Tsuyoshi Ikeno

Itinerant-Electron Weak Ferromagnetism in La-Based Filled Skutterudite LaFe4As12

High-quality samples of LaFe 4 As 12 with a residual resistivity ratio of 64 have been successfully synthesized under high pressure ∼4 GPa at 1100 K. The magnetization, specific heat, electrical resistivity, and Hall coefficient of the samples were measured. All the investigated properties exhibit evident anomalies at 5.2 K, thereby indicating a ferromagnetic transition. The large Rhodes–Wohlfarth ratio (≡ the ratio of the effective moment deduced from the Curie constant to the saturation moment per magnetic ion) of 32 suggests that LaFe 4 As 12 is the first La-based filled skutterudite that exhibits itinerant-electron weak ferromagnetism, which is realized by the high density of states at the Fermi level D ( E F ). The ferromagnetism is unexpected within the simple Stoner-type argument, because D ( E F ) lies between those of an ordinary Pauli paramagnet LaFe 4 P 12 and an enhanced Pauli paramagnet LaFe 4 Sb 12 in the LaFe 4 X 12 (X = P, As, Sb) series. We discuss this unexpected ferromagnetic transition...

Origin of Weak Ferromagnetism in YbxFe4Sb12, Relationship between Weak Ferromagnetism and Filling Ratio x

The magnetization and specific heat of single-crystalline samples of filled-skutterudite compounds Yb x Fe 4 Sb 12 were measured. Six single crystals with the filling ratio of Yb, x , from 0.875 to 0.910 were prepared. The weak ferromagnetism of Yb x Fe 4 Sb 12 appearing at low temperatures was found to strongly correlate with x ; with increasing x from 0.875 to 0.910, the transition temperature, T C , decreases from 17 to 5 K and the weak-ferromagnetic moment, m 0 , decreases from 0.066 to 0.005 µ B /Fe. From these trends, both T C and m 0 were expected to reach zero at the critical concentration, x c =0.93. In the temperature dependence of the specific heat, no anomaly at T C was observed, reflecting that m 0 is extremely small compared with the effective Bohr magneton number. With the application of pressure, the T C of the sample with x =0.882 increases rapidly, with the ratio of d T C /d P =+5.27 K/GPa. This weak ferromagnetism of Yb x Fe 4 Sb 12 originates from the itinerant 3d electrons of Fe in an...

Mössbauer Effect and Magnetization Studies of YbFe4Sb12 and LaFe4Sb12

We prepared two YbFe 4 Sb 12 samples (#1 and #2) and a LaFe 4 Sb 12 sample. Mossbauer and magnetization measurements were performed on the samples at various temperatures and in applied magnetic fields. The experimental results indicate that #1 is in a weakly ferromagnetic state below 13 K, while #2 is in an itinerant paramagnetic state even at 2 K, and that quadrupole splitting for #1 at each temperature is slightly less than that for #2. Therefore, we conclude that YbFe 4 Sb 12 is an intermediate material between a nearly ferromagnetic material and a weakly ferromagnetic material.

Sb NQR Studies of Ferromagnetism in YbxFe4Sb12 with x 0:89

121,123 Sb nuclear quadrupole resonance (NQR) measurements have been performed for a polycrystalline sample of Yb x Fe 4 Sb 12 with x ≈0.89. Five well-separated NQR lines were observed above 18 K, ...

Dramatic evolution of magnetic properties induced by electronic change in Ce(Pd1-xAgx)2Al3

By measuring the lattice parameters (a, c), the magnetic susceptibility χ(T) and magnetization M(H), the specific heat C(T), and the electrical resistivity ρ(T) for Ce(Pd1−xAgx)2Al3, we found that with increasing x the system undergoes a variation from antiferromagnetism to ferromagnetism at x~0.05 then back to antiferromagnetism at x~0.45. The magnetic evolution, together with the Kondo temperature TK that is inferred to decline with x, closely resembles that of Ce(Pd1−xCux)2Al3. The two systems have the same electronic but opposite volume change; resemblance between them indicates the dominant effect in the substitution is caused by electronic change but not volume change. Modification of the RKKY interaction may not account for the unusual magnetic evolution.

Relationship between quadrupole splitting and filling ratio x in YbxFe4Sb12

In a previous paper, the authors studied two samples of YbxFe4Sb12 (#1 and #2) by the Fe Mossbauer effect; it was found that the quadrupole splitting (QS 1⁄4 eqQ=2) for YbxFe4Sb12 (#1) at each temperature was slightly less than that for YbxFe4Sb12 (#2), thereby leading the authors to conjecture that the filling ratio x of Yb atoms in YbxFe4Sb12 (#2) was smaller than x in YbxFe4Sb12 (#1). However, in more recent papers, Ikeno et al. concluded that the ferromagnetic-transition temperature TC decreased with increasing x. From the relationship of x versus TC, the filling ratios in YbxFe4Sb12 (#1) and YbxFe4Sb12 (#2) are estimated at x 0:89 and x & 0:92, respectively. This indicates that x in YbxFe4Sb12 (#2) is larger than x in YbxFe4Sb12 (#1). In this paper, we explain the causation of the fact that QS increases with increasing x, and obtain the value of x in YbxFe4Sb12 (#2). The electric-field gradient eq at the Fe site is expressed as follows:

Temperature Dependence of Quadrupole Splitting and Spectra for YbxFe4Sb12 Studied by 57Fe Mössbauer Effect

The temperature dependence of quadrupole splitting for Yb x Fe 4 Sb 12 deviates from the T 3/2 law because of the itinerant electron contribution to the electric field gradient (EFG). Each of the 57 Fe Mossbauer spectra of Yb 0.89 Fe 4 Sb 12 is a double peak with similar asymmetries above and at 45 K; however, the asymmetry of the double peak varies with temperature below 45 K. The asymmetry variation in the temperature range from 45 to 13 K is explained by the increasing valence of the Yb ion in cages adjacent to cages with Yb deficiency, and the asymmetry in the spectra below 13 K is explained by the angle between the principal axis of the EFG tensor and the internal magnetic field.

Raman scattering study of filled skutterudite compounds

Raman scattering of skutterudite compounds RT4X12 (R=La, Ce, Pr, Nd, Sm and Yb, T=Fe, Ru and Os, X=P and Sb) have been measured. All first-order Raman active phonons are observed and are assigned as the pnicogen vibrations. At the low energy region, the second-order phonons, due to the vibration of the rare earth ions with a flat phonon dispersion, are observed in the spectra of RRu4P12 (R=La and Sm) and ROs4Sb12 (R=La, Ce, Pr, Nd, and Sm). The appearance of the second-order phonons in the spectra is caused by an anharmonic vibrations of rare earth ions in large cage space and a large density of state due to the flat phonon dispersion. However, in spite of the similar cage space, the 2nd-order phonons are hardly observed for RFe4Sb12 and RRu4Sb12. Thus, these results suggest that the dynamics of the rare earth ion is closely related to not only the cage size but also the electronic state due to the transition metals. Raman spectra of PrRu4P12 show the drastic spectral change due to the metal-insulator transition. The phonon spectra and crystal field excitations due to the structural change have been assigned above and below the transition temperature.