Crystal field effect in the thermal expansion of hexagonal rare earth compounds

The low temperature thermal expansion in hexagonal rare earth materials exhibits crystal field effects. These are quantitatively explained with a magnetoelastic coupling of Γ₁-symmetry strains to a 1=2 quadrupolar operator of the magnetic ion. For PrNi₅ the temperature dependence of both c- and a-axis thermal expansion is correctly accounted for. For dhcp Pr both the cubic and the hexagonal sites of the Pr-ions contribute to the thermal expansion. The magnetoelastic coupling constants are an order of magnitude larger than for cubic compounds.     

Crystal field effect in the thermal expansion of cubic rare earth compounds

We report on evidence of crystal-electric-field effects in the low temperature thermal expansion of various rocksalt-type rare earth compounds (PrSb, SmSb, ErSb, TmSb, CeTe, TmTe). From our measurements we deduce the volume dependence of the energy splitting between the lowest crystal field levels and we calculate magnetoelastic coupling parameters involving the volume strain.     

Exchange in rare earth metals and intermetallic compounds

Analytical expressions of the isotropic and the anisotropic exchange integrals in rare earth (RE) metals and intermetallic compounds are derived using the augmented-plane-wave (APW) formalism. Numerical results for DyZn indicate that the d electrons contribute predominantly to both isotropic and anisotropic exchange at most points on the Fermi surface.     

Dependence of crystal field in rare earth garnets on the structure of coordination dodecahedron

The superposition model is used to relate the crystal field parameters reported in various rare earth garnets to the structure of the coordination dodecahedron. The results obtained confirm the overall consistency of the model, reported by other authors, but they differ in the dependence of intrinsic parameters upon distance. Special attention is paid to the approximations inherent to the procedure and to the utility of the model in quantitative predictions.     

Effective crystal fields in the rare earth pnictides

Various contributions to the effective 4f electron crystal fields in the rare earth pnictides are estimated and compared. These include the direct interaction with the ligand electrostatic field and effects associated with the 5d band electrons.     

Resonant energy transfer in stoichiometric rare earth compounds

A clear distinction is made between reversible and irreversible energy transfer. It is argued that low loss materials (stoichiometric compounds of rare earths) call for resonant, non-dissipative energy transfer describable in terms of spreading of excitonic wave packets rather than by Förster-Dexter theory. Difficulties concerning phonon collisions are recognized and suggestions for further theoretical and experimental work are made.     

Crystal field splitting of some rare earth intermetallic compounds with Cu3Au structure

Inelastic neutron scattering studies were performed in the paramagnetic phases of several rare earth compounds that crystallize in the cubic Cu₃Au structure: ErPb₃, ErTl₃, ErIn₃, HoPb₃, HoTl₃, HoIn₃, PrSn₃, PrPb₃, PrTl₃, PrIn₃, CeIn₃, La_1–c Pr _c Tl₃, and Pr(In_0.5Tl_0.5)₃. The energies, widths and intensities of the crystal field excitations are determined and discussed in terms of interactions between the rare earth ions. Variations of the crystal field parameters are observed across the series.Work partially supported by the BMFT     

Crystal field and exchange interactions in rare earth transition metal salen compounds containing bipyridine, pyrazole complexes

Crystal field parameters for Pr³⁺ in {[Ni(salen)Pr-(hfac)₃](H₂O)} (noted as NiPr) and {[Ni(salen)Pr(hfac)₃(pyr)]-(CHCl₃)} (noted as NiPrpyr) have been found from a fit to the thermal variation in the magnetic susceptibility of NiPr and NiPrpyr. The nature of exchange interaction in [Cu(salen)Pr(hfac)3(pyr)] (noted as CuPrpyr), {[{Cu(salen)Pr(hfac)₃}₂(pyz)](H₂O)₃} (noted as Cu₂Pr₂pyz) and {[{Cu(salen)Pr(hfac)₃}₂(bpy)]-(CHCl₃)₂} (noted as Cu₂Pr₂bpy, bpy=4,4_-bipyridine) have been found using the derived results for NiPr and NiPrpyr. All the exchange interactions give significant contribution to the thermal variation in magnetic susceptibility below 50 K. The contribution due to Pr-Cu interaction is positive while that of the Cu-Cu and Pr-Pr interactions are negative. The behaviors below 10 K for Pr-Cu and Pr-Pr are difficult to explain, and point to a possible change in structure of CuPrpyr, Cu₂Pr₂pyz and Cu₂Pr₂bpy below 10 K. The theoretical thermal variations in the magnetic specific heat of NiPr and NiPrpyr are computed and discussed.     

A phenomenological study of superconductivity in rare earth compounds

The effect of critical magnetic scattering is studied in paramagnetic phase of rare earth compounds showing superconductivity and magnetic order. If the exchange coupling between the conduction electrons and the magnetic ions is small the system becomes superconducting between two transition temperatures. The lower one is slightly higher than the magnetic ordering temperature and this is due to the critical magnetic scattering of the conduction electrons. The influence of the lifetime of one electron on the phase transition is briefly discussed.     

Magnetic properties of equiatomic rare earth cadmium compounds

The magnetization processes obtained in monocrystalline samples are presented for the ferromagnetic compounds of the equiatomic rare earth cadmium series. They are discussed in terms of crystalline electric field and bilinear and quadrupolar pair interactions. A close similarity appears with the isomorphous series with zinc. The importance of quadrupolar interactions is emphasized. They are, for instance, strong enough in DyCd to enforce the fourfold axis as easy magnetization direction instead of the threefold one which would be driven by the only crystalline electric field. A discussion of the magnetic properties of dysprosium compounds having the CsCl-type structure is then given.     

The electric field gradient in heavy rare earth metals

Estimates of the electric field gradient in heavy rare earth metals have been evaluated from experimental hyperfine interaction data. In addition, the magnetic hyperfine fields are analyzed. In the metals the effective radial integrals 〈r ^?3〉_4f of the magnetic and quadrupole hyperfine interaction are reduced at most by 10% compared with the free ion values. The electric field gradients due to the crystalline field have been found to be 200 times larger than those predicted from point charge calculations. This antishielding effect can be explained by an enhanced conduction electron density at the interstitial sites and an increase of the Sternheimer factor γ_∞ in the metallic environment.     

Renormalization of transversal phonons in mixed valent rare earth compounds

The possibility of a softening of transversal acoustic phonons in mixed valent rare earth compounds is demonstrated. The effect should be largest at the zone boundary.     

Susceptibility of rare earth compounds in the intermediate valence state

The thermodynamic potential of rare earth compounds in a mixed valent state is investigated by perturbation theoretical methods. Two types of contributions must be distinguished, the impurity terms and terms corresponding to interactions between different rare earth sites. For a large class of substances the latter are shown to be of minor importance. The susceptibility of the resulting theory is compared directly with the renormalization group calculations of Krishna-murthy, Wilson and Wilkins and with measured susceptibilities of-Ce, SmB₆, YbAl₃.     

Anomalous thermoelectric power of rare earth metals and their compounds

We propose a new mechanism for an anomalous thermoelectric power (ATP) in the paramagnetic state of certain rare earth metals and their compounds, in which the ions possess a nonmagnetic ground state in a given crystal field. The ATP is found to be due to higher order inelastic scattering (second Born approximation) of the conduction electrons by the crystal field split rare earth ions. It has a peak at a temperatureΔ/3 ~Δ/2, whereΔ is the splitting energy between the ground state and the first excited state. Our main result is that the appearance of an ATP requires interactions between the conduction electrons and the ions of other than the simple isotropic exchange type. This implies that the ATP may serve as a valuable tool to detect more complicated types of thek-f interaction than the isotropic exchange.     

Crystal field effects on the magnetic structures of the rare earth compounds with the FeB structure

In order to confirm the role of the crystalline electric potential on the stability of non collinear magnetic structures of the rare earth compounds with the FeB-type structure, the magnetic properties of the (Gd_0.5Y_0.5)Ni compound, where the rare earth orbital moment is nul, are studied. Below its Curie temperature (57 K) the compound is ferromagnetic. The spontaneous magnetization at 0 K reaches 7.05 μ_B per gadolinium atom. Yttrium and nickel atoms being not magnetic the gadolinium moments are parallel and the exchange interactions are positive. Then the non collinear magnetic structures observed when the alloyed rare earths have an orbital moment result from the competition between a multiaxial anisotropy due to the crystal field effects and isotropic exchange interactions of the Heisenberg type.     

Magnetic field induced crystallographic domains in rare earth vanadates and arsenates

Crystallographic domains in the Jahn—Teller distorted phase are observed directly in linearly polarised light. The dependence of their shape on magnetic fields is investigated, and the results are compared with magnetisation and spectroscopic measurements.     

Low temperature properties of valence fluctuating cerium impurity with spin-orbit and crystal field splittings

The Anderson Hamiltonian, with spin-orbit and cubic crystal-field splittings, is studied using Brillouin-Wigner perturbation theory. The ratio between the low temperature paramagnetic susceptibility and the electronic specific heat coefficient is reported for both intermediate valent and Kondo regimes. We find that finite spin-orbit effect is not negligible in Ce systems in the intermediate valent regime. In the nearly integral valent case, for small Kondo temperatureT _K, we also find that even for relatively large crystalfield splitting (10T _K), if the free ion ground state is of ₇ symmetry, the effect of the ₈ level is still large.     

Transverse fieldμ + SR investigations of rare earth substituted 1-2-3 compoundsSR investigations of rare earth substituted 1-2-3 compounds

Transverse field studies (0.3 T) in HoBa₂Cu₃O₇, DyBa₂Cu₃O₇ and ErBa₂Cu₃O₇ (x=6.40. 6.45, 6.53, 7) were undertaken with the aim of detecting a possible 4f-moment inducedμ ⁺ Knight shift which could elucidate the nature of the magnetic coupling among the 4f-moments and a possible interference with superconductivity. AboveT _c no shift could be detected in any of the systems except in HoBa₂Cu₃O₇ where a split signal appears involving a positive and a negative shift. Unexpected shifts may be present belowT _c .