Influence of spin fluctuations on the electrical resistance of UAl 2 and UPt 3 at high pressures and low temperatures

15 NOVEMBER 1984 VOLUME 30, NUMBER 10 PHYSICAL REVIEW B Influence of spin fluctuations on the electrical resistance of UA12 and Upt3 at high pressures and low temperatures M. S. Wire, J. D. Thompson, and Z. Fisk Los Alamos National Laboratory, Mail Stop G730, Los Alamos, New Mexico 87545 (Received 7 May 1984; revised manuscript received 23 July 1984) The compounds UA12 and UPt~ are thought to exhibit ferromagnetic spin fluctuations. We have measured the electrical resistance of both materials to over 1000 K at zero pressure and from room temperature to 1 K at pressures up to 18 kbar. The primary effect of pressure is to reduce the low- temperature slope BR/BT and thus shift the inflection point in the resistance to higher tempera- tures. We interpret these observations as arising from an increase in the spin-fluctuation tempera- ture T, with increasing pressure. Using a Fermi-liquid description of these materials, we calculate the pressure dependence of the T=O susceptibility from the pressure dependence of T, . We also resistance at various fixed pressures scales as T/T, (P) over a find that the temperature-dependent remarkably large temperature interval. I I. INTRODUCTION Recently, increased activity has focused on experimen- tal and theoretical descriptions of materials showing near- ly magnetic behavior. Within this class of materials are those that display signatures believed to be characteristic of ferromagnetic spin fluctuations, namely a Curie-Weiss susceptibility at high temperatures that evolves with de- a into enhanced paramagnetism, creasing temperature and a resistivity proportional to low-temperature T ln T contribution to the low-temperature heat capacity. Both UA12 (Refs. 2 and 3) and UPt3 (Refs. 4 and 5) are known to comply satisfactorily with all of these proper- ties. Of these two, UPt3 is particularly interesting because of the recent demonstration that bulk superconductivity and possibly spin fluctuations coexist in this material. Preliminary band-structure calculations indicate that in both UA12 (Ref. 6) and UPt3 (Ref. 7) the Sf electrons form a narrow band at the Fermi energy, a condition favoring' spin fluctuations. Decreasing the U-U separa- tion by the application of hydrostatic pressure should in- crease the 5f-wave-function hybridization and broaden the band causing a decrease in the density of states at the Fermi level. Provided pressure increases the f-f Coulomb repulsion less rapidly than it broadens the Sf-band width, decreasing the U-U separation should lower the magnetic susceptibility (X). Such behavior should be reflected as a systematic change with volume in physical properties, e.g. , the magnetic susceptibility and resistivity, that de- pend sensitively on Sf-conduction-band interactions. Evi- dence substantiating this view has been found in suscepti- on UA12 and through resistivity bility measurements measurements on UPt3 (Ref. 9) at pressures up to, respec- tively, 6. 65 and 4. 2 kbar. To study the effect of U-U separation on spin fluctua- the tions in UA12 and UPt3, we have measured resistance of polycrystalline UA12 temperature-dependent and single crystals of UPt3 (along both the c axis and in the a bplane) subjected to-hydrostatic pressure. We show that the suppression of spin fluctuations with pressure, as T, is reflected deduced from susceptibility measurements, resistance and that, directly in the temperature-dependent over an appropriate temperature interval, the resistance parameter which may be scales with a pressure-dependent identified with the spin-fluctuation temperature. II. EXPERIMENTAL PROCEDURE Our polycrystalline UA12 sample was prepared by arc melting the pure elements together on a water-cooled argon atmosphere. copper hearth in a zirconium-gettered Needle-shaped single crystals of UPt3 were grown from a bismuth flux. Both the UA12 and the UPt3 samples were shown to be single phase by x-ray diffraction and metallo- Because of geometrical considera- graphic techniques. tions, two separate UPt3 samples were studied, one on which the resistance was measured along the hexagonal c axis and one for measurements in the a bplane. As -a measure of the quality of our samples, the residual resis- tance ratios for UA12, UPt3 c axis, and UPt3 a were, respectively, 14, 130, and 47. The superconducting transition temperatures for UPt3 c axis and a were 0. 51 and 0. 38 K, respectively. This difference 'and that of the residual resistance ratios for the two UPt3 samples result from the annealing process given the c-axis sample. were made by a standard Resistance measurements four-lead ac technique at 23 Hz. Platinum leads were at- tached to the samples by spot welding (UA12) or by silver-filled epoxy (UPt3). The samples were subjected to hydrostatic pressures exceeding 17 kbar in a self-clamping ' which used a 1:1 mix- pressure cell, described elsewhere, ture of n-pentane and isoamyl alcohol as the pressure The pressure was determined from a lead medium. manometer situated adjacent to the sample. Resistance measurements in the cell were performed in the tempera- ture range 1 T & 300 K, measured by a calibrated carbon-glass thermometer embedded in the side of the bplane- bplane- cell. Resistance measurements made above room tempera- The American Physical Society