Multiferroics are of interest for memory and logic device applications, as the coupling between ferroelectric and magnetic properties enables the dynamic interaction between these order parameters. Here, we report an approach to control and switch local ferromagnetism with an electric field using multiferroics. We use two types of electromagnetic coupling phenomenon that are manifested in heterostructures consisting of a ferromagnet in intimate contact with the multiferroic BiFeO(3). The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BiFeO(3) film that leads to electric-field control of the antiferromagnetic order. The second is based on exchange interactions at the interface between a ferromagnet (Co(0.9)Fe(0.1)) and the antiferromagnet. We have discovered a one-to-one mapping of the ferroelectric and ferromagnetic domains, mediated by the colinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. Our preliminary experiments reveal the possibility to locally control ferromagnetism with an electric field.

Multifunctional materials have attracted increasing interest in recent years because of their potential applications in novel technological devices. [1][2][3][4][5][6][7][8][9][10][11] [12] The ferroelectric and magnetic properties as well as the degree of the coupling are critically dependent on the morphology of the nanostructures, including domain patterns and shapes as well as the interfaces. In order to pursue the enhanced multifunctionality, significant effort has been made on understanding the growth mechanism and controlling the morphology of the nanostructures. The morphology adopted by a crystalline material when it nucleates on a substrate surface is one of the fundamental issues of heteroepitaxy. Depending on the surface energy terms, i.e., substrate surface energy c 1 , interface energy c 12 , and surface energy of the crystalline phase c 2 , the equilibrium shape of a crystalline nucleus on a substrate can be determined using the Winterbottom construction.[13] The possible configuration of the crystalline nucleus on the substrate is a Wulff shape that has been cut off by the substrate, translated by the signed distance Dc from the origin. Dc is the wetting strength, which is the energy difference obtained by replacing the substrate surface with an interface, Dc = c 12 -c 1 . In the BiFeO 3 -CoFe 2 O 4 system, BiFeO 3 has a distorted perovskite structure (R3c) [14] and CoFe 2 O 4 has a cubic spinel structure (Fd3m). CoFe 2 O 4 is characterized by the lowest surface energy of {111} surfaces, which is reflected in an equilibrium shape of an octahedron bounded by eight {111} facets. [15,16] In contrast, most perovskite phases have the lowest energy surfaces of {001} surfaces and a corresponding equilibrium shape of a cube dominated by six {100} facets. [17][18][19][20]

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...

Elemental tellurium (Te) nanoparticles are increasingly important in a variety of applications such as thermoelectricity, photoconductivity, and piezoelectricity. However, they have been explored with limited success in their biomedical use, and thus a tremendous challenge still exists in the exploration of Te nanoparticles that can treat tumors as an effective anticancer agent. Here, we introduce bifunctional Te nanodots with well-defined nanostructure as an effective anticancer agent for photo-induced synergistic cancer therapy with tumor ablation, which is accomplished using hollow albumin nanocages as a nanoreactor. Under near-infrared light irradiation, Te nanodots can produce effective photothermal conversion, as well as highly reactive oxygen species such as •O and dismutated •OH via a type-I mechanism through direct electron transfer, thereby triggering the potent in vivo hyperthermia and simultaneous intracellular reactive oxygen species at tumors. Moreover, Te nanodots possess perfect resistance to photobleaching, effective cytoplasmic translocation, preferable tumor accumulation, as well as in vivo renal elimination, promoting severe photo-induced cell damage and subsequent synergy between photothermal and photodynamic treatments for tumor ablation. These findings provide the insight of elemental Te nanodots for biomedical research.