Quantized magnetization plateaux in the orthogonal dimer system SrCu₂(BO₃)₂

We carried out high-field magnetization measurements up to 57 T and down to 0.08 K on a single crystal of a two-dimensional spin-gap material SrCu₂(BO₃)₂. We successfully observed the predicted plateau at 1/3 of the total magnetization around 50 T, in which the magnetic superstructure is characterized by a novel stripe order of excited triplets. The 1/3 plateau is stable over much wider field region than the previously observed 1/4 and 1/8 plateaux. A sharp jump in the magnetization between 1/3 and 1/4 plateaux may be associated with a first-order transition. Furthermore, several anomalous behaviors were observed in a lower-field region including field-direction dependent magnetizations around 30 T. The obtained magnetizations are compared with theoretical calculations based on a hard-core boson model and on a perturbation expansion method. The magnetization measurements were also performed on 1% zinc-doped single crystal.