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

Adamo, C.

Bai, X. F.

Dkhil, B.

Goian, V.

Infante, I. C.

Kadlec, C.

Kadlec, F.

Kamba, S.

Schlom, D. G.

Skiadopoulou, S.

English

arXiv

International audience2 count the nearest and next-nearest neighbor exchange interactions, two Dzyaloshinskii-Moriya interactions and an easy-axis anisotropy. 31,35 The same model successfully described the low-energy inelastic neutron scattering spectra. 30 In contrast, Komandin et al. 15 showed IR transmission spectra with an excitation at approximately 47 cm −1 which had not been previously reported by experimental or theoretical studies. The intensive discussion in the literature concerning the nature of the spin ex-citations raises the question: which excitations are pure magnons (i.e., contribute only to the magnetic perme-ability µ) and which are electromagnons (i.e., influence at least partially the permittivity ε *)? It is worth noting that according to a recent symmetry analysis, 36 BiFeO 3 allows directional dichroism and therefore spin waves can be simultaneously excited by the electric and magnetic components of electromagnetic radiation. For more details on BiFeO 3 spin dynamics, see the review of Park et al. 37 In the current work, we report spin and lattice exci-tations in BiFeO 3 ceramics, as measured by the combination of IR reflectivity and time-domain THz transmission spectroscopy, in a temperature range from 10 to 900 K. All 13 IR-active phonon modes are observed, exhibiting softening on heating. Five low-frequency spin modes are detected from 5 K up to RT, the highest two appearing at 53 and 56 cm −1. This corresponds to the frequency range where such excitations were theoretically predicted, 31,32,35 but not experimentally confirmed up to now. 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. Additionally, a BiFeO 3 epitaxial thin film grown on an orthorhombic (110) TbScO 3 single crystal substrate was studied for the first time via IR reflectance spectroscopy. II. EXPERIMENTAL DETAILS BiFeO 3 ceramics were prepared by the solid-state route. A stoichiometric mixture of Fe 2 O 3 and Bi 2 O 3 powder oxides with a purity of 99.99% was ground and uniaxially cold-pressed under 20-30 MPa pressure into 8 mm diameter pellets. The pellets were then covered by sacrificial BFO powder to avoid bismuth oxide loss from the pellet, and sintered in a tube furnace at 825 • C for8 h in air. To avoid any secondary phase formation, the samples were quenched to RT. Polished disks with a diameter of 4 mm and thicknesses of ca. 600 and 338 µm were used for the IR reflectivity and THz transmission measurements, respectively. An epitaxial BiFeO 3 thin film with a thickness of 300 nm was grown by reactive molecular-beam epitaxy on a (110) single crystal substrate. The growth parameters were the same as for the samples reported in Ref. 38. Near-normal incidence IR reflectivity spectra of the BiFeO 3 ceramics and film were obtained using a Fourier-transform IR spectrometer Bruker IFS 113v in the frequency range 20-3000 cm −1 (0.6-90 THz) at RT; for the low and high temperature measurements the spectral range was reduced to 650 cm −1. Pyroelectric deuterated triglycine sulfate detectors were used for the room-and high-temperature measurements up to 900 K, whereas a He-cooled (operating temperature 1.6 K) Si bolometer was used for the low-temperature measurements down to 10 K. A commercial high-temperature cell (SPECAC P/N 5850) was used for the high-temperature experiments. The thermal radiation from the hot sample entering the interferometer was taken into account in our spectra evaluation. THz measurements from 3 cm −1 to 60 cm −1 (0.09-1.8 THz) were performed in the transmission mode with the use of a custom-made time-domain terahertz spectrometer. In this spectrometer, a femtosecond Ti:sapphire laser oscillator (Coherent, Mira) produces a train of femtosecond pulses which generates linearly polarized broadband THz pulses in a photoconduct-ing switch TeraSED (Giga-Optics). A gated detection scheme based on electrooptic sampling with a 1 mm thick [110] ZnTe crystal as a sensor allows us to measure the time profile of the electric field of the transmitted THz pulse. The same high-temperature cell as for the IR reflectivity was used for the THz range high-temperature measurements. Oxford Instruments Opti-stat optical cryostats with mylar and polyethylene windows were used for the low-temperature THz and IR measurements , respectively. THz experiments in an external magnetic field H ext ≤ 7 T were performed upon decreasing H with an Oxford Instruments Spectromag cryostat in the Voigt configuration, where the electric component of the THz radiation E THz was set perpendicular to H ext. The IR reflectivity and THz complex (relative) per-mittivity spectra ε * (ω) were carefully fit assuming the factorized form of the dielectric function based on a generalized damped-harmonic-oscillator model: 39 ε * (ω) = ε ∞ N j=1 ω 2 LOj − ω 2 + iωγ LOj ω 2 TOj − ω 2 + iωγ TOj , (1) where ω TOj and ω LOj are the frequencies of the j-th transverse optical (TO) and longitudinal optical (LO) phonons, γ TOj and γ LOj are the corresponding damping constants, and ε ∞ denotes the high-frequency (elec-tronic) contribution to the permittivity, determined from the RT frequency-independent reflectivity tail above the phonon frequencies. The reflectivity R(ω) is related to the complex dielectric function ε * (ω) by: R(ω) = ε * (ω) − 1 ε * (ω) + 1. (2) To evaluate the IR reflectance spectra of the BiFeO 3 /TbScO 3 thin film, a model corresponding to a two-layer optical system was used. 40 The IR reflectivity spectra of the bare TbScO 3 substrate were fit first at eac

Skiadopoulou, S

Goian, V

Kadlec, C

Kadlec, F

X.F., Bai

C.Infante, Ingrid

Dkhil, B

Adamo, C

Schlom, D.G.

Kamba, S

HAL-CentraleSupelec

Spin and lattice excitations of a BiFeO3 thin film and ceramics

Hal-Diderot

HAL Descartes

S. Skiadopoulou

V. Goian

C. Kadlec

F. Kadlec

X. F. Bai

I. C. Infante

B. Dkhil

C. Adamo

D. G. Schlom

S. Kamba

Crossref

Physical Review B

Spin and lattice excitations of a<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>BiFeO</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>thin film and ceramics

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Spin and lattice excitations of a BiFeO3 thin film and ceramics

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