[Show abstract][Hide abstract] ABSTRACT: Progress in nanotechnology requires new approaches to materials synthesis that make it possible to control material functionality down to the smallest scales. An objective of materials research is to achieve enhanced control over the physical properties of materials such as ferromagnets, ferroelectrics and superconductors. In this context, complex oxides and inorganic perovskites are attractive because slight adjustments of their atomic structures can produce large physical responses and result in multiple functionalities. In addition, these materials often contain ferroelastic domains. The intrinsic symmetry breaking that takes place at the domain walls can induce properties absent from the domains themselves, such as magnetic or ferroelectric order and other functionalities, as well as coupling between them. Moreover, large domain wall densities create intense strain gradients, which can also affect the material's properties. Here we show that, owing to large local stresses, domain walls can promote the formation of unusual phases. In this sense, the domain walls can function as nanoscale chemical reactors. We synthesize a two-dimensional ferromagnetic phase at the domain walls of the orthorhombic perovskite terbium manganite (TbMnO3), which was grown in thin layers under epitaxial strain on strontium titanate (SrTiO3) substrates. This phase is yet to be created by standard chemical routes. The density of the two-dimensional sheets can be tuned by changing the film thickness or the substrate lattice parameter (that is, the epitaxial strain), and the distance between sheets can be made as small as 5 nanometres in ultrathin films, such that the new phase at domain walls represents up to 25 per cent of the film volume. The general concept of using domain walls of epitaxial oxides to promote the formation of unusual phases may be applicable to other materials systems, thus giving access to new classes of nanoscale materials for applications in nanoelectronics and spintronics.
[Show abstract][Hide abstract] ABSTRACT: TbMnO3 films have been grown under compressive strain on (001)-oriented SrTiO3 crystals. They have an orthorhombic structure and display the (001) orientation. With increasing thickness, the structure evolves from a more symmetric (tetragonal) to a less symmetric (bulk-like orthorhombic) structure, while keeping constant the in-plane compression, thereby leaving the out-of-plane lattice spacing unchanged. The domain microstructure of the films is also revealed, showing an increasing number of orthorhombic domains as the thickness is decreased: we directly observe ferroelastic domains as narrow as 4 nm. The high density of domain walls may explain the induced ferromagnetism observed in the films, while both the decreased anisotropy and the small size of the domains could account for the absence of a ferroelectric spin spiral phase.
[Show abstract][Hide abstract] ABSTRACT: With shrinking device sizes, controlling domain formation in nanoferroelectrics becomes crucial. Periodic nanodomains that self-organize into so-called 'superdomains' have been recently observed, mainly at crystal edges or in laterally confined nanoobjects. Here we show that in extended, strain-engineered thin films, superdomains with purely in-plane polarization form to mimic the single-domain ground state, a new insight that allows a priori design of these hierarchical domain architectures. Importantly, superdomains behave like strain-neutral entities whose resultant polarization can be reversibly switched by 90°, offering promising perspectives for novel device geometries.
[Show abstract][Hide abstract] ABSTRACT: We investigate ferroelectric resistive switching in BiFeO3 thin films by performing local conductivity measurements. By comparing conduction characteristics at artificially up-polarized domains with those at as-grown down-polarized domains, the change in resistance is attributed to the modification of the electronic barrier height at the interface with the electrodes, upon the reversal of the electrical polarization. We also study the effect of oxygen vacancies on the observed conduction and we propose the existence of a different screening mechanism for up and down polarized domains.
[Show abstract][Hide abstract] ABSTRACT: In bulk TbMnO3 below 28K, the Mn sublattice orders as an
antiferromagnetic cycloidal spin structure. This breaks inversion
symmetry and induces a macroscopic electrical polarization:
TbMnO3 is a multiferroic material with a strong
magnetoelectric coupling. Contrary to the bulk, TbMnO3 thin
films grown on (001)-SrTiO3 substrates show
ferromagnetic-like behavior with a magnetic moment of
1.5μB/f.u. at 15K. However, the thickness dependence of
the magnetic moments is not consistent with magnetism homogeneously
distributed through the film. Additionally, epitaxial strain enables the
stabilization of different symmetries and particular domain
configurations at nanometric scales. Large strain gradients and/or
lowering of symmetry at the boundaries of these domains allow the
appearance of physical responses distinct from those of the domains. In
this work we investigate the contribution of the domain walls to the
[Show abstract][Hide abstract] ABSTRACT: Optimizing the piezoelectric performance at the nanoscale is one of the
main challenges for future piezoelectric applications, especially in the
field of vibrational energy harvesting. In this work, we have
investigated the combined influence of epitaxial strain, compositional
variation and size reduction on the crystallographic structure,
ferroelectric domain configuration and piezoelectric properties of
PbxSr1-xTiO3 thin films and
nanostructures epitaxially grown by Pulsed Laser Deposition on
SrRuO3-buffered (110)-DyScO3 substrates.
Theoretical predictions on the PbTiO3-SrTiO3 solid
solution show an interesting phase transition, expected to give rise to
enhanced piezoelectric properties, as a function of composition when the
films are grown under strain on (110)-DyScO3. A series of
high quality epitaxial thin films has been grown with various Pb/Sr
ratios. We have experimentally confirmed the predicted phase transition.
Highly periodic domains with purely in-plane polarization have been
observed by both X-ray diffraction and piezoresponse force microscopy.
The piezoelectric properties have then been studied as a function of
composition and of the lateral dimensions of nano-objects defined by
Electron Beam Lithography.
[Show abstract][Hide abstract] ABSTRACT: Materials in which structural polymorphs coexist are of great interest
in the design of magnetoelectric devices and piezoactuators at the
nanoscale. In BiFeO3, coexisting polymorphs are stabilized in
thin film form by strain resulting from film/substrate lattice mismatch
and/or thermal expansion differences. In films on LaAlO3
substrates, these polymorphic phases give rise to stripe patterns; they
are formed by the coexistence of the highly-strained (T') phase with an
intermediary polymorph (S') in samples devoid of the rhombohedral-like
relaxed (R') structure. Here, we investigate the local properties of the
stripe patterns by piezoresponse force microscopy and conductive atomic
force microscopy. This makes it possible to investigate the local
conductivity both of specific domains and of different domain walls, and
to compare the results to those obtained for R'-BiFeO3 films
(on SrTiO3 substrates). We show that patterns of locally
varying polarization and conductivity can be reversibly written and
erased at length scales determined by the phase stability of the
strain-induced structural polymorphs, and illustrate similarities and
differences between R' and T' BiFeO3.
[Show abstract][Hide abstract] ABSTRACT: We report on the thickness dependence of the ferroelastic domains of PbTiO3 films grown on (110)-DyScO3 with low thicknesses (up to 240 nm), which fall outside the validity range of the square root law proposed by Roytburd. For slow-grown films, the data reveal the linear thickness dependence predicted by Pertsev and Zembilgotov, using a complete elastic description, while a 2/3 scaling exponent is found for fast-grown films. Extremely long domains running all through the samples have been observed in the latter case, compared to the short domains observed in slow-grown films. These differences are ascribed to the in-plane anisotropy for domain wall nucleation, which is likely caused by the anisotropic elastic modulus of the substrate.
[Show abstract][Hide abstract] ABSTRACT: Using metal-ferroelectric junctions as switchable diodes was proposed several
decades ago. This was shown to actually work in PbZr(1-x)TixO3 (PZT) by Blom et
al. [P.W. M. Blom et al., Phys. Rev. Lett. 73, 2107 (1994)], who reported
switching in the rectification direction and changes of the current of about 2
orders of magnitude upon switching the polarization direction of the
ferroelectric layer. This form of resistive switching enables the read out of a
ferroelectric memory state at higher speed compared to the capacitive design,
without destroying the information in each reading cycle. Recently, Jiang and
coworkers have shown that these Schottky barrier effects are enormous in
BiFeO3, giving thousand times more switched charge than found by in PZT [A.Q.
Jiang. et al., Adv. Mat. 23, 1277 (2011)]. Here, by performing local
conductivity measurements, we attribute this to a large change of the Schottky
barrier height between the as-grown, down-polarized domains and the
up-polarized domains. These measurements allow to estimate the relative effect
of polarization charges and screening charges on the conduction through the
[Show abstract][Hide abstract] ABSTRACT: The sensitivity of spontaneous polarization to epitaxial strain for both 10 and 50 nm thick Ba0.5Sr0.5TiO3 (BSTO) ferroelectric thin films has been studied. Crystal truncation rod (CTR) profiles in the 00L directions at different wavelengths, and grazing incidence diffraction (GID) in the 0K0 direction on a single crystal have been recorded. Modeling of the CTR data gives a detailed picture of the strain and provides clear evidence of the film out-of-plane expansion at the surface, an increase of the polarization, as well as a contraction at the interface. GID data confirm the fitting of the CTR, showing an in-plane expansion of the BSTO film at the interface and a contraction at the surface.
physica status solidi (a) 11/2012; 209(11):2255-2259. · 1.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have used temperature-dependent impedance spectroscopy to study the dielectric response of thin films of ferromagnetic and ferroelectric (Bi0.9La0.1)2NiMnO6 oxide. This technique has allowed us to disentangle its intrinsic dielectric response and extract the dielectric permittivity of ∼220, which is in close agreement with bulk values but significantly smaller than early reported values for similar thin films. The permittivity is found to be temperature independent in the vicinity of the ferromagnetic transition temperature and independent of magnetic field. We have shown that the measured magnetocapacitance arises from the magnetoresistance of the films, thus indicating a negligible magnetoelectric coupling in these double perovskites, probably due to the different energy scales and mechanisms of ferroelectric and magnetic order.
[Show abstract][Hide abstract] ABSTRACT: BiFeO3 thin films epitaxially grown on SrRuO3-buffered (001)-oriented SrTiO3
substrates show orthogonal bundles of twin domains, each of which contains
parallel and periodic 71o domain walls. A smaller amount of 109o domain walls
are also present at the boundaries between two adjacent bundles. All as-grown
twin walls display enhanced conductivity with respect to the domains during
local probe measurements, due to the selective lowering of the Schottky barrier
between the film and the AFM tip (see S. Farokhipoor and B. Noheda, Phys. Rev.
Lett. 107, 127601 (2011)). In this paper we further discuss these results and
show why other conduction mechanisms are discarded. In addition we show the
crucial role that oxygen vacancies play in determining the amount of conduction
at the walls. This prompts us to propose that the oxygen vacancies migrating to
the walls locally lower the Schottky barrier. This mechanism would then be less
efficient in non-ferroelastic domain walls where one expects no strain
gradients around the walls and thus (assuming that walls are not charged) no
driving force for accumulation of defects.
Journal of Applied Physics 12/2011; · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Strain engineering enables modification of the properties of thin films using the stress from the substrates on which they are grown. Strain may be relaxed, however, and this can also modify the properties thanks to the coupling between strain gradient and polarization known as flexoelectricity. Here we have studied the strain distribution inside epitaxial films of the archetypal ferroelectric PbTiO(3), where the mismatch with the substrate is relaxed through the formation of domains (twins). Synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy reveal an intricate strain distribution, with gradients in both the vertical and, unexpectedly, the horizontal direction. These gradients generate a horizontal flexoelectricity that forces the spontaneous polarization to rotate away from the normal. Polar rotations are a characteristic of compositionally engineered morphotropic phase boundary ferroelectrics with high piezoelectricity; flexoelectricity provides an alternative route for generating such rotations in standard ferroelectrics using purely physical means.
Nature Material 12/2011; 10(12):963-7. · 36.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Understanding the magnetotransport properties of epitaxial strained thin films requires knowledge of the chemistry at the interface. We report on the change in Mn electronic structure at the epitaxially strained TbMnO3/SrTiO3 interface. Scanning transmission electron microscopy shows an abrupt interface with a bright contrast, indicating the presence of misfit strain. Electron energy loss spectroscopy displays a chemical shift of the Mn L2,3 edge together with a high white line intensity ratio revealing a reduction in the nominal Mn oxidation state in the first 3–4 monolayers. These observations indicate misfit strain significantly changes the electronic structure at the interface.
[Show abstract][Hide abstract] ABSTRACT: Local conduction at domains and domain walls is investigated in BiFeO(3) thin films containing mostly 71° domain walls. Measurements at room temperature reveal conduction through 71° domain walls. Conduction through domains could also be observed at high enough temperatures. It is found that, despite the lower conductivity of the domains, both are governed by the same mechanisms: in the low voltage regime, electrons trapped at defect states are temperature activated but the current is limited by the ferroelectric surface charges; in the large voltage regime, Schottky emission takes place and the role of oxygen vacancies is that of selectively increasing the Fermi energy at the walls and locally reducing the Schottky barrier. This understanding provides the key to engineering conduction paths in BiFeO(3).
[Show abstract][Hide abstract] ABSTRACT: BiFeO3 (BFO) is, at room temperature, a rhombohedrally distorted, ferroelectric perovskite. There are eight possible polarization directions (or domains) and three different types of domain walls, namely, 180 (ferroelectric) and 109 and 71 (ferroelectric and ferroelastic) domain walls. Recent works have shown that the domain walls of BFO can display functionalities different from those of the domains, generating photocurrents , inducing exchange bias  and displaying conductivity at room temperature . Conduction has been reported at 180 and 109 domain walls and it was proposed that the reduction of the band gap, associated with the suppression of ferroelectric distortions, at domain walls was responsible for the observed conduction. It is, however, not yet clear if and how other (extrinsic) mechanisms affect the conductivity at the walls. In order to help clarifying the origin of domain wall conductivity, we have performed temperature, thickness and orientation dependent local conductivity measurements in BFO thin films. The results will be discussed in this presentation. [4pt]  S.Y. Yang, Nature Nanotech. 5, 143 (2010);  L.W. Martin et al. Nano Letters 8, 2050 (2008); J. Seidel et al., Nature Mat. 8, 229 (2009).
[Show abstract][Hide abstract] ABSTRACT: The operation of resistive switches based on phase-separated blends of organic ferroelectrics and semiconductors depends significantly on the microstructure of such systems. A wide range of analysis techniques are used to characterize spin-coated films of the ferroelectric random copolymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], and the semiconducting polymer, regio-irregular poly(3-hexylthiophene) (rir-P3HT). The blend separates into amorphous rir-P3HT domains embedded in a crystalline P(VDF-TrFE) matrix. The rir-P3HT domains are continuous throughout the film, from the substrate/blend interface to the blend/air interface. We also investigate the rir-P3HT domain size and number as a function of composition and find – unexpectedly – a rather mono-disperse domain size distribution for a given rir-P3HT:P(VDF-TrFE) ratio. The domain size increases with rir-P3HT content, indicating that the solidification is not dominated by nucleation processes. Spinodal decomposition is therefore more likely to be responsible for the microstructure induced in the rir-P3HT:P(VDF-TrFE) blends. Since spinodal decomposition occurs spontaneously without the presence of a nucleation step, this can facilitate processing considerably, since the intricate control of nucleation processes (homogenous or heterogenous) is rendered unnecessary. Measurement of the lateral conductivity of the blends demonstrates that the rir-P3HT domains are electrically not connected, supporting the microstructural evidence. A perpendicular current through the film is measured using both Au and Ag electrodes as a function of blend composition. A model was used to interpret the electrical transport. The injection for Ag diodes poled into the ON-state preferentially occurs at the circumference of the rir-P3HT domains. An accumulation width over which the injection occurs is estimated to be of the order of a few hundred nm.
[Show abstract][Hide abstract] ABSTRACT: Epitaxial thin films of biferroic (Bi1−xLax)2NiMnO6 have been grown on SrTiO3 (001) substrates. High resolution electron microscopy, energy-loss spectroscopy and synchrotron radiation have been used to demonstrate that, under appropriate growth conditions, stoichiometric, and fully oxidized thin films with long-range order of Ni2+ and Mn4+ ions can be obtained, despite the presence of randomly distributed dissimilar cations (Bi, La) at the A-site. This ordering leads to Ni2+–O–Mn4+ ferromagnetic interactions and its preservation in thin films is key for implementation of these biferroic materials in practical devices.
Journal of Applied Physics 01/2011; · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A short review of the study of thermal properties of the nanostructured ferroelectrics is given. An application of the modern calorimetric technique to a comparative study of phase transitions in the ferroelectric nanostructured materials (polycrystalline and epitaxial thin films, superstructures, ferroelectric heterostructures, substrate-induced ferroelectricity) is considered.
[Show abstract][Hide abstract] ABSTRACT: Thin films of orthorhombic TbMnO$_3$, as well as other orthorhombic manganites, epitaxially grown on cubic SrTiO$_3$ substrates display an induced magnetic moment that is absent in the bulk (antiferromagnetic) counterpart. Here we show that there is a clear correlation between the domain microstructure and the induced magnetic moment in TbMnO$_3$ films on SrTiO$_3$. In addition, the distinct dependence of the magnetization with the film thickness is not consistent with domain magnetism and indicates that the domain walls, rather than the domains, are the origin of the net magnetic moment. Since the orientation of the domain walls can be designed by the film-substrate relationship and its density can be tuned with the film thickness, these results represent a significant step forward towards the design of devices based on domain wall functionality. Comment: 5pages, 3figures (color online). Revised version including estimation of domain wall magnetization; reference to Salje et al. and Malozemoff added; title revised. Other minor changes made overall