大型放射光施設 SPring-8

日   時 ： 2004年1月29日 15:00 - 16:30

講演者 ： Sudesh Kumar Dhar 教授
所   属 ： Condensed Matter Physics & Materials Science Dept.,Tata Institute of Fundamental Research

講演要旨
　　The (CePd₃)₈M compounds are derived from the parent CePd₃. A periodic distribution of the M atoms at the interstitial body-center position in one out of every eight CePd₃ unit cells results in the formation of cubic superstructure (CePd₃)₈M, closely related to the AuCu₃-type structure of the parent CePd₃. In the superstructure, while the CePd₃ skeleton is periodically distorted in space, the distortion causing M atoms themselves form a regular simple-cubic sub-lattice with nearest neighbor M-M distance of ~ 8.5 Å. The consequences of the superstructure formation are i) The Ce-Ce separation, on an average, increases by 2-3 % over its value in CePd₃ weakening the strength of s-f hybridization and ii) the outer-shell electrons of the M atom contribute to the conduction band leading to minor alteration in the electronic density of states at the Fermi level. Due to these changes, (CePd₃)₈M compounds exhibit dense Kondo lattice behavior (M =Al, Ga and In), ordering antiferromagnetically at low temperatures with the exception for M = Mn, in contrast to valence fluctuating, paramagnetic behavior of CePd₃. (LaPd₃)₈Mn and (CePd₃)₈Mn are two new dilute Mn based compounds with unusually large Mn-Mn separation of ~ 8.5 Å and Mn concentration of ~ 3 at.%. At such large near neighbor distance, the direct Mn-Mn 3d overlap is practically negligible. While the Mn ions order antiferromagnetically in (LaPd₃)₈Mn (TN = 46 K) with the Mn ions in a 2+ valence state, a ferromagnetic transition of the Mn ions is observed in the Ce compound (Tc = 36K). Interestingly, dilution of Mn sublattice with Al even by 25 % and the substitution of Pd with Rh and Ag at ~ 8 % level in (LaPd₃)₈Mn changes the valence of Mn from 2+ to 3+ and the nature of magnetic ordering from antiferromagnetic to ferromagnetic in the Rh substituted alloy. The valence change indicates varying s-d hybridization. Preliminary investigations show Kondo-lattice like behaviour in the magnetic resistivity of (LaPd₃)₈Mn and Al diluted alloys, indicating that this may well be the first instance of a 3d-based dense Kondo lattice compound. In (CePd₃)₈Mn the Kondo effect due to Ce ions dominates the thermal variation of the resistivity and the Ce ions are paramagnetic at least down to 1.5 K.