We report electron spin resonance (ESR) measurements in the Gd3+ doped semiconducting filled skutterudite compound Ce1-xGd xFe4P12 (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 with 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 JfdS.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 (Fe 2P3)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. © 2011 American Physical Society.

The electron spin resonance (ESR) spectra of Eu2 +(4 f7, S = frac(7, 2)) in LaB6 single crystal show a single Dysonian resonance for the localized Eu2 + magnetic moments. It is shown that the Eu2 + ions are covalent exchange coupled to the (B) 2p-like host conduction electrons. © 2007 Elsevier B.V. All rights reserved.

Electron spin resonance of Eu2+ (4 f7, S=7 2) in a La hexaboride (La B6) single crystal shows a single anisotropic Dysonian resonance. From the observed negative g shift of the resonance, it is inferred that the Eu2+ ions are covalent exchange coupled to the B 2p -like host conduction electrons. From the anisotropy of the spectra (linewidth and field for resonance), we found that the S ground state of Eu2+ ions experience a cubic crystal field of a negative fourth order crystal field parameter (CFP), b4 =-11.5 (2.0) Oe, in agreement with the negative fourth order CFP, A4, found for the non- S ground state R hexaborides. These results support covalency as the dominant contribution to the fourth order CFP for the whole R hexaboride family. © 2007 The American Physical Society.