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arXiv · 2406.15574

Polar phonons and magnetic excitations in the antiferromagnet CoF$_2$

R. M. Dubrovin·A. Tellez-Mora·A. C. Garcia-Castro·N. V. Siverin·N. N. Novikova·K. N. Boldyrev·E. A. Mashkovich·Aldo H. Romero·R. V. Pisarev
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Abstract

The coupling between antiferromagnetic spins and infrared-active phonons in solids is responsible for many intriguing phenomena and is a field of intense research with extensive potential applications in the modern devices based on antiferromagnetic spintronics and phononics. Insulating rutile antiferromagnetic crystal CoF$_2$ is one of the model materials for studying nonlinear magnetophononics due to the strong spin-lattice coupling as a result of the orbitally degenerate ground state of Co$^{2+}$ ions manifested in the plethora of static and induced piezomagnetic effects. Here we report results on the complete infrared spectroscopy study of lattice and magnetic dynamics in CoF$_2$ in a wide temperature range and their careful analysis. We observed that infrared-active phonons demonstrate frequency shifts at the antiferromagnetic ordering. Furthermore, using first-principles calculations, we examined the lattice dynamics and disclosed that these frequency shifts are rather due to the spin-phonon coupling than geometrical lattice effects. Next we found that the low-frequency dielectric permittivity demonstrates distinct changes at the antiferromagnetic ordering due to the spontaneous magnetodielectric effect caused by the behavior of infrared-active phonons. In addition, we have observed magnetic excitations in the infrared spectra and identified their magnetodipole origin. To strengthen our conclusions, we analyze the theoretical phonon-magnon coupling overall phonons at the $Γ$ point. We conclude that the largest effect comes from the $A_{1g}$ and $B_{2g}$ Raman-active modes. As such, our results establish a solid basis for further investigations and more deeper understanding of the coupling of phonons with spins and magnetic excitations in antiferromagnets.

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R. M. Dubrovin, A. Tellez-Mora, A. C. Garcia-Castro, N. V. Siverin, N. N. Novikova, K. N. Boldyrev, E. A. Mashkovich, Aldo H. Romero, R. V. Pisarev. 2024-06-21. Polar phonons and magnetic excitations in the antiferromagnet CoF$_2$. https://doi.org/10.1103/physrevb.109.224312

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