  We propose a first-principles formulism for system with spin fluctuations and
apply it to the ordered Fe3Pt to uncover the Invar anomalies, including
negative thermal expansion and spontaneous magnetization. The theory has
coherently predicted the finite temperature intermixing between the fully
ferromagnetic configuration and the spin-flipping configurations. We also
discover a tri-critical point at which a high-temperature second-order phase
transition, between the fully ferromagnetic configuration and the spin-flipping
configurations, becomes first-order at low temperatures.
