Observation of nanotwinning and room temperature ferromagnetism in sub-5 nm BiFeO₃ nanoparticles: a combined experimental and theoretical study

Abstract

Particle size significantly affects the properties and therefore the potential applications of multiferroics. However, is there special particle size effect in BiFeO, which has a spiral modulated spin structure? This is still under investigation for sub-5 nm BiFeO. In this report, the structural, electronic and magnetic properties are investigated for chemically synthesized BiFeO nanoparticles with an average size of 3 nm. We observed nanotwinning features in the specific size regime of the nanoparticles (2-4 nm). A weak Bi-O-Fe coordination and weak covalent nature has been observed in the nanoparticles through high-resolution electron energy loss spectroscopy and theoretical analysis, confirming that BiFeO nanoparticles a retain rudimentary R3c phase even at sub-5 nm dimensions. The R3c phase of sub-5 nm BiFeO nanoparticles has also been confirmed using Raman spectroscopy and Raman mapping of the vibrational modes. The nanoparticles display cluster spin glass, room temperature ferromagnetism, and a metamictization-davidite phase. The observation of weak magnetic entropy features confirmed the presence of a weak correlation between the magnetic and ferroelectric components. To support our experimental observations, we have simulated a sub-5 nm BiFeO nanocluster. Using density functional theory, the ferromagnetic ground state and the presence of a weak covalent nature in the nanocluster is established considering the first Brillouin zone, thus confirming our experimental results. Finding of new physicochemical features in sub-5 nm BiFeO would be beneficial for the understanding of the fundamental physical and chemical science as well as potential device development.

Electronic structure of BiFeO3 in the presence of strong electronic correlations

“…Room-temperature Mössbauer spectrum of BiFeO 3 nanoparticles reveal two quadrupole doublets, which were assigned to coexisting Fe 3+ and Fe 2+ valence states [23]. The degree of amorphization (or metamictization) in BiFeO 3 nanoparticles was found to be Fe 3+ 0.838 Fe 2+ 0.162 [23], a ratio consistent with x-ray photoelectron spectroscopy (XPS) were 19% Fe-ions are in the d 6 (Fe 2+ ) electronic configuration [19].…”

“…Room-temperature Mössbauer spectrum of BiFeO 3 nanoparticles reveal two quadrupole doublets, which were assigned to coexisting Fe 3+ and Fe 2+ valence states [23]. The degree of amorphization (or metamictization) in BiFeO 3 nanoparticles was found to be Fe 3+ 0.838 Fe 2+ 0.162 [23], a ratio consistent with x-ray photoelectron spectroscopy (XPS) were 19% Fe-ions are in the d 6 (Fe 2+ ) electronic configuration [19]. The changes in the Fe-ionic content in BiFeO 3 might be due to the multiple valences possible for Bi and Fe, particularly if the Bi ions acquire an average formal valence of 4+, leaving the Fe ion with a 2+ electronic state [8].…”

“…Due to the large orbital radius of the Bi-6s 2 lone pairs, the crystal structure of Bi perovskites is usually distorted to low symmetries, producing spontaneous ferroelectric polarization along certain crystallographic directions [6]. Among the Bi perovskites, BiFeO 3 is the most systematically studied system both from theory [7][8][9][10][11][12][13][14] and experimental [3,4,[15][16][17][18][19][20][21][22][23][24] perspectives.…”

“…Importantly, the latter study is providing direct evidences that the formation of interface ferromagnetism is related to electronic orbital reconstruction. Theoretically, magnetically induced electronic reconstruction in BiFeO 3 has been investigated using firstprinciples density functional theories (DFT) [4,8,10,23]. Particularly relevant in this context is the recent density functional dymanical mean-field theory (DFDMFT) study by S. Paul et al [14] (see our discussion below), where the spectral properties of paramagnetic and antiferromagnetic phases of BiFeO 3 were investigated and compared to photoemission spectra.…”

“…The electronic structure, band gap, and magnetic properties of bulk and BiFeO 3 films were investigated by different ab initio density functional theories implemented in the GGA or local density approximation (LDA) as well as (GGA/LDA)+U and local spin density approximation plus U (LSDA+U ) schemes [4,[8][9][10]12,22,23]. A perusal of these DFT+U studies, which explicitly take into account the effect of local electron-electron interactions, reveal that the on-site Coulomb interaction U (effective or not) for BiFeO 3 could be in the range between 2 and 9 eV [8,10,22].…”