Speaker
Description
The total energy of real magnetic materials always includes, in addition to magnetic energy, the static lattice energy and the energy of thermal atomic vibrations. Despite this, most of the theoretical work on magnetic systems considers only the magnetic energy, completely ignoring lattice energy contributions. To clarify the influence of lattice energy on the magnetic properties of localized quantum spin systems, we assume that the total Helmholtz free energy includes the static lattice energy in the form of a volume-dependent Morse potential, the vibrational energy in the form of Grüneisen quasi-harmonic modification of Einstein phonon theory, and the magnetic contribution, represented by the quantum isotropic Heisenberg model with distance/volume-dependent nearest-neighbors exchange interactions. Using these basic inputs, we derived an analytical formula for the total energy, utilizing the methodology previously applied to Ising systems with magnetoelastic interactions [1]. Having obtained the Helmholtz free energy of the system, we calculated equations of state and all relevant thermodynamic quantities, using standard methods of statistical mechanics. Subsequently we performed extensive numerical calculations for a special case of a cubic lattice. Numerical results indicate that the region of stability of different magnetic phases varies significantly with the strengthening/weakening of the magnetoelastic coupling in the system. We discuss in detail the dependence of critical boundaries on freely adjustable model parameters and show that, in addition to second-order phase transitions, the isotropic Heisenberg model with strong magnetoelastic coupling also exhibits first-order phase transitions. This phenomenon is not observed in the traditional isotropic Heisenberg ferromagnet without magnetoelastic coupling. We support our conclusions by investigating the temperature dependences of magnetization, thermal expansion coefficients, and the Helmholtz free energy of the system.
Acknowledgements
Funded by the EU NextGenerationEU through the Recovery and Resilience Plan for Slovakia under the project No. 09I03-03-V02-00021, and by the Ministry of Education, Science, Research and Sport of the Slovak Republic under the project VEGA 1/0695/23.
References
[1] T. Balcerzak et al., “Self-consistent model of a solid for the description of lattice and magnetic properties,” Journal of Magnetism and Magnetic Materials, vol. 426. Elsevier BV, pp. 310–319, Mar. 2017. https://doi.org/10.1016/j.jmmm.2016.11.107
