Unusual Enhancement of Magnetization by Pressure in the Antiferro-Quadrupole-Ordered Phase in CeB₆

Abstract

The effect of pressure on CeB₆ was investigated by the measurement of the magnetization (M) under pressure, and we obtained the following results. The effect of pressure on M in phase I is very small. By applying pressure, T_Q is enhanced, but T_N and the critical field from the antiferromagnetic (AFM) phase III to the antiferro-quadrupole (AFQ) phase II (H_c^III–II) are suppressed, as previously reported. The magnetization curve in phase III shows the characteristic shoulder at H∼H_c^III–II⁄2 at ambient pressure. This shoulder becomes much more pronounced by applying pressure. Both H_c^III–II and the magnetic field, where a shoulder is seen in the magnetization curve in phase III, are largely suppressed by pressure. In phase II, the M–T curve at a low magnetic field exhibits an unusual concave temperature dependence below T_Q down to T_N. Thus, we found that the lower the magnetic field, the larger the enhancement of M in both phases III and II. To clarify the origin of the unusual pressure effect of M, we performed a mean-field calculation for the 4-sublattice model using the experimental results of dT_Q⁄dP>0 and dT_N⁄dP<0 and assuming the positive pressure dependence of the T_xyz-antiferro-octupole (AFO) interaction. The characteristic features of the pressure effect of M obtained by the experiments could be reproduced well by the mean-field calculation. We found that the origin of the characteristic effect of pressure on CeB₆ is the change in the subtle balance between the AFM interaction and the magnetic field-induced-effective FM interaction induced by the coexistence of the O_xy–AFQ and T_xyz–AFO interactions under pressure.