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 dis ordered and infinitely degenerate, but in an applied magnetic field th e degeneracy is broken and ordering can occur. We find that the degree of degeneracy breaking is highly dependent on the field direction, an d one direction in particular leads to a complete and spontaneous lift ing of the degeneracy. We show that this occurs via a new type of magn etic 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].