Ordering mechanism and spin fluctuations in a geometrically frustrated heavy-fermion antiferromagnet on the Kagome-like lattice CePdAl: A $^{27}$Al NMR study
Akira Oyamada, Satoru Maegawa, Masahide Nishiyama, Hideaki Kitazawa, and Yosikazu Isikawa

$^{27}$Al NMR measurements have been performed to investigate the ordering mechanism and spin fluctuations in a geometrically frustrated heavy-fermion antiferromagnet on the Kagome-like lattice CePdAl. This compound shows a partially ordered state below $T_{\rm N}$ (2.7 K), in which 1/3 of the Ce moments remain paramagnetic. Whether the remaining paramagnetic moments order at lower temperature or not has been controversial. Our NMR measurements show that the nuclear magnetic relaxation rate $T_{1}^{-1}$ is proportional to the temperature below 0.2 K and there is no other phase transition down to 30 mK. The simulation of the spin echo spectra shows that the observed spectra are compatible with the partially ordered state and an incommensurate $k$-vector along $c$-axis down to the lowest temperature. These results demonstrate that the paramagnetic Ce moments in the partially ordered state are in a heavy-fermion state and consequently the partially ordered state is considered to be stable down to 0 K. The temperature dependence of $T_{1}^{-1}$ also shows that antiferromagnetic correlations develop from much higher temperatures above $T_{\rm N}$.

© 2008 The American Physical Society.