Subterahertz and terahertz spin and lattice dynamics of the insulating ferromagnet PbMnBO4

DOI:https://doi.org/10.1103/PhysRevResearch.4.013004

Orthorhombic ${\mathrm{PbMnBO}}_4$ is a rare example of an insulating ferromagnet in which the magnetic properties, in particular the strong magnetic anisotropy, are determined by the ${\mathrm{Mn}}^{3+}$ Jahn-Teller ion. Here, we report on the detailed investigation of magnetic excitations in the subterahertz and terahertz frequency ranges, as well as lattice excitations in the terahertz range. Using polarized azimuthally resolved Raman spectroscopy, four branches of magnetic excitations were observed and assigned to one acoustic and three optical one-magnon modes. The coupling between ferromagnetic ordering at $T_C=31K$ and the Jahn-Teller distortions of $\left[{\mathrm{MnO}}_6\right]$ octahedra due to the spin-phonon interaction was directly observed for some specific phonon modes involved in these distortions. A spin-wave Hamiltonian was developed that allowed us to determine a unique set of exchange and effective anisotropy parameters. The intensity ratio of the anti-Stokes to Stokes acoustic magnon peaks is found to be anomalous and directly indicates the involvement of the magneto-optical parameters in the magnetic Raman scattering process.