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Pt nuclear-magnetic-resonance measurements were performed on aligned single-crystal whiskers of UPt3 for temperatures from 4 to 50 K. A special alignment technique allows us to produce samples with very good c-axis alignment and to work with the field parallel to the c axis with an excellent signal-tonoise ratio. Combining the Knight shift and spin-lattice-relaxation data, we show a continuous crossover from paramagnetic behavior above 40 K toward a degenerate-Fermi-liquid behavior below 5 K.Antiferromagnetic spin fluctuations appear to be responsible for this crossover process, and dominate all low-frequency magnetic properties of this compound. The anisotropy ratios of both the spin-lattice and the spin-spin relaxation rates are found to be temperature independent. Analyzing these ratios we find that the imaginary part of the dynamic spin susceptibility is slightly stronger in the basal plane. I. INTRQDUCTIQN II. SAMPLE PREPARATION AND MEASUREMENTSHeavy-fermion systems have attracted considerable attention since they exhibit unusual physical properties and a variety of ordered states at low temperature. ' For example, in UPt3, superconductivity is found to coexist with antiferromagnetism as inferred from neutronscattering experiments.The static antiferromagnetic order is reported to set in below 5 K. Other neutronscattering experiments ' have revealed the presence of spin fluctuations at higher temperatures. Short-range antiferromagnetic (AFM) correlations were observed between uranium atoms in adjacent basal planes, as were ferromagnetic (FM) correlations between uranium atoms in the same basal plane. These correlations are thought to be responsible for the formation of a coherent electronic state leading to a degenerate Fermi-liquid regime. In light of the possible significance of antiferromagnetic coupling in the superconducting pairing mechanism, it is important to study the temperature region where the crossover to the Fermi-liquid behavior occurs.Nuclear magnetic resonance (NMR) has been an important probe of electronic structure and the low-energy excitations of heavy-fermion metals through the measurements of Knight shift, spectrum linewidth, and nuclearspin relaxation rates. However, ' Pt NMR on a powder sample of UPt3 is difficult to interpret due to the enormous broadening from Knight shift anisotropy. ' We have discovered a way to align the UPt3 whiskers, each of which is a high-quality single crystal, and we have measured anisotropic Knight shifts and relaxation rates in a temperature range of 4 -50 K. These measurements show a continuous crossover from paramagnetic behavior above 40 K toward a degenerate Fermi-liquid behavior below 5 K.Single-crystal whiskers were grown by a method reported previously.The whiskers were less than 40 pm thick and their principal geometric axis is the crystalline c axis. The small thickness of the whiskers allows complete rf penetration at our NMR frequency of 11 MHz throughout the measurement temperature range. The super conducting onset temperature and the transit...

We present the preparation and characterization of a new metallomacrocycle-based molecular conductor, (triazatetrabenzoporphyrinato)copper(II) iodide, Cu(tatbp)I. The material crystallizes with two formula units in space group Dih-P4/mcr of the tetragonal system in a cell of dimensions a = 13.998 (5) A and c = 6.426 (3) 8, (120 K). The structure has been refined to a value of R ( F ) of 0.085 for 862 data and 65 variables. This material is isostructural with (phthalocyaninato)copper(II) iodide, Cu(pc)I, and has analogous physical properties. However, Cu(tatbp) exhibits subtle metrical and electronic differences from Cu(pc) because the tatbp macrocycle has a methine carbon in place of one bridging nitrogen atom of pc. These differences produce distinct charge-transport and magnetic properties for each of the iodinated compounds. Cu(tatbp)I is a ring-oxidized organic conductor with metallic behavior that contains a dense array of localized Cu2+ moments embedded in the "Fermi sea" of carriers. I3C NMR spectroscopy at 13 MHz of enriched and natural-abundance I3C nuclei in Cu(tatbp)I show a distribution of positive and negative spin densities. The temperature dependence of the shifts identifies their origin as an isotropic contact hyperfine interaction transferred from Cu(I1). EPR and magnetic susceptibility measurements show that the local and itinerant spin systems are coupled and that the local moments are exchange-coupled to one another by direct and carrier-mediated mechanisms. This unusual situation results in two novel transitions of the coupled systems: for T < T, = 20 K, g,, increases anomalously as T is decreased; for T < Tb 6-8 K the EPR line width begins to broaden sharply, but the signal of Cu(tatbp)I remains detectable to T < 2 K. Comparisons of X-band and Q-band data show that gl is dependent on magnetic field at liquid-helium temperatures. Magnetic moments localized on the Cu2+ metal spine of Cu(tatbp)I also have a dramatic effect on the conductivity and dielectric constant. From -90 to 5 1 0 K, both four-probe (27 Hz) and microwave (13 GHz) conductivities decrease by 3 orders of magnitude and both slightly increase with magnetic field. For T < 6 K the microwave conductivity is enhanced in comparison to the four-probe conductivity and decreases with field; over the same range the dielectric constant increases with field. These effects correlate with a relaxation of the local moments observed in EPR spectroscopy and reflect a dielectric loss associated with an unusual coupling between magnetic and dielectric properties. (1) (a) Ogawa, M. Y.; Martinsen, J.; Palmer, S. M.; Stanton, J. L.; Tanaka, J.; Green, R. L.; Hoffman, B. M.; Ibers, J. A. (3) (a) Quirion, G.; Poirier, M.; Ogawa, M. Y.; Hoffman, B. M. Solid State Commun. 1987,64,613-616. (b) Quirion, G.; Poirier, M.; Liou, K. K.; Ogawa, M. Y.; Hoffman, B. M. Phys. Rev. B 1988, 37, 4272-4275. (4) Martinsen, J.; Stanton, J. L.; Greene, R. L.; Tanaka, J.; Hoffman, B. M.; Ibers, J. A. J . Am. Chem. SOC. 1985, 107, 6915-6920. (5) (a) Martinsen, J.; Greene, ...

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