Fabrication and Raman scattering of a core–shell structure based on Mn doped ZnO and barium titanate
Graphical abstract
Introduction
Research on the multiferroic materials has drawn increasing interest because of their technological applications such as information storage and sensors taking advantage of coupled ferroic properties. Usually, multiferroic materials are those in which ferromagnetic and ferroelectric orders coexist and are coupled [1], [2], [3]. The magnetoelectric (ME) coupling in multiferroic heterostructures is interfacial and can originate from charge, elastic strain and exchange bias interactions [4]. The low dimensional composite materials can provide tighter coupling between ferroelectric and ferromagnetic phases, and offer additional degrees of freedom in controlling the size, interface, and epitaxial strain to enhance the ME coupling [5]. Recent theoretical analysis indicates that composite multiferroic nanofibers exhibit ME responses higher than thin films of similar compositions [6], due to the substantially reduced influence of the substrate upon them. For the future developments on spintronics technology is important to control magnetization with electric field rather than with electric current or magnetic field in order to avoid the problems of heat dissipation and stray magnetic field. Recent developments in the electric field control of magnetism in multiferroic heterostructures shown in [7] might inspire possible applications in low-power spintronics. It was observed that a small voltage across a ferroelastic barium titanate (BaTiO3) substrate changed magnetic order in a thin film of ferromagnetic FeRh grown on this substrate. The low voltage applied on BaTiO3 substrate deforms its crystal structure via a ferroelastic effect, creating a strain. This strain is transferred to the FeRh film and influences its magnetic order. The strain in BaTiO3 substrate modified the transition temperature of FeRh, a characteristic temperature which separates antiferromagnetic order from ferromagnetic order.
The efficiency of the generated strain transfer across the shared interfaces determines the control degree of the ferroic order parameters in magnetoelectric composites. Some characteristics like the intrinsic surface stress, the curvature of the surface [8], the contact area and the characteristics of the interfaces at the atomic scale are important factors for the enhancement of the magnetoelectric coupling. One shortcoming is that the imperfect interface will reduce the transfer of the elastic strain between the magnetostrictive and ferroelectric phases. This can lead to a decrease of the magnetoelectric response of the nanocomposites. Besides, the formation of undesired phases as a result of the interfusion and/or chemical reactions during the annealing, thermal expansion mismatch between the two phases, as well as the presence of voids, residual grains, phase boundaries, porosity, dislocations and clamping effects [9], [10] could hinder the stabilization of the elastic coupling between the constituent phases.
ZnO is a wide bandgap semiconductor that was intensively studied as a perspective material for creation of diluted magnetic semiconductors. These semiconductors doped with magnetic transition metal ions in small concentration, in which the spin is a new degree of freedom for the electron, show a ferromagnetic phase at room temperature and there are candidates for designing spintronic devices. In this context, Mn doped ZnO nanostructures have been investigated especially for the use in magnetic and spintronic devices. The existence of the room temperature ferromagnetism in Mn doped ZnO nanorods for manganese concentrations of 1.3 and 2% [11] has been previously reported. Even high quality 8%-Mn-substituted ZnO film grown by pulsed laser deposition on single phase (0 0 1) sapphire substrate with strong ferromagnetism and large coercivity has been obtained [12].
Defect-induced room temperature ferromagnetism was observed for Zn0.995Mn0.005O film prepared by sol–gel technique [13] and even for undoped ZnO [14] prepared by electrospinning and annealing. Recently, Zn1−xMnxO/ZnO coaxial nanocables showing room temperature ferromagnetic properties were fabricated by sputtering Zn1−xMnxO onto the surface of ZnO nanowires [15]. Similarly, coupling of the Zn1−xMnxO ferromagnetic thin film with a ferroelectric material, like BaTiO3 film could permit the electrical control of the magnetism. BaTiO3 thin films have been extensively studied due to their interesting dielectric [16], ferroelectric [17], piezoelectric and electro-optical [18] properties and recently, BaTiO3 was used for the preparation of multiferroic heterostructures [7], [19].
In this report we present a route to prepare arrays of ZnO/Zn1−xMnxO/BaTiO3 core–shell nanorods. As the quality of the magnetoelectric coupling depends on the physical and chemical properties of this interface, the characteristics of the interface between ZnO/Zn1−xMnxO nanorods and BaTiO3 film deposited on these nanorods are studied by transmission electron microscopy and Raman scattering.
Section snippets
Experimental details
ZnO nanorod array was prepared by combined hydrothermal–electrochemical processes on the surface of FTO (fluorine doped SnO2 glass substrate) using the aqueous solution containing 5 mM Zn(NO3)2 and 5 mM hexamethylenetetramine at 95 °C and current density of −0.25 mA cm−2 [20]. Prior to the ZnO-nanorod array deposition process, the FTO substrate with sheet resistance of 15 Ω/square and optical transmission >80% from 400 to 700 nm was thoroughly cleaned by ultrasonic treatments, first in acetone and
ZnO/Zn1−xMnxO core/shell nanorods
A typical SEM morphology of ZnO nanorod array is shown in Fig. 1a. The length of ZnO nanorods deposited for 2000 s at −0.25 mA cm−2 was about 2 μm. These nanorods are thinner to the top end and form a sharp edge at the tip. The SEM images (Fig. 1b) of ZnO/Zn1−xMnxO nanorods show the presence of Mn doped ZnO layer on the surface of ZnO nanorods. The EDX measurements of the Zn1−xMnxO thin film deposited on FTO substrate have indicated a value of 0.07 for x (Fig. 2), which is larger than the x value
Conclusions
Large area of ZnO/Zn1−xMnxO/BaTiO3 nanorods was prepared on FTO substrate using successive depositions by spin coating on ZnO nanorods of Zn1−xMnxO and BaTiO3 precursors prepared by sol–gel technique.
XPS, photoluminescence and Raman scattering measurements indicate the existence of the intrinsic defects, especially oxygen vacancies in the ZnO/Zn1−xMnxO nanostructure. This can be associated with the presence of the ferromagnetic order in this sample.
In the case of the ZnO/Zn1−xMnxO/BaTiO3
Acknowledgements
This work was financially supported by Romanian Ministry of Education and Research (project with the code PNII-ID-PCE-2011-2-0006).
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