J. Rouchy

Magnetic and quadrupolar properties of DyCu and related dysprosium cubic compounds

Abstract The study of the magnetoelastic properties of DyCu allows us to show unambiguously that the magnetic structure is of the triple-q type. It consists of 4 pairs of ferromagnetic spins pointing along each of the threefold axes. The lack of any spontaneous magnetostriction in spite of a large magnetoelastic coupling agrees with the cubic symmetry of triple-q structure, which is shown to be stabilized by negative quadrupolar interactions of trigonal symmetry. A coherent analysis of the complex magnetization processes for magnetic fields applied along the three main crystallographic directions is then proposed. A comparison with other related compounds within the CsCl-type structure, DyAg, DyZn and DyCd reveals the important role of the quadrupolar interactions, in connection with the crystalline electric field level schemes. The crystal field favours the threefold axes as the easy magnetization direction for all the four compounds. In DyAg and DyCu, the addition of negative quadrupolar interactions of trigonal symmetry leads to observed non-collinear structures; in DyZn and DyCd, the positive tetragonal quadrupolar interactions are strong enough to establish a collinear arrangement along a fourfold axis.

Elastic properties of ferromagnetic single crystal with cubic symmetry

A group theoretical description is given of the first and second order magnetoelastic coupling for cubic ferromagnets. First, this coupling is expanded as a series of the Lagrangian tensor componentsηij, which definesN coefficients. An alternative expansion is given in terms of the symmetrical (εij) and antisymmetrical (ωij) components of the homogeneous strainsuij, which definesN(>N) coefficients. The conditions for rotational invariance of the energy are then written: they provide relations which reduce fromN toN the number of independent coefficients in the 2nd description. These relations were previously derived in the framework of the localised model and some features of this model are briefly discussed here: in particular, the exchange contributions to the elastic constants are corrected for volume effects and differ significantly from Fuchs and Satos formulae. In the next step, the magnetostriction is taken into account to derive the internal energy of a distorted crystal. It is then possible to analyse the ultrasonic waves propagation under high magnetic fields in terms of three main magnetic contributions, namely an isotropic exchange effect, an anisotropic morphic effect and a field dependent Simon effect.

Magnetic and magnetoelastic properties of GdZn single crystals

Abstract The magnetic and magnetoelastic properties of GdZn have been investigated by various ways: magnetization, magnetic susceptibility, magnetocaloric effect, pressure dependence of the Curie temperature, spontaneous magnetostriction, parastriction and sound velocity measurements. The γ-mode strain is large for an S-state ion, and two orders of magnitude larger than the ϵ-one. Both the first-order B γ,2 and the three isotropic second-order magnetoelastic coefficients exhibit a m 2 (1 − 0.5 m 2 ) thermal variation (where m ( T, H ) is the reduced magnetization), which differs from the classical behaviour. All the results suggest a noticeable contribution of the conduction band to the magnetism of GdZn.

Magnetoelasticity and quadrupolar interactions in TmGa3

Abstract Magnetoelastic and magnetic studies in the paramagnetic phase of TmGa3 have allowed us to determine both the magnetoelastic coefficients and the strength of the ferroquadrupolar pair interactions. For the tetragonal and trigonal symmetries, these interactions are too weak to explain the first-order transition observed at 4.29 K as a ferroquadrupolar ordering. On the contrary, antiferroquadrupolar interactions appear to be dominant and to stabilize a multiaxial spin structure. In comparison with previous studies on CsCl-type rare earth intermetallics, the magnetoelasticity and the quadrupolar pair coupling appear to be smaller in the AuCu3-type compounds.

Magnetic and magnetoelastic properties of

The rare-earth oxide compound (tetragonal zircon structure) is investigated using the extended susceptibility formalism, which considers at any temperature all of the features of the crystalline electric field within the ground-state multiplet in the joint analysis of the magnetic, magnetoelastic and elastic properties. The magnetoelastic coefficients are determined from third-order magnetic susceptibility, parastriction and elastic constant measurements for the different symmetry modes. The dominant magnetoelastic coupling is associated with the orthorhombic -symmetry; however, tetragonal and symmetry-lowering modes are also observed to give rise to noticeable effects, in particular in the thermal expansion. The close coherency between the magnetoelastic coefficients in and is then emphasized: the tetragonal magnetoelastic coefficients are sizeable and all of the magnetoelastic couplings keep the same sign and order of magnitude in the two phosphates. This coherency appears to be valid throughout the large family of rare-earth zircons. Through these two examples, the occurrence of a quadrupolar ordering is then discussed as being governed by the crystalline electric field, and the role of undercritical quadrupolar interactions is emphasized in .

A 6 T ultrasonic study of the magnetoelastic coupling in ferromagnetic Co—Pt alloy

Abstract The theoretical description of the sound-wave propagation in ferromagnets is briefly summarized. An original apparatus is then described which allows a 6 T magnetic field to be applied either paralle1 to the sound propagation k or rotating in the plane perpendicular to k ; this provides a convenient method for studying Simon, morphic and rotational effects. Some preliminary experiments have been performed on single crystals of disordered Co—Pt alloy, and the six morphic coefficients determined.

Magnetoelastic properties of PrAg

Abstract Parastriction and ultrasonic velocity experiments have been performed in order to have a better knowledge of the magnetic and quadrupolar interactions in PrAg, an intermetallic compound (CsCl-type structure) which exhibits puzzling magnetic properties, extensively studied in the recent past. As a first result, the existence of a ferromagnetic transition at T c = 7.3 K below the Neel temperature ( T N = 10.5 K ) is confirmed on monocrystalline samples. In all the ordered range, the magnetic structure is proved to be multiaxial: first the knowledge of the magnetoelastic coefficients allows us to rule out the collinear structure, explaining the lack of any spontaneous strain. On the other hand, the quadrupolar interactions, found weakly negative, favour an antiferroquadrupolar arrangement in agreement with a multiaxial spin ordering. The stability of the CsCl-type lattice at low temperature is then discussed in connection with the instability features observed in neighbouring compounds LaAg and CeAg. It appears that in these three compounds many origins may be simultaneously proposed for the instability tendency: an electronic deformation potential coupling, a ∈ 3 strain coupling with zone boundary phonon and the magnetoelastic coupling. PrAg appears to be clearly more stable than the other two compounds.

Magnetic and quadropolar properties of the TmMg compound

Abstract The Crystalline Electric Field, the bilinear and quadropolar pair interactions as well as the magnetoelasticity have been studied in the CsCl-type structure compound TmMg by means of neutron spectroscopy, magnetization, parastriction and elastic constants experiments. Each of these couplings is then analysed in comparison with previous determinations for other isomorphous thulium compounds.

The magnetoelastic coupling in nickel

Abstract The thermal variations of the first-order magnetoelastic coupling coefficients. Bγ,2 and Bϵ2, have been measured accurately in the whole ferromagnetic range. Bϵ,2 is found to vary as m2 + m8 where m is the reduced magnetization: the magnetization of nickel as given by various authors is discussed and found to be somewhat inaccurate above room temperature. B γ,2 m 2 remains constant up to 340 K and decreases drastically above: this unexpected behaviour has been carefully checked and raises new questions about the band structure of nickel; we compare these data with the results observed in a Ni 3.3 at% Ru single crystal. The second-order magnetoelastic coefficients have been also investigated: the pulse echo method under a magnetic field up to 60 kOe rotatable in any direction allowed us to observe for the first time the morphic effect in nickel. Anharmonic contributions to this effect have been shown to be far from negligible.

Interplay between quadrupolar and bilinear interactions in rare earth intermetallics

A survey of the properties of 4f quadrupolar moments in rare earth intermetallic compounds is proposed. The double nature, one-ion magnetoelasticity and pair interactions, of the quadrupolar couplings as well as the different types of ferro-and antiferroquadrupolar orderings are first reviewed. The quadrupolar consequences on the magnetic properties, such as the order of the magnetic transition and the nature of the magnetic structure are then discussed. This interplay between quadrupoles and spins is illustrated by the competitions between collinear and multiaxial structures or between commensurate and modulated ones, sometimes in a same compound.