Crystal fields and exchange parameters in LaAl2: Eu2+ and LaAl2: Gd3+ single crystals

Reported are 10- and 35-GHz ESR measurements on single crystals of GdxLa1-xAl2 and EuxLa1-xAl2, x=100-1000 ppm. For the Gd alloys no effects of fine structure could be detected, which, under the prevalent experimental conditions implies b4<10 G. In contrast the Eu alloys exhibit a large, b4=+112 G, b6=-8 G, crystal field, the largest in a metal to date and corresponding to an overall crystal-field splitting of 480 mK. This system is the first to show resolved fine structure and a bottleneck. A computer analysis using the full Barnes-Plefka theory shows excellent agreement between theory and experiment. As is usually the case, the exchange obtained from the g shift does not agree with that from the linewidth. We show that this cannot be explained in terms of the current "spherically symmetric partial-wave-expansion" approach but is reconcilable with the crystal-field split-band theory of Narath. Finally, we show that, including the ferromagnetic Kondo effect, our g-shift exchange parameter is consistent with the theoretical value of Harmon and Freeman for Gd3+ scaled to Eu2+.