Drastic Change of the Magnetic Phase Diagram of Ce_xLa_1-xB₆ between x = 0.75 and 0.5

抄録

We have studied the magnetoresistance and magnetic phase diagram of Ce_xLa_1-xB₆(x = 0.65, 0.7). The antiferromagnetic (AF) phase IV recently discovered in Ce_0.75La_0.25B₆, where it exists only in a narrow temperature and magnetic field region, extends down to T = 0 K for x = 0.7. The magnetoresistance in phase IV is much smaller than that in the AF phase III dominated by the antiferro-quadrupolar (AFQ) ordering. A first-order phase transition takes place at a critical magnetic field from phase IV to III and that field for H // ‹ 100 › is smaller than that for H // ‹ 110 ›. These can be explained by considering the difference of the free energy in magnetic fields between phases IV and III. A tetra-critical point seems to exist for x = 0.75 but it is separated into two tri-critical points by the IV-II boundary for H // ‹ 110 › in Ce_0.65La_0.35B₆.