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Studies performed at the LNCMI-Toulouse

Activity on heavy-fermion physics has been strongly developed during the last years at the LNCMI-Toulouse. Among recent works, studies performed on the antiferromagnet CeRh2Si2, on the “hidden-ordered” paramagnet URu2Si2 and on the weak itinerant ferromagnet UIr are summarized below.

URu2Si2 is surely one of the most mysterious of the heavy-fermion compounds. Despite more than 20 years of experimental and theoretical works, the order parameter of the transition at T0 = 17.5 K is still unknown. The state below T0 is called the “hidden-order phase,” and the stakes are still to identify the energy scales driving the system to this phase. A new generation of magnetoresistivity and magnetization measurements on very-high-quality single crystals of URu2Si2 has been performed up to 60 T at the LNCMI-Toulouse. It permitted to show that the transition to the hidden-order state is initially driven by a high-temperature crossover at around 40–50 K, which is a fingerprint of intersite electronic correlations. In a magnetic field applied along the easy-axis c, the vanishing of this high-temperature scale precedes the polarization of the magnetic moments, as well as driving the destabilization of the hidden-order phase. Strongly impurity-dependent magnetoresistivity confirms that the Fermi surface is reconstructed below T0 and is strongly modified in a high field applied along c, i.e., at a sufficiently high magnetic polarization. These new results will help for a future understanding of the hidden-order in URu2Si2.

Fig. 1 (left) phase diagram and (right) magnetization and magnetoresistivity of URu2Si2 in a magnetic field applied along c.

A careful study of the antiferromagnet CeRh2Si2 has been made at the LNCMI-T using a combination of transport, torque, and magnetostriction experiments. This heavy-fermion system becomes polarized via two successive first-order transitions at around 26 T (at low temperature) and its (H,T) magnetic phase diagram (at ambient pressure). At zero-field, an antiferromagnetic-to-paramagnetic instability is also reached at a pressure of 11 kbar. As in many heavy-fermion systems, superconductivity is induced in the vicinity of the pressure-driven magnetic instability, suggesting that critical magnetic fluctuations play a role for the development of superconductivity. We have noticed that the application of a magnetic field (along c) or a pressure induce similar enhancements of the quadratic coefficient A extracted from the temperature dependence of the resistivity. The electronic effective mass m*, which is proportional to the square root of A in a Fermi liquid description, varies thus in a similar manner at the magnetic field- and pressure-induced instabilities. Assuming that m* is dressed by the quantum magnetic fluctuations, both field- and pressure-induced transitions are thus associated with similar enhancements of the critical magnetic fluctuations, although their nature are expected to be different (critical antiferromagnetic fluctuations under pressure and critical ferromagnetic fluctuations in high field).

Fig. 2 : (H,T) phase diagram of the antiferromagnet CeRh2Si2 in high magnetic fields H || c and magnetic field and pressure dependences of the A coefficient.

Another work concerned the study of the itinerant ferromagnet UIr. Its magnetization was measured in high magnetic fields up to 60 Tesla (see Fig.3). Critical behavior, which surprisingly extends up to several Tesla, was observed at the Curie temperature TC = 45 K and was analyzed using Arrott and Maxwell relations. Critical exponents were found that do not match with any of the well-known universality classes. We found that the low-temperature magnetization Ms = 0.5 µB below 3 T rises towards higher fields and converges asymptotically around 50 T with the magnetization at TC. From complementary magnetostriction measurements, the uniaxial pressure dependences of TC was extracted using a new method (which is an alternative to the Ehrenfest relation), as well as uniaxial pressure dependences of Ms. These results may serve as a basis for understanding spin fluctuations in anisotropic itinerant ferromagnets.

aimantation UIr Fig. 3 : Magnetization of UIr in high magnetic field

Members of the lab implied in this activity :

Géraldine Ballon, William Knafo, Marc Nardone, Gernot Scheerer, David Vignolles, Abdelaziz Zitouni

Former members of the lab implied in this activity :

Evert Hannapel

Collaborations :

- Dai Aoki, Jacques Flouquet, Daniel Braithwaite, Georg Knebel, SPSMS/INAC/CEA-Grenoble

- Rikio Settai, Niigata University

- Pascal Lejay, Pierre Haen, Institut Néel, Grenoble (CNRS)

- Christoph Meingast, Frédéric Hardy, IFP/KIT-Karslruhe

- Tatsuma Matsuda, JAEA Tokai

Selected publications :

- W. Knafo, T. D. Matsuda, D. Aoki, F. Hardy, G.W. Scheerer, G. Ballon, M. Nardone, A. Zitouni, C. Meingast, and J. Flouquet, High-field moment polarization in the ferromagnetic superconductor UCoGe, Physical Review B, 86, 184416 (2012).

- G. W. Scheerer, W. Knafo, D. Aoki, G. Ballon, A. Mari, D. Vignolles, and J. Flouquet, Interplay of magnetism, Fermi surface reconstructions, and hidden order in the heavy-fermion material URu2Si2, Phys. Rev. B 85, 094402 (2012).

- High-field metamagnetism in the antiferromagnet CeRh2Si2, W. Knafo, D. Aoki, D. Vignolles, B. Vignolle, Y. Klein, C. Jaudet, A. Villaume, C. Proust, and J. Flouquet, Phys. Rev. B 81, 094403 (2010).

- Critical scaling of the magnetization and magnetostriction in the weak itinerant ferromagnet UIr, W. Knafo, C. Meingast, S. Sakarya, N.H. van Dijk, Y. Huang, H. Rakoto, J.-M. Broto, and H. v. Löhneysen, J. Phys. Soc. Jpn. 78, 043707 (2009).

- Antiferromagnetic criticality at a heavy-fermion quantum phase transition, W. Knafo, S. Raymond, P. Lejay, and J. Flouquet, Nature Phys. 5, 753 (2009).