Unusual Magnetic Phase Diagram of Ce_xLa_1-xB₆ ( x= 0.5, , 0.75) Studied by the Magnetoresistance

Abstract

We have studied the magnetoresistance of Ce_xLa_1-xB₆ (x=0.25, , 0.5, , 0.75) single crystals under the magnetic fields up to 15, T and in a temperature range between 0.4, K and 15, K. In Phase I (paramagnetic), in all the samples the negative magnetoresistance is observed, whose magnitude becomes larger with decreasing temperature. However, in Ce_0.5La_0.5B₆, the magnetoresistance below T-- 1, K becomes very small neverthless the negative magnetoresistance is observed above -- 1, K. This is completely different from the magnetoresistance in Ce_0.25La_0.75B₆ where the magnitude of the negative magnetoresistance becomes larger with decreasing temperature. The weak magnetic field dependence of the electrical resistivity is observed also in Phase IV in Ce_0.75La_0.25B₆. These results indicate that the ground state of Ce_0.5La_0.5B₆ is not the Kondo singlet but the AF (antiferro) magnetic state called Phase VI^′ is realized below -- 1, K. Phase IV^′ is similar to Phase IV in Ce_0.75La_0.25B₆. The anisotropy of the phase boundaries between I, II (AFQ (antiferroquadrupolar) ordered), III (antiferromagnetic ordered) and IV^′ in Ce_0.5La_0.5B₆ 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 discussed from the stand point that the magnetic anisotropy and the intereference effect between the AFQ and AF interaction are important. We propose the mechanism of the phase transition from IV^′ to III or II.