Abstract
BiFeO3 (BFO) nanoparticles (NPs) were synthesized using the sol-gel method at different calcination temperatures from 400 °C to 600 °C. XRD studies have confirmed that all BFO NPs show distorted rhombohedral crystals that match the R3c space group. We found evidence of local structural strain that develops with increasing particle size as suggested by TEM and Raman spectroscopy measurements. Magnetic measurements suggest that NPs have two distinct regimes: a ferromagnetic-like one at low temperatures and a superparamagnetic-like one at room temperature. The crossover temperature increases with NPs size, suggesting a size-dependent blocking magnetic regime. Similarly, local piezoelectric measurements at room temperature in single NP have confirmed a ferroelectric order with a NP size-dependent d33 coefficient. An analysis of both the ferroelectric and the magnetic results suggest that ferromagnetism and ferroelectricity coexist at room temperature in NPs. Our results lead to the possibility of tailoring the ferroic order in multifunctional materials by means of NP size.
Highlights
We report the synthesis of BFO NPs by sol-gel method at different calcination temperatures
We show the A1 − 4 mode red shift evolution as a function of Tcal; such mode is associated to Fe-O1 and Fe-O2 bonds[21,22] (see Fig. 1(f)). From these results we can identify a possible distortion of Fe-O bonding distances or Fe-O-Fe bonding angle and relate them with the calcination. Such red shifts observed in Raman spectra and the analysis of the Williamson-Hall equation from X-Ray diffraction (XRD) results could be related to the strain of NPs and the size confinement
The strain increases with decreasing calcination temperature, suggesting smaller particles have stronger strain
Summary
We found evidence of local structural strain that develops with increasing particle size as suggested by TEM and Raman spectroscopy measurements. Magnetic measurements suggest that NPs have two distinct regimes: a ferromagnetic-like one at low temperatures and a superparamagnetic-like one at room temperature. Local piezoelectric measurements at room temperature in single NP have confirmed a ferroelectric order with a NP size-dependent d33 coefficient.
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