Anomalous Magnetic Phase Diagrams of HoB_<b>2</b>C_<b>2</b>: Antiferroquadrupolar Ordering in the Magnetic Phase

Abstract

Magnetic properties of the single-crystalline tetragonal HoB 2 C 2 compound were investigated. An antiferromagnetic transition occurs at 5.8 K. At 5.0 K, a spin-reorientation transition of the first order occurs, and spontaneous magnetizations appear along the a - and [1 1 0]-axes. Magnetization processes below 5.0 K are well scaled down from those of the antiferroquadrupolar ordering compound DyB 2 C 2 and are well interpreted by postulating the magnetic structure similar to that of DyB 2 C 2 . The phase below 5.0 K is the one with competitive coexistence of the antiferromagnetic and antiferroquadrupolar order. The result implies that the antiferroquadrupolar ordering transition occurs below the antiferromagnetic transition temperature and that a pure antiferroquadrupolar ordered phase only in the magnetic fields exists along the [1 1 0]-axis.