Role of thermal vibrations in magnetic phase transitions

TL;DR

This paper demonstrates that thermal vibrations are crucial for accurately modeling magnetic phase transitions, resolving a long-standing contradiction by showing they make exchange interactions temperature-dependent and ensure consistent experimental and theoretical order parameters.

Contribution

It is the first to show that neglecting thermal vibrations causes multiple-valued order parameters, emphasizing their essential role in magnetic phase transition models and neutron diffraction analysis.

Findings

Ignoring thermal vibrations leads to inconsistent order parameters.

Including thermal vibrations results in temperature-dependent exchange interactions.

Thermal vibrations are essential for accurate magnetic phase transition modeling.

Abstract

We address and resolve the fundamental contradiction that has existed from the earliest studies on magnetic phase transitions between theoretical models that ignore the role of thermal vibrations and represent the exchange interaction as a constant, Jij(0), and analysis of neutron diffraction data that always incorporates thermal vibrations even though it is also possible to analyze the same data by ignoring them. Of the two possibilities, ignoring thermal vibrations in both theoretical models and analysis of diffraction data leads to the latter giving different magnetic order parameters for different reciprocal lattice lines. This appears to be the first report of a unique consequence, viz. the assumption to neglect a physical phenomenon turns a single-valued experimental observable into a multiple-valued one where all values are equally valid. This assumption is clearly unacceptable and must be rejected. The second possibility of incorporating thermal vibrations in both leads to single-valued theoretical and experimental order parameters. Thus, analysis of neutron diffraction data constrain the exchange interaction in all theoretical models to be temperature dependent and represented as Jij(T). Additional experimental and theoretical evidences in support of this conclusion are presented.