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

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We measured DC magnetization of the antiferromagnet Yb2Pt2Pb (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.
2012-10-15