Low Temperature Magnetization of Yb2Pt2Pb with the Shastry--Sutherland Type Lattice and a High-Rank Multipole Interaction

We measured DC magnetization of the antiferromagnet Yb 2 Pt 2 Pb ($T_{\text{N}} \sim 2.1$ K) with the Shastry--Sutherland lattice for two in-plane directions $H\parallel [100]$ and [110] down to 0.08 K. Multiple metamagnetic transitions are found to develop in the field dependence of the magnetization $M(H)$ near the I--II phase boundary. Substantial anisotropy observed in $M(H)$ is well explained on the basis of a two-sublattice model with strong Ising anisotropy along the orthogonal [110] and [$1\bar{1}0$] directions arising from the wave function of the ground state Kramers doublet predominantly comprised of $|{\pm}7/2\rangle$, as recently proposed by Ochiai et al. In the high-field region of phase II, however, $M(H)$ increases nearly linear to $H$ and the full saturation is reached by a second-order transition even at the base temperature of 0.08 K, just like a spin flop state of an ordinary antiferromagnet. The results are interpreted by a pseudospin-flop state, which can be stabilized by a high-rank multipole, possibly a rank-7 octacosahectapole (128 pole), interaction between the Yb ions.