Juraschek et al. [1,2] theoretically showed that resonant pumping of double- or triple-degenerate phonons using circularly polarized THz radiation can induce circular motion of ions, which in turn can generate a magnetic moment measurable via the magneto-optical Faraday effect. This phenomenon can, in theory, occur even in non-magnetic materials such as the quantum paraelectrics SrTiO₃ and KTaO₃. Based on the assumption that the time-dependent polarization P associated with the circular motion of polar phonons generates a magnetization M ∝ P × ∂ₜP, the concept was termed dynamical multiferroicity.1
Transient THz-field-induced magnetization in SrTiO₃ was recently observed by Basini et al. [3], who pumped the soft mode at room temperature using circularly polarized THz radiation. However, the detected magnetization was surprisingly four orders of magnitude higher than predicted by theory.2 Several theoretical explanations have been proposed for this unexpectedly strong effect, but these require further experimental validation.
We decided to study the THz-field-induced Faraday effect (TFE) in KTaO₃. Compared to SrTiO₃, KTaO₃ has the advantage of remaining cubic at all temperatures, enabling low-temperature measurements where soft mode damping is significantly reduced compared to room temperature. In this regime, the effective charge of the soft phonon is large, allowing efficient coupling with the THz field. This leads to greater phonon amplitudes and higher values of the phonon magnetic moment.
For our measurements, we used intense circularly polarized pulses at 0.7 THz, generated by an accelerator-based source with an undulator at the TELBE beamline of the Helmholtz-Zentrum Dresden-Rossendorf. The transient magnetic moment on the picosecond timescale was measured via the magneto-optic Faraday effect using a probe wavelength of 800 nm. For comparison with Ref. 3, we also performed measurements at room temperature using a 3 THz pump beam generated by an optically nonlinear crystal on a tabletop spectrometer.
We found that the TFE signal is always mixed with the electro-optic THz-field-induced Kerr effect (TKE) signal, which we also measured directly using a linearly polarized THz pump beam. This allowed us to quantitatively compare the TKE contribution to the overall signal containing the TFE response. Our measurements convincingly demonstrate the magnetic moment of a circularly polarized soft phonon in KTaO3.
[1] D. M. Juraschek, M. Fechner et al. Dynamical multiferroicity, Phys. Rev. Mat. 1 (2017) 014401.
[2] D.M. Juraschek, N.A. Spaldin, Orbital magnetic moments of phonons, Phys. Rev. Mat., 3 (2019) 064405.
[3] M. Basini et al. Terahertz-field driven dynamical multiferroicity in SrTiO3, Nature 628, 2024, 534.