Changes in spin and lattice dynamics induced by magnetic and structural phase transitions in multiferroic ${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$

${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$ is a recently synthesized homolog of multiferroic ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$. Upon cooling, ${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$ undergoes a series of structural and magnetic phase transitions from cubic to rhombohedral symmetry, and to an incommensurately modulated crystal structure, which is connected with charge and orbital ordering of the Mn cations. We report infrared, terahertz, and Raman spectra of ${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$ ceramics reflecting corresponding changes in phonon selection rules, including new phonons appearing in spin-order-induced ferroelectric phases. The observed phonon activities are compared with the predictions from the factor-group analysis. In the high-temperature phase, more phonons are observed than the number predicted for the cubic symmetry. This is explained by the presence of rhombohedral clusters in the cubic phase. The strongest variations occur in THz spectra near the two magnetic phase transitions, at $T_{\mathrm{N}1}=87\mathrm{K}$ and $T_{\mathrm{N}2}=63\mathrm{K}$. These activate new modes in the spectra, with resonance frequencies and intensities changing with temperature and magnetic field. Below $T_{\mathrm{N}2}$, we observed a transfer of oscillator strengths from low-frequency phonons to these excitations, which we assign to electromagnons.

Figure 1

Temperature dependence of IR reflectivity spectra of ${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$ ceramics.

Figure 2

Temperature dependence of complex refractive index of ${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$ ceramics obtained by THz time-domain spectroscopy.

Figure 3

(a) Temperature dependence of the low-frequency excitations observed in THz and IR spectra of ${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$. The open symbols correspond to spin excitations or phonons activated by breaking of the inversion center, whereas the remaining phonons are marked by solid symbols. (b) Temperature dependence of the mode plasma frequencies ${\mathrm{\Omega }}_j=\sqrt{\mathrm{\Delta }{\varepsilon }_j}{\omega }_j$. The symbols correspond to those used for the modes in (a).

Figure 4

Magnetic field dependence of complex refractive index of ${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$ ceramics at selected temperatures. At 10 K, the mode frequencies are marked by arrows. The spectra were measured in the Voigt geometry, where the electric vector ${\mathbf{E}}^{\omega }$ of the THz beam was parallel to external magnetic field $\mathbf{H}$. The ${\mathbf{E}}^{\omega }\perp \mathbf{H}$ spectra (not shown) were the same within the experimental error.

Figure 5

Temperature dependence of Raman scattering spectra of ${\mathrm{SrMn}}_7{\mathrm{O}}_{12}$ ceramics with the parallel polarizer and analyzer.