No abstract

La 1Àx Y x )Mn 4 Al 8 (0 x 1) is a very unique itinerant electron system, in which the spin pseudogap can be controlled continuously and nearly uniformly in a wide range of the gap width from Á ' 250 to 500 K by the anisotropic volume shrinkage induced by the chemical pressure. The hydrostatic pressure on LaMn 4 Al 8 also increases Á but tends to fill the gap. The strong and anisotropic volume dependence of Á strongly supports the idea that the origin of the gap formation is associated with the one-dimensional geometry of the Mn spin arrangement.The effect of geometry is now known to be important in describing magnetism even in itinerant electron magnets, provided that electron correlations are sufficiently strong. For example, geometric frustration plays a nonnegligible role in determining the physical properties of strongly correlated itinerant electron magnets such as the Laves phase compounds RMn 2 (R ¼ rare earth) 1) and one of allotropes of manganese -Mn. 2) On the other hand, spin-gap formation in correlated electron systems is one of the extensively investigated topics in a wide variety of fields in the magnetism. Among itinerant electron magnets, a unique correlated-electron gap system, FeSi, attracts much attention, and the origin of the spin compensation at the ground state is still a matter of debate. 3) Recently, we have confirmed a susceptibility maximum at T max ' 500 and 200 K for the itinerant electron compounds YMn 4 Al 8 and LaMn 4 Al 8 , respectively, and interpreted this as due to the presence of a narrow pseudogap in the spin excitation spectrum, 4,5) but the origin of the gap remains open for question. The crystal structure of YMn 4 Al 8 and LaMn 4 Al 8 is of the tetragonal CeMn 4 Al 8 type (space group I4=mmm) derived from the ThMn 12 type, in which Mn atoms at the 8 f site form linear chains along the c axis. The large difference in intrachain and interchain Mn-Mn interatomic distances (d intra ' 2:6 and d inter ' 4:4 # A 6,7) ) suggests a quasi-onedimensional (1D) nature of magnetic interaction. YMn 4 Al 8 and LaMn 4 Al 8 show relatively large electronic specific heat coefficients, . 8) Therefore, RMn 4 Al 8 has appeared as another candidate for strongly correlated itinerant electron systems in which the effect of geometry plays a leading role in determining the magnetism. Thus far, no indication of phase transitions has been reported, at least, for YMn 4 Al 8 . For example, the C=T vs T 2 relation (C: specific heat) shows a good linearity down to 1.5 K for both compounds. 8) Although Yamasaki et al. 9) did not find any appreciable evidence of a phase transition in an 27 Al NMR experiment of LaMn 4 Al 8 down to 4.2 K, our recent experiments suggested that magnetic correlation develops below $ 50 K. 5) A recent zero-field SR experiment detected the appearance of static internal fields in LaMn 4 Al 8 at low temperatures, and suggested an antiferromagnetic order below 4.5 K. 10) In this paper, we report the concentration dependences of lattice volume, susceptibility and resistivity for poly...

No abstract

We have carried out Mössbauer spectroscopy and nuclear resonant inelastic scattering to elucidate the lattice dynamics in filled-Skutterudite compounds, especially phosphides. The second-order Doppler shift obeys the Debye model in RFe 4 P 12 . Nuclear quadrupole interaction reveals an unusual temperature dependence in these compounds. An anomaly is observed in 57 Fe nuclear resonant inelastic scattering of these compounds. The energy where the anomaly observed in SmFe 4 P 12 agrees with the phonon excitation energy observed by 149 Sm nuclear resonant inelastic scattering. We have also performed the 99 Ru Mössbauer measurements of SmRu 4 P 12 .

We have carried out X-ray diffraction and 57 Fe nuclear forward scattering using synchrotron radiation to study structural and magnetic properties of Fe 2 P under high pressure at low temperature. The lattice parameter ratio changes at 1.8 and 7.0 GPa as a function of pressure at 24 K without structural change, where the transition at 1.8 GPa corresponds to a ferromagnetic-to-antiferromagnetic one. We deduced the magnetic structures of Fe 2 P at 3 K and 2.5 GPa from refined hyperfine interaction parameters extracted from 57 Fe nuclear forward scattering spectra. Without external magnetic field where Fe 2 P is in the antiferromagnetic state, the magnetic structure is a complex one with a propagation vector along h100i. The distinct change of 57 Fe nuclear forward scattering spectra with external magnetic field along [001] indicates that a metamagnetic-like transition occurs below 5 kOe at 3 K and 2.5 GPa. With external magnetic field, the magnetic structures are not simple collinear ones. Furthermore, the magnetic-field-induced change in the magnetic structure occurs at around 40 kOe.