Vibrational spectra of multiferroics with Y- and Z-type hexaferrite structures

We investigated polarized infrared and Raman spectra of the multiferroic hexaferrites BaSrCoZnFe11AlO22 (Y-type) and (Z-type) on cooling from room temperature down to 4 K, and we compared the observed numbers of phonons with factor-group analyses of their lattice vibrations. All phonons can be explained within non-polar space groups and for Y- and Z-type hexaferrites, respectively. Thus, the vibrational spectra provide no evidence of ferroelectric order in both kinds of hexaferrites, although their multiferroic properties with anomalously strong magnetoelectric coupling were reported; consequently, the crystal structure should be non-centrosymmetric. The ferroelectric distortions and breakings of the inversion symmetry are probably very small, therefore polar phonons become very weak in the Raman spectra, and Raman modes originally active in non-polar phases are also weak in the infrared spectra. In contrast, electromagnons are both infrared- and Raman-active, as expected for polar (ferroelectric) phases. Based on the electromagnon activities in infrared and THz spectra, polar point groups of both hexaferrite systems are proposed.