Based on ab initio Monte Carlo approach, finite-temperature magnetism of tetragonal Fe is studied. It is shown that the ground state magnetic structure changes from ferromagnetic to antiferromagnetic as the structure transforms from bcc to fcc. The Curie temperature (TC) or Náel temperatures (TN) are very sensitive to the lattice distortion, which decreases from bcc to fcc and then increases over the fcc range. There are a maximum of TC at the bcc and a minimum of TN around the fcc limit due to the change of the coupling distance and symmetry consideration.
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