Magnetic Phase Diagram of Ce_xLa_1-xB₆ Studied by Static Magnetization Measurement at Very Low Temperatures

DC magnetization of single crystals of Ce_XLa_1−XB₆, 0.5 ≤ x ≤ 1, have been measured at low temperatures (T) down to 40 mK in magnetic field H∥[100], and the H-T-x phase diagrams were obtained. At zero field, the antiferro-quadrupolar (AFQ) transition temperature T_Q rapidly decreases with decreasing x and vanishes abruptly between x = 0.75 and 0.7. The Néel temperature T_N, which is lower than T_Q in CeB₆, varies more slowly with x and exceeds T_Q for x ≳ 0.8. A clear peak observed in the susceptibility strongly suggests that a new antiferromagnetic (AF) state having no AFQ moment (phase IV) exists for 0.8 ≳ x ≳ 0.5, in addition to the two ordered phases II (AFQ) and III (AF+AFQ). As H increases, a first-order phase IV-III transition is observed with a metamagnetic jump in the magnetization. Successive I (paramagnetic)TV-III phase transitions are observable in a very narrow region of the H-T-x space.