American physicists from Yale University and the Los Alamos National Laboratory have discovered a special metal alloy, the properties of which resemble superfluids. This substance is a kind of spin ice, inside which analogs of quantum vortices appear. The article of scientists was published in the journal Nature Physics.
Spin ice is a substance in which the orientation of the magnetic moments of charged ions resembles the arrangement of hydrogen atoms (protons) in water ice. When water freezes, the atoms inside the tetrahedral cell of the crystal are arranged in such a way that the oxygen atom of one water molecule is surrounded by four protons. In this case, two protons are farther than others, because they belong to two other water molecules. Similarly, in spin ice, the magnetic moments of two ions are directed inward of the tetrahedron, and the other two - outward. In essence, spin ice is made up of tiny nanomagnets.
Physicists have studied a type of spin ice formed by the Shakti lattice. It allows many such configurations of magnetic moments at which the interaction energy in the cells is minimized. However, some of the configurations are in an excited state and their appearance in spin ice is inevitable. As a result, geometric frustration occurs - a phenomenon in which the entire system cannot be completely frozen (even at absolute zero), since it does not have a single base state. This behavior is typical for all spin ices.
During the study, the scientists performed spin ice photoemission electron microscopy (PEEM) made from permalloy, an alloy of iron and nickel (Ni81Fe19). Irradiation of the sample with X-rays made it possible to record changes in the magnetic moments that occur with decreasing temperature. The spin ice sample was first cooled from 290 Kelvin (K) to 220 K, and then to 180 K and below (1 K corresponds to -272.15 degrees Celsius).
Although other types of spin ice rebuild their lattices with decreasing temperature in order to reach the lowest possible energy state, it turned out that the Shakti lattice "gets stuck" at a certain level. This happens even if a large-scale restructuring of the system could minimize energy. According to the conclusions of physicists, this indicates that this spin ice has a global topological order, and excitations are topologically protected from scattering and persist for a long time.
Topologically ordered phases have previously been described for quantum mechanical systems that can take on different states with the same energy (degenerate states). In other words, stable excitations in the Shakti lattice in a sense resemble quantum vortices in superfluids, which are also topologically protected due to the degeneracy of the ground states.
