Orbital frustration at the origin of the magnetic behavior in LiNiO2

We report on the ESR, magnetization and magnetic susceptibility measurements performed over a large temperature range, from 1.5 to 750 K, on high-quality stoichiometric LiNiO2. We find that this compound displays two distinct temperature regions where its magnetic behavior is anomalous. With the help of a statistical model based on the Kugel'-Khomskii Hamiltonian, we show that below T_of ~ 400 K, an orbitally-frustrated state characteristic of the triangular lattice is established. This then gives a solution to the long-standing controversial problem of the magnetic behavior in LiNiO2.Comment: 5 pages, 5 figures, RevTex, accepted in PR

The effect of uniaxial pressure on the magnetic anomalies of the heavy-fermion metamagnet CeRu2Si2

The effect of uniaxial pressure (P_u) on the magnetic susceptibility (X), magnetization (M), and magnetoresistance (MR) of the heavy-fermion metamagnet CeRu2Si2 has been investigated. For the magnetic field along the tetragonal c axis, it is found that characteristic physical quantities, i.e., the temperature of the susceptibility maximum (T_max), the pagamagnetic Weiss temperature (Q_p), 1/X at 2 K, and the magnetic field of the metamagnetic anomaly (H_M), scale approximately linearly with P_u, indicating that all the quantities are related to the same energy scale, probably of the Kondo temperature. The increase (decrease) of the quantities for P_u || c axis (P_u || a axis) can be attributed to a decrease (increase) in the nearest Ce-Ru distance. Consistently in MR and X, we observed a sign that the anisotropic nature of the hybridization, which is believed to play an important role in the metamagnetic anomaly, can be controlled by applying the uniaxial pressure. PACS numbers: 75.20.Hr, 71.27.+a, 74.62.FjComment: 7 pages, ReVTeX, 6 EPS figures : Will appear in Phys. Rev.

Variation avec la pression des points de transition magnétiques des composés TCo2 entre le cobalt et les terres rares

An apparatus for the determination of the variation with pressure of the magnetic ordering temperatures in the one hundred kilobar range is described. Temperature can between 4 and 300 °K. This apparatus is used for the determination of the effect of pressure on the first order magnetic transition temperature between ferro- or ferri-magnetic state and paramagnetic state incountered in the Laves phase compounds TbCo2, DyCo2, NdCo 2, HoCo2 and ErCo2.On décrit un dispositif expérimental permettant de déterminer la variation avec la pression des points de transition magnétiques situés entre 4 et 300 °K, dans le domaine des pressions jusqu'à 100 kbar. On utilise ce dispositif pour l'étude de la variation avec la pression des transitions magnétiques du premier ordre entre les états ferro- ou ferrimagnétique et paramagnétique dans les composés à structure de Laves TbCo2, DyCo2, NdCo2, HoCo2 et ErCo2

Développement d'un logiciel d'acquisition et de traitement pour l'imagerie spirale

L'imagerie spirale par sa faible sensibilité aux artefacts de flux et de mouvement est la technique de choix pour l'imagerie de flux et pour l'imagerie fonctionnelle. Un logiciel a été développé, qui gère la modélisation de la forme des gradients, l'acquisition (intégrée au logiciel d'exploitation du système), la reconstruction des images ainsi que les corrections des trajectoires d'échantillonnage et des inhomogénéités du champ magnétique. Sa facilité d'utilisation en fait un outil adapté aux applications médicales