Itinerant-Electron Weak Ferromagnetism in La-Based Filled Skutterudite LaFe_4As_<12>(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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High-quality samples of LaFe_4As_<12> with a residual resistivity ratio of 64 have been successfully synthesized under high pressure 〜4 GPa at 1100K. The magnetization, specific heat, electrical resistivity, and Hall coefficient of the samples were measured. All the investigated properties exhibit evident anomalies at 5.2K, 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_4As_<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_4P_<12> and an enhanced Pauli paramagnet LaFe_4Sb_<12> in the LaFe_4X_<12> (X=P, As, Sb) series. We discuss this unexpected ferromagnetic transition on the basis of the difference in the spin-fluctuation strength between As- and Sb-based systems.
2008-03-15