Competition between the antiferro-quadrupolar and antiferro-exchange interactions in ${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{B}}_6$

We have studied the magnetic phase diagram of ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{La}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{B}}_6$ (x=1, 0.75, 0.5), focusing on the anisotropy of the phase boundary. The phase diagrams for x=1 and 0.5 were determined from the specific-heat measurements under the magnetic fields up to 15 T. In ${\mathrm{Ce}}_{0.5}$ ${\mathrm{La}}_{0.5}$ ${\mathrm{B}}_6$, a sharp peak of the specific heat was observed at the transition temperature ${\mathrm{T}}_{\mathrm{Q}}$ from the antiferro-quadrupolar ordered phase (II) to the paramagnetic phase (I) even at 15 T, while the phase boundary I–II was not recognized above ∼6 T in the previous results of the magnetization. The clear anisotropy of ${\mathrm{T}}_{\mathrm{Q}}$ depending on the applied field direction was observed. In ${\mathrm{Ce}}_{0.75}$ ${\mathrm{La}}_{0.25}$ ${\mathrm{B}}_6$, the existence of a phase called phase IV recently discovered by Goto and Sakakibara was confirmed by the measurements of the electrical resistivity ρ. The results of the thermal expansion showed that the lattice distortion in phase IV is very small, being different from the large lattice distortion in phase III. It was found that the phase boundary I–IV is insensitive but that of III–II is sensitive to the applied field direction. The phase boundary III–II for H∥〈001〉 shows the unusual increase with magnetic field in a low-field region. These results are discussed from the standpoint that the interference effect between the quadrupolar and exchange interactions which depends on the field direction is important.