Magnetic, structural, and electronic properties of the multiferroic compound FeTe2O5Br with geometrical frustration

We report electron spin resonance (ESR), Raman scattering, and interband absorption measurements of the multiferroic FeTe2O5Br with two successive magnetic transitions at T-N1 = 11.0 K and T-N2 = 10.5 K. ESR measurements show all characteristics of a low-dimensional frustrated magnet: (i) the appearance of an antiferromagnetic resonance (AFMR) mode at 40 K, a much higher temperature than T-N1, and (ii) a weaker temperature dependence of the AFMR linewidth than in classical magnets, Delta H-pp(T) proportional to T-n with n = 2.2-2.3. Raman spectra at ambient pressure show a large variation of phonon intensities with temperature while there are no appreciable changes in phonon numbers and frequencies. This demonstrates the significant role of the polarizable Te4+ lone pairs in inducing multiferroicity. Under pressure at P = 2.12-3.04 GPa Raman spectra undergo drastic changes and absorption spectra exhibit an abrupt drop of a band gap. This evidences a pressure-induced structural transition related to changes of the electronic states at high pressures.