  The ordered phase of the FeNi system is known to have promising magnetic
properties as a rare-earth-free permanent magnet. Understanding the parameter
space that controls the order-disorder transformation is important to find
growth conditions that stabilize the $L1_0$ phase. Magnetic properties and
chemical order-disorder transformation of FeNi are investigated as a function
of lattice expansion using first-principles theory. Thermodynamic and magnetic
calculations are performed using long-range order parameters, which is used to
find the ordering temperature. Negative pressure promotes ordering and thus
synthetic routes involving an increase in volume is expected to expand the
stability field of the $L1_0$ phase.
