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Ms Styliani Skiadopoulou


Institute of Physics of the Czech Academy of Sciences
Department of Dielectrics
Na Slovance 1999/2, 182 21, Prague





Multiferroics for Tunable THz Devices



Among the novel materials studied, an intense effort is concentrated on the research in multiferroics, materials that present at least two of the ferroic properties: ferroelectricity, ferromagnetism and ferroelasticity (Figure 1).[1-3] A wide range of applications, such as information storage, sensing, actuation and spintronics, await pioneering materials and/or strategies that would produce robust magnetoelectric coupling. The magnetoelectric multiferroics' ability of magnetization manipulation via electric fields can be extremely promising for such applications, due to the simplicity and cost-efficiency of the use of the electric field.[4]


Figure 1 Schematic representation of the definition of multiferroics.[1]

In addition to the static magnetoelectric coupling, a dynamic magnetoelectric coupling can occur at the presence of elementary magnetoelectric excitations (Figure 2). Such excitations are also known as electromagons: electro-active magnons, which can be tuned by electric or magnetic fields.[5] The possibility of modulation of the index of refraction could promote the design of novel optoelectronic devices.

Due to the fact that the electromagnons very often lie at the THz range of the electromagnetic spectrum, THz spectroscopy is an essential tool for the detection of such excitations. However, a combination of spectroscopic techniques is required to be able to account for the nature of the detected excitations, since those can be pure magnons (i.e. contribute only to the magnetic permeability μ) or electromagnons (i.e. influence at least partially the permittivity ε). Thus, Raman spectroscopy, obeying to different selection rules, can be employed for the assignment of such modes. The simultaneous detection of spin excitations by both THz and Raman spectroscopies manifests the presence of electromagnons.


Figure 2 Artistic animation of a spin excitation (please click on it). (O. Waldmann, Freiburg University)

Here, two examples of materials with dynamical magnetoelectric coupling detected by THz spectroscopy are presented: bismuth ferrite BiFeO3 and nickel telluride Ni3TeO6.

BiFeO3, as one of the few single-phase RT magnetoelectric multiferroics, is the center of attention, as it presents a ferroelectric phase transition at approximately 1100 K and an antiferromagnetic one at 643 K.[6] The knowledge of lattice and spin excitations in BiFeO3 is essential for the understanding of the underlying mechanisms that induce its multiferroic behavior. A series of Raman and Infrared (IR) spectroscopy studies have presented controversial results concerning the assignment of the magnon and phonon modes, as well as of the highly acclaimed electromagnons. Our THz spectroscopy studies revealed five low frequency spin modes for a temperature range from 10 K up to RT, the highest two appearing at 53 and 56 cm-1 (Figure 3).[7] This corresponds to the frequency range where such excitations were theoretically predicted,[8,9] but not experimentally confirmed up to now. In Figure 3, one can also see the simultaneously Raman and IR active modes in BiFeO3, suggesting the presence of electromagnons.[10-12] In addition, at 5 K, the low-energy spin dynamics in the THz range were also studied in a varying magnetic field of up to 7 T. Softening of the (electro)magnon frequencies upon increasing the magnetic field was observed.


Figure 3 Temperature dependence of the THz spin excitation frequencies of BiFeO3,[7] compared with frequencies obtained from far IR spectra [8-10] and Raman scattering [11,12].


Ni3TeO6 presents a collinear antiferromagnetic order below 52 K, giving rise to spin-induced-ferroelectricity. Among the spin-order driven multiferroics, only Ni3TeO6 exhibits non-hysteretic colossal magnetoelectric effect near 8.5 T and 52 T, where spin-flop and metamagnetic phase transitions occur, respectively.[12,13] The lack of hysteretic behavior in the magnetic field dependence of magnetization and dielectric constant precludes losses for a series of magnetoelectric applications. In the current work, we investigated the spin and lattice excitations of Ni3TeO6 ceramics and single crystals. Infrared, time-domain THz and Raman spectroscopy experiments were conducted for a temperature range of 5 to 300 K. Time-domain THz spectroscopy at external magnetic field was carried out at selected temperatures below and close to the antiferromagnetic phase transition. The THz spectra revealed dynamic magnetoelectric coupling, i.e. tuning of THz spectra with magnetic field. Simultaneous Raman and IR active spin excitations correspond to electromagnons, highly sensitive on magnetic field (Figure 4(a) and (b)).

Figure 4 (a) Raman z(xy)z spec­tra for temperatures from 4 to 45 K. (b) Extinction coefficient from the THz spectra for selected temperatures and for Hext of 0 and 7 T. Two excitations are simultaneously seen in both Raman and THz spectra.


[1]     Spaldin & Fiebig, Science 309, 391 (2009)
[2]     M. Fiebig, J. Phys. D: Appl. Phys. 38, R123 (2005)
[3]     Y. Tokura, et al., Rep. Prog. Phys. 77, 076501 (2014)
[4]     W. Eerenstein, et al., Nature 442, 759 (2006)
[5]     A. Pimenov, et al., Nat. Phys. 2, 97 (2006)
[6]     G. Catalan and J. F. Scott, Adv. Mater. 21, 2463 (2009)
[7]     S. Skiadopoulou, et al., Phys. Rev. B 91, 174108 (2015)
[8]     U. Nagel, et al., Phys. Rev. Lett. 110, 257201 (2013)
[9]     R. S. Fishman, Phys. Rev. B 87, 224419 (2013)
[10]    D. Talbayev, et al., Phys. Rev. B 83, 094403 (2011)
[11]    M. Cazayous, et al., Phys. Rev. Lett. 101, 037601 (2008)
[12]    P. Rovillain, et al., Phys. Rev. B 79, 180411 (2009)
[13]    Y. S. Oh, et al., Nature Communications 5, 3201 (2014)
[14]    J.W. Kim, et al., Phys. Rev. Lett. 115, 137201 (2015)



List of Publications



  • Thin-Film Porous Ferroic Nanostructures: Strategies and Characterization, Alichandra Castro, Paula Ferreira, Stella Skiadopoulou, Liliana P. Ferreira, Margarida Godinho, Brian J. Rodriguez and Paula M. Vilarinho, In: Miguel Algueró, J. Marty Gregg, Liliana Mitoseriu, Nanoscale Ferroelectrics and Multiferroics: Key Processing and Characterization Issues, and Nanoscale Effects, Wiley, pp. 147-162 (2016)
The Chapter "Thin-Film Porous Ferroic Nanostructures: Strategies and Characterization" of the book "Nanoscale Ferroelectrics and Multiferroics" describes various strategies of synthesis of porous thin films, aiming at novel functionalizable nanostructures.
  • Optical and vibrational properties of (ZnO)k In2O3 natural superlattice nanostructures, Samuel Margueron, Jan Pokorny, Stella Skiadopoulou, Stanislav Kamba, Xin Liang and David R. Clark, J. Appl. Phys., 119, 195103 (2016)
Study of superlattice structures of ZnO-In2O3 by photoluminescence, optical and IR reflectivities, resonant and non-resonant Raman scattering as well as low-frequency Raman scattering.
  • Pb2MnTeO6 Double Perovskite: An Antipolar Anti-ferromagnet, Maria Retuerto, Stella Skiadopoulou, Man-Rong Li, Artem M. Abakumov, Mark. Croft, Alexander Ignatov, Tapati Sarkar, Brian M. Abbett, Jan Pokorný, Maxim Savinov, Dmitry Nuzhnyy, Jan Prokleška, Milinda Abeykoon, Peter W Stephens, Jason P. Hodges, Přemysl Vaněk, Craig J. Fennie, Karin M. Rabe, Stanislav Kamba, and Martha Greenblatt, Inorg. Chem., 55 (9), pp 4320–4329 (2016)

The article is presenting the novel double perovskite Pb2MnTeO6, a rare antipolar antiferromagnet.
  • Comment on “Interesting Evidence for Template-Induced Ferroelectric Behavior in Ultra-Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates”, S. Skiadopoulou, S. Kamba, J. Drahokoupil, J. Kroupa, N. Deepak, M. E. Pemble and R. W. Whatmore, Adv. Funct. Mater., 26, 642-646 (2016)

        This paper contradicts previous evidence of ferroelectricity in anatase-TiO2 strained thin films, emphasizing
        that piezoresponse force microscopy (PFM) is not sufficient for claiming ferroelectricity.

  • Spin and lattice excitations of thin film and ceramics, S. Skiadopoulou, V. Goian, C. Kadlec, F. Kadlec, X. F. Bai, I. C. Infante, B. Dkhil, C. Adamo, D. G. Schlom, and S. Kamba, Phys. Rev. B, 91, 174108 (2015)


This paper supports the existence of electromagnons in BiFeO3, due to simultaneous detection of spin 
excitations by infrared and Raman spectroscopies.