A Dönni

Structural and magnetic properties of RFe4P12 (R=Pr, Nd) studied by neutron diffraction

Abstract In this work we present a neutron powder diffraction study of the ternary rare-earth phosphides PrFe 4 P 12 and NdFe 4 P 12 . High-resolution measurements of the structural temperature dependence reveal large-amplitude thermal vibrations of the rare-earth ions, consistent with speculations that the ion is weakly bound in an oversized atomic ‘cage’ formed in the skutterudite crystal structure. In addition, low temperature neutron diffraction measurements of RFe 4 P 12 (R=Pr, Nd) were performed. Whereas the proposed long-range antiferromagnetic ordering in PrFe 4 P 12 could not be confirmed, NdFe 4 P 12 orders ferromagnetically below T C =1.94 K with an ordered magnetic Nd 3+ moment of 1.61(7) μ B at 1.5 K.

Spin-glass behaviour with extended short-range ferromagnetic order in U2RhSi3

For U2RhSi3, which crystallizes in a disordered derivative of the hexagonal AlB2-type structure, we present a detailed investigation of magnetic properties by measurements of AC and DC susceptibility, magnetization, magnetic relaxation, specific heat and electrical resistivity. The results provide evidence for the formation of extended short-range ferromagnetic order at the temperature TC25 K, as indicated by a specific heat anomaly, and for a low-temperature spin-glass state with a freezing temperature Tf = 19.6 K. According to systematic specific heat experiments on isostructural spin-glass systems, short-range ferromagnetic correlations above the spin-freezing temperature are observed in U2RhSi3 (TC25 K, Tf = 19.6 K) and U2CoSi3 (TC10 K, Tf8 K), but not in U2PdSi3 (Tf = 13.5 K) and U2NiSi3 (Tf22 K). Different degrees of structural disorder of transition metal and silicon atoms, both on the same crystallographic site, seem to be the origin for the distinct different magnetic behaviour.

Evidence for an isostructural phase transition in the metastable high-temperature modification of TbPdAl

Abstract The intermetallic rare-earth aluminide TbPdAl crystallizes in the hexagonal ZrNiAl-type structure (metastable high-temperature modification, HTM) and in the orthorhombic TiNiSi-type structure (stable low-temperature modification, LTM). Based on neutron diffraction, magnetic susceptibility and electrical resistivity data for TbPdAl we provide clear evidence for a first-order isostructural phase transition (from HTM I to HTM II), which is absent in the LTM phase, and determine the structural parameters for all three phases. For TbPdAl HTM jumps in the hexagonal lattice parameters a (decrease of 1.0%) and c (increase of 1.9%) occur at T sh =106 K (heating) and T sc =91 K (cooling), and the two phases HTM I and HTM II coexist with a volume fraction ≥20% in a temperature interval of 17 K. The structural phase transition is likely to be of electronic origin.

Low-temperature antiferromagnetic moments at the 4a site in Ce3Pd20Ge6

We have performed powder neutron diffraction experiments to examine the magnetic ordering in Ce3Pd20Ge6, which exhibits ferroquadrupolar ordering at TQ1≈1.2 K and antiferromagnetic ordering at TN2 = 0.75 K. In the series of isostructural cubic rare-earth (R) compounds R3Pd20X6 (X = Si, Ge; space group Fmm), it is common that at the Neel temperature TN1 the magnetic moments at the 8c site (simple cubic sublattice) order in an antiferromagnetic sequence with propagation vector k1 = [1,1,1] and at TN2 the magnetic moments at the 4a site (face-centred cubic sublattice) order with a propagation vector k2 = [0,0,1]. In Ce3Pd20Ge6 the Neel temperature TN1 is replaced by TQ1 and at the 8c site a possible ordered cerium moment is found to be reduced to an undetectable small value down to the lowest measured temperature T = 0.05 K. In contrast, at the 4a site the cerium moments order below TN2 with the expected antiferromagnetic structure. The moments align perpendicular to the propagation vector k2 = [0,0,1] with a saturation value µ2(4a) = (1.1±0.1) µB/Ce that is reasonably large for the crystal field ground-state quartet Γ8. Our results provide indirect experimental evidence that only the quadrupolar moments of cerium at the 8c site are involved in the phase transition at TQ1.

Commensurate-incommensurate magnetic phase transitions of CePtAl

The magnetic properties of CePtAl with orthorhombic TiNiSi-type chemical structure were investigated. Single-crystal experiments of electrical resistivity, specific heat and magnetic susceptibility reveal the existence of three magnetic phase transitions at T1=5.9 K (second order), T2=4.3 K (first order) and T3=2.2 K (first order). Powder neutron diffraction measurements detected complex magnetic structures consisting of two coexisting, partly incommensurate magnetic propagation vectors: k1=0 and k3=(0, 0.463(8), 0) for T2<or=T<or=T1; and k1=0 and k2=(0, 1/2, 0) for T<or=T2. The predominantly ferromagnetic k1 component and the antiferromagnetic k2 component of the magnetic low-temperature structure of CePtAl were refined at 1.5 K. Magnetism in CePtAl is dominated by a ferromagnetic component (1.35(4) mu B at 1.5 K) oriented parallel to the magnetically easy a axis. The complexity of magnetic structures in CePtAl appears to be caused by competing anisotropic magnetic exchange interactions in a network, where Ce-Ce nearest neighbours (3.56 AA) form chains parallel to the b axis and Ce-Ce second-nearest neighbours (3.77 AA) form chains along the a axis.

Successive magnetic ordering of the Tb sublattices in Tb3Pd20Si6

The chemical and magnetic structures of the intermetallic compound were studied by means of bulk magnetic measurements and neutron diffraction. The crystal structure of was confirmed to be isotypic with that of , space group , No 225, with Tb being located at the two crystallographic sites 4a and 8c. The compound undergoes two successive second-order magnetic phase transitions at the Neel temperatures and . Below , the 8c sites order in an antiferromagnetic sequence with a magnetic propagation vector , leaving the 4a sites disordered down to , where additional antiferromagnetic ordering of the 4a sites sets in according to .

Magnetic order and crystal-field excitations in

The magnetic properties of the new cubic intermetallic compound have been investigated by neutron scattering and low-temperature specific heat experiments. The results include the refinement of the crystal structure at 10 K (space group , No 225), the observation of three successive magnetic phase transitions (at and ), the determination of the antiferromagnetic structure between and (antiferromagnetically ordered Nd moments on site 8c coexist with disordered Nd moments on site 4a) and the measurement of the crystalline-electric-field (CEF) level scheme in the paramagnetic state. An extrapolation of the CEF parameters obtained for predicts the CEF ground-states to be the triplet in and the quartet in .

Crystalline electric field excitations in R3Pd20Ge6 (R=Ce, Pr, Nd)

Abstract The ternary intermetallic compounds R3Pd20Ge6 (R=Ce, Pr, Nd) were studied by inelastic neutron scattering. Energy splittings within the electronic ground-state J-multiplet due to crystalline electric field interaction at the R3+ sites were directly observed and rationalized in terms of CEF parameters of cubic symmetry.

Magnetic fluctuations and superconductivity in YbPd2Sn

Abstract We report muon spin relaxation and inelastic neutron measurements on the Heusler system YbPd2Sn. Localized anisotropic and quasi-elastic Yb magnetic fluctuations are observed below T=150 K . Both μSR and neutron data indicate a slowing-down of the spin fluctuations upon lowering the temperature, similar to that observed in Kondo lattices. The temperature dependence of the quasi-elastic neutron signal is compatible with a crystal-electric field scheme having a Γ7CEF ground state. The muon depolarization rate exhibits an additional contribution upon decreasing the temperature below Tc suggesting a close interplay between magnetic fluctuations and the superconducting state.

Magnetic ordering and crystalline electric field splitting in Nd3Pd20Si6

Abstract At the temperatures T N1 ≈2.4 K and T N2 ≈0.7 K, the cubic intermetallic compound Nd 3 Pd 20 Si 6 undergoes second order magnetic phase transitions. Here we report on the magnetic structures obtained from low-temperature neutron diffraction data by using a He dilution refrigerator, and on the crystalline electric field splitting of the Nd 3+ ground state multiplet 4 I 9/2 obtained from inelastic neutron scattering data.