Spin ice

A spin ice is a substance that does not have a single minimal-energy state. It has "spin" degrees of freedom, i.e., it is a magnet, with frustrated interactions that prevent it from completely freezing. Spin ices show low-temperature properties, residual entropy in particular, closely related to those of crystalline water ice. The most prominent compounds with such properties are dysprosium titanate and holmium titanate. The magnetic ordering of a spin ice resembles the positional ordering of hydrogen atoms in conventional water ice.