Juraschek et al.[1,2] theoretically predicted that circularly polarized phonons should carry orbital magnetic moment even in diamagnetic or paramagnetic materials. The macroscopic orbital magnetic moment should be induced by the excitation of two perpendicularly polarized degenerate polar phonons using circularly polarized THz radiation.
We decided to pump the ferroelectric soft mode in quantum paraelectric KTaO₃ single crystal using extremely intense pulses with frequency 0.7 THz. The pulses with a duration of about 10 ps were generated by accelerator-based source with undulator at the TELBE beamline in the Helmholz Zentrum in Dresden-Rossendorf. The transient magnetic moment on the picosecond time scale was measured via magneto-optic Faraday effect at a probe wavelength of 800 nm. A possible breaking of inversion symmetry was sensed by measuring the second harmonic generation at 400 nm. The experiments were performed between 10 and 300 K.
At low temperatures, the THz pump frequency was close to the soft-mode frequency, and a strong transient birefringence was observed. However, THz pump-induced transient birefringence was detected not only using a circularly polarized THz beam, but also upon pumping with a linearly polarized beam. We explain this by the THz-field-induced ionic electro-optic Kerr effect which was recently observed also in the quantum paraelectric SrTiO₃.[3] In KTaO₃, a transient polarization and the related broken inversion symmetry were proved also by measuring the transient second harmonic generation.
In summary, we demonstrated that a transient polarization and magnetization can be induced simultaneously, by a strong resonant excitation of polar phonons in a diamagnetic and quantum paraelectric crystal KTaO₃. Similar effects were recently observed in SrTiO₃.[3,4]

1. D.M. Juraschek 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. X. Li, T. Qiu, J. Zhang, E. Baldini, J. Lu, A.M. Rappe, and K.A. Nelson. Terahertz field-induced ferroelectricity in quantum paraelectric SrTiO₃, Science 364 (2019) 1079
4. M. Basini, M. Pancaldi, B. Wehinger, M. Udina, T. Tadano, M.C. Hoffmann, A.V. Balatsky, S. Bonetti, THz electric-field driven dynamical multiferroicity in SrTiO₃, arXiv:2210.01690

Alkali tantalates, such as KTaO₃ (KT) and LiTaO₃, are members of the ferroic family of lead-free compounds with potential applications for electronic components and tissue engineering, respectively. Their solid solution, (K,Li)TaO₃, wherein displacements of small Li ions on K sites generate strong local dipole moments, which couple electrostatically to the KT polar soft mode, has also been intensively studied. Therefore, both dielectric relaxations and ferroelectric phase transition were reported for heavily Li-doped KT. Only the relaxation was, however, reported for (K,Li)TaO₃ with Li content < 0.008 as well as for weakly (0.01–0.3%) Mn-doped KTaO₃ single crystals. Analogically, the origin of the relaxation was shown to be the off-centre displacement of dopant ions on the K site in one of the six <001>-type crystal directions. In addition to the single crystal results, low-frequency dielectric relaxations were further observed for Li- or Mn-doped KT ceramics. However, in contrast to K₀.₉₈Li₀.₀₂TaO₃ ceramics, no ferroelectric phase transition was detected in polycrystalline K₀.₉₈₅Mn₀.₀₁₅TaO₃. Moreover, the low-temperature dielectric relaxation strength or permittivity amplitude in Li-doped KT was reported to be enhanced by weak Cu or Ca donor co-doping, respectively, while the ferroelectric order and dipole-glass phase were reported to coexist in 1.2% Nb and 0.14% Li co-doped KT single crystals. However, no research on the dielectric response has been undertaken for Li and Mn co-doped KT. Therefore, KT single crystals with 0.27% Li and 0.15% or 0.31% Mn contents determined by inductively coupled plasma spectroscopy analysis are studied here using dielectric spectroscopy in a frequency range of 10²-10⁶ Hz at low temperatures. Both Li and Mn doping are found to induce separate low-frequency dielectric relaxations of comparable strength in contrast to other co-doped KTaO₃ single crystals. The relaxation dynamics follow the Arrhenius law with activation energy values of ~ 77 and 107 meV, attributing the relaxation origin to the dipoles formed by the off-centre Li and Mn ions, respectively.

The lack of knowledge about short-range order in sophisticated, hardened piezoelectric ceramics blurs the global understanding in how the microscopic world influences the material’s properties [1]. Even when certain symmetries are present on the large scale, depicting the local environment of atoms becomes particularly challenging, especially in the presence of disorder in the range of 1–4 Å [2]. In this context, we aim to use solid-state Nuclear Magnetic Resonance (ssNMR) to monitor structural short-range order changes occurring in Li0.16Na0.84NbO3 (LNN) solid solutions during systematically aging of the material as a function of temperature. Results using 1D and 2D 23Na Spectroscopy have demonstrated the existence of phases that have, so far, gone undetected by Pair Distribution Function [3]. Sodium local phases were quantified and interpreted as coexisting in P and Q orthorhombic phases and one amorphous phase. Furthermore, changes in the local environment encountered in ongoing experiments probing 93Nb surroundings have proven to be a fundamental tool for tracking the formation of intra- or intergranular precipitates in the aging process. In conclusion, solid-state NMR spectroscopy proves to be a powerful tool to fill the missing gap between short- and long-range order, mainly in the presence of local disorder.

[1] Egert S.; Zhang M.; Koruza J.; Groszewicz P. B.; Buntkowsky G. 23Na NMR Spectroscopic Quantification of the Antiferroelectric–Ferroelectric Phase Coexistence in Sodium Niobate. J. Phys. Chem. C. 2020, 124, 43, 23852–23858. DOI: 10.1021/acs.jpcc.0c07202

[2] Zhao C.; Prosandeev S.; Bellaiche Laurent.; Li F.; Zhang S.; Li S.; Jones J. L. Bridging the gap between the short-range to long-range structural descriptions of the lead magnesium niobate relaxor. Acta Mater. 2023, 258, 119171, DOI: 10.1016/j.actamat.2023.119171

[3] Zhao C.; Gao S., Kleebe H. J.; Tan X.; Koruza J.; Rödel J. Coherent Precipitates with Strong Domain Wall Pinning in Alkaline Niobate Ferroelectrics. Acta Mater. 2022, 34, 38, 2202379. DOI: 10.1016/j.actamat.2023.119171