320 Journal of Magnetism and MagneUc Matermls 63 & 64 (1987) 320-322 North-Holland, Amsterdam A POLARIZED HEAVY FERMION SYSTEM: CeRu2Si2 P HAEN, J FLOUQUET, F LAPIERRE, P LEJAY, J -M MIGNOT, A PONCHET and J VOIRON* CRTBT- CNRS, BP 166 X, 38042 Grenoble- C~dex, France *Laboratotre Louts Ndel, CNRS, BP 166 X, 38042 Grenoble-C~dex, France High-pressure magnetoreslstance and suscepUbdlty measurements have been performed on CeRu2SI2 single crystals The metamagneuc-hke transmon at 4 2 K shifts rapidly from 85 kOe at P = 0 to ~ 180 kOe at 5 4 kbar Th~s variation correlates with the pressure decrease of X42, suggesting that the transmon occurs for a crmcal value of the magnetization, in contrast to the predlcUons of an almost locahzed Ferm~ hqmd theory Cermm-based mtermetalhc compounds with the ThCr2S12-type tetragonal structure nave attracted a great deal of interest since the dis- covery of so-called "heavy-fermlon supercon- ductwlty" m CeCu2SI2 [1] CeRuzSI2 is another Interesting case, although all attempts to detect a superconducting transmon down to 40mK [2] and now 20 mK [3] have been unsuccessful Near room temperature, x(T) has a Curie-Weiss de- pendence with an effective moment close to the Ce ~÷ value [2] and crystal field excitations are well resolved in the specific heat data [4] At low temperatures, no long-range magnetic ordering occurs and the large linear term in the spectfic heat (y~350mJ/molK 2) [4,5] characterizes a Fermi hqmd state of heavy quasi particles An- other outstanding feature of this compound is ~ts extremely large anisotropy (Xc/X(a,b)~-15 at 4 2 K [3,6,7]) which makes it necessary to work with single-crystals The electrical resistivity p decreases continuously on cooling with a linear regime between 1 and 10K and a power-law dependence AT n (n -~ 1 75 rather than 2 in most speomens) with A = 0 3 to 0 7 ixfl cm K-" below 1 K [3] The unusually strong pressure response of p at low temperatures [5] suggests that the system may be close to a valence instability In a prewous study [6,7], the magnetization at 4 2 K measured with H parallel to the c-axis was found to display a metamagnetic-hke transmon with a very steep increase around t4(x) = 85 kOe "'M Furthermore, the transverse magnetores~stance (/411 c) exhibits a large posmve contribution in low 0304-8853/87/$03 50 © Elsevier Science Publishers (North-Holland Physics Pubhshing Dwlsion) fields, followed by a pronounced maximum for H= H~ )~H~ ) This paper describes an attempt to correlate these intrtguing new features of CeRuzS12 with the Fermi-liquid parameters of this system, by varying the temperature and the magnetic field, and also by applymg an external pressure Magnetization and restsUwty measure- ments will be reported, all performed for H ap- plied along the c-axis, since only weak mono- tonous effects are obtained for other d~rections of the field [7] Small single-crystalline platelets with faces perpendicular to the c-ax~s were extracted from ingots of polycrystalhne material One of them was used for transport experiments by the Van der Pauw method (sample 1) and a small paral- lehplped (~ 0 9 × 0 5 x 0 2 mm 3) (sample 2) cut from a different one for conventional 4-termmal measurements In both cases, the current was parallel to the basal plane Magnetoresistance curves were recorded in a large range of tem- peratures, from 0 07 to 4 2 K for sample 1 and from 1 35 to 70K for sample 2 The absolute reslstivities at room temperature were 85 and 115 I~f~cm respectively and the vanauons ~p(H) at 4 2K were ldenUcal for the two samples A selected set of data is presented in fig 1 The main features of the curves, 1 e the initial posmve contribution and the maxtmum In high fields, disappear only at 70 K The magnetoresis- tance anomaly is particularly sharp for T < 1 5 K and the saturation value In high fields is then larger than the zero-field resistivity Surprisingly BV