We present results of a multimethod investigation of the polar antiferromagnets Ni2CoTeO6 and NiCo2TeO6, inspired by the colossal magnetoelectric effect present in Ni3TeO6. Both compounds crystalize in the same polar space group R3 as Ni3TeO6, preserving the crystal symmetry at least from room temperature down to 2 K. Ni2CoTeO6 and NiCo2TeO6 undergo antiferromagnetic phase transitions at T-N = 55 and 52 K, and spin-flop transitions at an external magnetic field of approximately 8 and 4 T, respectively. Both compounds present an incommensurate antiferromagnetic helical structure with spins lying in the ab plane, in contrast to the collinear one along the c axis in Ni3TeO6. Moreover, dielectric anomalies are observed at their antiferromagnetic phase transitions, suggesting a magnetoelectric behavior. Spin and lattice dynamics studies by a combination of infrared, Raman, and terahertz spectroscopies were performed. Below T-N, in both Ni2CoTeO6 and NiCo2TeO6, low-frequency spin excitations extremely sensitive to external magnetic field were observed. At least one of these magnons was simultaneously seen in Raman and THz spectra of both compounds, therefore we propose to assign them to electromagnons.

Employing a combination of infrared, Raman and time-domain THz spectroscopies, we studied spin and lattice excitations in Ni 3 TeO 6 and its isostructural compounds obtained by substitution of Ni with Mn or Co. Low-temperature THz spectra revealed THz-range excitations sensitive to magnetic field; some of them can be assigned to electromagnons.

We present a structural and spectroscopic study of the compound Ni2MnTeO6, closely related to the polar antiferromagnet Ni3TeO6 known to show a colossal magnetoelectric effect and pronounced elementary magnetoelectric excitations. We prepared single crystals and polycrystalline samples of Ni2MnTeO6 showing the same polar structure as Ni3TeO6 from room temperature down to 4 K with the R3 space-group symmetry. Magnetic and dielectric measurements have indicated an antiferromagnetic phase transition at T-N approximate to 70 K, almost 20 K higher than that of Ni3TeO6. Extensive infrared, Raman, and terahertz spectroscopy experiments were employed for investigating lattice and spin excitations, revealing all phonons predicted by the factor group analysis. Terahertz spectra below TN reveal one new excitation, which is strongly influenced by external magnetic field, thus assigned to a magnon.

Maria Retuerto,1,2 Stella Skiadopoulou,3,4 Fedir Borodavka,3 Christelle Kadlec,3 Filip Kadlec,3 Jan Prokleška,5 Zheng Deng,1 Jose A. Alonso,6 Maria T. Fernandez-Diaz,7 Felix O. Saouma,8 Joon I. Jang,9 Dominik Legut,4 Stanislav Kamba,3 and Martha Greenblatt1,* 1Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, USA 2Grupo de Energía y Química Sostenibles, Instituto de Catálisis y Petroleoquímica, CSIC. C/Marie Curie 2, 28049, Madrid, Spain 3Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 18221 Prague 8, Czech Republic 4IT4Innovations Center, VSB Technical University Ostrava, 17 listopadu 15, CZ-708 33 Ostrava-Poruba, Czech Republic 5Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic 6Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, Spain 7Institut Laue Langevin, BP 156X, Grenoble F-38042, France 8Kaimosi Friends University College, P.O. Box 385, Kaimosi-50309, Kenya 9Department of Physics, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 04107, South Korea

The spin-order-induced ferroelectric antiferromagnet Ni3TeO6 transcends the magnetoelectric performance of all other single-phase multiferroics because it exhibits nonhysteretic colossal magnetoelectric coupling [Y. S. Oh, S. Artyukhin, J. J. Yang, V. Zapf, J. W. Kim, D. Vanderbilt, and S.-W. Cheong, Nat Commun. 5, 3201 (2014)]. We investigated spin and lattice excitations in Ni3TeO6 by a combination of infrared, Raman, and THz spectroscopies. Two spin excitations (near 13 and 35 cm(-1)) were observed simultaneously in Raman and time-domain THz spectra below the Neel temperature T-N = 53 K. We propose to assign them to electromagnons, which are activated by the dynamic magnetoelectric coupling. A third magnon is seen only in the Raman spectra near 206 cm(-1).

Pb2MnTeO6, a new double perovskite, was synthesized. Its crystal structure was determined by synchrotron X-ray and powder neutron diffraction. Pb2MnTeO6 is monoclinic (I2/m) at room temperature with a regular arrangement of all the cations in their polyhedra. However, when the temperature is lowered to ∼120 K it undergoes a phase transition from I2/m to C2/c structure. This transition is accompanied by a displacement of the Pb atoms from the center of their polyhedra due to the 6s2 lone-pair electrons, together with a surprising off-centering of Mn2+ (d5) magnetic cations. This strong first-order phase transition is also evidenced by specific heat, dielectric, Raman, and infrared spectroscopy measurements. The magnetic characterizations indicate an anti-ferromagnetic (AFM) order below TN ≈ 20 K; analysis of powder neutron diffraction data confirms the magnetic structure with propagation vector k = (0 1 0) and collinear AFM spins. The observed jump in dielectric permittivity near ∼150 K implies possible anti-ferroelectric behavior; however, the absence of switching suggests that Pb2MnTeO6 can only be antipolar. First-principle calculations confirmed that the crystal and magnetic structures determined are locally stable and that anti-ferroelectric switching is unlikely to be observed in Pb2MnTeO6.

A thermodynamically stable series of superlattices, (ZnO)kIn2O3, form in the ZnO-In2O3 binary oxide system for InO1.5 concentrations from about 13 up to about 33 mole percent (m/o). These natural superlattices, which consist of a periodic stacking of single, two-dimensional sheets of InO6 octahedra, are found to give rise to systematic changes in the optical and vibrational properties of the superlattices. Low-frequency Raman scattering provides the evidence for the activation of acoustic phonons due to the folding of Brillouin zone. New vibrational modes at 520 and 620 cm−1, not present in either ZnO or In2O3, become Raman active. These new modes are attributed to collective plasmon oscillations localized at the two-dimensional InO1.5 sheets. Infrared reflectivity experiments, and simulations taking into account a negative dielectric susceptibility due to electron carriers in ZnO and interface modes of the dielectric layer of InO2, explain the occurrence of these new modes. We postulate that a localized electron gas forms at the ZnO/InO2 interface due to the electron band alignment and polarization effects. All our observations suggest that there are quantum contributions to the thermal and electrical conductivity in these natural superlattices.

The semiconductor industry has been characterized for more than five decades by a fast rate of development in its products. A greater degree of creativity at the level of materials and, as a consequence, of functions will be possible to be implemented. This is the case, for example, of multiferroic materials that with the coexistence of at least two ferroic orders, ferroelectric, ferromagnetic, or ferroelastic, may have great potential applications as multifunctional devices. Ferroic is the general name given to a material exhibiting the property of ferroelasticity, ferromagnetism, and ferroelectricity. The degradation of the electrical properties may result from the decrease of the film thickness with consequent reduction of the tetragonality. Bismuth ferrite is the only single-phase multiferroic presenting simultaneously magnetic and ferroelectric order at room temperature, thus considered as the multiferroic archetype and the most adequate recruit for the pursuit of solutions in multiferroic's perplexities.

We present a comprehensive study of polar and magnetic excitations in BiFeO3 ceramics and a thin film epitaxially grown on an orthorhombic (110) TbScO3 substrate. Infrared reflectivity spectroscopy was performed at temperatures from 5 to 900 K for the ceramics and below room temperature for the thin film. All 13 polar phonons allowed by the factor-group analysis were observed in theceramic samples. The thin-film spectra revealed 12 phonon modes only and an additional weak excitation, probably of spin origin. On heating towards the ferroelectric phase transition near 1100 K, some phonons soften, leading to an increase in the static permittivity. In the ceramics, terahertz transmission spectra show five low-energy magnetic excitations including two which were not previously known to be infrared active; at 5 K, their frequencies are 53 and 56 cm-1. Heating induces softening of all magnetic modes. At a temperature of 5 K, applying an external magnetic field of up to 7 T irreversibly alters the intensities of some of these modes. The frequencies of the observed spin excitations provide support for the recently developed complex model of magnetic interactions in BiFeO3 (R.S. Fishman, Phys. Rev. B 87, 224419 (2013)). The simultaneous infrared and Raman activity of the spin excitations is consistent with their assignment to electromagnons.