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2009 METALS, SUPERCONDUCTORS...High-field metamagnetism in the antiferromagnet CeRh 2 Si 2CeRh 2 Si 2 is a heavy-fermion antiferromagnet which canbe driven to a magnetic instability either by applying pressureor in a high magnetic field. It orders antiferromagneticallyat a second-order phase transition T N = 36 K, afirst-order phase transition occurring at a lower temperatureT 1,2 = 26 K, below which the antiferromagnetic structure ismodified [Graf et al., Phys. Rev. B 57, 7442 (1998)]. Applicationof hydrostatic pressure induces a quantum phasetransition to a paramagnetic Fermi liquid regime at around11 kbar, and unconventional superconductivity was evidencedin the vicinity of the quantum phase transition,below a critical temperature going up to TSCmax ≈ 0.4 K[Movshovich et al., Phys. Rev. B 53, 8241 (1996)]. Here,we present a careful study of the magnetic field-temperaturephase diagram of CeRh 2 Si 2 when a magnetic field is appliedalong the easy-axis c. This study was made combiningtransport, torque and dilatometry experiments using thepulsed magnetic fields generated at the LNCMI-Toulouse.Figure 81 shows a plot of the field-derivative of the torqueversus magnetic field of CeRh 2 Si 2 , at temperatures between4.2 and 24 K. The torque signal is proportional to MH sinθ,where M is the magnetization and θ the small angle betweenthe magnetic field H and the easy axis c of the sample.The field-induced polarization of the system is accompaniedat 4.2 K by two successive minima in the fieldderivativeof the torque, which are the characteristics oftwo first-order transitions, separated by 0.5 T, at H 2,3 andH c ≃ 26 T. From figure 81, it is clear that the two transitionsH 2,3 and H c merge at about 20 K into a single first-ordertransition H c . In our data, a first-order-like anomaly at H ccan be seen up to 23 K, which is characterized by a symmetricpositive anomaly in the field-derivative of the torque.Above 24 K, a second-order-like anomaly has replaced thefirst-order-like anomaly, and the torque versus field data arecharacterized by an asymmetric step-like anomaly in thefield-derivative of the torque.Our torque data are in good agreement with our transport,thermal expansion, and magnetostriction measurements(not shown here), but also with studies performedby Settai et al. [J. Phys. Soc. Jpn. 66, 2260 (1997)], Abeet al. [J. Phys. Soc. Jpn. 66, 2525 (1997)], and Demuer,Sheikin et al. [to be published]. This study permitted todraw the magnetic field-temperature phase diagram of thesystem, indicating the presence of three distinct antiferromagneticphases (see figure 82). The possibility of a tetracriticalpoint at around (24 T, 20 K), where the four antiferromagnetictransition lines could merge, was suggestedhere. It should be further checked using continuous highmagnetic fields. The temperature dependence of the resistivity(not shown here) was also extracted from our pulsedfield scans. This permitted to show that the quadratic coefficientA of the resistivity is enhanced in a rather large Hwindow, of about 10 T, which contrasts with the additionalsharp enhancement of A through the first-order metamagnetictransitions H 2,3 and H c , which are only separated by0.5 T. Finally, a drop of resistivity observed at H c is compatiblewith the idea of a Fermi surface reconstruction.Figure 81: Magnetic field-derivative of the torque versus magneticfield of CeRh 2 Si 2 , for temperatures T ≤ 24 K and magneticfields along c.Figure 82: Magnetic field versus temperature phase diagram ofCeRh 2 Si 2 , with H ‖ c, obtained from resistivity, torque, and thermalexpansion. The insert focuses on the low-temperature part ofthe phase diagram.W. Knafo, D. Vignolles, B. Vignolle, Y. Klein, C. Jaudet, C. ProustD. Aoki, A. Villaume, J. Flouquet (Commissariat à l’Energie Atomique, Grenoble)61