Thermodynamic Fluctuations in Magnetic Phase Transition: Invar as a Prototype

TL;DR

This paper introduces a first-principles theoretical framework to analyze thermodynamic fluctuations in magnetic phase transitions, successfully explaining Invar anomalies in Fe3Pt, including negative thermal expansion and phase transition behaviors.

Contribution

It develops a novel first-principles formulism for spin fluctuations and applies it to Fe3Pt, revealing key magnetic and thermodynamic phenomena with predictive accuracy.

Findings

Predicted finite temperature mixing of magnetic configurations.

Identified a tri-critical point in the phase transition.

Explained Invar anomalies such as negative thermal expansion.

Abstract

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.