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

TL;DR

This study investigates how thermal activation influences the electron spin resonance (ESR) fine structure in Gd3+-doped Ce1-xGdxFe4P12, revealing a combination of carrier activation, exchange relaxation, and ion rattling effects.

Contribution

It presents new insights into the thermally activated exchange narrowing mechanism affecting ESR in skutterudite compounds, highlighting the role of ion rattling and conduction carrier dynamics.

Findings

ESR fine and hyperfine structures coalesce into a broad resonance between 150 K and 165 K.

Line narrows and becomes homogeneous at temperatures above 200 K.

Rattling of Gd3+ ions contributes to the ESR line narrowing via motional averaging.

Abstract

We report electron spin resonance (ESR) measurements in the Gd3+ doped semiconducting filled skutterudite compound Ce1-xGdxFe4P12 (x = 0.001). As the temperature T varies from T = 150 K to T = 165 K, the Gd3+ ESR fine and hyperfine structures coalesce into a broad inhomogeneous single resonance. At T = 200 K the line narrows and as T increases further, the resonance becomes homogeneous with a thermal broadening of 1.1(2) Oe/K. These results suggest that the origin of these features may be associated to a subtle interdependence of thermally activated mechanisms that combine: i) an increase with T of the density of activated conduction-carriers across the T-dependent semiconducting pseudogap; ii) the Gd3+ Korringa relaxation process due to an exchange interaction, J_{fd}S.s, between the Gd3+ localized magnetic moments and the thermally activated conduction-carriers and; iii) a relatively weak confining potential of the rare-earth ions inside the oversized (Fe2P3)4 cage, which allows the rare-earths to become rattler Einstein oscillators above T = 148 K. We argue that the rattling of the Gd3+ ions, via a motional narrowing mechanism, also contributes to the coalescence of the ESR fine and hyperfine structure.