Magnetic properties of layered borocarbides RB 2 C (RϭDy, Ho, and Er͒ have been studied by the magnetization and specific heat measurements at 1.8 -300 K under the field up to 5 T. ErB 2 C has a basic twosublattice antiferromagnetic order below T N ϭ16.3 K. HoB 2 C and DyB 2 C show the three-dimensional magnetic order below T C ϭ7.0 K and 8.5 K, respectively. However, they have still large magnetic fluctuations below T C . The magnetic ordering temperatures of RB 2 C (RϭDy, Ho, and Er͒ do not

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... llow the de Gennes relation, where the ordering temperatures of HoB 2 C and DyB 2 C are suppressed. The suppressions of ordering temperature and the unconventional fluctuating ground states of HoB 2 C and DyB 2 C originate from the spin frustration effects. The instability of the unconventional magnetic phase associated with the frustration is significant in this series; the fluctuating ground state results from the complex spin-spin interactions or spin-quadrupole interactions.