Korrelation von Phononen, Elektromagnonen und der magnetischen Ordnung in multiferroischen Oxiden
Zusammenfassung der Projektergebnisse
We performed detailed investigations by Raman spectroscopy on the composition-, temperature- and magnetic-field dependent properties of the orthorhombic Eu1−x Hox MnO3 (0 ≤ x ≤ 0.5) compounds, as well as the delafossite ACrO2 (A= Cu, Pd, Ag) compounds. For the RMnO3 compounds with R = Eu:Ho, we observed an interesting difference when compared to the Eu:Y series: For Eu:Y, the SPC strength decreases with increasing Y content. In contrast, the Eu:Ho samples show a relative maximum in SPC for x = 0.35, which coincides with the reorientation of the spin cycloid plane and show also the maximum electric polarization. We observed SPC effects up to 100 K, which indicates pronounced spin interactions at temperature far above TN ≈ 50 K. Together with the non-zero spectral weight of the EM, this phenomenon was interpreted as the onset of the local formation of spin cycloids even at this elevated temperature. In this scenario, the transition to the fully ordered cycloidal phase is not regarded as a sequence of abrupt transitions from the paramagnetic to the sinusoidal and eventually to the cycloidal phase, but can be explained by an orderdisorder transition. Besides, we observed and identified for the first time the B3g (1) mode. With this additional information, we derived an equation, which enabled for the first time the quantitative evaluation of the plane-to-plane SPC coupling constant along the b-direction. The magnetic-field dependent results on phonons and the EM show, that the effect of an external magnetic field is small, yet not negligible. The ACrO2 compounds show an interesting anomaly of phonon frequencies in the PdCrO2 sample. The behavior of these samples can not be integrated in a systematic way with respect to A-site mass, ionic radius or bond angle dependence. In comparison, the SPC effects of the ACrO2 compounds are much weaker than in the RMnO3 series (about one order of magnitude). We could show that the satellite peak of the Eg mode is continuously shifting away from the original Eg peak with decreasing temperature and coincides with the Eg mode peak at room temperature. A promising approach might therefore be to investigate the satellite peak of the Eg mode, as its appearance could be linked to a deformation of the nearly isotropic triangular lattice. The magnetic field dependence of the phonons shows an unexpected pronounced increase of phonon frequency with magnetic fields until 2-3 T. Further clarification by complementary experimental methods is needed to investigate if this originates from magnetic or lattice effects, or a possible cross-coupling of both. Eventually, it was not possible yet to detect any EM activity in the investigated ACrO2 samples. Further work is required, mainly focused on how signal strength could be optimized. The investigation of the EM and phonons with Raman- as well as infrared/THz spectroscopy will prove crucial to understand how spin- and lattice degrees of freedom and conversely magnetization and polarization couple in the multiferroic ACrO2 system.
Projektbezogene Publikationen (Auswahl)
- Optical phonons in multiferroic CuCrO2 studied by Raman and far-infrared spectroscopy. DPG Frühjahrstagung, Regensburg (2013)
M. Eichberger, J. Geurts, M. Schmidt, V. Tsurkan, J. Deisenhofer
- Raman spectroscopy and spin-phonon coupling of multiferroic Eu1-xHo2MnO3. DPG Frühjahrstagung, Berlin (2015)
S. Elsässer, V. V. Glushkov, A. M. Balbashov and J. Geurts
- Raman spectroscopy with spin-phonon coupling as a quasi-local probe of magnetic order in multiferroic Eu1-xHoxMnO3 compounds. Phonons 2015, Nottingham (2015)
S. Elsässer and J. Geurts
- Impact of temperature-dependent local and global ordering in AMnO3 for electromagnons and spin-phonon coupling. DPG Frühjahrstagung, Regensburg (2016)
S. Elsässer, M. Schiebl, A. Shuvaev, A. A. Mukhin, J. Geurts and A. Pimenov
- Lattice dynamics and spin-phonon coupling in orthorhombic Eu1−x Hox MnO3 (x ≤ 0.3) studied by Raman spectroscopy. Phys. Rev. B 93, 054301 (2016)
S. Elsässer, A. A. Mukhin, A. M. Balbashov and J. Geurts
(Siehe online unter https://doi.org/10.1103/PhysRevB.93.054301) - Spin-phonon coupling in ACrO2 (A=Cu, Ag, Pd) studied by Raman spectroscopy. DPG Frühjahrstagung, Regensburg (2016)
S. Elsässer, Anna Pimenov and J. Geurts
- Impact of temperatures-dependent local and global spin order in AMnO3 compounds for spin-phonon coupling and electromagnon activity. New Journal of Physics 19, 013005 (2017)
S. Elsässer, M. Schiebl, A. A. Mukhin, A. M. Balbashov, A. Pimenov and J. Geurts
(Siehe online unter https://doi.org/10.1088/1367-2630/aa55ed) - Magnetic correlations in Eu1-xHoxMnO3 (0 ≤ x ≤ 0.5) probed by Raman spectroscopy. DPG Frühjahrstagung, Dresden (2017)
S. Elsässer, A. A. Mukhin, A. M. Balbashov and J. Geurts
