Oxygen influence in the magnetic and the transport properties of ferroelectric/ferromagnetic heterostructures
Introduction
Manganites compounds have been studied for many decades [1], [2]. Nevertheless, a renewed interest in these systems took place in the early 90s. The origin of this interest was based essentially in two important properties of manganite systems: the colossal magneto-resistance effect [3] and the interplay between magnetic, transport and structural properties, which gives place to many intriguing phenomena. On the other hand, manganites and oxides with perovskite structure have become the key component in the development of an important number of devices, going from magnetic sensors, memory systems, new electronic devices, multiferroic tunnel junctions to field effect transistors. The possibility to tune the physical properties of these systems by different “extrinsic” effects like stress or chemical composition has given place to a novel area of study in basic physics and technological applications known as functionalized oxides and surface engineered systems (e. g. bilayer and multilayer nanostructures).
However, the growth and control of the physical properties of oxides could be difficult. In particular, the oxygenation of the samples is important since oxygen vacancies are a common defect in oxides nanostructures [4]. Usually a post deposition annealing processes is needed in order to achieve the correct stoichiometry. It is worth noticing that controlling and understanding the role of oxygen vacancies in the physical properties of oxides is critical since they are the origin of many fundamental and novel phenomena [4]. These phenomena include induced ferromagnetism [5], [6], two dimensional electron gases in LaAlO3/SrTiO3 interfaces [7] and oxygen vacancies could contribute to the fatigue and aging effects in ferroelectric materials [4], [8]. Additionally, this kind of defects are responsible of some interesting technological applications, such as memristors devices [9], [10]. Ferroelectric barriers has been used in the last years to induce additional resistance states in multiferroic tunnel junctions due to the interplay of the current flow and the polarization of the barrier which modifies the tunneling energy barrier [11].
The aim of this work is to study the oxygen influence in the physical properties of multiferroic heterostructures. There has been little experimental work done to understand the role of oxygen vacancies in the tunneling process across thin oxide barriers. In this context, it is particularly interesting the study of the electrical transport through a ferroelectric layer. Conductive atomic force microscopy (CAFM) has been used to study La0.8Ba0.2MnO3/Ba0.25Sr0.75TiO3 bilayers, using different oxygen pressures during the post deposition cooling of the samples. We have developed a phenomenological method and a characterization analysis using CAFM that allows to study the transport and morphological properties of thin insulting layers over conducting electrodes [12]. This model was used to obtain critical information for the development of tunnel junctions like devices, i.e. information about the energy barrier, the attenuation length and the width of the barrier thickness distribution.
Section snippets
Experimental details
La0.8Ba0.2MnO3 (LBMO) and Ba0.25Sr0.75TiO3 (BSTO) bilayers were grown on SrTiO3 (100) single crystal substrates by DC and low power (5000 W/m2) RF magnetron sputtering, respectively. The substrate temperature was kept at 720 °C in an argon (90%) / oxygen (10%) atmosphere at a pressure of 53 Pa for the whole sputtering process. We have deposited thin LBMO electrodes (30 ± 1 nm) in order to reduce the roughness of the surface, increasing the quality of the BSTO barrier as deduced from previous works [12]
Results and discussion
X-ray diffraction patterns of the multiferroic bilayers show that the samples present a good crystalline growth, textured in the direction perpendicular to the substrate. The bilayers present good magnetic properties, independently of the oxygenation pressure during the cooling down of the samples. The ferromagnetic transition temperature (Tc) of the electrodes, obtained from magnetization measurements is around 245 K and the coercive field of the samples is around 12 mT. Previous results
Conclusions
We have studied the influence of the oxygen pressure in the tunneling properties of a ferroelectric barrier grown on top of a ferromagnetic electrode. A phenomenological approach was used to obtain critical information about the structure and electrical properties of ultra-thin Ba0.25Sr0.75TiO3 (BSTO) layers, grown over a ferromagnetic electrode, using conductive atomic force microscopy. The tunneling of the current carriers is probably the main conduction mechanism for samples with higher
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