J. Voiron

Metamagnetic CeRu2Si2: New perspectives for heavy-fermion studies in high fields

Abstract Interesting magnetic effects have recently been reported to occur in the normal phase of heavy-fermion systems. Among them are the metamagnetic properties of UPt 3 and CeRu 2 Si 2 ( H M = 20 and 8 T). High-pressure magnetization and magnetoresistance, as well as magnetostriction measurements have been performed on CeRu 2 Si 2 . The magnetostriction data imply a remarkable scaling behavior for the magnetization as a function of P and thus a relation between H M and the characteristic Fermi-liquid energy k B T ∗ . The magnetoresistance suggests that the field enhancement of the effective mass derived from the specific-heat γ( H ) leads to the observed transition at H M for a critical value of m ∗ which is pressure independent.

The magnetic instability in the heavy fermion compounds Ce1?x La x Ru2Si2

The magnetization and the specific heat of Ce1−xLaxRu2Si2 with x≤0.13 are reported with special attention to the effect of magnetic field and the role of lanthanum doping. Evidence is given of differences between the undoped (x=0) and the solid solution (x≠0) cases. A common feature is the occurrence of well-defined anomalies in the magnetization at the “metamagnetic” fieldHM independently of whether the ground state is one of long-range order or Pauli paramagnetism. Forx=0, the ground state appears to be a Pauli paramagnet for any strength of the magnetic field; quantum fluctuations or deviations from an ideal lattice may prevent the occurrence of a true static magnetic transition.

High-field magnetostriction in the pseudo-metamagnetic heavy-fermion system CeRu2Si2

Low-temperature magnetostriction experiments have been performed on CeRu2Si2 single crystals in magnetic fields up to 15T. WhenH is applied parallel to the tetragonalc-axis, a very large expansion is observed in all lattice directions. A pronounced anomaly occurs at a field close to the “metamagnetic” threshold. The results are discussed with use of thermodynamic relations and support a very simple scaling behavior for the pressure dependence of the magnetization.

The magnetic properties of FexCo1-x and FexTi1-x alloys

Abstract High pressures and high magnetic field behaviour give indications on the nature of magnetism encountered in transition metal alloys. We present experiments performed under pressure up to 7 kbar, in fields up to 150 kOe, between 1.4 K and 300 K. The results are used to discuss the occurence of ferromagnetism in Fe x Co 10 x Si and Fe x Co 1- x Ti systems. These systems exhibit two critical concentrations for the onset of ferromagnetism. Their magnetic properties are very sensitive to magnetic fields and high pressures and this allows us to characterize them as weak itinerant ferromagnets. A more localized behaviour is evidenced for the iron rich alloys and the appearance to ferromagnetism is different for cobalt rich side and for iron rich one.

HIGH MAGNETIC-FIELD BEHAVIOR OF MEM-(TCNQ)2

Abstract The charge-transfer complex MEMue5f8(TCNQ)2 is a spin-Peierls system with a non-magnetic, singlet ground state at T=0. We report high-field magnetization data which provide some evidence for a new magnetic spin-Peierls phase at fields above 190 kOe. The experimental results are compared to those for TTFue5f8CuBDT.

First order magnetic transition induced by pressure in CeNi

Abstract It is shown that the intermediate valence compound CeNi exhibits a γ → α first-order transition associated with a huge and anisotropic change of lattice parameters. This transition, previously observed by susceptibility and resistivity, is strongly anisotropic because of the covalent character of the Ce-Ni hybridization responsible for the Ce intermediate valence state.

Magnetoelastic effects in the spin reorientation region of single crystal NdCo5

Abstract The temperature dependence of the single crystal elastic constants of NdCo 5 reveal the presence of a significant magnetoelastic effect associated with the magnetisation reorientation. The effect of uniaxial and hydrostatic pressures on this reorientation is also reported and reveals the strong dependence of the exchange interaction or c -axis atomic separation.

Resistivity and thermoelectric power of YbCu4.5 under very high pressure

Transport properties of YbCU4.5 at zero pressure and under high pressure are reported. The resistivity is strongly perturbed by the disorder of the crystallographic structure, showing notably high residual resistivity and anisotropy at low temperature. Several samples have been measured at zero pressure and results are dominated by the Kondo effect: resistivity anomaly and giant thermoelectric power. Crystal field effect is taken into consideration to account for resistivity data. Under pressure, the decrease of the Kondo temperature have been observed with two experiments, up to 17 and 93 kbar. The possible appearance of a magnetic ground state under pressure is discussed.

Pressure effects on the metamagnetic transition of ThCO5

Abstract We present high pressure magnetization measurements performed at low temperature on a single crystal of ThCo 5 . This compound exhibits under an applied magnetic field a sharp itinerant electron metamagnetic transition. A large increase of the critical field is observed with the applied pressure. Results are well described within the itinerant electron model

A polarized heavy fermion system: CeRu2Si2

Abstract High-pressure magnetoresistance and susceptibility measurements have been performed on CeRu2Si2 single crystals. The metamagnetic-like transition at 4.2 K shifts rapidly from 85 kOe at P = 0 to ≈180 kOe at 5.4 kbar. This variation correlates with the pressure decrease of X4.2, suggesting that the transition occurs for a critical value of the magnetization, in contrast to the predictions of an almost localized Fermi liquid theory.