L. Keller

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.

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 structure and field-dependent properties of CeCu5

A study of the magnetic properties of the hexagonal compound CeCu5 reveals antiferromagnetic order below TN=3.9 K. The magnetic structure comprises moments along the c-axis with a propagation vector k=(0,0, 1/2 ). The presence of the Kondo interaction reduces the magnitude of the moments from about 0.42 mu B, expected for a mod +or- 1/2 ) doublet, to about 0.36 mu B, observed from elastic neutron scattering experiments.

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.

Sample quality and the magnetic phase transitions in Nd3Pd20Ge6: a neutron diffraction study

Abstract For the cubic intermetallic compound Nd 3 Pd 20 Ge 6 (space group Fm 3 m ) magnetic and transport properties are known to be highly sensitive to sample quality. We have performed powder neutron diffraction experiments down to 100 mK to study the magnetic structures of an inhomogeneous Nd 3 Pd 20 Ge 6 sample, which is the only known 3–20–6 system with three successive magnetic phase transitions. The onset of magnetic Nd order at the 8 c site (simple cubic sublattice) is observed in two phase transitions with a dominant commensurate antiferromagnetic component ( k 1 =[1,1,1] below T N1 =1.75 K) and an additional weak incommensurate perturbation ( k 1i =[ τ , τ , τ ], τ =0.79, below T N1i =1.40 K). The Nd moments at the 4 a site (face-centered cubic sublattice) remain disordered down to T N2 =0.58 K and align perpendicular to the propagation vector k 2 =[0,0,1]. The phase transition at T N1i appears because of atomic disorder inside the cubic lattice of the 3–20–6 structure. High-quality Nd 3 Pd 20 Ge 6 samples show only two magnetic phase transitions at T N1 and T N2 .

Magnetic Neutron Scattering from the Rare-Earth Intermetallic Compound System R3Pd20Si6 (R = Rare Earth)

The low-temperature magnetic properties of the rare-earth intermetallic compound system R3Pd20Si6 (R = rare earth) were studied by elastic and inelastic neutron scattering. The compounds show due to the two rare-earth sublattices built from the 4a and 8c sites in the cubic crystal structure (space group Fm-3m) successive magnetic ordering with two different propagation vectors. In this report, we give an overview of the results obtained for the magnetic structures and the crystalline electric fields in the 3-20-6-system.

Structural study of U(Pd1−xFex)2Ge2 at high pressure

The crystal structure of the U(Pd1−xFex)2Ge2 compounds with Fe content x = 0– 0.03 and the crystal and magnetic structure of U(Pd0.98Fe0.02)2Ge2 at high external pressures up to 4.5 GPa were studied by means of powder neutron diffraction in the temperature range 1.5–300 K. With increasing Fe content the values of the lattice parameters and interatomic distances change only slightly, but it is known from previous experiments that the magnetic structure changes drastically for x ≥ 0.015. In contrast to this, high external pressure modifies the crystal structure more significantly while the magnetic structure remains unchanged. The results obtained allow one to infer that drastic changes in the magnetic structure of the U(Pd1−xFex)2Ge2 compounds with increasing Fe content are a consequence of modification of the RKKY-type (RKKY standing for Ruderman, Kittel, Kasuya and Yosida) indirect exchange interaction due to the variation of the number of conduction electrons per U atom rather than a result of lattice contraction.

Magnetic properties of PrPdAl and NdPdAl

Abstract Magnetic phase transitions of the ternary intermetallic compounds PrPdAl and NdPdAl with hexagonal ZrNiAl type crystal structure were investigated by means of specific heat and neutron diffraction experiments. The triangular coordination symmetry of the magnetic rare-earth atoms on site 3f gives rise to geometrical frustration and leads to incommensurate antiferromagnetic structures. PrPdAl undergoes magnetic phase transitions at T N,1 = 42 K and T N,2 ≈ 1.5 K. Themagnetic structure at 1.5 K corresponds to a propagation vector k = [ 1 2 , 0, τ], τ = 0.398 , and the ordered Pr(2) moments of 1.9 μ B are significantly reduced below the value 3.1 μ B for Pr(1) and Pr(3). NdPdAl shows magnetic phase transitions at T N,1 = 5 K and T N,2 = 4 K. The magnetic propagation vector shows a pronounced temperature dependence above T N,2 and locks in to k = [ 1 4 ,0,τ], τ = 0.444 , below T N,2 .