Tetragonal-like BiFeO3 Research Articles

Strong spin-lattice coupling in tetragonal-like BiFeO3 films with thermal expansion anomalies

The evidence for a spin-lattice coupling in a tetragonal-like BiFeO3 (BFO) film is derived from thermal expansion measurements. Taking the advantage of La doping, the Néel temperature (TN) of the BFO film can be further adjusted over a broad range of temperatures. The lattice parameters exhibit anomalies near the Néel temperatures. The Bi0.8La0.2FeO3 film has a spin order-to-disorder transition at the TN without a structural phase transition, suggesting that a spin-lattice coupling drives the thermal expansion anomalies. The spin-lattice coupling can be enhanced by the coexistence of phases in the thicker BFO films. The density functional theory calculations show a smaller lattice constant of the c axis in the spin-order state than that in the spin-disorder state for the same tetragonal-like phase, which supports that the thermal expansion anomalies are a consequence of the spin-lattice coupling. Our findings may have application prospects for functional materials with controllable thermal expansions.

Polarization tunable and enhanced photovoltaic properties in tetragonal-like BiFeO3 epitaxial films with graphene top electrode

Ferroelectric photovoltaic materials have attracted intensive interest due to their intriguing above-bandgap photovoltage, while the low photocurrent limits their further device applications. In this work, both enhanced and tunable photovoltaic effect (PVE) are demonstrated in sandwiched structure of Graphene/tetragonal-like BiFeO3/Ca0.96Ce0.04MnO3 (Graphene/T(-like) BFO/CCMO). The epitaxial BFO film is grown on the CCMO buffered LaAlO3 substrate by pulsed laser deposition and the mechanically exfoliated graphene is transferred directly onto the BFO film as top electrode. The optimized open circuit voltage (Voc) and short circuit current density (Jsc) are measured to be −0.88 V and 2.56 mA/cm2, respectively. Moreover, the photovoltaic response can be modulated by controllable ferroelectric polarization, whereby both the Voc and Jsc show piezoresponse-like hysteresis behaviors against voltage. The notably enhanced PVE is likely a result of the combination effects of large polarization in the T(-like) BFO film, partially unscreened depolarization field, high transmittance and conductivity of the graphene top electrode. This work clearly indicates the potential of Graphene/ferroelectric photovoltaic devices for memory and energy conversion applications.

Exchange bias in tetragonal-like BiFeO3/Sr2FeMoO6 bilayer

Exchange bias in the bilayer of half-metallic La0.7Sr0.3MnO3 and multiferroic BiFeO3 has been applied for the electric field control of magnetization, which is strongly limited by the much low blocking temperature of about 100 K (Phys. Rev. Lett. 105, 027201 (2010)). One possible solution is to strengthen the interfacial exchange coupling, thus another half-metallic oxide Sr2FeMoO6 is selected. Tetragonal-like BiFeO3 has been successfully fabricated on (0 0 1) SrTiO3 substrate using Sr2FeMoO6 as buffer layer. By replacing the exchange coupling between Fe and Mn ions in BiFeO3/La0.7Sr0.3MnO3 bilayer, stronger exchange couplings between Fe and Fe ions are established at the interface, and the blocking temperature has been significantly increased to 160 K. A second blocking temperature at 110 K due to the exchange coupling between Fe and Mo ions is also observed. Detailed magnetization measurements confirm the conventional mechanism of exchange coupling between ferromagnetic and antiferromagnetic layers and exclude the contribution from surface spin glass. Furthermore, BiFeO3 still retains excellent ferroelectric properties with Sr2FeMoO6 acting as ferromagnetic conducting electrode, which may have important applications in multiferroic spintronics devices.

Microstructure and temperature stability of highly strained tetragonal-like BiFeO3 thin films

A highly strained tetragonal (T)-like BiFeO3 (BFO) phase interspersed between two rhombohedral (R)-like phase regions in BFO films grown on LaAlO3 substrates was confirmed by transmission electron microscopy (TEM). The typical width of the T-like phase was about 5–35nm and the areal fraction of the T-like phase was estimated to be 35%. A lattice misalignment of 3.8° between adjacent R- and T-like phases was observed in both high-resolution TEM images and selected-area electron diffraction patterns. Superlattice reflection spots were observed along the [110] direction in the R-like phase, which were attributed to the antiphase tilting of oxygen octahedra about the pseudocubic [111] axis of the R-like phase. A structural phase transition from the T-like phase to the R-like one at around 435°C was observed by TEM during in situ heating and cooling experiments.

Strain evolution of epitaxial tetragonal-like BiFeO3 thin films on LaAlO3(001) substrates prepared by sputtering and their bulk photovoltaic effect

The structural evolution of high-quality 3.3–73.2-nm-thick tetragonal-like BiFeO3 (T-BFO) thin films grown on LaAlO3(001) substrates and the bulk photovoltaic effect of the films were investigated. The T-BFO films were grown by rf magnetron sputtering, showing the Peudellösung fringes around the T-BFO (001) diffraction peak in X-ray diffraction θ–2θ patterns. These indicate the structural coherence between the surface and the interface in the surface normal direction of the films. High-resolution synchrotron X-ray diffraction analysis and transmission electron microscopy reveal that the lattice relaxation behavior from the MA monoclinic to MC monoclinic structure occurs as the film thickness increases. The domain structure was partly controlled by using a vicinal LAO (001) substrate along [100]. Regarding the current–voltage characteristics of the Pt/T-BFO/Pt coplanar capacitor under violet laser illumination, T-BFO films show an anomalous photovoltaic effect with an open-circuit voltage of 6.1 V and a short-circuit current of −290 pA along the [100]T-BFO direction.

The valence band electronic structure of rhombohedral-like and tetragonal-like BiFeO3 thin films from hard X-ray photoelectron spectroscopy and first-principles theory

We investigate the electronic structure of rhombohedral-like (R) and tetragonal-like (T) BiFeO3 thin films using high energy X-ray photoelectron spectroscopy and first-principles electronic structure calculations. By exploiting the relative elemental cross sections to selectively probe the elemental composition of the valence band, we identify a strong Bi 6p contribution at the top of the valence band in both phases, overlapping in energy range with the O 2p states; this assignment is confirmed by our electronic structure calculations. We find that the measured occupied Bi 6p signal lies closer to the top of the valence band in the T phase than in the R phase, which we attribute, using our electronic structure calculations, to lower Bi–O hybridization in the T phase. We note, however, that our calculations of the corresponding densities of states underestimate the difference between the phases, suggesting that matrix element effects resulting from the different effective symmetries also contribute. Our results shed light on the chemical nature of the stereochemically active Bi lone pairs, which are responsible for the large ferroelectric polarization of BiFeO3.

Optical properties of epitaxial BiFeO3 thin films grown on LaAlO3

Highly strained and nearly pseudomorphic BiFeO3 epitaxial films were deposited on LaAlO3 and TbScO3 substrates, respectively. The symmetry of the tetragonal-like BiFeO3 films is discussed based on polarisation dependent Raman measurements and on the comparison with Raman spectra measured for rhombohedral films deposited on TbScO3. The evaluation of ellipsometric spectra reveals that the films deposited on LaAlO3 are optically less dense and the features in complex dielectric function are blue-shifted by 0.3 eV as compared to the rhombohedral films. Optical bandgaps of 3.10 eV and 2.80 eV were determined for the films deposited on LaAlO3 and TbScO3, respectively. The shift in the optical bandgap and dielectric function is nearly preserved also for thicker films, which indicates that the compressive strain is retained even in films with thicknesses above 100 nm as was confirmed also by XRD investigations.

The absence of exchange bias with (001)-oriented tetragonal-like BiFeO3films

Single-phase tetragonal-like BiFeO3 (T-BFO) films with thickness up to 480 nm have been successfully prepared on (001) LaAlO3 substrates using NdAlO3 (NAO) as a buffer layer. The monoclinic distortion increases on increasing the thickness of T-BFO films. Negligible small exchange bias field (HE) (<8 Oe) has been observed in T-BFO/NiFe bilayers. The insertion of a 2-nm thick La0.67Sr0.33MnO3 (LSMO) film between NAO and BFO can keep the structure of single-phase T-BFO. Strongly enhanced HE and coercivity has been observed below 100 K in LSMO/T-BFO bilayer, which has been attributed to the phase separation and coexistence of antiferromagnetic and ferromagnetic phases in the ultrathin LSMO layer due to the compressive strain exerted by the NAO buffer layer. Our results clearly demonstrate that T-BFO cannot provide an effective exchange bias field on the adjacent ferromagnetic layer, and the possible explanation has been provided.

Strain dependence of transition temperatures and structural symmetry of BiFeO3 within the tetragonal-like structure

The influence of strain-imposed in-plane lattice symmetry on the structural and magnetic properties of tetragonal-like BiFeO3 is investigated by x-ray and elastic neutron scattering. We find that an increase in the in-plane distortion results in an increase of the Néel temperature from 313 ± 5 K to 324 ± 3 K for films grown on YAlO3 and LaAlO3, respectively. The change in magnetic ordering temperature is reproduced in three-dimensional Heisenberg Monte-Carlo simulations. These results show that strain cannot be treated as a single scalar number or simply as a direct consequence of the lattice mismatch between the film material and the substrate.

Temperature-driven evolution of hierarchical nanodomain structure in tetragonal-like BiFeO3 films

Transmission electron microscopy study of tetragonal-like BiFeO3 films reveals a hitherto unreported hierarchical nanodomain structure. The 30-50 nm wide stripe domains with {110} domain walls consist of a substructure of lamellar nanodomains of 8-10 nm width in a herringbone-like arrangement. In situ heating and cooling reveal a reversible transition from the hierarchical nanodomain structure to a tweed-like domain structure which is accompanied by a first-order phase transition near 120 °C with a thermal hysteresis.