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Significance A unifying mechanism for the origin of ferromagnetism which takes into account both well-known and polar extreme theories, namely, the localized- and itinerant-electron regimes, remains a longstanding mystery. Here, we study a particularly interesting system of FeGa 3− y Ge y , within which the magnetic orderings can be adjusted by the Ge-electron filling control, turning from itinerancy to adequate localized through a ferromagnetic quantum critical transition with increasing y . By involving the effects of the spin fluctuations in general, the theoretical estimations of the magnetic properties at the intermediate state of magnetic orderings formulated in terms of the correlation between itinerancy electrons and the spin fluctuations are in quantitative agreements with experimental observations. Our analysis shows a potential universality to all itinerant ferromagnets.

A unifying mechanism for the origin of ferromagnetism which takes into account both well-known and polar extreme theories, namely, the localized- and itinerant-electron regimes, remains a longstanding mystery. Here, we study a particularly interesting system of FeGa3−yGey, within which the magnetic orderings can be adjusted by the Ge-electron filling control, turning from itinerancy to adequate localized through a ferromagnetic quantum critical transition with increasing y. By involving the effects of the spin fluctuations in general, the theoretical estimations of the magnetic properties at the intermediate state of magnetic orderings formulated in terms of the correlation between itinerancy electrons and the spin fluctuations are in quantitative agreements with experimental observations. Our analysis shows a potential universality to all itinerant ferromagnets.

We have performed a photoemission and inverse photoemission spectroscopic study of a narrow-gap semiconductor FeGa_3, in order to characterize the occupied and unoccupied electronic states. The energy-gap size was found to be ～0.4 eV, and the valence-band maximum (VBM) was located around the A point of the Brillouin zone. We observed a dispersive Ga 4sp derived band near the Fermi level (E_F), and Fe 3d narrow bands located at -0.5 and -1.1 eV away from E_F. In contrast to the case of FeSi, there was no temperature-dependent peak enhancement at the VBM on cooling. The observed density of states and band dispersions were reasonably reproduced by the LDA+U calculation with the on-site effective Coulomb interaction U_ ～3 eV to the Fe 3d states. Present results indicate that, in spite of sizable U_ /W ～0.6 (W: band width), electron correlation effects are not significant in FeGa_3 compared with FeSi since the VBM consists of the dispersive band with the reduced Fe 3d contribution, and the energy gap is large.

To study the effect of double filling of the skutterudite iron antimonides on the thermal conductivity, we have synthesized dense samples of Ca1-xLaxFe4Sb12 (0 les times les 1) and Ca0.3La0.6Fe4-yCoySb12 (y = 1, 1.5) by the spark plasma sintering technique. Electron-probe microanalysis revealed that the double-filled samples with x = 0.25, 0.5, and 0.75 consist of three phases of micrometer scale; CaFe4Sb12, Ca-rich phase, and La-rich phase, respectively. The lattice thermal conductivity at 150 K is smaller by 10-20% than that for the end compounds. This is attributable to the scattering of heat conducting phonons by the local fluctuations in both mass and charge of Ca2+ and La3+ ions. In the full range of x from 0 to 1, the thermopower at 300 K stays at 70 muV/K. This value was increased to 120 muV/K by partial substitution of Co for Fe. The dimensionless figure of merit ZT for Ca0.3La0.6Fe3CoSb12 is estimated to be 0.9 at 800 K, which is comparable to the value reported for p-type Ce0.44Yb0.32Fe3CoSb12.

We have synthesized Yb14MnSb11 and Yb14ZnSb11 to study the thermoelectric properties at low temperature (4