Multiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. In this work, we demonstrate the first observation of electrical control of antiferromagnetic domain structure in a single-phase multiferroic material at room temperature. High-resolution images of both antiferromagnetic and ferroelectric domain structures of (001)-oriented multiferroic BiFeO3 films revealed a clear domain correlation, indicating a strong coupling between the two types of order. The ferroelectric structure was measured using piezo force microscopy, whereas X-ray photoemission electron microscopy as well as its temperature dependence was used to detect the antiferromagnetic configuration. Antiferromagnetic domain switching induced by ferroelectric polarization switching was observed, in agreement with theoretical predictions.

We report a discovery that self-assembled perovskite-spinel nanostructures can be controlled simply by selecting single-crystal substrates with different orientations. In a model BiFeO(3)-CoFe(2)O(4) system, a (001) substrate results in rectangular-shaped CoFe(2)O(4) nanopillars in a BiFeO(3) matrix; in contrast, a (111) substrate leads to triangular-shaped BiFeO(3) nanopillars in a CoFe(2)O(4) matrix, irrespective of the volume fraction of the two phases. This dramatic reversal is attributed to the surface energy anisotropy as an intrinsic property of a crystal.

With an ever-expanding demand for data storage, transducers, and microelectromechanical (MEMS) systems applications, materials with superior ferroelectric and piezoelectric responses are of great interest. The lead zirconate titanate (PZT) family of materials has served as the cornerstone for such applications up until now. A critical drawback of this material, however, is the presence of lead and the recent concerns about the toxicity of lead-containing devices. Recently, the lead-free ferroelectric BiFeO 3 (BFO) has attracted a great deal of attention because of its superior thin-film ferroelectric properties, [1,2] which are comparable to those of the tetragonal, Ti-rich PZT system; therefore, BFO provides an alternate choice as a "green" ferro/piezoelectric material. Another advantage of BFO is its high ferroelectric Curie temperature (T c = 850°C in single crystals), [3,4] which enables it to be used reliably at high temperatures. The ferroelectric domain structure of epitaxial BFO films are typically discussed in the context of the crystallographic model of Kubel and Schmid; [5] however, by suppressing other structural variants in BFO, we can obtain periodic domain structures that may open additional application opportunities for this material. Ferroelectrics with periodic domain structures are of great interest for applications in photonic devices [6] and nanolithography.[7] Such a periodic polarization could be obtained by applying an external electric field while utilizing lithographically defined electrodes or by a direct writing process. [8,9] To obtain sub-micrometer feature sizes, however, domain engineering using a scanning force microscope with an appropriate bias voltage must be used to fabricate the patterned domain structures.[10] Unfortunately, this method works only on small areas and is limited by its slow scanning rate. Theoretical models predict the feasibility of controlling the domain architecture in thin films through suitable control over the heteroepitaxial constraints. [11] In the case of BFO thin films, we have found that such a control is indeed possible, mainly through control over the growth of the underlying SrRuO 3 electrode. Using this approach, we demonstrate the growth of highly ordered 1D ferroelectric domains in 120 nm thick BFO films. On the (001) C perovskite surface there are eight possible ferroelectric polarization directions corresponding to four structural variants of the rhombohedral ferroelectric thin film. (For simplicity, the c and o subscripts refer to the pseudocubic structures for BFO and orthorhombic structures of SrRuO 3 (SRO) and DyScO 3 (110) O (DSO), respectively.) Domain patterns can develop with either {100} C or {101} C boundaries for (001) C -oriented rhombohedral films. [12] In both cases, the individual domains in the patterns are energetically degenerate and thus equal-width stripe patterns are theoretically predicted. When the spontaneous polarization is included in the analysis, the {100} C boundary patterns have no normal component of the net po...

Control over ferroelectric polarization variants in BiFeO 3 films through the use of various vicinal SrTiO 3 substrates is demonstrated. The ferroelectric polarization variants in these films are characterized by piezoelectric force microscopy and the corresponding structural variants are carefully analyzed and confirmed by X-ray diffraction. Implementation of this approach has given us the ability to create single domain BiFeO 3 films on (001), (110), and (111) surfaces. The piezo/ ferroelectric properties of the BiFeO 3 films, in turn, can be tailored through this approach. Such results are very promising for continued exploration of BiFeO 3 films and provide a template for detailed multiferroic-coupling studies in the magnetoelectric BiFeO 3 system. Magnetoelectric coupling in multiferroic materials has attracted much attention because of the intriguing science underpinning this phenomenon. Additionally, there is an exciting potential for applications and devices that take advantage of these materials with multiple order parameters. [1][2][3][4] BiFeO 3 (BFO) is a room temperature, single-phase magnetoelectric multiferroic with a ferroelectric Curie temperature of ∼ 1103 K [5] and an antiferromagnetic Néel temperature of ∼ 643 K.[6] Recent studies of BFO thin films have shown the existence of a large ferroelectric polarization, as well as a small net magnetization of the Dzyaloshinskii-Moriya type resulting from a canting of the antiferromagnetic sublattice. [7,8] The ferroelectric polarization in BFO can have orientations along the four cube diagonals (<111>), and the direction of the polarization can be changed by ferroelectric and ferroelastic switching. [9] Our previous studies have shown coupling between ferroelectricity and antiferromagnetism in BFO thin films resulting from the coupling of both antiferromagnetic and ferroelectric domains to the underlying ferroelastic domain switching events.[10] Such a study was a crucial first step in the exploration of approaches to control and manipulate magnetic properties using an electric field. It was also noted, however, that these films exhibit a very complicated domain structure, which complicates the interpretation of the fundamental properties of this system as well as the interactions across hetero-interfaces. The lack of large single crystals of the desired crystallographic orientation provokes another motivation to explore approaches to create "single crystalline" epitaxial films that are free of ferroelectric/ferroelastic domains. Recent studies have explored the ability to control the ferroelectric domain configuration, which is formed after the phase transformation, through substrate engineering. [11,12] In this study, we demonstrate an approach to control the ferroelectric domain structure in BFO films through the use of vicinal SrTiO 3 (STO) substrates. This has enabled us to create thin films that "mimic" the primary crystal facets of the pseudo-cubic unit cell, namely single domain (100), (110), and (111) surfaces. The ferroelectric domain structu...

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We have grown BiFeO3 thin films on SrRuO3∕SrTiO3 and SrRuO3∕SrTiO3∕Si using liquid delivery metalorganic chemical vapor deposition. Epitaxial BiFeO3 films were successfully prepared through the systematic control of the chemical reaction and deposition process. We found that the film composition and phase equilibrium are sensitive to the Bi:Fe ratio in the precursor. Fe-rich mixtures show the existence of α-Fe2O3, while Bi-rich mixtures show the presence of β-Bi2O3 as a second phase at the surface. In the optimized films, we were able to obtain an epitaxial single perovskite phase thin film. Electrical measurements using both quasistatic hysteresis and pulsed polarization measurements confirm the existence of ferroelectricity with a switched polarization of 110–120μC∕cm2, ΔP(=P*−P̂). Out-of plane piezoelectric (d33) measurements using an atomic force microscope yield a value of 50–60pm∕V.

Ferroelectric size effects in multiferroic BiFeO 3 have been studied using a host of complementary measurements. The structure of such epitaxial films has been investigated using atomic force microscopy, transmission electron microscopy, and x-ray diffraction. The crystal structure of the films has been identified as a monoclinic phase, which suggests that the polarization direction is close to ͗111͘. Such behavior has also been confirmed by piezoforce microscopy measurements. That also reveals that the ferroelectricity is down to at least 2 nm.

We have studied the stability of domains and domain walls in multiferroic BiFeO3 thin films using a combination of piezoelectric force microscopy and phase-field simulations. We have discovered that a film-substrate misfit strain may result in a drastically different thermodynamic stability of two parallel domain walls with the same orientation. A fundamental understanding of the underlying physics, the stress distribution in a domain structure, leads to a novel approach to control the ferroelastic domain stability in the multiferroic BiFeO3 system.