Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3

Abstract

Terahertz and far-infrared electric and magnetic responses of hexagonal piezomagnetic YMnO 3 single crystals are investigated. Antiferromagnetic resonance is observed in the spectra of magnetic permeability μ a [H (ω) oriented within the hexagonal plane] below the Néel temperature T N . This excitation softens from 41 to 32 cm −1 upon heating and finally disappears above T N . An additional weak and heavily-damped excitation is seen in the spectra of complex dielectric permittivity ε c within the same frequency range. This excitation contributes to the dielectric spectra in both antiferromagnetic and paramagnetic phases. Its oscillator strength significantly increases upon heating toward room temperature, thus providing evidence of piezomagnetic or higher-order couplings to polar phonons. Other heavily-damped dielectric excitations are detected near 100 cm −1 in the paramagnetic phase in both ε c and ε a spectra, and they exhibit similar temperature behavior. These excitations appearing in the frequency range of magnon branches well below polar phonons could remind electromagnons, however their temperature dependence is quite different. We have used density functional theory for calculating phonon dispersion branches in the whole Brillouin zone. A detailed analysis of these results and of previously published magnon dispersion branches brought us to the conclusion that the observed absorption bands stem from phonon-phonon and phonon-paramagnon differential absorption processes. The latter is enabled by strong short-range in-plane spin correlations in the paramagnetic phase.

Writing and reading of an arbitrary optical polarization state in an antiferromagnet

“…This demonstrates that the light → magnetism → light one-to-one mapping is indeed applicable to arbitrary polarization states. Note that the damping factor of less than 0.03 is consistent with previously obtained values 22,23,25 , which should be low enough for ultrafast operation using femtosecond light pulses.…”

“…Figure 3b shows their spectrum, obtained by Fourier transform (see Methods). The peak at 1.3 THz with a width of 0.2 THz is a magnetic mode [22][23][24][25] , whereas the peaks at 4.2, 7.0, 7.5 and 9.2 THz are E 2 phonon modes [25][26][27] . As expected, the Z mode was not observed because S 3 = 0 and thus m z = 0.…”

“…1c. The frequencies of the in-plane 'X' and 'Y' modes are degenerate, with Ω X /2π = Ω Y /2π = 1.3 THz (refs [22][23][24][25]. The frequency of the out-of-plane 'Z' mode is Ω Z /2π = 56 GHz (ref.…”

Spin-phonon coupling and high-temperature phase transition in multiferroic material YMnO3

“…It was concluded that YMnO 3 is an improper ferroelectric. Various theoretical and experimental attempts have been made to understand the anomalous properties of YMnO3 as well as to understand the mechanism of phase transitions, multiferroicity, structure and dynamics of the compounds [33][34][35][36][37][38][39][40][41][42][43][44].…”

Magnons and crystal-field transitions in hexagonalRMnO3(R= Er, Tm, Yb, Lu) single crystals

“…The recent studies have been mostly limited to YMnO 3 and HoMnO 3 . 19,20 This paper presents spectra of magnons and crystal field (CF) excitations for a number of hexagonal manganites with (RE= Er, Tm, Yb, and Lu).…”