Article pubs.acs.org/cm
Interface Magnetic Coupling in Epitaxial Bilayers of La0.92MnO3/ LaCoO3 Prepared by Polymer-Assisted Deposition José Manuel Vila-Fungueiriño,† Beatriz Rivas-Murias,† Benito Rodríguez-González,‡ and Francisco Rivadulla*,† †
Centro de Investigación en Química Biológica y Materiales Moleculares (CIQUS), Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain ‡ Departamento de Química Física, Universidad de Vigo, 36310 Vigo, Spain S Supporting Information *
ABSTRACT: We report the synthesis of epitaxial bilayers of ultrathin La 0.92MnO3/LaCoO3 (LMO/LCO) on SrTiO3 (STO) by polymer solution deposition. We have achieved an excellent control over the thickness (from ∼4 to ∼20 nm) and roughness of the films, which allows the fabrication of the sharp interfaces required for their functionality. We suggest that the large increase in the coercive field observed in the bilayer with respect to isolated films of LMO or LCO is due to a strong ferromagnetic Mn−O−Co superexchange interaction at the interface. Our results demonstrate that abrupt interfaces can be obtained in epitaxial multilayers by a simple and affordable chemical deposition technique, with the quality required for fundamental studies or highly demanding applications.
■
INTRODUCTION Thin films play an important role in science and technology; the development of sophisticated deposition techniques allowed the microelectronic revolution of the last decades, which demanded smaller and more reliable components with a lower consumption of energy and materials. In this respect, deposition by physical methods (sputtering, MBE, PLD, etc.) is applicable to a wide range of compositions (nitrides, oxides, sulfides, etc.), which are produced with the quality required for highly demanding applications. From the scientific point of view, epitaxially engineered stress in ultrathin films opens a new way for tuning the competition between different phases.1 On the other hand, growth of multilayers with atomic-control offers the possibility of studying the emergent phenomena that occur at interfaces between two materials with different symmetries, which can be designed to create new functionalities.2,3 In the past few years, different methods based on the deposition from a chemical solution emerged as a cheaper alternative to physical methods.4−10 Although the control of thickness, homogeneity, and stoichiometry is still not competitive with physical techniques, the quality of the films is improving very fast. Particularly, spin-coating deposition of aqueous solutions of cations coordinated to polyethyleneimine (PEI) produced thin-films of very high quality in a wide range of compositions.5,11−14 However, deposition of epitaxial multilayers or superlattices by chemical methods is still extremely difficult because of the roughness of the film surface, which obstructs uniform layer-onlayer growth. This limits the applicability of chemical © 2014 American Chemical Society
deposition techniques in highly demanding applications, like multilayer tunnel junction devices,15 or in fundamental studies of electric/magnetic control of exchange interactions across the interface,2 which require ultrathin films with smooth and uniform boundaries. The purpose of this paper is to demonstrate the suitability of polymer solution deposition for the production of high quality heteroepitaxial bilayers. In particular, we have attempted the fabrication of an ultrathin ferromagnetic insulating (FI) film on top of a ferromagnetic metallic (FM) layer. Growing ultrathin films of FI on top of a FM could be an important step forward toward the fabrication of advanced devices, like spin-filtering tunnel barriers, by an affordable chemical method. For the FI film we have selected, LaCoO3 (LCO), we previously showed that FI thin films of LCO can be grown by chemical assisted deposition under tensile stress on STO.14 This atypical combination of ferromagnetism and high electronic resistivity is attributed to a spin-state transition of Co3+, induced by epitaxial tensile stress.16,17 We wondered whether it is possible to keep these properties on chemically deposited ultrathin layers (
