Interactions In Rare-earth Compounds Research Articles

New excitement with crystal-field excitations

The crystal field is one of the major interactions in rare-earth compounds. Neutron spectroscopy has become the key tool to measure the crystal-field transitions in metallic systems. This has been demonstrated for almost 1000 metallic rare-earth compounds in the past 30 years which resulted in a detailed understanding of the various physical effects caused by the crystal-field splitting. One may conclude that the determination and description of crystal fields in metallic rare-earth systems is now well established and has become a standard technique. Yet the past years have seen exciting developments in different applications where the crystal-field concept attained increasing and sometimes even crucial importance. This is exemplified for two applications: Firstly, the novel principle for cooling by adiabatic pressure application which is based on the occurrence of a pressure-induced structural and/or magnetic phase transition where the point symmetry at the rare-earth site is changed involving a change in the degeneracy of the crystal-field states. Secondly, the observation of anomalies in the linewidth of crystal-field transitions in high-temperature superconductors which reveals direct information on the doping and isotope dependence of the pseudogap.

Intersublattice interactions in rare earth compounds of the types R 2(Co,Al) 17 and R 2(Fe,Al) 17

The field dependence of magnetization at 4.2 K was studied in fields up to 35 T for the pseudobinary compounds R 2Co 17− x Al x (R = Gd, Tb, Dy, Ho and Er) and Er 2Fe 17− x Al x . Values for the intersublattice coupling strength were derived from a mean field analysis of the magnetic isotherms. For the series Er 2Fe 17− x Al x , we determined the concentration dependence of the intersublattice coupling strength.

Exchange interactions in rare earth compounds

The development of studies on rare earth compounds is reviewed by focusing on the exchange mechanism and its various aspects. Two fundamental interactions, namely intra-atomic Coulomb-exchange and interatomic mixing, are introduced and it is shown how they differ in character. Their different effects in metals, semimetals and insulators are ascribed to differences in the electronic structure. Some new developments are also briefly described.

Theory of phonon linewidths caused by magnetoelastic interactions in rare-earth compounds

A theory is presented of phonon linewidths in rare-earth insulators where the broadening is caused by interactions with the crystal-field levels of the 4f electrons. The projection-operator method is used to obtain results for any crystal-field level scheme to fourth order in the coupling coefficients. For the special case of two singlet levels a comparison is made with previous work on paramagnetic salts. The present theory is applied to a Gamma 7, Gamma 8 level scheme to illustrate the results.

Magnetic phase transitions in presence of bilinear and quadrupolar interactions in rare earth compounds

A Landau expansion of the free energy in the non-ordered phase of cubic rare earth intermetallics is obtained from an analytical susceptibilities formalism, taking into account the exact crystalline electric field. The deduced magnetic dipolar and quadrupolar phase diagram closely agrees with experimental observations.

Third-order magnetic susceptibility as a new method for studying quadrupolar interactions in rare-earth compounds

We present a new method based on the detailed analysis of the magnetization curve in the paramagnetic range for studying the quadrupolar interactions in cubic rare-earth intermetallic compounds; the third-order magnetic susceptibility characterizes the anisotropic curvature of the magnetization curve. It corresponds to the ${H}^{3}$ coefficient in the odd field development of the magnetization. This high-order coefficient receives a contribution from the induced quadrupolar moment which varies as ${H}^{2}$. Thus studied in the nonordered (cubic and paramagnetic) range with magnetic fields applied along the [001] and [111] directions, the third-order magnetic susceptibility provides information on the quadrupolar interactions associated with the two tetragonal and trigonal symmetry-lowering modes. Using perturbation theory and mean-field approximation, we give analytical expressions for the third-order magnetic susceptibility in the presence of a crystalline electric field, bilinear exchange, and quadrupolar (exchange and magnetoelastic) interactions. Application to ${\mathrm{Tm}}^{3+}$ cubic intermetallic compounds, for which large quadrupolar interactions have been shown to exist, is then given, illustrating the reliability of the method.

Parastriction: A new probe for quadrupolar interactions in rare-earth compounds

We present a new method called parastriction for investigating the quadrupolar interactions in cubic rare-earth intermetallic compounds. Parastriction measures the rare-earth quadrupolar moment induced by an applied magnetic field through the anisotropic deformation of the lattice. The magnetoelastic coefficients associated with the two principal strain modes (tetragonal and rhombohedral) are thereby obtained. The quadrupolar exchange parameters may be evaluated without any ambiguity for compounds with low magnetic-ordering temperature. Application to the heavy rare-earth---zinc compounds is then given.

Multipolar interactions in metallic rare-earth compounds

The existence of multipolar pair interactions in rare-earth compounds is well established but it has been difficult to determine the origins of these couplings in specific systems. Here we review some of the evidence and origins of these interactions.

Large Quadrupolar Interactions in Rare-Earth Compounds

Several series of rare-earth compounds have been found to undergo structural phase transitions. For most of them the cause has been the cooperative Jahn-Teller effect. Here we point out that for TmZn and TmCd these transitions are not due to the cooperative Jahn-Teller effect. Rather they are driven by quadrupolar pair interactions. In these systems the lattice follows the quadrupoles and distorts in such a way as to minimize the Jahn-Teller coupling energy.

Ion-lattice interactions in rare earth compounds

A study is made using a variational method, of the interaction of a dysprosium ion in an axially symmetric crystal field with a lattice distortion which gives a coupling of type epsilon Q(J+2+J-2). It is shown that a low energy configuration can be obtained in which the dysprosium moment lies in, or near, the perpendicular plane is accompanied by static strain in the lattice. Several orientations of moment plus strain have equal energies. It is then shown that in a magnetically concentrated crystal there will be mutual energies coming from the strains, and it is suggested that the unusual properties of dysprosium vanadate can be understood as arising from ordering of the lattice strains at low temperatures.

Optical investigation of hyperfine interactions in rare earth compounds

A Fabry-Pérot-interferometer is used for the study of optical transitions in La(Ho)Cl 3 and Ca(Eu)F 2. A magnetic hyperfine splitting, a quadrupole splitting and an isotope shift are observed.