UNUSUAL MAGNETIC PHASE-DIAGRAM OF CEXLA1-XB6 (X=0.5, 0.75) STUDIED BYTHE MAGNETORESISTANCE

We have studied the magnetoresistance of CexLa1-xB6 (x = 0.25, 0.5, 0. 75) single crystals under the magnetic fields up to 15 T and in a temp erature range between 0.4 K and 15 K. In Phase I (paramagnetic): in al l the samples the negative magnetoresistance is observed, whose magnit ude becomes larger with decreasing temperature. However, in Ce0.5La0.5 B6, the magnetoresistance below T similar to 1 K becomesvery small nev ertheless the negative magnetoresistance is observed above similar to 1 K. This is completely different from the magnetoresistance in Ce0.25 La0.75B6 where the magnitude of the negative magnetoresistance becomes larger with decreasing temperature. The weak magnetic field dependenc e of the electrical resistivity is observed also in Phase IV in Ce0.75 La0.25B6. These results indicate that the ground state of Ce0.5La0.5B6 is not the Kondo singlet but the AF (antiferro) magnetic state called Phase VI' is realized below similar to 1 K. Phase IV' is similar to P hase IV in Ce0.75La0.25B6. The anisotropy of the phase boundaries betw een I, II (AFQ (antiferroquadrupolar) ordered): III (antiferromagnetic ordered) and IV' in Ce0.5La0.5B6 has been studied. The phase boundary I-IV' does not show the anisotropy but that of III-II shows the large anisotropy depending on the applied field direction. These are discus sed from the stand point that the magnetic anisotropy and the interefe rence effect between the AFQ and AF interaction are important. We prop ose the mechanism of the phase transition from IV' to III or II.