in the temperature range 10–300 K. The pressure dependences of structural parameters are obtained. Ferromagnetic ordering of the Co sublattice is observed at normal pressure below T _C ～ 140 K, and ferrimagnetic ordering of the Co and Nd sublattices with an antiparallel direction of magnetic moments appears at T _F ～ 40 K. The magnetic moment of Co and the temperature T _C change slightly under pressure, which points to the stability of the initial intermediate-spin (S = 1) state of Co³⁺ ions. This behavior differs considerably from the characteristic behavior of cobaltites that are close in chemical composition and structure and exhibit ferromagnetic ordering of only the Co sublattice. In these cobaltites, the magnetic moment of Co is substantially suppressed and T _C decreases under pressure, which is related to the change in the state of Co³⁺ ions from the intermediate spin state to the nonmagnetic low-spin state (S = 0). The interplay between the appearance of the magnetic interaction of the R-Co sublattices and the stability of the spin state of Co³⁺ ions in the Nd_0.78Ba_0.22CoO₃ cobaltite is discussed.     

Manifestation of resonance dipole-dipole interaction in spectra of low-temperature molecular mixtures of C2F6 in CF4 and CF4 in C2F6

We have measured IR absorption spectra of solutions C₂F₆ in CF₄ (T = 178 K) and CF₄ in C₂F₆ (T = 173 K) in the overtone range of the spectrum. We have studied how the resonance dipole-dipole interaction affects the formation of contours of bands that correspond to transitions to states involving the vibrations ν₁₀(C₂F₆) and ν₃(CF₄), which are strong in the dipole absorption. For the system C₂F₆ in liquid CF₄, the state ν₂ + ν₁₀(C₂F₆) resonantly interacts with the state ν₂(C₂F₆) + ν₃(CF₄), and, for the system CF₄ in liquid C₂F₆, the state ν₁ + ν₃(CF₄) resonantly interacts with the state ν₁(CF₄) + ν₁₀(C₂F₆). The contours of the bands ν₂ + ν₁₀ (C₂F₆) in the spectrum of the mixture with CF₄ and of the bands ν₁ + ν₃(CF₄) in the spectrum of the mixture with C₂F₆ have been calculated.     

The classification of superconductors using muon spin rotation

In this paper we explore the role of muon spin rotation (μSR) techniques in the characterization and classification of superconducting materials. In particular we focus upon the Uemura classification scheme which considers the correlation between the superconducting transition temperature,T _c, and the effective Fermi temperature,T _F, determined from μSR measurements of the penetration depth. Within this scheme strongly correlated “exotic” superconductors, i.e, hightT _C cuprates, heavy fermions, Chevrel phases and the organic superconductors, form a common but distinct group, characterized by a universal scaling ofT _C withT _F such that 1/100 <T _C/T _F<1/10. For conventional BCS superconductorsT _C/T _F<1/1000. The results of new μSR measurements of the penetration depth in superconducting Y(Ni_1?x Co _x )₂B₂C and YB₆ are also presented. In Y(Ni_1?x Co _x )₂B₂C the decrease ofT _C with increasing Co concentration is linked to a marked decrease in the carrier density from 2.9·10²⁸ m^?3 atx=0 to 0.6·10²⁸ m^?3, atx=0.1, while the carrier mass enhancement remains almost constant at approximately 10. For YB₆ we find evidence of a modest enhancement of the carrier mass (m ^*/m=3), and a relatively low carrier density of 0.24·10²⁸ m^?3. These results are discussed within the Uemura classification scheme. It is found that neither Y(Ni_1?x Co _x )₂B₂C withT _c/T _F>>1/250 nor YB₆ withT _C/T _F>>1/340 can be definitively classified as either “exttic” or “conventional”, but instead the compounds display behavior which interpolates between the two regimes.     

Modification of the magnetic structure of UAs — Appearance of an incommensurate phase below TN in UAs0.97Se0.03, studied by neutron diffraction from a single crystal

The magnetic structure of UAs_0.97Se_0.03 has been studied by neutron diffraction from a single crystal in zero applied magnetic field. It is found to be antiferromagnetic, of type-IA (++--) below T_o = 75.6 K and of type-I (+-+-) above T_o. The type-I persists till T_IC = 113.5 K, while above it and up to T_N = 122 K an incommensurate phase appears, thereby modifying the magnetic structure in pure UAs. The k-value of the wavevector $\text{K}$ (along cubic axes) is changing from 0.642 at T_N to 0.652 at T_IC. The transitions at T_o and T_IC are first-order while the transition at T_N is second-order. The ordered magnetic moment is 2.15 μ_B at T = 4.2 K, it varies smoothly to 1.95 μ_B at T = 75.4 K and drops drastically to 1.47 μ_B at T = 76 K.     

Scanning tunneling microscopy of fluorinated fullerene molecules on silicon surfaces

Scanning tunneling microscopy (STM) under ultra-high vacuum conditions is used to study the initial stages of adsorption of C₆₀F₁₈ and C₆₀F₃₆ fluorofullerene molecules on Si(111)-7 x 7 and Si(100)-2 x 1 surfaces. Spatially resolved STM images of individual molecules and ab initio calculations show that the fluorofullerene molecules interact with an Si surface, with the F atoms oriented toward the surface. The large electric dipole moment of the molecules induces strong polarization on the surface, but the charge transfer is weak. The presence of C₆₀F₃₆ isomers with different symmetry—T, C ₃, and C ₁—is revealed in STM images for the first time.     

Sensitivity of polarization observables in elastic ed scattering to the neutron form factors

The deuteron elastic form factors are calculated within the Bethe-Salpeter approach with separable interaction. The charge, quadrupole, and magnetic form factors [F _C(q ²), F _Q(q ²), and F _M(q ²), respectively]; the structure functions A(q ²) and B(q ²); and the tensor polarization components T ₂₀(q ², T ₂₁(q ²), and T ₂₂(q ²) are investigated up to ?q ²=50 fm^?2. The role of relativistic effects is discussed, and a comparison with nonrelativistic calculations is performed. The effect of the neutron form factors on the deuteron form factors and especially on tensor polarization components is discussed too.     

Occurrence of a tetracritical line in the x-H-T phase diagram of the random 2-component antiferromagnetic system K2Mn1−xFexF4

The H-T phase diagram of the antiferromagnetic compound K₂Mn_.978Fe_.022F₄ system K₂Mn_1-xFe_xF₄ has been studied by neutron scattering. The results can be explained by means of a 3-dimensional X-H-T phase diagram, which can be constructed on basis of the x-T diagram previously found for K₂Mn_1?xFe_xF₄ (H=0) and the H-T diagram of an antiferromagnetic with weak uniaxial anisotropy (x=0).     

Effect of antiferromagnetic ordering on the optical transitions in MnF2

Two optical absorption bands (viz. C and F) in MnF₂, whose line positions are nearly temperature-independent above T_N = 67.3 K, are investigated for the effect of magnetic ordering. Both bands are blue-shifted by ΔE (≌90 cm^?1 for C and 120 cm^?1 for F at 10 K) and the lines show additional narrowing on cooling through T_N. It is found that ΔE varies as the sublattice lattice magnetization below T_N and nearly as the magnetic energy above T_N.     

Spin correlations in Ni-Mn-Ga

Results of measuring small-angle neutron scattering and neutron depolarization in a Ni_49.1Mn_29.4Ga_21.5 single crystal in the temperature range 15<T<400 K and in the range of magnetic fields 0<H<4.5 kOe are presented. The characteristic temperatures of the alloy were found to be as follows: T _C=373.7 K and the martensite transition temperature T _m=301–310 K. The magnetic critical scattering at T _C and the scattering at T<T _C were adequately described by the relationship I _m=A(q ²+κ²)^?2 (q is the transferred wave vector and R _c=1/κ is the correlation radius), and the temperature dependences of the A and R _c scattering parameters were determined. Left-right asymmetry was observed at 150<T< T _m in the scattering of neutrons polarized along or opposite to the applied field. This asymmetry was due to the inelastic magnetic interaction of neutrons in the sample. The magnetization of the alloy at T _m, critical scattering at T?T _C, anomalies in scattering, and the softening of magnetic excitations at 150 <T<T _m are discussed.