We report specific-heat and magnetocaloric-effect studies on single-crystalline Kondo helical magnet YbNi${}_3$Al${}_9$. Molecular field analysis of a Schottky peak due to the Zeeman splitting of the Yb-ion doublet crystalline-field ground state demonstrates that the interlayer antiferromagnetic exchange interactions are 2 orders of magnitude smaller than that of the intralayer ferromagnetic coupling among Yb ions, reflecting realization of magnetically well separated Yb layers. The Sommerfeld coefficient $\gamma$, which is 110 mJ/K${}^2$mol in zero field, decreases smoothly with increasing field without any noticeable anomalies at the helical magnetic phase boundary. This fact confirms that heavy quasiparticles are formed on a part of the Fermi surface away from “hot sheets” that have nesting instabilities responsible for the helical magnetic structure. These results indicate that YbNi${}_3$Al${}_9$ is a novel system where heavy quasiparticles are confined within the two-dimensional Yb layers.

• Received 26 July 2012

DOI:https://doi.org/10.1103/PhysRevB.86.155106