Session P32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - BiFeO3

Inelastic neutron scattering study of spin-wave from single crystal BiFeO3

BiFeO$_3$ is one of the most promising multiferroic materials for device applications in spintronics and memory devices. There have been a number of studies on electric field tuning of antiferromagnetic domains, as well as possible E-field control of spin-waves in this material. The potential of controlling spin dynamics using electric field is extremely appealing. However, so far there have been very limited work on the direct measurements of spin-waves in BiFeO$_3$, mostly due to lack of large size single crystals. We will present our recent inelastic neutron scattering studies on a single crystal BiFeO$_3$, showing the full spin-wave spectrum in three-dimensions. A classical spin-wave model can be used to describe the results in details. The coupling parameters and spin-wave velocities have been obtained, and are in good agreements with those obtained in Raman measurements.   

Local Raman spectroscopic study of BiFeO$_3$ strained states

Among single-phase multiferroic materials, BiFeO$_3$ (BFO) has relatively high Curie and N$'{e}$el temperatures, which possesses ferroelectric and antiferromagnetic couplings at room temperature, so is motivated for novel device applications. Recent studies had shown piezoelectric and magnetic properties of BFO in strained states varied significantly. For BFO epitaxial films grown on LaAlO$_3$ substrate, high piezoelectric coefficient and spontaneous ferromagnetic moments had been demonstrated in a new kind of morphotropic tetragonal-rhombohedral phase boundary driven by substrate strain. In this study, we used Raman spectrum to investigate the local BFO distorted structure under substrate strain or strain caused by external electric fields. The crystal structure of BFO under compressive substrate strain is monoclinically distorted. The ordering of the monoclinic structures could also be controlled by electric field. These two kinds of strained states were locally studied by atomic force microscopy (AFM) equipped with on-axis Raman measurement. This study provided the basic physical insight of unique physical properties depended on distorted structures.   

Unambiguous phonon mode assignment in multiferroic BiFeO$_3$ single crystals

In Bismuth ferrite (BiFeO$_3$) antiferromagnetic and ferroelectric order parameters coexist at room temperature, making this material an excellent candidate for new functionalities, such as electrical control of magnetism. Despite extensive reports on Raman scattering experiments on single crystals and thin films, controversy still remains in the observation and assignment of the phonon mode symmetries. We present polarized micro-Raman spectroscopy of single crystals ((1 0 0)$_{cubic}$ surface) with uniform ferroelectric polarization. Careful examination of the Raman spectra upon crystal rotation enables us to unambiguously assign the (A$_1$, E$_x$ and E$_y$) modes. We will show that ambiguity is easily introduced by slight misalignment of the crystal and that the crystal rotation is necessary to reach unambiguous mode assignment. Our method not only results in proper Raman mode assignment, which is necessary to describe the phonons critical for the multiferroic behavior, it also allows study of symmetry breaking and may provide a way to non-invasively check the ferroelectric polarization direction.   

Multiple Phase Transitions in the model multiferroic BiFeO3

Bismuth ferrite BiFeO3 (BFO) is commonly considered a model system for multiferroics, and is perhaps the only material that is both magnetic and a ferroelectric with a strong electric polarization at 300K [1]. Despite numerous investigations, the crystal structures of BFO as a function of temperature and pressure are still not established and lead to ongoing controversial reports in the literature [1,3]. Besides being a model multiferroic, BFO is also one of the very few materials that present both octahedra tilts and strong cation displacements at room temperature. Here we report the high-pressure phase transitions in BFO by both synchrotron x-ray diffraction and Raman spectroscopy, namely a surprising richness of six phase transitions in the 0--60 GPa range [2-3]. At low pressures, 4 transitions are evidenced at 4, 6, 7 and 11 GPa. In this range, the crystals display in that range unusual large unit cells and complex domain structures, which suggests a competition between complex tilt systems and possibly off-center cation displacements. The non polar Pnma phase remains stable over a large pressure range between 11 and 38 GPa. The two high pressure phase transitions at 38 and 48 GPa are marked by the occurrence of larger unit cells and an increase of the distortion away from the cubic parent perovskite cell. The previously reported insulator-to-metal transition appears to be symmetry breaking. Finally, we will present a new schematic P-T phase diagram for BFO and discuss the recently reported phase transition in highly strained BFO films [4,5] in the light of our high-pressure findings. \\[4pt] [1] G. Catalan, J. F. Scott, Advanced Materials 21, 1 (2009).\\[0pt] [2] R. Haumont et al., Phys. Rev. B 79, 184110 (2009).\\[0pt] [3] M. Guennou et al., Phys. Rev. B 2011, accepted http://arxiv.org/abs/1108.0704.2011\\[0pt] [4] J. Kreisel et al. J. Phys.: Cond. Matt. 23, 342202 (2011).\\[0pt] [5] W. Siemons et al. Appl. Phys. Express 4 (2011).   

Phase Transitions in Epitaxial (-110) BiFeO$_3$ Films from First Principles*

The effect of misfit strain on properties of epitaxial BiFeO$_3$ films that are grown along the pseudo-cubic [$\bar{1}$10] direction, rather than along the ``usual'' [001] direction, is predicted from density functional theory. These films adopt the monoclinic $Cc$ space group for compressive misfit strains smaller in magnitude than $\simeq$1.6\% and for any investigated tensile strain. In this $Cc$ phase, both polarization and the axis about which antiphase oxygen octahedra tilt rotate {\it within} the epitaxial plane as the strain varies. Surprisingly and unlike in (001) films, for compressive strain larger in magnitude than $\simeq$1.6\%, the polarization vanishes and two orthorhombic phases of $Pnma$ and $P2_12_12_1$ symmetry successively emerge via strain-induced transitions. The $Pnma$-to-$P2_12_12_1$ transition is a rare example of a so-called pure ``gyrotropic'' phase transition, and the $P2_12_12_1$ phase exhibits original interpenetrated arrays of ferroelectric vortices and antivortices. This work is mostly supported by ONR Grants N00014-08-1-0915 and N00014-07-1-0825 (DURIP). *S. Prosandeev, Igor A. Kornev, and L. Bellaiche, Phys. Rev. Lett. 107, 117602 (2011).   

Nonlinearity in the high-electric-field piezoelectric response of epitaxial BiFeO3

The multiferroic material BiFeO3 provides the means to understand the piezoelectric coupling between lattice strain and ferroelectric polarization. Little is known about the piezoelectric properties of BiFeO3 under high electric fields. In our study, the transient high-electric-field piezoelectricity of BiFeO3 was measured at electric fields up to about 300 MV/m using time-resolved x-ray microdiffraction. A linear strain-electric field response with a piezoelectric coefficient of 55 pm/V was observed at electric fields up to 150 MV/m. At higher electric fields, the strain is larger than the value anticipated from the low-field regime, reaching 0.02 at 281 MV/cm. The integrated intensity near the BiFeO3 (002) Bragg reflection is unchanged in large electric fields, showing that the enhanced piezoelectricity occurs without producing a field-induced phase transition. We also observe a relative increase of diffuse x-ray intensity at high electric fields. We will discuss a model in which the increase of piezoelectricity and diffuse scattering originates from the softening of phonon modes at high electric fields.   

Ultrafast photostriction in BiFeO$_{3}$ thin films

The ultrafast dynamics of BiFeO$_{3}$ (BFO) thin films was studied by dual-color transient reflectivity measurements ($\Delta R/R$) from 80 K to room temperature. Based on the thickness-dependent propagating time of the photoinduced strain pulse, the sound velocity along [110] direction of BFO is 4.76 km/s. Anisotropic photostriction effect in BFO generated within short time scale of 5 ps is enhanced by the optical rectification effect. Furthermore, the anomalous changes of the temperaure-dependent $\Delta R/R$ at 130 K and 210 K may reveal the spin-orbital coupling and magnetoelastic effect in BFO.   

Photoinduced structural dynamics in BiFeO$_3$ thin films studied by ultrafast x-ray diffraction

We have used time-resolved x-ray diffraction to study the temporal response of multiferroic BiFeO$_3$ to laser excitation. Above-bandgap light pulses, with 400 nm central wavelength and 50 fs duration, were used to photoexcite 35-nm thick BiFeO$_3$ films grown by molecular beam epitaxy on SrTiO$_3$ (001) substrates. The angular shifts of BiFeO$_3$ Bragg peaks vs.\ time were recorded with $\sim$100 ps resolution and used to determine the out-of-plane strain in the film. Observed strains range up to several tenths of a percent after excitation and relax on a several-ns timescale. Strains of such magnitude are too large to be explained by thermal expansion alone, but rather appear to be due to screening of the depolarization field by photoexcited carriers. At higher laser fluences, the integrated intensity of the Bragg peak decreases due to transient rearrangement of the atomic lattice on the scale of $\sim$0.1 \AA.   

The valence electronic structure of multiferroic BiFeO$_{3}$ from high energy X-ray photo-electron spectroscopy and first principles theory

BiFeO3 (BFO) is a multi-functional material with high ferroelectric and magnetic ordering temperature. Here we have investigated the electronic structure of (001) oriented 100nm rhombohedral BFO thin films using high energy X-ray photoelectron spectroscopy (XPS). By making use of the energy dependence of the relative cross sections for different states, we were able to selectively probe the elemental contributions to the valence band . At high energies, states with high main quantum number will have a higher relative probability for photo-ionization, i.e., the Bi 6s and 6p contributions in the valence region are enhanced relative to the Fe 3d and O 2p. We find that the Bi 6p states hybridize strongly with the valence band dominated by the Fe 3d and O 2p states, resulting in a splitting of the 3d states due to bonding and anti-bonding combinations with the Bi 6p. Our results thus suggest that a previously relatively ignored electronic interaction needs to be considered for BFO and related Bi-TMOs. Ab initio calculations indicate the importance of screened Coulomb correlations to describe Bi and Fe electronic states.   

Direct characterization of the surface layer of BiFeO3 Single Crystals

A surface layer different from the bulk was found in single crystals of BiFeO3, and was analyzed with different techniques such as surface impedance and grazing incidence x-ray diffraction, showing an specific phase transition at T* $\sim $ 275 $^{\circ}$C. Local physical characterization studies have been performed with different AFM techniques, such as Piezoelectric Force Microscopy (PFM), Scanning Kelvin Probe Microscopy (SKPM) and Force Modulated Microscopy (FMM) at different temperatures up to 300$^{\circ}$C. The thin superficial skin layer is found to be an electrically ``dead'' layer with a thickness of 6 +/- 1 nm and different thermal expansion coefficient with respect to the bulk. The sharp thermal expansion of the surface layer at T* facilitates its mechanical declamping from the underlying crystal at the transition temperature, enabling direct access to the sub-surface region using scanning probe microsocopy techniques. A distribution of near-surface ferroelectric domains is found in a region of less than 200 nanometers deph under the surface. These nanodomains organize in a hierarchical metastructure on top of the existing bulk domains. The symmetry and properties of these sub-surface nanodomains will be discussed.   

First-principles investigation of phase-change effects in multiferroics

Using first-principles calculations we have characterized new phases of bulk multiferroic materials that are close in energy to the ground state, but that display very different properties. This suggests that the application of electric fields could induce phase changes that would involve large effects of different kinds. In particular, (i) we have found stable supertetragonal bulk phases for the prototype multiferroic bismuth ferrite [Di\'eguez {\em et al}, Phys.~Rev.~B {\bf 83}, 094105 (2011)], and (ii) we propose to use a solid solution of bismuth ferrite and bismuth cobaltite to create a material where it is possible to switch between two very different phases in a way that involves strong piezoelectric, electric, and magnetoelectric effects [Di\'eguez and \'I\~niguez, Phys.~Rev.~Lett. {\bf 107}, 057601 (2011)].   

Magnetic and structural properties of BiFeO$_{3}$ thin films grown epitaxially on SrTiO$_{3}$/Si substrates

Commensurate growth of SrTiO$_{3}$ (STO) on Si using molecular beam epitaxy (MBE) has been achieved. STO on Si is used as a virtual substrate to enable the growth of BiFeO$_{3}$ (BFO). Having a crystalline oxide surface on Si is an enabler for deposition of various functional oxides that would not have been possible directly on silicon. A systematic study of the dependence of the magnetic and structural properties of BFO on the growth conditions, such as O$_{2}$ plasma pressure and film thickness, is performed. The crystalline nature of the BFO film has been confirmed by X-Ray diffraction showing the expected peak positions for (100) oriented oxide films with no additional, unidentified peaks. The BFO/STO/Si films exhibit antiferromagnetic behavior with high transition temperatures, thus leading to the possibility of room temperature magnetoelectric coupling-based devices integrated onto Si CMOS circuitry. Thinner films at lower O$_{2}$ plasma pressures exhibit stronger magnetic characteristics.   

Magnetoelectric and Ferroelectric Properties of BiFeO3/Ni film Deposited by Pulsed Laser Deposition

To fabricate a layer by layer (2-2) magnetoelectric (ME) sensor, ferroelectric (FE) BiFeO3 film was directly deposited on ferromagnetic (FM) nickel foil by pulsed laser deposition (PLD) without oxide or noble metal buffer layer, which significantly lowers the cost of ME and FE devices, and makes it possible to deposit longer ME and FE bendable band by PLD. X-ray diffraction and transmission electron microscopy analysis confirmed that the BiFeO3 film was successfully deposited on the top of nickel foil. The BiFeO3 film had a saturation polarization and a piezoelectric d33 coefficient of 69 $\mu $C/cm2 and 52 pm/V respectively. The ME coefficient of the sample was 4mV/cmOe which was measured under 1 Oe AC magnetic field at 1 kHz frequency.