Amritendu Roy

Room temperature nanoscale ferroelectricity in magnetoelectric GaFeO3 epitaxial thin films.

We demonstrate room temperature ferroelectricity in the epitaxial thin films of magnetoelectric gallium ferrite. Piezoforce measurements show a 180° phase shift of piezoresponse upon switching the electric field indicating nanoscale ferroelectricity in the thin films. Further, temperature-dependent impedance analysis with and without the presence of an external magnetic field clearly reveals a pronounced magnetodielectric effect across the magnetic transition temperature. In addition, our first principles calculations show that Fe ions are not only responsible for ferrimagnetism as observed earlier but also give rise to the observed ferroelectricity, making gallium ferrite a unique single phase multiferroic.

Electronic structure, Born effective charges and spontaneous polarization in magnetoelectric gallium ferrite.

We present a theoretical study of the structure-property correlation in gallium ferrite, based on first-principles calculations followed by a subsequent comparison with experiments. The local spin density approximation (LSDA + U) of the density functional theory has been used to calculate the ground state structure, electronic band structure, density of states and Born effective charges. The calculations reveal that the ground state structure is orthorhombic Pc 2(1)n having A-type antiferromagnetic spin configuration, with lattice parameters matching well with those obtained experimentally. Plots of the partial density of states of constituent ions exhibit noticeable hybridization of Fe 3d, Ga 4s, Ga 4p and O 2p states. However, the calculated charge density and electron localization function show a largely ionic character of the Ga/Fe-O bonds which is also supported by a lack of any significant anomaly in the calculated Born effective charges with respect to the corresponding nominal ionic charges. The calculations show a spontaneous polarization of ∼ 59 µC cm( - 2) along the b-axis which is largely due to asymmetrically placed Ga1, Fe1, O1, O2 and O6 ions.

First-principles calculations of Born effective charges and spontaneous polarization of ferroelectric bismuth titanate

In this study, we present the results of our first-principles calculations of the band structure, density of states and the Born effective charge tensors for the ferroelectric (ground state B 1a 1) and paraelectric (I4/mmm) phases of bismuth titanate. The calculations are done using the generalized gradient approximation (GGA) as well as the local density approximation (LDA) of the density functional theory. In contrast to the literature, our calculations on the B 1a 1 structure using GGA and LDA yield smaller indirect bandgaps as compared to the direct bandgaps, in agreement with the experimental data. The density of states shows considerable hybridization among Ti 3d, Bi 6p and O 2p states indicating the covalent nature of the bonds leading to the ferroelectric instability. The Born effective charge tensors of the constituent ions for the ground state (B 1a 1) and paraelectric (I4/mmm) structures were calculated using the Berry phase method. This is followed by the calculation of the spontaneous polarization for the ferroelectric B 1a 1 phase using the Born effective charge tensors of the individual ions. The calculated value for the spontaneous polarization of ferroelectric bismuth titanate using different Born effective charges was found to be in the range of 55 ± 13 µC cm( - 2) in comparison to the reported experimental value of (50 ± 10 µC cm( - 2)) for single crystals. The origin of ferroelectricity is attributed to the relatively large displacements of those oxygen ions in the TiO(6) octahedra that lie along the a axis of the bismuth titanate crystal.

Effect of site-disorder on magnetism and magneto-structural coupling in gallium ferrite: A first-principles study

We report a first-principles study of the magnetic properties, cation site disorder effect on magnetism and magneto-structural coupling in multiferroic gallium ferrite (GFO) using the local spin density approximation (LSDA+U) of the density functional theory. The calculation of the ground state A-type antiferromagnetic structure predicts magnetic moments consistent with the experiments while consideration of spin-orbit coupling yields a net orbital moment also in good accordance with the experiment. We find that though cation site disorder is not spontaneous in the ground state, interchange between octahedrally coordinated Fe2 and Ga2 sites is most favored in the disordered state. The results show that ferrimagnetism in GFO is primarily due to Ga-Fe site disorder such that Fe spins at Ga1 and Ga2 sites are antiferromagnetically aligned while maintaining ferromagnetic coupling between Fe spins at Ga1 and Fe1 sites as well as between Fe spins at Ga2 and Fe2 sites. Thus we are able to explain the origin of f...We report a first-principles study of the magnetic properties, site disorder and magneto-structural coupling in multiferroic gallium ferrite (GFO) using local spin density approximation (LSDA+U) of density functional theory. The calculations of the ground state A-type antiferromagnetic structure predict magnetic moments consistent with the experiments whilst consideration of spin-orbit coupling yields a net orbital moment of ~ 0.025 Bohr magneton/Fe site also in good accordance with the experiments. We find that though site disorder is not spontaneous in the ground state, interchange between Fe2 and Ga2 sites is most favored in the disordered state. The results show that ferrimagnetism in GFO is due to Ga-Fe site disordering such that Fe spins at Ga1 and Ga2 sites are antiferromagnetically aligned while maintaining ferromagnetic coupling between Fe spins at Ga1 and Fe1 sites as well as between Fe spins at Ga2 and Fe2 sites. The effect of spin configuration on the structural distortion clearly indicates presence of magneto-structural coupling in GFO.

Engineering polarization rotation in ferroelectric bismuth titanate

Here, we report a combined experimental-theoretical study showing that collective application of rare earth doping on A-site and epitaxial strain to ferroelectric bismuth titanate does not lead to a very large c-axis polarization as reported previously. Further first principles calculations based on the examination of polarization tensor suggest that simultaneous Bi and Ti site doping could result in moderate polarization along c-axis of bismuth titanate which is typically a preferential axis of film growth and thus enabling c axis oriented films to have appreciable polarization. This approach could also be applicable to other ferroic oxides where one can correlate the doping, epitaxial strain, and polarization to design materials compositions resulting in epitaxial films grown along desired directions yielding substantial polarization.

Effect of isovalent non-magnetic Fe-site doping on the electronic structure and spontaneous polarization of BiFeO3

We report the results of our first-principles calculations on the effect of isovalent, non-magnetic, Al3+ ion doping on the electronic structure and spontaneous polarization of multiferroic BiFeO3. Our calculations reveal that Al3+ doping in BiFeO3 results in the reduction of Fe–O–Fe bond angle, leading to the weakening of antiferromagnetic superexchange interaction, further substantiated by the reduction of exchange interaction constant with increasing doping level. Lowering of well-depth is suggestive of reduced switching potential and improved P-E loop with lowered coercivity. Chemical bonding analysis by electron localization function shows that cation–oxygen bonding is of mixed ionic–covalent character, with marginal increase in the covalent character with increasing doping concentration. Large spontaneous polarization of undoped BiFeO3 is retained with lower doping level (6.25%), while for higher doping content (31.25%), the spontaneous polarization is reduced, primarily due to larger c/a ratio at h...

Effects of site disorder, off-stoichiometry and epitaxial strain on the optical properties of magnetoelectric gallium ferrite.

We present a combined experimental-theoretical study demonstrating the role of site disorder, off-stoichiometry and strain on the optical properties of magnetoelectric gallium ferrite. Optical properties (bandgap, refractive indices and dielectric constants) were experimentally obtained by performing ellipsometric studies over the energy range 0.8-4.2 eV on pulsed laser deposited epitaxial thin films of stoichiometric gallium ferrite with b-axis orientation and the data were compared with theoretical results. Calculations on the ground state structure show that the optical activity in GaFeO(3) arises primarily from O 2p-Fe 3d transitions. Further, inclusion of site disorder and epitaxial strain in the ground state structure significantly improves the agreement between the theory and the room temperature experimental data substantiating the presence of site disorder in the experimentally derived strained GaFeO(3) films at room temperature. We attribute the modification of the ground state optical behavior upon inclusion of site disorder to the corresponding changes in the electronic band structure, especially in Fe 3d states leading to a lowered bandgap of the material.

Optical anisotropy in bismuth titanate: An experimental and theoretical study

We report experimental and theoretical investigation of anisotropy in optical properties and their origin in the ferroelectric and paraelectric phases of bismuth titanate. Room temperature ellipsometric measurements performed on pulsed laser deposited bismuth titanate thin films of different orientations show anisotropy in the dielectric and optical constants. Subsequent first-principles calculations performed on the ground state structures of ferroelectric and high temperature paraelectric phases of bismuth titanate show that the material demonstrates anisotropic optical behavior in both ferroelectric and paraelectric phases. We further show that O 2p to Ti 3d transition is the primary origin of optical property of the material while optical anisotropy results from the asymmetrically oriented Ti-O bonds in TiO6 octahedra in the unit cell.

Structure and Properties of Magnetoelectric Gallium Ferrite: A Brief Review

Here, we review the advances in understanding the structure and properties of magnetoelectric gallium ferrite (GaFeO3 or GFO). Special significance of GFO lies in its compositional tunability over a large range of Ga:Fe ratio without forming a second phase which also renders prospective room temperature magnetoelectricity in the material. Detailed structural studies show noncentrosymmetric orthorhombic Pc21n symmetry without any structural phase transition between 4 K and 700 K. The material shows a ferrimagnetic behavior driven by cation site disorder, corroborated both experimentally as well as theoretically, with transition temperature dependent upon Ga:Fe ratio. GFO exhibits magnetostructural and magnetoelectric coupling indicating coupling among the structural, magnetic and electrical degrees of freedom which could possibly be utilized to strain engineer electrical and magnetic properties in the thin film structures. However, the biggest challenge remains to reduce the leakage in the material in both thin film and bulk form which hinders its potential as a device material.

Flexible nano-GFO/PVDF piezoelectric-polymer nano-composite films for mechanical energy harvesting

Owing to the persistent quest of renewable energy technology, piezoelectric energy harvesters are gathering considerable research interest due to their potential in driving microelectronic devices with small power requirement. Electrical energy (milli to microwatt range) is generated from mechanical counterparts such as vibrations of machines, human motion, flowing water etc. based on the principles of piezoelectricity. Flexible high piezoelectric constant (d33) ceramic/polymer composites are crucial components for fabricating these energy harvesters. The polymer composites composed of gallium ferrite nanoparticles and polyvinylidene fluoride (PVDF) as the matrix have been synthesized by solvent casting method. First, 8 wt. % PVDF was dissolved in DMF and then different compositions of GaFeO3 or GFO (10, 20, 30 wt. %) (with respect to PVDF only) nanocomposites were synthesized. The phase of the synthesized nanocomposites were studied by X- Ray diffraction which shows that with the increase in the GFO concentration, the intensity of diffraction peaks of PVDF steadily decreased and GFO peaks became increasingly sharp. As the concentration of GFO increases in the PVDF polymer matrix, band gap is also increased albeit to a small extent. The maximum measured output voltage and current during mechanical pressing and releasing conditions were found to be ~ 3.5 volt and 4 nA, respectively in 30 wt % GFO-PVDF composite, comparable to the available literature.