Chapter One Magnetic Properties of Filled Skutterudites

Abstract

Abstract Filled skutterudite compounds with a chemical formula AT4X12 (A= rare earth, actinide, alkaline, or alkaline earth elements; T =Co, Ni, Rh, etc.; X= pnictogen) have been intensively investigated, since they exhibit variety of attractive features by only replacing the constituent elements within the same crystal structure. For A=rare earth, generally the hybridization between conduction and 4f-electrons (c-f hybridization) is quite strongly enhanced by the unique crystal structure of A ions placed inside an icosahedral cage made of twelve X-ions. Thereby unprecedented phenomena are realized even in the system containing rare-earth ions with plural number of 4f-electrons. The metal-insulator transition in PrRu4P12, the field induced heavy fermion state above the multipolar ordered state in PrFe4P12, and the heavy fermion superconductivity in PrOs4Sb12 are typical examples. The effect of X-replacement appears most apparently as a change in c-f hybridization strength; the strongest hybridization is realized for X=P with the smallest lattice constant. As additional important characteristics realized by the filled skutterudite structure, unique crystalline electric field (CEF) level scheme with higher order term in CEF Hamiltonian and the higher order multipoles are recognized to play key roles in many of the features found in the filled skutterudite compounds. On the other hand, in AFe4X12, 3d-electrons from Fe play decisive roles in the magnetic properties for nonmagnetic A-ions, and cooperate with 4f-electons to exhibit unusual magnetic behavior for A = magnetic rare earths. As a result of intensive studies, the origins of many of the discovered features have been elucidated, although there still remain quite a few interesting puzzles to be clarified. In this chapter, we try to provide a thorough review on the filled skutterudites, putting emphasis on their basic physical properties other than the application oriented thermoelectric properties.