Introducing an extended spin–lattice coupling model with application to thermodynamic properties
This topic stems from our project, “Theoretical study of the effects of spin–lattice coupling in various geometrical frustrated magnets”. Pursuing a theoretical study of spin–lattice–coupling (SLC) from an experimentalist’s perspective, we have come to recognize that the two canonical frameworks, the bond-phonon model and the site-phonon model, each have strengths and limitations. Specifically, the bond-phonon model, which assumes independent modulation of each bond, can describe macroscopic magnetostriction, yet it can fail to reproduce magnetic long-range order and complex magnetic phase transitions observed in real materials. Conversely, the site-phonon model, which assumes independent displacements at each lattice site, incorporates effective further-neighbor interactions arising from SLC and thus excels at capturing magnetic long-range order, although it does not take into account macroscopic magnetostriction. Against this backdrop, it is necessary to develop a unified theoretical framework to provide a consistent description of a broader range of experimental observations related to SLC.
Accordingly, we introduced an “extended SLC model” that fuses the bond-phonon and site-phonon models. In addition to the overall strength of the SLC, we introduced a phenomenological parameter η that characterizes the site-phonon contribution relative to the bond phonon, and we investigated in detail the theoretical phase diagram and thermodynamic responses of a pyrochlore antiferromagnet. Comparison with high-field thermodynamic measurements on the model compound CdCr2O4 reveals that a variety of phenomena, including negative thermal expansion and a sharp specific-heat peak in the 1/2-magnetization plateau phase as well as two-step metamagnetic transitions below saturation, are all reproduced with a single parameter value, η = 0.6. These results suggest that the extended SLC model provides an effective, unified description of thermodynamic properties in real materials. The proposed framework is broadly applicable not only to the pyrochlore lattice but also to a wide range of lattice geometries, and further validation of its utility is anticipated.
Accordingly, we introduced an “extended SLC model” that fuses the bond-phonon and site-phonon models. In addition to the overall strength of the SLC, we introduced a phenomenological parameter η that characterizes the site-phonon contribution relative to the bond phonon, and we investigated in detail the theoretical phase diagram and thermodynamic responses of a pyrochlore antiferromagnet. Comparison with high-field thermodynamic measurements on the model compound CdCr2O4 reveals that a variety of phenomena, including negative thermal expansion and a sharp specific-heat peak in the 1/2-magnetization plateau phase as well as two-step metamagnetic transitions below saturation, are all reproduced with a single parameter value, η = 0.6. These results suggest that the extended SLC model provides an effective, unified description of thermodynamic properties in real materials. The proposed framework is broadly applicable not only to the pyrochlore lattice but also to a wide range of lattice geometries, and further validation of its utility is anticipated.
関連論文
[A] M. Gen, H. Suwa et al., arXiv:2508.13535