Low-energy spin fluctuations in filled skutteruditesYbFe4Sb12 andLaFe4Sb12 investigatedthrough 121Sb nuclearquadrupole and 139La nuclear magnetic resonance measurements

Abstract

We have elucidated low-energy spin fluctuations in the new filled skutteruditesYbFe4Sb12 andLaFe4Sb12 synthesized at highpressures, through 121Sb nuclear quadrupole resonance (NQR) and139La nuclear magnetic resonance (NMR) measurements. The longitudinal spin–lattice relaxation rate1/T1 of121Sb inYbFe4Sb12 provides evidence thatupon cooling below ∼20 K, the compound transforms from the localized 4f electron state ofYb3+ ions to a nonmagnetic heavy Fermi liquid state, originating from the mixing of 4felectrons with conduction electrons. Whereas, the Curie–Weiss type behaviour of the139La Knightshift and 121Sb- 1/T1 in LaFe4Sb12 indicate that the compound remains in the localized electron statedown to 1.4 K, it in fact originates from 3d electrons of Fe in[Fe4Sb12 ] anions. In both compounds, the transversal nuclear spin–spin relaxation rate1/T2 exhibits a clear peak at and K, respectively. The origin of the1/T2 peak is discussed in terms of the freezing of the thermal vibration of Sb cages or rare-earthions filled in each Sb cage. By comparing the experimental results of the presentstudy with those previously reported for the compounds synthesized at ambientpressure, it is pointed out that both the strongly correlated electron properties andthe thermal vibrations are greatly modified with the increase of rare-earth atomdeficiency.