KONDO-LIKE RESISTIVITY IN SINGLE CRYSTALS OF dhcp Pr AT MILLIKELVIN TEMPERATURES

Abstract

Resume._ La resistance electrique de Pr dhcp a ete mesure e entre 15 m K et 5 K dans la direction C. En-dessous de 0,4 K nous avons observe une contribution de type Kondo. L'ordre nucleaire, comme l'indique le temps de relaxation thermique, est observe vers 25 m K dans un cristal, mais parait ab ­sent dans un autre. Abstract- Electrical resistance of dhcp Pr has been measured between 15 m K and 5 K in the c-direc-tion. Below 0.4 K a Kondo-like behaviour is observed. Cooperative ordering indicated by the thermal time constant, which has been observed near 25 m K in one crystal, seeme d to b e absent in the presen t sample. INTRODUCTION.- In dhcp Pr the crystal fiel d ground states of the ions are singlets, i.e., non-magnetic. The ratio between effective exchange interactions and crystal field level splitting, however, appears to be close to the critical valu e for spontaneous magnetic order , sinc e polycrystalline materia exhil ­bits a n ordere d state with transition temperature above 20 K /l/, while no such transition has bee n found in magnetic studies of single crystals down to 0.4K 111. Murao /3/ an d Triplett an d White /4/ predicte d that at low enoug h temperatures a subcritical system would order as a consequency of an effective nuclear exchange interaction mediated by hyperfine induced ionic moments and conduction electrons. In specific heat measurements, Lindelof et al./5/ have approa­ched this nuclear cooperative ordering and estima­ted a transistion temperature T„ between 25 and 30 mK. Since the transition involves a polarization of ions by nuclei it woul d be interesting to detect possible effects in the resistivity . RESISTANCE MEASUREMENTS.- Our sample wa s a single crystal of dhcp Pr, approximate lenght 7 m m an d cross section 1mm , cu t alon g the c-direction. Cur ­rent and potential lead s were tinned, thin copper wires attached by ultrasonic soldering. The crystal was mounted directly in the mixin g chamber ofe thdilution refrigerator and supported only by the leads so as to minimiz e strain. Temperature was mea ­sured with Speer carbon resistors also immersed in the liquid. Work supported by the Danis h Natural Science Re ­search Council.