Magnetic and nonmagnetic impurities in some A-15 structure systems. II. Magnetic properties

We present measurements of the susceptibility of the A-15 structure systems NbyPt_100–y–x M _x where M = Fe and Ru (y = 73–78). These measurements correlated with those of the critical temperature T _c permit us to conclude that there is a saturation of the effective number of magnetic impurity atoms and that these atoms are situated on the chains. We find also that the magnetic moment of the Fe atoms depend on the value of y. The high value of the Curie temperature and its variation is discussed with the aid of a model of pairs with antiferromagnetic interaction. In order to describe the formation of the localized moments we have taken into account the effect of the environment around each impurity. Depending on the number and the nature of the nearest neighboring atoms, we deduce a magnetic or nonmagnetic behavior of the impurity. The variation of the moment as a function of the nearest neighbors is explained by supposing electronic redistribution between the atoms, which is the same as supposing an interatomic charge transfer.     

Possible coexistence of superconductivity and magnetism in layered compounds

The possibility of a coexistent superconducting and magnetic phase in layered transition metal dichalcogenides of the type MX ₂ A _x is investigated (M =transition metal; X = S, Se; A = magnetic ion; x 0.25). Describing such systems with a model in which a highly anisotropic electron gas interacts with a quasi-two-dimensional Heisenberg magnet, we find the following conditions for coexistence: (1) The coupling between adjacent layers of ferromagnetically ordered magnetic ions needs to be antiferromagnetic (S ^z = 0) in order to have no pair-breaking internal fields. (2) The exchange interaction between conduction electrons and magnetic ions must be very small ( 0.003 eV), since otherwise, due to the high concentration of localized magnetic moments, spin-flip scattering processes and spin fluctuations would destroy superconductivity. The theoretical prediction for the persistence of superconductivity up to concentrations of x 0.25 of magnetic ions is compared with recent experiments on Eu-intercalated TaS ₂ and NbS ₂ and related compounds showing a tendency for ferromagnetically ordered layers of Eu impurities and antiferromagnetic coupling between neighboring layers but no superconductivity for x exceeding a few percent. Reasons for the quick disappearance of superconductivity in these systems and criteria for possible observation of high-magnetic-impurity-concentration superconductivity in other layered compounds are given.     

Sintering and crystallization behaviour of a glass powder and block with a composition of anorthite and the microstructure dependence of its thermal expansion

Two kinds of glass specimens were prepared in the form of powders and blocks with anorthite compositions, and an examination was made of the sintering and crystallization behaviour of the specimens and the microstructure dependence of their thermal expansion. It was found that the addition of a few wt% of Ti0₂ was necessary to obtain a good sintered body by heat treatment at 1050°C. Specimens with low expansion coefficients were obtained when no TiO₂ was added and when the grain size of the starting glass powder was large (ca. less than 180 m). It was estimated that this was mainly due to the presence of orthorhombic and hexagonal anorthite in the central part of each grain. In order to explain the abnormally low expansion coefficient of the sintered specimens, the crystallization behaviour of glass blocks with the same composition was examined. The crystallization proceeded by two mechanisms: the triclinic phase precipitated from the surface, and the orthorhombic and the hexagonal phase precipitated from the central part of the specimens. The orthorhombic phase showed a strong tendency for a preferred orientation, and the thermal-expansion measurements showed strong anisotropy. The addition of TiO₂ to the mother glass suppressed the precipitation of the orthorhombic and hexagonal phases, and the resultant pieces were composed mainly of the triclinic phase; the thermal-expansion coefficient became isotropic.     

The behavior of magnetic and nonmagnetic impurities in the system Nb76Pt24−x Fe x

We find that Fe is magnetic when introduced as an impurity in the A-15 super-conductor Nb ₇₆ Pt ₂₄ . The ternary diagram of the system Nb-Pt-Fe is presented. The Fe impurities show a magnetic behavior when they are on the linear chains (Nb sites) and a nonmagnetic behavior when they are on the cubic sites (Pt sites). Supposing that the Fe atoms have a pair-breaking effect as well as a nonmagnetic effect on the superconducting state and determining the latter by adding nonmagnetic impurities (Ru), we are able to fit the measured critical field as a function of temperature to the WHH theory generalized to include magnetic impurities. The analysis shows that the exchange interactionJ between conduction electrons and the Fe atoms is negative. A Jaccarino-Peter compensation effect takes place at about 2 at % Fe. The inverse Fe susceptibility curves are fitted using a model of short-range interactions between impurities.     

Magnetic and nonmagnetic impurities in some A-15 structure systems. I. Superconducting properties

We present measurements of the critical temperature T _c and the critical field H _c2 of the A-15 structure systems Nb _y ,Pt_100–y–x M _x where M = Fe and Ru (y = 73–78). T _c is influenced by two effects: one due to the magnetic Fe atoms, which causes a strong decrease, and the other due to the nonmagnetic effect of the Fe atoms, which causes a smaller but non-negligible effect. For the study of the nonmagnetic effect we introduced Ru impurities instead of Fe. Anomalous high critical field values are found as T 0, and the curves H _c2 versus T do not follow the WHH theory generalized to include magnetic impurities. The introduction of an empirical parameter permits us to fit our experimental results. This abnormal behavior of H _c2 is discussed in terms of strong-coupling and anisotropy effects.