Liquid-gas critical behavior in a frustrated pyrochlore ferromagnet

The newly discovered frustrated pyrochlore ferromagnet ("spin ice") is investigated using Monte Carlo simulations. The ground state is disordered and infinitely degenerate, but in an applied magnetic field the degeneracy is broken and ordering can occur. We find that the degree of degeneracy breaking is highly dependent on the field direction, and one direction in particular leads to a complete and spontaneous lifting of the degeneracy. We show that this occurs via a new type of magnetic phase transition that does not involve a change in symmetry, and is related to the liquid-gas transition. These results are relevant to a wide number of pyrochlore compounds. [S0031-9007(98)07616-9].