  Based on results of density functional theory (DFT) calculations with the
local spin density approximation (LSDA) and the generalized gradient
approximation (GGA), we propose a new magnetic material, CsCl-type FeSe. The
calculations reveal the existence of ferromagnetic (FM) and antiferromagnetic
(AFM) states over a wide range of lattice constants. At 3.12\,{\AA} in the GGA,
the equilibrium state is found to be AFM with a local Fe magnetic moment of
$\pm 2.69\,\mu_\mathrm{B}$. A metastable FM state with Fe and Se local magnetic
moments of $2.00\,\mu_\mathrm{B}$ and $-0.032\,\mu_\mathrm{B}$, respectively,
lies 171.7\,{meV} above the AFM state. Its equilibrium lattice constant is
$\sim 2$\,{\%} smaller than that of the AFM state, implying that when the
system undergoes a phase transition from the AFM state to the FM one, the
transition is accompanied by volume contraction. Such an AFM-FM transition is
attributed to spin-density $z$-reflection symmetry; the symmetry driven AFM-FM
transition is not altered by spin-orbit coupling. The relative stability of
different magnetic phases is discussed in terms of the local density of states.
We find that CsCl-type FeSe is mechanically stable, but the magnetic states are
expected to be brittle.
