  High-temperature magnetic measurements have been carried out in
hydrothermally synthesized greigite (Fe3S4). We show that the Curie temperature
of greigite is significantly lower than that for its iron oxide counterpart
Fe3O4. The lower TC value (about 677 K) of greigite is in quantitative
agreement with that calculated using the exchange energy (3.25 meV) and the
spin values of the two sublattices, which are inferred from the neutron and
magnetization data of high-quality pure greigite samples. We further show that,
with an effective on-site Hubbard energy Ueff = 1.16 eV, the lattice constant
and two sublattice spins predicted from ab initio density-function theory are
in nearly perfect agreement with the measured values. The parameter Ueff = 1.16
eV ensures Fe3S4 to be an excellent half-metallic material for spintronic
applications.
