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.
Highlights
These results demonstrate that doped BFO films with compositions near a phase boundary may be promising candidates for multifunctional applications due to the simultaneous possession of the maxima in ferromagnetic and ferroelectric properties
The measured acoustic velocity in the BPFMO films with different concentrations of Pr deposited on LAO substrate (Table 3) demonstrates highly non-monotonic variation in the vicinity substitution deposited on LAO substrate (Table 3) demonstrates highly non-monotonic variation in of the rhombohedral–tetragonal structural phase transition at around 12.5% of Pr substitution the vicinity of the rhombohedral–tetragonal structural phase transition at around 12.5% of Pr (Figure 8), similar to the one revealed earlier in films deposited on Si substrate
It is worth mentioning here that, in Ref. [34], an attempt was reported to access this information in Bi1−x Dyx FeO3 (BDFO) films even thinner than ours, by extracting from the time-domain Brillouin scattering (TDBS) signals the information on the transmission of the coherent acoustic pulse (CAP) at the interface of the film with STO substrate
Summary
Multiferroic materials, where different ferroic orders coexist, are of paramount importance for the generation of multifunctional devices for spintronics and memory technologies, since the material state in one of the ferroic orders could be controlled by the others. X-ray diffraction and Raman spectra results confirmed that a phase transition from rhombohedral to tetragonal structure occurs at about 15% Pr substitution These results demonstrate that doped BFO films with compositions near a phase boundary may be promising candidates for multifunctional applications due to the simultaneous possession of the maxima in ferromagnetic and ferroelectric properties. [31], the sound velocity in the 360 nm rhombohedral BFO film grown on SrTiO3 (STO) (110) single-crystal substrates by pulsed laser deposition was estimated as νS ∼. The elastic moduli of rhombohedral, tetragonal, and rare-earth-doped BFO were determined from the measured Brillouin frequencies in conjunction with variable-angle spectroscopic ellipsometry for the determination of the complex refractive index of thin films. TDBS was applied for the first time for the characterization of the doping-induced structural transition from the rhombohedral to the orthorhombic phase of BFO in Ref. Measurements of the refractive indices were not performed and the anomalies of the sound velocity and of the refractive index across the phase transition were not disentangled [34]
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