Spin ice

A spin ice is a magnetic substance that does not have a single minimal-energy state. It has magnetic moments (i.e. "spin") as elementary degrees of freedom which are subject to frustrated interactions. By their nature, these interactions prevent the moments from exhibiting a periodic pattern in their orientation down to a temperature much below the energy scale set by the said interactions. Spin ices show low-temperature properties, residual entropy in particular, closely related to those of common crystalline water ice.^[1] The most prominent compounds with such properties are dysprosium titanate (Dy₂Ti₂O₇) and holmium titanate (Ho₂Ti₂O₇). The orientation of the magnetic moments in spin ice resembles the positional organization of hydrogen atoms (more accurately, ionized hydrogen, or protons) in conventional water ice (see figure 1).

Experiments have found evidence for the existence of deconfined magnetic monopoles in these materials,^[2][3][4] with properties resembling those of the hypothetical magnetic monopoles postulated to exist in vacuum.