Not a Simple Ferro-quadrupole Order in DyB₆

We performed detailed measurements of the thermal and magnetic properties in the ferroquadrupole (FQ) ordered state of DyB₆, which exhibits the antiferromagnetic (AFM) order at T_N = 26 K and the O₂ ⁰½111 -type FQ order at T_Q = 32 K. We also performed the mean-field calculation for the two-sublattice model in which the crystalline electric field (CEF) effect and AFM and FQ interactions are taken into account, focusing on the FQ ordered state. We could estimate the CEF level scheme to reproduce the temperature dependence of the magnetic susceptibility and specific heat. We could also reproduce the temperature dependence of the specific heat in the ordered state by the calculation quite well. However, the thermal and magnetic properties in magnetic fields, i.e., a decrease in the magnetic susceptibility below T_Q, a nearly temperature-independent behavior of the electrical resistivity between T_N and T_Q, the magnetic phase diagram for H ∥ [111] and so forth could not be reproduced by the calculation where the dependence of the domain distribution on the magnetic field direction is taken into account.