Showing papers on "Transition temperature published in 1972"

Low-Spin-High-Spin Transitions in Transition-Metal-Ion Compounds

Abstract: Models for low-spin-high-spin transitions in transition-metal-ion compounds are presented. Within a molecular-field scheme, the transition between, and coexistence of, high-spin (Hund's-rule) and low-spin (crystal field) states are described. One- and two-sublattice-spin structures and the influence of magnetic interactions are considered. It is pointed out that the one-sublattice model, which was considered by Chestnut for the singlet-triplet system, applies to the transitions observed in certain organic complexes. We have found that the inclusion of magnetic interactions between ions can yield a magnetically ordered state above a certain transition temperature. This "heat magnetization" is not due to the coupling between levels but rather to a self-consistently determined crystal field splitting. The two-sublattice-spin-structure case exhibits a rich variety of behaviors. These include a low-spin to two-sublattice spin to high-spin transition with the order of the transitions varying with the parameters of the model. Applications to certain ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Co}}^{3+}$ compounds are noted.

A comprehensive X-ray study of the ferroelectric–ferroelastic and paraelectric–paraelastic phases of Gd2(MoO4)3

Abstract: The crystal structure of Gdz(MoO4)a has been investigated both at room temperature and at elevated temperatures below and above the ferroelectric-ferroelastic transition temperature (~ 160°C). Space group, unit-cell dimensions, and formula units per unit cell are" Pba2 (C8v), a= 10"3881 +0"0003, b= 10"4194+0.0004, c= 10"7007+0.0006/~,, Z = 4 at 25°C and PS421m (D~), a=7.393 +0.002, c= 10.670 + 0.004/~, Z= 2 at 183 °C. Full-matrix least-squares refinements have been carried out with anisotropic thermal parameters with two single-crystal diffractometer data sets measured at 25 and 183 °C, yielding conventional R values of 0.032 (3000 reflections) and 0-028 (880). The high-temperature structure comes close to the average structure of the two ferroelectric-ferroelastic orientations. The differences in interatomic distances between the two modifications are all less than 0.05 .~. Ferroelectric-ferroelastic switching can be accomplished through two macroscopically equivalent switching mechanisms. Changes in interatomic distances of nearest neighbors upon switching are all less than 0.05 ,~,. The main difference between the two ferroelectric-ferroelastic orientations results through movements of several oxygen atoms by as much as 0.7.~. The spontaneous polarization was calculated assuming point charges Gd 3+ and [MOO4]'from positional parameters: Ps = 0.175/tC.cm -2, in good agreement with experiment. Since most of the dipole moments cancel out within the unit cell, Gd2(MoO4)3 can be described as a canted antiferroelectric. The temperature dependence of intensities and peak widths of superstructure reflections has been monitored through the transition temperature. Below the transition temperature, temperature-dependent physical properties can be accounted for by gradual changes in positional parameters towards the high-temperature structure. The mechanism of the phase transition is discussed in terms of the 'positional order-disorder' and the 'soft mode' model. The results obtained in the refinement of the room-temperature structure are compared with results of an independent study of this structure by Keve, Abrahams & Bernstein.

Crystal Growth and Observation of the Ferroelectric Phase of PbZrO3

Abstract: High-purity single crystals of PbZrO3 suitable for electrical measurements were grown from solutions in the PbO-ZrO2-PbF2-B2O3 system. Dielectric, hysteresis-loop, and optical measurements confirmed the existence of an intermediate ferroelectric (FE) form, previously observed only in ceramics, just below the transition temperature (Tc#232°C). The temperature stability range of the FE phase, which cooling curve measurements show to vary from 12° to 26°C, appears to be related to crystal stoichiometry. The maximum value of spontaneous polarization is P8(max)#24 μC/cm2, assuming that the FE phase possesses rhombohedral symmetry.

Superconductivity at high pressure in the A15 compounds

Abstract: The superconducting transition temperature for a number of A15 compounds has been determined as a function of hydrostatic pressure up to 25 kbar. The observed pressure dependences are compared with the predictions of a number of correlations between Tc and volume derived from zero-pressure data. The limitations of the theoretical models used to calculate Tc for the transition metals and their alloys when applied to a description of the pressure dependence are discussed.

Theory for Superconductivity in d‐Band Metals

Abstract: Superconductivity in transition metals, in transition metal alloys, in A‐15 compounds and in U6X compounds is studied within the framework of the B.C.S. theory. The electron‐phonon coupling constant λ is expressed in terms of measureable normal‐state quantities and an atomic parameter. Then, using the McMillan formula for the superconducting transition temperature Tc, the variation of Tc in the three transition metal series is discussed. Calculated values for Tc are given for several transition metal alloys, U6X compounds and for Sc, Y, Lu, and La. Also, it is shown that in A15 compounds Cooper pair fluctuations may occur even at temperatures several degrees Kelvin above Tc.

Temperature and frequency dependence of exchange anisotropy effects in oxidized NiFe films

Abstract: Hysteresis loops have been measured for thin NiFe films between 5 and 300 K using the magneto‐optic Kerr effect. Films that are free of any surface oxide do not show a displacement of the easy‐axis loop along the field axis, and the coercive force and anisotropy field are very weak functions of the temperature. Oxidized films show striking exchange anisotropy effects; displaced loops appear below a transition temperature ranging from 30 to 90 K as the film is more heavily oxidized. Loop width and anisotropy field increase with decreasing temperature, exhibiting a change in slope in the neighborhood of the transition temperature. The steady‐state parameters have characteristic frequency dependences; with increasing frequency the loop displacement and anisotropy field increase and the loop width decreases in the range 0.05–100 Hz. The interpretation of these results in terms of thermal aftereffect taking place in the antiferromagnetic oxide is presented in a companion paper.

ELECTRICAL CHARACTERISTICS OF rf-SPUTTERED SINGLE-CRYSTAL NIOBIUM FILMS.

Abstract: It is shown from measurements of the resistivity ratio (RR ≡ resistivity at room temperature divided by resistivity at 10°K), superconducting transition temperature, and transition‐temperature width that high‐quality single‐crystal niobium films can be fabricated by rf sputtering. The observed dependence of RR on sample thickness indicates that the intrinsic electron mean free path increases with thickness. Superconducting characteristics are essentially thickness independent for films thicker than approximately 1000 A.

Coupling of Hydrogen Bonding to Orientational Fluctuation Modes in the Liquid Crystal PHBA

Abstract: In the liquid state, the molecules of the carboxylic acids are associated in pairs by hydrogen bonds connecting their acid groups. If these dimers are long and rigid, they can form a nematic phase. The dimer molecules of the liquid crystal p-hexyloxybenzoic acid have been deuterated on the hydrogen bonds, and the nuclear quadrupolar spin-lattice relaxation rate 1/T 1 of these deuterons has been studied. In the nematic phase, 1/T 1 is frequency dependent between 2.7 and 13.8 MHz; its temperature behavior is identical with that of the parameter S describing the orientational order of the nematic phase. In the isotropic phase, 1/T 1 behaves in the same way as in conventional nematics: at high nuclear frequencies, it is still frequency dependent, and rather unsensitive to temperature in the vicinity of the nematic-isotropic transition; at low frequencies, it has no frequency dependence, but diverges when the transition temperature T c is approached from above: The “critical frequency” which separates...

Ordered Alloys in the Cobalt-Palladium System

Abstract: Ordered phases in the Co-Pd alloy system are studied by electron diffraction, using evaporated single crystal films of the alloys with Pd content extending from 20 to 100 at.%. Two types of the ordered states are found in this system; one is the Ll 2 type (Cu 3 Au type) and exists over the range of composition of Pd between 60 and 90 at.%. The temperature of order-disorder transition is 830°C for the 80 at.% Pd alloy. The other has the structure of Ll 0 type (CuAuI type) and exists within a narrow region of composition around CoPd. This phase is accompanied by two-phase regions over fairly wide ranges at both sides of it. The transition temperature is estimated to be higher than 850°C.

Impurity Effects on the Transition Temperature of Magnetite

Abstract: Influence of oxidization and various impurities on the transition temperature of the Verwey order in magnetite was studied experimentally. A small amount of impurities such as Li, Ni, Co, Cr, Al, Ti, Mg, Zn and Ga was doped and the transition temperature was determined by magnetic measurement. The decreases of the transition temperature of the specimens doped with the impurities are not simply related to the amount of the ferrous ions on the B sites but become larger as the ionic configurations deviate from the stoichiometric one. Oxidation of magnetite also decreases the transition temperature and this effect is smaller for the specimens sintered at lower temperature. For the Ga-doped specimens, the concentration dependence of the transition point is found to show a sharp change at a small (0.3%) Ga concentration.

Polarized Raman Spectra of β‐Quartz

Abstract: The Raman spectra of single crystal quartz were measured over the temperature range from a few degrees below the α‐β transition temperature (573°C) to 700°C. The polarization characteristics of the fundamental optical modes of the β phase were obtained, and the temperature dependent behavior of the bands in the region below 200 cm−1 was examined. A complete assignment of the Raman active optical modes of β‐quartz was determined from the polarization measurements.

Fluctuation-induced conductivity of superconductors above the transition temperature: regularization of the Maki diagram

Abstract: The Maki contribution to the conductivity above the superconducting transition temperature is regularized within the framework of the BCS theory. This is achieved through the renormalization of the impurity-scattering vertex by inclusion of the effects of pair fluctuations. The conductivity is evaluated for a thin film. It depends only on the reduced temperature and the normal resistance per square. Fair agreement is found with Al films over a wide temperature range. Agreement is not found with experiments on Bi, Pb, and Ga films, which apparently contain a strong additional pair-breaking effect. The temperature range in which interactions among fluctuations become important in the Maki conductivity is generally larger than that given by the Ginzburg criterion.

A Theory of the Phase Transition in ADP

Abstract: A four-sublattice theory is presented to explain the phase transition of ADP, where the dipole-dipole interaction is taken into account. The theory shows that the phase transition is of first order and the sublattice polarization is saturated below the transition temperature. These conclusions seem to be in good agreement with experimental observations.

Magnetic Behavior and Microstructure of Vanadium Phosphate Glasses

Abstract: The magnetic properties and microstructures of the vanadium phosphate glass system over the composition range 60 to 90 mol% V2O5 were investigated to study magnetic ordering in the glass and the effect of microstructure on its magnetic properties. Direct antiferromagnetic coupling between V4+ ions in the glassy matrix exists, and a transition temperature near - 70°C was observed. As-cast glasses with high V2O5 concentrations separated into two glassy phases; this separation increased the ESR line width as a result of inhomogeneity broadening. The separation, which concentrated the vanadium ions in a vanadium-rich phase, caused a hysteresis in the plot of ESR line intensity vs temperature at the transition temperature. Reduction of the vanadium ions by dextrose added to the melt enhanced phase separation and resulted in weak antiferromagnetic transitions at +70° and -120°C, the Neel temperatures of VO2 and V2O3, respectively.

Electric Potentials near a Superconducting-Normal Boundary

Abstract: Measurements have been made within a few micrometers of a current-carrying superconducting-normal boundary, which indicate that an electric potential exists in the superconductor in this region Near the transition temperature the experimental results are consistent in magnitude and temperature dependence with a theory recently proposed by Rieger, Scalapino, and Mercereau

Pair tunneling as a probe of order-parameter fluctuations in superconductors: zero magnetic field effects

Abstract: Thermal fluctuations of the order parameter in a superconductor above its transition temperature result in manifestations of the ordered state which appears below the transition. In a Josephson junction consisting of two metals with different transition temperatures there is associated with the incipient superconductivity an enhanced conductivity across the junction at temperatures between the transition temperatures of the two metals. The enhanced conductivity is observed as a current which flows in excess of the junction quasiparticle current. The excess current has been investigated using Sn-Sn x O y -Pb and Al-Al x O y -Pb junctions masked to eliminate effects of graded film edges. Details of the variation of the excess current with voltage and temperature are in quantitative agreement with calculations in the Al-Al x O y -Pb junctions. The excess current-voltage characteristic is a direct measure of the imaginary part of the generalized susceptibility above T c . The pair relaxation frequency in aluminum based on the data is consistent with the theoretical value.

Theory of magnetic impurities in superconductors

Abstract: The jump of the specific heat at the transition temperature is calculated for a superconductor with magnetic impurities. The calculation includes the Kondo effect and is exact within the Nagaoka-approximation.

Properties of Water in Macromolecular Gels. IV. Proton Magnetic Resonance Studies of Water iu Macromolecular Gels

Abstract: Electrochemical and dilatometric studies have indicated that at least three states of water exist in macromolecular gels. The properties of each state of water in gels such as agarose gels were investigated by highresolution and broadline PMR over the temperature range from +50 to −60°C. The linewidth of water protons in gels was found to depend markedly on the water content of gels, and the linewidth vs. water content curve gave two inflection points. One of the inflection points can be attributed to the so-called bound water, but the other cannot be explained unless three states of water are assumed in gels. Three kinds of transition temperatures were confirmed by anomalous changes in the PMR linewidth below 0°C. The so-called bound water exhibited no transition temperature between 0 and −20°C, and free water gave the transition temperature at 0°C. Moreover, the transition temperature of a quantity of water was distributed in the range from 0 to −20°C. On the basis of the transition temperature, the pro...

INVESTIGATION OF THE MAGNETIC PHASE TRANSFORMATION IN Mn3Sn

Abstract: The transformation between a long‐period antiferro‐magnetic and a triangular‐type weak ferromagnetic phase was investigated by magnetic, x‐ray and neutron diffraction methods on polycrystalline and single‐crystal samples of non‐stoichiometric Mn3Sn with hexagonal DO19 structure The transition is of first‐order, being accompanied by a thermal hysteresis A slight change in the electrical resistivity was observed during the transformation, but no change in the lattice parameters could be detected The transition temperature depended on the conditions of annealing and varied between 150° and 300°K In spite of the fact that the samples quenched from 800°C and those annealed in the range 650–750°C have the same crystal structure, the transformation occured only in the annealed samples The phase transformation can be switched on and off by appropriate heat treatment

Anomaly of the Proton Spin-Lattice Relaxation Time near the Critical Temperature of Dicalcium Lead Propionate Ca2Pb(C2H5COO)6

Abstract: The precise temperature dependence of the proton spin-lattice relaxation time T 1 is observed in the vicinity of the transition temperature of dicalcium lead propionate (Ca 2 Pb(C 2 H 5 COO) 6 ). Measurements are carried out by using 90°-t-90° pulse method and the boxcar integrator is used to reduce an experimental error of T 1 . The anomalous part of T 1 -1 , ( T 1 -1 ) fluc , seems to display a logarithmic divergence at the transition temperature T c . The interpretation of the anomalous ( T 1 -1 ) fluc is formulated in terms of the time dependent correlation functions in the order-disorder phase transition system described by the Ising operators, and the relation to the recent theory of the critical phenomena is shown. Some possible interpretations of this divergence are discussed.