Magnetic devil's staircase in UAgBi${}_2$

Materials characterized by competing interactions often exhibit a large number of nearly degenerate periodic states. Here we show that layered UAgBi${}_2$ hosts a cascade of field- and temperature-induced magnetic transitions. Based on specific heat, thermal expansion, and neutron diffraction, we construct a phase diagram that reveals at least seven nearly degenerate magnetic states in UAgBi${}_2$. The observed multi-step magnetization process can be understood by square-wave structures with distinct propagation vectors $\mathbf{k}$=(0, 0, $\mathit{\text{k}}$) in the presence of strong easy-axis anisotropy that confines the moments along the $\mathit{\text{c}}$ axis. Our findings are consistent with a magnetic devil’s staircase described by the axial next-nearest neighbor Ising (ANNNI) model and place UAgBi${}_2$ as a rare realization of the devil’s staircase in a 5$\mathit{\text{f}}$-electron system.