Evaluation of the Structural Phase Transition in Multiferroic (Bi1−x Prx)(Fe0.95 Mn0.05)O3 Thin Films by A Multi-Technique Approach Including Picosecond Laser Ultrasonics

Abstract

Picosecond laser ultrasonics is an experimental technique for the generation and detection of ultrashort acoustic pulses using ultrafast lasers. In transparent media, it is often referred to as time-domain Brillouin scattering (TDBS). It provides the opportunity to monitor the propagation of nanometers-length acoustic pulses and to determine acoustical, optical, and acousto-optical parameters of the materials. We report on the application of TDBS for evaluating the effect of Praseodymium (Pr) substitution on the elasticity of multiferroic (Bi1−xPrx)(Fe0.95Mn0.05)O3 (BPFMO) thin films. The films were deposited on Si and LaAlO3 (LAO) substrates by a sol-gel method. X-ray diffraction and Raman spectra revealed earlier that a phase transition from rhombohedral to tetragonal structure occurs at about 15% Pr substitution and is accompanied by the maxima of remnant magnetization and polarization. Combining TDBS with optical spectral reflectometry, scanning electron microscopy, and topographic measurements by atomic force microscopy, we found that the structural transition is also characterized by the maximum optical dielectric constant and the minimum longitudinal sound velocity. Our results, together with earlier ones, suggest that BiFeO3-based films and ceramics with compositions near phase boundaries might be promising materials for multifunctional applications.

Evaluation of Optical and Acoustical Properties of Ba1−xSrxTiO3 Thin Film Material Library via Conjugation of Picosecond Laser Ultrasonics with X-ray Diffraction, Energy Dispersive Spectroscopy, Electron Probe Micro Analysis, Scanning Electron and Atomic Force Microscopies

“…It is presented here for the convenient comparison of the lateral variations in thickness of the BSTx film with the lateral variations of the parameters accessed by PLU, and for the appreciation of the fact that the uncertainty in the determination of the sound velocities and refractive indices by PLU is dominantly controlled by the uncertainty in thickness measurements. It is commonly considered that the positioning of the interface with SEM can be achieved with the uncertainty of δZ ≈ 10 nm [43]. Relating the uncertainty in the film thickness to the uncertainty in positioning of its two surfaces by δd = (δZ ) 2 + (δZ ) 2 = √ 2δZ, we estimated that depending on the Y coordinate, it varies between ±5% and ±6%.…”

“…In the studies of thin films of the room temperature multiferroic Bi 1−x−y Dy x La y FeO 3 [42], the elastic moduli of rhombohedral, tetragonal, and rare-earth doped BiFeO 3 were determined by PLU in conjunction with spectroscopic ellipsometry. In the recent experiments, for evaluating the effect of Praseodymium (Pr) substitution on the elasticity of multiferroic (Bi 1−x Pr x )(Fe 0.95 Mn 0.05 )O 3 (BPFMO) thin films [43], PLU was combined with optical spectral reflectometry, scanning electron microscopy and atomic force microscopy.…”

“…However, the examples for the thin films prepared by other techniques are rather common. For example, in [43] the difference in the sound velocities of (Bi 1−x Pr x )(Fe 0.95 Mn 0.05 )O 3 films deposited by sol-gel method on Si and LaAlO 3 (LAO) substrates is up to 8% even for the x = 0.2 composition, which is sufficiently far from the structural phase transition around x = 0.12-0.16, where the differences in sound velocities are much more significant. The dependences of the optical refractive index and of the sound velocity in the thin films on the structure/substrate chosen for their deposition could both contribute to the differences in the Brillouin frequencies in the thin films deposited on different structures/substrates.…”

Special Issue on Laser Ultrasonics

“…Catalini et al [8] studied the propagation of ultrasonic waves in lysozyme solutions characterized by different degrees of aggregation and networking. Raetz et al [9] report the variation in the longitudinal acoustic velocity and the optical refractive index in thin films deposited on different substrates across the rhombohedral to tetragonal structural phase transition induced by element substitution. Finally, Li et al [10] demonstrate how semiconductor saturable absorber mirrors can be characterized non-destructively by their coherent acoustic response in the sub-THz range.…”

Simulation of multiwavelength conditions in laser picosecond ultrasonics

“…The ultrafast optical experimental set-up usually involved in the laboratory to perform such picosecond acoustic studies is presented. [1][2][3] Several basic quantities are afterwards detailed, such as the interaction length and the optical absorption parameters. The acoustic velocities are next calculated, using the elastic stiffness coefficients and the so-called Burenkov equations.…”