Ferromagnetic epitaxial Cr2O3 thin films grown on oxide substrates by Pulsed Laser Deposition
Graphical abstract
Introduction
Chromium trioxide (Cr2O3) is a widely studied oxide, being an important technological material because of its intrinsic properties. It is an antiferromagnetic (AFM, Néel Temperature (TN) = 307 K) and the first discovered magnetoelectric material above room temperature [1], [2], [3]. Bulk Cr2O3 also presents uniaxial magnetic anisotropy along the [1 1 1] direction. Additionally, it is a good insulator. The combination of these properties makes this material suitable as a building block in different technological devices. In particular, Cr2O3 has been employed in devices which involve exchange bias effect of coupled ferromagnetic - antiferromagnetic systems [1]. Furthermore, as a result of its good insulating properties, it is also suitable for magnetic tunnel junctions [2]. Cr2O3 is a potential candidate for heteroestructures based on epitaxial thin films as it can be easily prepared using a simple growth process. For instance, it has been already implemented as a buffer layer for the growth of other thin films, and has been employed as anode material for Li–ion batteries. However, special attention should be paid to the modification of its intrinsic properties due to the accumulated epitaxial strain. Previous results revealed that the presence of strain induces defects, such as oxygen vacancies, which can infer dramatic changes of its magnetic properties. For instance, unexpected ferromagnetism was found to occur in highly strained Cr2O3 thin films [3], [4]. Also, an enhancement of TN has been directly related to in–plane lattice contraction [5]. Thus, the ability of Cr2O3 to tune its intrinsic properties by artificial strain through epitaxial growth opens the way to the development of complex technological devices. For that, it is mandatory to explore the use of new substrates as templates for the epitaxial growth of Cr2O3 thin films with exotic macroscopic response. Several substrates have been already used, such as sapphire (α - Al2O3) [6], [7], graphene [8], Co [9], garnet [10], Cr (1 1 0) [11], YAlO3 (0 0 1) [12], TiO2 [13], LiNbO3(0 0 0 1) [14] or Ni(1 1 1) with a graphene buffer [15]. However, the use of the well-known SrTiO3 (STO) substrate has been scarcely explored. An attempt of epitaxial growth has been achieved on STO (0 0 1) using CeO2 as buffer layer [16]. However, direct growth of Cr2O3 on STO surface has not been reported yet. The successful epitaxial growth directly on SrTiO3 is of special importance as this material has been widely used for applications in microelectronics due to its high charge storage capacity, chemical stability and its excellent insulating properties [17], [18]. Hence, the combination of Cr2O3 and STO intrinsic properties provides a unique possibility to obtain different macroscopic responses on the same heterostructure. In the present work we have successfully grown for the first time epitaxial Cr2O3 thin films on STO (1 1 1) substrates. A complete morphological, structural, electronic and magnetic characterization is presented based on a comparison with epitaxial thin films grown on sapphire using equivalent conditions.
Section snippets
Experimental section
Cr2O3 thin films were grown on SrTiO3 (1 1 1) and α - Al2O3 (0 0 0 1) substrates (5x5 mm2) by Pulsed Laser Deposition (PLD). A Nd:YAG laser (λ = 355 nm) with 10 Hz and 1 J/cm2 irradiance power was used to ablate a polycrystalline Cr metal target. The deposition was carried out in an oxygen atmosphere of 10−4 mbar and at a surface temperature of 350 °C. High surface crystallinity was determined by in-situ Reflection High-Energy Electron Diffraction (RHEED) using a primary electron beam of 29 keV. The
Results and discussion
During the thin film growth by PLD, in-situ RHEED was used to monitor the evolution of the layers crystallinity. Fig. 1a-d shows representative RHEED patterns of the clean substrates, STO (1 1 1) and α - Al2O3 (0 0 0 1) (Fig. 1a and c), and the evaporated Cr2O3 layers (Fig. 1b and d). The RHEED patterns show smooth diffraction stripes, representative of a smooth, homogeneous and crystalline layer. The morphology of the film surface of the different samples has been studied by AFM. Fig. 2a and b show
Conclusions
The successful epitaxial growth of single phase and single oriented Cr2O3 thin films on SrTiO3 (1 1 1) has been achieved for the first time. We demonstrate that the use of SrTiO3 (1 1 1) substrate is favourable for the growth of high quality Cr2O3 epitaxial thin films. Low surface roughness and absence of oxygen vacancies are found for the layer grown on SrTiO3 while a moderately rough surface coupled to the presence of oxygen defects is found for the layer grown on α - Al2O3 (0 0 0 1). However larger
Authorship contributions
Category 1 - Conception and design of study: María Vila, Juan Rubio – Zuazo, Germán Rafael Castro. Acquisition of data: María Vila, Juan Rubio – Zuazo, Irene Lucas, César Magén, Alicia Prados, Eduardo Salas-Colera, Icíar Arnay and Germán Rafael Castro. Analysis and/or interpretation of data: María Vila, Juan Rubio – Zuazo, Irene Lucas, César Magén, Alicia Prados, Eduardo Salas-Colera, Icíar Arnay and Germán Rafael Castro.
Category 2 - Drafting the manuscript: María Vila, Juan Rubio – Zuazo,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The ESRF, MCIU and CSIC are acknowledged for the provision of synchrotron radiation facilities under Project No. PIE 2010-6-OE-013. This work has received financial support from Ministerio de Economía y Competitividad (Spain) under Project No. MAT2017-82970-C2-R and MAT2017-82970-C2-2-R. The microscopy works was conducted in the Laboratorio de Microscopías Avanzadas (LMA) at Instituto de Nanociencia de Aragón (INA)-Universidad de.
Zaragoza. The authors acknowledge the LMA-INA for offering access.
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María Vila and Juan Rubio-Zuazo are shared first authors.