Low-temperature properties of pseudoternary R(Rh1−x Ir x )4B4 (R=Ho and Dy) systems

Pseudoternary Ho(Rh_1–x Ir_x)₄B₄ and Dy(Rh_1–x Ir_x)₄B₄ systems were investigated with respect to the interplay between superconductivity (SC) and long-range magnetic ordering by means of specific heat (C) and upper critical magnetic field (H _c2) measurements. For Ho(Rh_1–x Ir_x)₄B₄ withx0.2 and Dy(Rh_1–x Ir_x)₄B₄ withx0.15, ferromagnetic (FM) ordering, reminiscent of spin-1/2 mean-field theory, was observed. In particular, FM ordering in Ho(Rh_1–x Ir_x)₄B₄ with 0.07x0.2 was found to destroy SC at a second critical temperatureT _c2 below the superconducting transition temperatureT _c, and was accompanied by a spike-shaped feature inC atT _c2. Coexistence of antiferromagnetic (AFM) ordering with SC was observed in both systems for 0.2x0.7. The compounds withx0.5 exhibited Néel temperaturesT _N that were smaller thanT _c and enhanced values ofH _c2 belowT _N. TheT _c andT _N undergo an abrupt decrease and increase, respectively, nearx0.5, and then cross atx0.6, above whichT _N>T _c. For Ho(Rh_1–x Ir_x)₄B₄ with 0.275x0.4, two different AFM transitions were detected in a double-peak feature inC.     

Transition from antiferromagnetism to ferromagnetism in the superconducting mixed ternary system (Sm1−x Er x )Rh4B4

The mixed ternary system (Sm_1–x Er _x )Rh₄B₄ has been investigated by means of measurements of ac electrical resistance in various applied magnetic fields, ac magnetic susceptibility, and specific heat. The low-temperature superconducting and magnetic transition temperature versus Er concentrationx phase diagram reveals the occurrence of a single transition to long-range magnetic order for all concentrations, changing from antiferromagnetism for SmRh₄B₄ to ferromagnetism for ErRh₄B₄ atx 0.2. The upper critical magnetic field versus temperature curves for various Er concentrations exhibit combined features characteristic of both SmRh₄B₄ and ErRh₄B₄, rather than a gradual evolution from one type to the other. A model for such a mixed magnetic system is discussed, and comparisons with other mixed ternary systems are made.Research supported by the U.S. Department of Energy under Contract No. DE-AT03-76ER70227.     

Superconducting and normal state properties of Li1+x Ti2−x O4 spinel compounds. II. Low-temperature heat capacity

Heat capacity data are reported which confirm as a bulk effect the previously reported superconductivity in LiTi ₂ O ₄ . These data also establish LiTi₂O₄ as a weak couplingd-band superconductor with superconducting state properties well described by the Bardeen—Cooper—Schrieffer theory of superconductivity. The properties of LiTi ₂ O ₄ are compared with those of other superconducting spinel compounds, and the composition dependence ofT _c for Li _1+x Ti _2–x O ₄ is discussed. The disappearance of superconductivity forx0.1 was found to be correlated with a rapid decrease in the normal-state linear heat capacity coefficient.Research sponsored by the U.S. Energy Research and Development Administration under Contract No. ERDA E(04-3)-34 PA227.Research sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under AFOSR Contract Number AFOSR/F-44620-72-C-0017.     

Non fermi liquid behavior in strongly correlated f-electron materials

Evidence for non Fermi liquid (NFL) behavior in y_1–x U_xPd₃ and related systems is reviewed and discussed within the context of possible microscopic mechanisms. Low temperature electrical resistivity, specific heat, and magnetic susceptibility measurements on the Th_1–xU_xPd₂Al₃ system reveal unconventional Kondo behavior with NFL low temperature characteristics. Magnetic susceptibility measurements on UCu_3.5Pd_1.5, which has previously been shown to exhibit NFL behavior, are presented. Some systematics of the NFL low temperature behavior observed in several f-electron materials include a linear temperature dependence of the electrical resistivity 1–aT with either positive or negative coefficient a, a logarithmically diverging specific heat C/T –lnT, and T^1/2 asymptotic behavior of the magnetic susceptibility 1 – T^1/2.     

Oxidation kinetics of cobalt and cobalt—iron alloys at high temperatures

The oxidation kinetics for cobalt and two cobalt—iron alloys alloys, CoFe and CoFe₃, were determined between 1073 and 1280 K in 1 atm. of oxygen. The oxidation of cobalt and CoFe follows a parabolic law, while the oxidation of CoFe₃ is better described by a direct logarithmic law. The oxidation rates for the cobalt—iron alloys alloys are lower than those expected from combining oxidation kinetics of pure iron and pure cobalt, thus suggesting a significant cobalt enrichment occurs at the oxide surface. This cobalt enrichment is observed using energy dispersive analyses. The logarithmic kinetics of CoFe₃ during oxidation is discussed, and a model is proposed to explain this behavior.     

Aging Effects in Gold Thermocompression Bonds to Complex Metallizations

The times to failure of Au thermocompression bonds to Au-Mo-Al and to Au-Ti-Al multilayer films were determined as a function of temperature over the range of 125-400°C and compared with the time to failure of gold bonds to aluminum films. A 500-? Ti intermediate film was found to be ineffective as a barrier to reaction between gold and aluminum. On the other hand, a 500-? Mo film was effective, providing increased time to failure over that observed for bonds to Au-Ti-Al and Al films. No failures were observed after 2000 hours at 400°C when gold wire bonds were made to an Au/Mo/Al metallization having 1000 ? of molybdenum as the intermediate layer.     

Superconducting and normal state properties of ErRh4B4 and LuRh4B4

Detailed heat capacity measurements of the ternary compounds ErRh₄B₄ and LuRh₄B₄ have been performed between 0.5 and 36 K and in magnetic fields up to 4 kG, yielding new information on crystal field effects in these materials and on the influence of externally applied magnetic fields on the coupled superconducting-ferromagnetic reentrant transition in ErRh₄B₄. Static magnetic susceptibility data on LuRh₄B₄ are presented which allow qualitative conclusions to be drawn regarding the magnitude of exchange enhancement and orbital paramagnetic effects in the RERh₄B₄ compounds. The electrical resistivity of ErRh₄B₄ has also been determined between 4 K and room temperature.This research was supported by the Department of Energy under Contract Number Ey-76-S-03-0034-PA227-3 (LDW, HBM, RWM, MBM) and by the National Science Foundation under Grant Number NSF/DMR77-08469 (DCJ). One of us (RWM) thanks the National Science Foundation for a postdoctoral fellowship.