Competition between the antiferro-quadrupolar and antiferro-exchange interactions in CexLa1-xB6

We have studied the magnetic phase diagram of CexLa1-xB6 (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 Ce0.5La0.5B6, a sharp peak of the specific heat was observed at the transition temperature T-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 similar to 6 T in the previous results of the magnetization. The clear anisotropy of T-Q depending on the applied field direction wats observed. In Ce0.75La0.25B6, the existence of a phase called phase IV recently discovered by Goto and Sakakibara was confirmed by the measurements of the electrical resistivity rho. 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 parallel to[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.