Role of thermal vibrations in magnetic phase transitions

This article addresses the contradiction that exists in studies on magnetic phase transitions between theoretical models that ignore the role of thermal vibrations and analysis of neutron diffraction data that always incorporates them. 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 leads to a unique consequence, 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. Diffraction data constrain all theoretical models to incorporate thermal vibrations and represent the exchange interaction as temperature dependent, J_ij (T), instead of the current practice, J_ij (0).