Distinct field-induced ferroquadrupolar states for two different magnetic-field directions in DyNiAl

The hexagonal Dy-based compound DyNiAl undergoes ferromagnetic and antiferromagnetic-type magnetic phase transitions at T_C = 30 K and T₁ = 15 K, respectively. To investigate the 4f -electronic state and quadrupole interactions in DyNiAl, we carried out ultrasonic measurements versus temperature and applied magnetic field. The transverse elastic moduli C₄₄ and C₆₆ show a prominent elastic softening originating from an interlevel ferroquadrupolar-type interaction between the ground state and excited Kramers doublets, clarified by a crystal field analysis. In magnetic fields applied along the [100] and [001] axes, we observed a field-induced phase transition. Because the quadrupole interaction is enhanced in high magnetic fields according to our calculations, we suggest a magnetic-field-induced ferroquadrupolar ordering of the electric quadrupoles O_xy and O_yz for fields applied along [100] and [001], respectively, with different quadrupolar order parameters depending on the field direction.