Nearly Ferromagnetic Metals AFe₄Sb₁₂(A = Ca, Sr, and Ba)

Abstract

The magnetic, transport and thermal properties of alkaline-earth-filled skutterudites AFe 4 Sb 12 (A = Ca, Sr, Ba) are reported. All three compounds show maximum magnetic susceptibility and thermopower at 50 K and a large electronic specific heat coefficient of 100 mJ/mol K 2 . The electrical resistivity exhibits a shoulder at 70 K and follows quadratic temperature dependence below 9 K. These properties are the characteristics of a nearly ferromagnetic metal. The suppression of ferromagnetic spin fluctuations of the Fe 3d band by applying magnetic fields was manifested as a negative magnetoresistance between 50 and 80 K.The concept of itinerant magnetism has been developed over the past three decades. 1) In weakly or nearly ferromagnetic systems, the temperature dependence of the amplitude of local spin density results in the Curie-Weiss behavior of magnetic susceptibility 1 at high temperatures. In an itinerant ferromagnet, the ordered moment can be much smaller than the effective moment. Low-energy spin fluctuations (SFs) significantly influence the transport and thermal properties at low temperatures. For example, SF scattering gives rise to quadratic T dependence of the electrical resistivity, and the temperature variation of SFs leads to the significant enhancement of the electronic specific heat. Furthermore, SFs in ZrZn 2 and Sr 2 RuO 4 are believed to act as the glue for electron pairing in unconventional superconducting states. 2,3) NaFe 4 Sb 12 is a newly discovered weak itinerant ferromagnet with a rather high T c of 85 K and a small ordered moment of 0.25 " B /Fe atom. 4) This compound belongs to the family of filled skutterudites with the chemical formula MT 4 X 12 , where M is an alkali, alkaline-earth or a rare-earth metal, T is Fe, Ru or Os, and X is P, As, or Sb. The T atom is located at the center of an X 6 octahedron, and the cornersharing TX 6 octahedra form a tilted array. Within this family, rare-earth filled compounds have been extensively studied because of their novel physical properties, such as the unconventional superconductivity in PrOs 4 Sb 12 . 5) However, the role of d electrons of T atoms in the physical properties has not been well understood so far. The discovery of itinerant ferromagnetism in NaFe 4 Sb 12 with Na þ ions has revealed the strong dependence of the d electron magnetism on the valence of the filler ion. In fact, LaFe 4 Sb 12 with La 3þ ions remains in an enhanced paramagnetic state. 6) These contrasting magnetic behaviors in turn suggest that the ground state of AFe 4 Sb 12 with divalent A ions should be close to a magnetic critical point at which an itinerant ferromagnetic state emerges. In this paper, we report the magnetic, transport, and thermal properties of AFe 4 Sb 12 (A = Ca, Sr, and Ba). The results show that all three are in a nearly ferromagnetic state with strong SFs of the Fe 3d electrons.The first investigation of the magnetic properties of AFe 4 Sb 12 (A = Ca, Sr, Ba) was performed by Danebrock et al. 7) The Curie-Weiss behavior with an effective magn...

Iron-based heavy quasiparticles inSrFe4Sb12: An infrared spectroscopic study

“…The optical results indicate that the suppression of the spin fluctuations and the creation of the heavy quasiparticles observed in σ(ω) are coincident, or in the other words, the origin of these phenomena is the hybridization between the Fe 3d spins and the charge carriers. This is consistent that the ratio between A of ρ = AT 2 and γ in SrFe 4 Sb 12 is close to the Kadowaki-Woods value as pointed out before 7 .…”

“…A high-density polycrystalline SrFe 4 Sb 12 sample was synthesized using a spark-plasma sintering technique previously reported 7 . The near-normal incident optical reflectivity spectra [R(ω)] of SrFe 4 Sb 12 were acquired from well polished samples by using 0.3 µm-grain-size Al 2 O 3 wrapping film sheets.…”

“…In addition, σ(ω) for this system roughly reflects the unoccupied electronic states of the Fe 4 Sb 12 frame 14,21 . However, the previous paper did not clarify the origin of the heavy-quasiparticle-like character appearing in the thermodynamical properties 7 .…”

Application of spark plasma sintering to the fabrication of binary and ternary skutterudites

“…A large charge carrier spin polarization up to 67% was found experimentally by point contact spectroscopy [7]. Alkaline-earth metals and Yb-filled skutterudites are paramagnets but several experimental and theoretical observations demonstrate that they are close to ferromagnetic ordering [4,[5][6][7][8]. In addition, compounds exhibit a pseudogap [9,10] in the infrared optical conductivity which appears at temperatures below 100 K. This phenomenon can be consistently explained by certain sharp peaks in the band structure above E F [9].…”

“…Together with compounds of divalent non-magnetic cations like Ca, Sr, Ba and Yb these skutterudites are well-suited candidates to study the influence of the [Fe 4 Sb 12 ] polyanion on physical properties of filled skutterudites [4][5][6]. From the chemical point of view, the filled skutterudites are iron antimonides of, e.g., alkali metals.…”