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]

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.

We demonstrate the room-temperature control of magnetization reversal with an electric field in an epitaxial nanostructure consisting of ferrimagnetic nanopillars embedded in a ferroelectric matrix. This was achieved by combining a weak, uniform magnetic field with the switching electric field to selectively switch pillars with only one magnetic configuration. On the basis of these experimental results, we propose to use an electric field to assist magnetic recording in multiferroic systems with high perpendicular magnetic anisotropy.

We report the study of the magneto-optical properties of composite multiferroic thin films composed of CoFe2O4 nanopillars embedded in a BiFeO3 matrix. The magneto-optical Kerr rotation and Kerr ellipticity in these films have been measured and are in good agreement with magnetization measurements. The Kerr signal has been studied as a function of film composition and nanopillar diameter confirming that the magneto-optical signal is due solely to the CoFe2O4 nanopillars.