  By means of ab initio calculations based on the density functional theory we
investigated magnetic phase diagram of ordered FePd$_3$ alloy as a function of
external pressure.
  Considering several magnetic configurations we concluded that the system
under pressure has a tendency to non-collinear spin alignment. Analysis of the
Heisenberg exchange parameters $J_{ij}$ revealed strong dependence of iron-iron
magnetic couplings on polarization of Pd atoms. To take into account the latter
effect we built an extended Heisenberg model with higher order (biquadratic)
terms. Minimizing the energy of this Hamiltonian, fully parameterized using the
results of ab initio calculations, we found a candidate for a ground state of
compressed FePd$_3$, which can be seen as two interpenetrating "triple-Q"
phases.
