Thermally activated exchange narrowing of the Gd3+ ESR fine structure in a single crystal of Ce1-xGdxFe4P12 (x = 0.001) skutterudite

摘要

We report electron spin resonance (ESR) measurements in the Gd3+ dopedsemiconducting filled skutterudite compound Ce1-xGdxFe4P12 (x = 0.001). As thetemperature T varies from T = 150 K to T = 165 K, the Gd3+ ESR fine andhyperfine structures coalesce into a broad inhomogeneous single resonance. At T= 200 K the line narrows and as T increases further, the resonance becomeshomogeneous with a thermal broadening of 1.1(2) Oe/K. These results suggestthat the origin of these features may be associated to a subtle interdependenceof thermally activated mechanisms that combine: i) an increase with T of thedensity of activated conduction-carriers across the T-dependent semiconductingpseudogap; ii) the Gd3+ Korringa relaxation process due to an exchangeinteraction, J_{fd}S.s, between the Gd3+ localized magnetic moments and thethermally activated conduction-carriers and; iii) a relatively weak confiningpotential of the rare-earth ions inside the oversized (Fe2P3)4 cage, whichallows the rare-earths to become rattler Einstein oscillators above T = 148 K.We argue that the rattling of the Gd3+ ions, via a motional narrowingmechanism, also contributes to the coalescence of the ESR fine and hyperfinestructure.