Appearance of the octupole ordered phase IV in CexLa1−xB6

Abstract

We investigated the physical properties of ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{B}}_{6}$ at $x\ensuremath{\sim}0.8$, below which the ${T}_{\ensuremath{\beta}}$-type antiferro-octupole (AFO) ordered phase IV appears as a result of the larger suppression rate of ${T}_{\mathrm{Q}}$ than ${T}_{\mathrm{N}}$ by La doping. The most important result is that while the peak of the specific heat at ${T}_{\mathrm{Q}}$ is rapidly suppressed and broadened by La doping, that at ${T}_{\mathrm{IV}}$ is sharp and large. This indicates that although the ${T}_{\ensuremath{\beta}}$-AFO order in the phase IV is robust against the local lattice distortion induced by La doping, the ${O}_{xy}$-type antiferroquadrupole (AFQ) ordered phase II is very weak. The ${T}_{xyz}$-AFO interaction is robust against La doping from the observation of the pronounced enhancement of ${T}_{\mathrm{Q}}$ even in a small $x$ region. Based on these La-doping effect of the multipole interactions, we carried out the mean-field calculation for the four-sublattice model to reproduce the magnetic phase diagrams of ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{B}}_{6}$. Based on the calculated results, we propose that the small splitting of the quartet is induced by La doping in phase I to explain the magnetic phase diagram for $x<0.65$. We could obtain the calculated results roughly consistent with the experimental results, although there appear new problems. We classified the mechanisms of the four different types of the competition among the four interactions with roughly the same magnitude, which induce the interesting and complicated properties in ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{B}}_{6}$.