Specific heat and magnetization measurements were performed precisely on the antiferroquadruplar (AFQ) ordering compound DyB2C2 in order to reexamine its magnetic B-T phase diagrams. We find new phase boundaries in both diagrams with B ∥ [1 0 0] and B ∥ [1 1 0], and revise the field dependency of some boundaries. In the resultant diagrams, all the phase have closed regions in the B-T space. Two phase boundaries start from TN in both diagrams with B ∥ [1 0 0] and B ∥ [1 1 0]. The one decreases its transition temperature with increasing field which is the same behavior just to normal antiferromagnets. The other gradually increases its transition temperature with increasing fields, which seems to satisfy behavior demanded for magnets with spontaneous magnetizations. The new-found boundary divides the previously reported AFQ phase into two phases. The one has magnetically ordered moments on the AFQ ordered quadrupoles and exists only in magnetic fields. The other is a pure AFQ phase with paramagnetic moments on the ordered quadrupoles.
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