Thermodynamic aspects of epitaxial self-assembly and magnetoelectric response in multiferroic nanostructures
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Zhuopeng Tan Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899; and Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742
Alexander L. Roytburd Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742
Igor Levinb) Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 31 December 2006; accepted 15 March 2007)
A thermodynamic approach was used to describe the formation and magnetoelectric response of composite multiferroic films. Experimental and theoretical results that address the origins of different phase morphologies in epitaxial spinel-perovskite nanostructures grown on differently oriented substrates are presented. A theoretical model of magnetoelectric coupling in multiferroic nanostructures that considers a microscopic mechanism of magnetization in single-domain magnetic nanorods is described. This model explains a discontinuous electromagnetic coupling, as observed experimentally, and predicts a hysteretic behavior of magnetization under external electric fields.
I. INTRODUCTION
Bulk laminate magnetoelectric composites that consist of piezoelectric and magnetostrictive layers exhibit magnetoelectric (ME) effects—an induction of electric polarization by a magnetic field or an induction of magnetization by an electric field—that are orders of magnitude larger than those exhibited by the best single-phase multiferroic materials.1–7 The ME effects in these composites are strain-mediated and largely depend on the efficiency of strain transfer across the interfaces separating the two components. The basic problem in implementing layered ME composites in thin films is the strong clamping effect of the substrate, which effectively suppresses the ME response. The clamping effect in thin film heterostructures can be minimized if the orientations of the interfaces between the piezoelectric and magnetostrictive components are perpendicular to the substrate surface. Recently, such transversely modulated nanostructures have been synthe-
Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2007.0286 a)
J. Mater. Res., Vol. 22, No. 8, Aug 2007
http://journals.cambridge.org
Downloaded: 15 Mar 2015
sized via the epitaxial self-assembly of immiscible by lattice matched ferroelectric perovskite (e.g., BaTiO3, PbTiO 3 , and BiFeO 3 ) and ferrimagnetic spinel (CoFe 2 O 4 , NiFe 2 O 4 ) phases on single-crystal substrates.8–15 These heterostructures exhibited substantial ME coupling, as was evidenced, for example, by the switching of a magnetization direction in the individual magnetic nanorods under external electric field.11 Despite a significant interest in multiferroic nanostructures, the driving forces for their epitaxial self-assembly and the underlying microscopic mechanism of their ME response remain unclear and need further investigation. Self-assembly of the n
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