Crystal electric field excitations in ferromagnetic CeTX compounds

A ferromagnetic ground state was identified for the compounds CeCuGe (T_C=10 K), CeCuSi (T_C=15 K) [F. Yang, et al., J. Appl. Phys. 69 (1991) 4705] and CeAuGe (T_C=10 K) [R. Pottgen, J. Magn. Magn. Mater. 152 (1996) 196]. The observed saturation magnetic moment values at low temperatures for all three compounds are considerably less than the theoretically expected value g_JJ=2.14μ_B for the free Ce³⁺ ion involving the entire six-fold J=5/2 multiplet, and thus provide a first indication of partial lifting of the f-electron level degeneracy in these compounds. Specific heat data yield crystal electric field (CEF) excitation energies (Δ_Sch) equivalent to 140 K for CeCuGe, 110 K for CeCuSi and 280 K for CeAuGe. To confirm the presence of CEF excitations directly, we have carried out inelastic neutron scattering (INS) measurements on all three compounds, using the HET spectrometer at ISIS Facility. Here, we present a detailed analysis of the INS spectra of CeCuSi on the basis of a CEF model and the detailed analysis of the INS of the other two compounds will be reported elsewhere.     

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.     

Crystal field in some light rare-earth compounds with the cubic CsC1 structure

Neutron spectroscopy experiments were performed on the cubic CsC1-type rare-earth intermetallics Pr_.25La_.75Mg, Pr_.25La_.75Zn, Nd_.25La_.75Mg and Nd_.2La_.8Zn. Crystal field parameters were deduced, but some ambiguities remain. A neutron diffraction study of PrMg is also reported : PrMg is an antiferromagnet with a moment μ_AF = 2.7 μ_B parallel to the propagation vector $\text{q}\text{= [0, 0,}\text{1}\text{2}\text{]}$ ; a strong quadratic distortion occurs in the ordered state, reaching $\text{c}\text{a}\text{? 1 = 1.4\%}$ at 4.2 K. The crystal field determinations on the four compounds are analyzed in relation to their magnetic properties.     

Crystal field in RPdIn (R=Ce, Pr, Nd) compounds

The RPdIn compounds (R = rare earth) crystallise in the hexagonal ZrNiAl-type crystal structure. The compounds from this family exhibit a great variety of interesting magnetic properties including heavy fermion behaviour. In order to get a deeper insight into nature of magnetism of RPdIn with light rare earths elements (La–Nd) an inelastic neutron scattering experiment was performed. For compounds with Pr and Nd excitations due to crystal field were clearly distinguished. On the other hand, interesting behaviour for the CePdIn sample was observed. The sample exhibits no signs of crystal field excitations, likely due to highly delocalised Ce 4f states leading to its heavy fermion behaviour.     

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 effects on the saturation magnetic moment of Sm3+ ion in ferronagnetic samarium compounds

The effects of cubic crystal fields on the saturation magnetic moment of Sm³⁺ ion in ferromagnetic compounds have been investigated. In samarium compounds with magnetic elements, the exchange fieldH _ex acting on Sm³⁺ ion is taken to be proportional to the sublattice magnetization of the magnetic element, while in compounds with nonmagnetic elementsH _ex is taken to be proportional to the spin average of the Sm³⁺ ion and is determined self-consistently. In both types of compoundsH _ex is assumed to be along [001] direction. The saturation magnetic moment is calculated by taking into account the admixture of excited (J=7/2 andJ=9/2) levels into the ground (J=5/2) level of Sm³⁺ ion by crystal fields and exchange fields. It is shown that depending upon the strength, the crystal fields quench or enhance the magnetic moment from the free ion value, and in some cases force Sm³⁺ ion to behave effectively like an (L+S) ion rather than an (L−S)ion. The crystal fields may have important bearing on the performance of samarium compounds as permanent magnet materials.     

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.     

Crystal field interaction in erbium garnets

A crystal field analysis has been performed for erbium aluminum garnet (ErAG) and erbium gallium garnet (ErGG). Good agreement between the measured and calculated energy levels was obtained.     

Crystal field splitting in γ-cerium

Results are reported on an inelastic neutron scattering experiment to determine the crystal field splitting and level scheme for the ²F_{$\text{5}\text{2}$} multiplet in γ-cerium. For the first time in a metallic cerium system the inelastic peak is well resolved and the splitting, Δ, can be accurately determined. As a next logical step in improving the point charge model we consider the screened point charge model. We find a Γ₇ ground state and Δ = 67 K.     

Crystal field effects in superconducting tunneling

A comparison of the tunnel curves of superconducting $\text{La}$Pr, $\text{La}$Y and $\text{La}$Gd reveals that the character of the tunneling density of states of $\text{La}$Pr is between that of the BCS- and that of the Abrikosov-Gorkov theory. This is expected on theoretical grounds for pair-breaking caused by ions in a singlet ground state. The predicted crystal field induced structures in the tunnel curves could not be detected.     

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.     

Electric field gradients in interhalogenic compounds

The time differential perturbed angular distribution me thod has been applied to investigate the electric field gradient in the interhalogenic compunds C1F, BrF₃ and 1F₅ Measurements at different temperatures revealed an unexpected behaviour of the modulation amplitude. Since for this behaviour no consistent explanation can be given yet, data are reported only.     

Crystal field effects in TmCu2 compound

The splitting of the³H₆ multiplet has been estimated for the Tm³⁺ ion in the crystal electric field of the orthorhombic TmCu₂ compound. Using the energy levels and appropriate eigenfunctions the crystal field only susceptibility has been calculated along the principal orthorhombic axes at temperatures 10 to 300 K. The obtained results are compared with our measurements of specific heat and paramagnetic susceptibility on polycrystalline sample.Dedicated to Dr. Svatopluk Krupika on the occasion of his 65th birthday.     

Ferromagnetic ordering of iron impurities in the valence-fluctuating semiconductor SmB6

The electron paramagnetic resonance (EPR) of the valence-fluctuating semiconductor SmB₆ doped by 1 at % Fe is studied. The EPR measurements are performed on a SmB₆ single crystal in a temperature range of 1.6–300.0 K. A number of resonance lines whose g factors indicate the presence of iron ions in the Fe⁰, Fe⁺, Fe²⁺, and Fe³⁺ states have been detected. The iron ions are ferromagnetically ordered below a Curie temperature T = 100 K, and this ordering can be caused by the exchange interaction of impurity ions due to matrix polarization (a similar mechanism is observed in PdFe alloys). This exchange interaction is estimated to be significantly higher than that in PdFe; this fact can result from a very high density of states in the narrow f band, which is characteristic of a valence-fluctuating material.     

Crystal field states in CeCu4Al

Heavy fermion CeCu₄Al compound has been studied by inelastic neutron diffraction (INS), heat capacity and magnetic susceptibility measurements. A single Crystal Electric Field (CEF) peak has been detected in the INS spectra, which may be explained by a quasi-quartet state suggested by the analysis of the Schottky anomaly contributing to the magnetic part of the specific heat. The Kondo interactions have been included in the analysis of the magnetic part employing the simplified resonance level model. The resulting Kondo temperature of about 5–10 K is somewhat larger than in the previous studies. The magnetic susceptibility confirms the CEF level scheme of the type 0–93 K and provides the values of the CEF parameters.     

Crystal field effects in terbium iron garnet

The infra-red absorption spectrum of terbium iron garnet, (TbIG), has been recorded at liquid helium temperature. The energy level scheme was reduced and simulated within the frame of crystal field (cf) theory. The cf parameters of TbIG are not significantly different from those of terbium aluminum garnet, (TbAlG).