D. Schmitt

Magnetic properties of RNi2Si2 compounds (R= rre earth)

Abstract We report an extensive study of the magnetic properties of tetragonal RNi 2 Si 2 compounds (R=Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm), through resistivity, neutron diffraction, susceptibility and magnetization experiments. All compounds exhibit complex incommensurate antiferromagnetic structures, while a transition occurs in TbNi 2 Si 2 between a modulated phase and a simple antiferromagnetic structure, stable at low temperature. The magnitude of the bilinear exchange interactions deviates from the Gennes law and the direction of the ordered magnetic moments presents anomalies across the series, including the probable existence of other types of interactions between the rare earth ions.

Magnetic and quadrupolar properties of PrPb3

Abstract The cubic rare earth intermetallic compound PrPb 3 (AuCu 3 -type structure) undergoes a transition at 0.35 K: as it is a Van Vleck paramagnet, the low temperature phase was assumed to be quadrupolarly ordered. In order to specify this possibility, we first discuss the cubic level scheme using specific heat and first-order magnetic susceptibility results; afterwards we present an extensive study of the two (tetragonal and trigonal) symmetry lowering modes, investigated in the cubic paramagnetic phase by means of parastriction and third-order magnetic susceptibility. For the tetragonal symmetry, the quadrupolar pair interactions appear as negative, dominating the magnetoelastic coupling; as a result, the low temperature ordering may be antiferroquadrupolar, a new situation among rare earth intermetallics, the TmZn and TmCd ordering being ferroquadrupolar.

Experimental study of super-rotation in a magnetostrophic spherical Couette flow

We report measurements of electric potentials at the surface of a spherical container of liquid sodium in which a magnetized inner core is differentially rotating. The azimuthal angular velocities inferred from these potentials reveal a strong super-rotation of the liquid sodium in the equatorial region, for small differential rotation. Super-rotation was observed in numerical simulations by Dormy et al. (Dormy, E., Cardin, P. and Jault, D., MHD flow in a slightly differentially rotating spherical shell, with conducting inner core, in a dipolar magnetic field, Earth Planet. Sci. Lett., 1998, 160, 15--30). We find that the latitudinal variation of the electric potentials in our experiments differs markedly from the predictions of a similar numerical model, suggesting that some of the assumptions used in the model – steadiness, equatorial symmetry, and linear treatment for the evolution of both the magnetic and velocity fields – are violated in the experiments. In addition, radial velocity measurements, using ultrasonic Doppler velocimetry, provide evidence of oscillatory motion near the outer sphere at low latitude: it is viewed as the signature of an instability of the super-rotating region.

Rotating spherical Couette flow in a dipolar magnetic field : experimental study of magneto-inertial waves

The magnetostrophic regime, in which Lorentz and Coriolis forces are in balance, has been investigated in a rapidly rotating spherical Couette flow experiment. The spherical shell is filled with liquid sodium and permeated by a strong imposed dipolar magnetic field. Azimuthally travelling hydromagnetic waves have been put in evidence through a detailed analysis of electric potential differences measured on the outer sphere, and their properties have been determined. Several types of waves have been identified depending on the relative rotation rates of the inner and outer spheres: they differ by their dispersion relation and by their selection of azimuthal wavenumbers. In addition, these waves constitute the largest contribution to the observed fluctuations, and all of them travel in the retrograde direction in the frame of reference bound to the fluid. We identify these waves as magneto-inertial waves by virtue of the close proximity of the magnetic and inertial characteristic time scales of relevance in our experiment.

Competition between multi-q antiferromagnetic structures in cubic rare earth-copper compounds

Abstract The cubic (CsCl-type structure) compounds HoCu, ErCu and TmCu have been investigated by means of specific heat and neutron diffraction experiments. In the three compounds, there exists a competition between various multi- q antiferromagnetic structures (modulated, multiaxis, or simple collinear). At low temperature, they order within the (Π, Π, 0)-type structure. In ErCu and TmCu, this structure is replaced, near the Neel temperature, by a more complex one, which results from the superimposition of the preceding one and of an amplitude-modulated structure; the incommensurate propagation vector is q mod = ( 1 2 ± τ, 1 2 , 0) 2Π a with τ = 0.04 ( ErCu ) and 0.06 (TmCu). The situation is different HoCu where two changes in the magnetic moment direction successively occur between the [111] and [101] directions. A first analysis of these peculiar magnetic behaviours is given starting from the crystalline electric-field level scheme and may indicate the possible existence of exchange interactions other than the isotropic bilinear ones.

CeCu4Ga: A high γ heavy fermion compound

Abstract In the search for new Ce intermetallics which display unusual physical properties we have identified CeCu4Ga as a new example of a heavy fermion compound. CeCu4Ga is characterized by an extremely enhanced electronic contribution to the specific heat which exceeds 3000 mJ/mol K2 at 0.9 K. The Ce ion in this hexagonal compound is nearly trivalent. The Kondo temperature estimated from low temperature susceptibility and specific heat data is of the order of a few kelvin.

Metamagnetism and thermodynamical properties in modulated systems modelisation and application to PrNi2Si2

Abstract A periodic field model taking into account explicitly the crystal field anisotropy is presented, in order to describe the thermodynamical properties and the magnetization process of incommensurate modulated magnetic systems. An application is made to the tetragonal PrNi 2 Si 2 compound, where the crystal field and exchange parameters have been unambiguously determined, from a careful joint analysis of specific heat, magnetization, magnetic susceptibility and inelastic neutron scattering experiments. The ground state is a nonmagnetic singlet well isolated from another singlet and a magnetic doublet as the first two excited states. The existence of an amplitude modulated magnetic structure persisting from T N = 20 K down to 0 K is then well explained. It is shown that the periodic field model does not require any additional parameter to account quantitatively very well for the observed properties in the modulated ordered phase, in particular the variation of the specific heat and the metamagnetic process along the [001] easy magnetization direction.

Magnetic properties of PrNi2Si2

Abstract Resistivity and magnetic measurements as well as neutron diffraction studies have been performed on the tetragonal PrNi2Si2 compound. Below TN = 18 K a colinear modulated antiferromagnetic structure with Qm = (0, 0, 0.87) is observed down to the lowest temperature investigated. Mi is non-magnetic and Pr moments are along the c-axis with a maximum value of 2.6 ± 0.1μB at 5.5 K. Magnetic measurements performed on a single crystal reveal a large uniaxial magnetocrystalline anisotropy. From the anisotropy of the paramagnetic susceptibility, the second-order crystal field parameter V 2 0 = + 190 K . is deduced. The stability of the modulated structure at low temperature should be associated with a crystal field splitting of the Pr3+ multiplet which gives rise to a non magnetic singlet ground state.

FIELD INDUCED MAGNETIC STRUCTURES IN TBNI2SI2

Abstract A neutron diffraction study under magnetic field has been performed on a TbNi 2 Si 2 single crystal. At low temperature, two intermediate magnetic structures have been determined between the zero-field simple antiferromagnetic structure and the field-induced ferromagnetic one. The first one is an antiphase incommensurate structure, the second an amplitude modulated one. A complex phase diagram H-T has been evidenced, revealing just below T N a zone with a devils staircase-type behaviour. These quite exciting properties underline the competition between crystal field and magnetic interactions.

A crossover from the magnetic Kondo compound CeCu5 to the heavy Fermion compounds CeCu4Al and CeCu3Al2

Abstract We present measurements on the isostructural series CeCu 5 , CeCu 4 Al and CeCu 3 Al 2 , which demonstrate the supression of the antiferromagnetic phase transition at 4 K in CeCu 5 , due to the substitution of Cu by Al in CeCu 4 Al and CeCu 3 Al 2 . This substitution, however, is linked to a dramatic enhancement of properties reflecting the electronic density of states.