LAUSR

Looking for a crystalline 2D spin ice Spin ices—materials in which local magnetic spins respect “ice rules” similar to those in water ice—are typically three-dimensional. Two-dimensional (2D) ice rules can also be formulated and have been found to be satisfied in engineered nanomagnetic systems, usually referred to as artificial spin ices. Zhao et al. used neutron scattering and thermodynamic measurements to study a crystalline candidate for a 2D spin ice, the intermetallic compound HoAgGe. They found that at low temperatures, the local spins on the distorted kagome planes of this quasi-2D material respect 2D ice rules. Increasing the temperature led to a series of transitions consistent with theoretical expectations. Science, this issue p. 1218 Neutron scattering and thermodynamic measurements suggest that HoAgGe is a crystalline two-dimensional spin ice. Spin ices are exotic phases of matter characterized by frustrated spins obeying local “ice rules,” in analogy with the electric dipoles in water ice. In two dimensions, one can similarly define ice rules for in-plane Ising-like spins arranged on a kagome lattice. These ice rules require each triangle plaquette to have a single monopole and can lead to different types of orders and excitations. Using experimental and theoretical approaches including magnetometry, thermodynamic measurements, neutron scattering, and Monte Carlo simulations, we establish HoAgGe as a crystalline (i.e., nonartificial) system that realizes the kagome spin ice state. The system features a variety of partially and fully ordered states and a sequence of field-induced phases at low temperatures, all consistent with the kagome ice rule.