  A first-principles approach to the construction of concentration-temperature
magnetic phase diagrams of metallic alloys is presented. The method employs
self-consistent total energy calculations based on the coherent potential
approximation for partially ordered and noncollinear magnetic states and is
able to account for competing interactions and multiple magnetic phases.
Application to the Fe$_{1-x}$Mn$_x$Pt "magnetic chameleon" system yields the
sequence of magnetic phases at T=0 and the c-T magnetic phase diagram in good
agreement with experiment, and a new low-temperature phase is predicted at the
Mn-rich end. The importance of non-Heisenberg interactions for the description
of the magnetic phase diagram is demonstrated.
