Author: Cheng, Wang-fai
Title: Oxide-based giant magnetoresistive heterostructures
Degree: M.Phil.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Heterostructures -- Magnetic properties.
Department: Department of Applied Physics
Pages: ix, 118 leaves : ill. (some col.) ; 30 cm.
Language: English
Abstract: The giant magnetoresistance (GMR) effect was discovered in 1988 in metal multilayer structures. The effect showed a large magnetoresistance (MR) ratio up to 80% at room temperature in Fe/Cr multilayers. Since then a number of alternative structures were developed, but they invariably relied on the changes of relative magnetization directions in the magnetic layers or components to achieve high and low resistive states. The discovery of GMR effect had a strong impact on the development of computer memory technology. At present, GMR read heads are used extensively in hard disk read heads. In my research project, oxide-based spin valve structures were deposited and characterized. La0.7Sr0.3MnO3 (LSMO) thin films were fabricated by pulsed laser deposition. The project consists of two parts. In the first part, the effects of post annealing on LSMO films, under different oxygen pressures, were examined. During multilayer fabrication processes, different growth conditions may be necessary for various layers. Different annealing procedures, sometimes ex situ, have to be performed. Therefore, the stability of the layers has to be studied, in order to obtain multilayers with desirable properties. This part of study provided useful information for investigations on epitaxial oxide multilayers. LSMO thin films were deposited by pulsed laser deposition on (100) LaAlO3 (LAO) substrates at 650oC, with the film thicknesses varying from 20 to 50 nm. The oxygen pressures used to fabricate the films were 150 (or 100) mTorr. In situ annealing processes were then performed at 100 (150) mTorr. Curie temperatures (Tc) of the films were estimated from the peaks of the resistance-temperature (RT) graphs. For LSMO films deposited at 150 mTorr and annealed at 100 mTorr, Tc of the samples dropped with an increasing annealing time until about 30 minutes, beyond which the TC started to increase. For LSMO films deposited at 100 mTorr and annealed at 150 mTorr, no signs of metal-insulation transition were observed even after 30 minutes of annealing. Ex situ annealing processes were also performed, testing the stability of LSMO films under post-deposition heat treatment processes after exposure to ambient conditions. The Tc of films were found to be strongly dependent on the annealing procedures. In the second part of the project, pseudo spin valves were fabricated by using LSMO and Co33Fe67 (CoFe) as ferromagnetic layers. These layers had very different coercivities, an important criterion for observing GMR effect. In order to reduce magnetic coupling between the two ferromagnetic layers, a non-magnetic and conducting spacer layer is usually required. Recent research has however shown that pseudo spin valves could be fabricated without using spacer layers. These spacerless pseudo spin valves, formed by direct deposition of polycrystalline metallic layers on epitaxial oxides, greatly reduced the difficulty in the fabrication process. In this part of the project, the magnetotransport property of deposited LSMO\CoFe structure was investigated. Magnetoresistances of samples were measured in current-perpendicular-to-plane mode. For single-layer LSMO samples grown on (100) LAO, a negative magentoresistance behaviour was obtained. After depositing the CoFe, the magnetoresistance behaviour was changed from negative to positive. One of possible reasons for the observation is the interfacial layer formed between the metal and oxide layers, which can be treated as a non-magnetic spacer. The thickness of the interface layer was sufficiently large to decouple the two ferromagnetic layers. Relative magnetization directions of the two ferromagnetic layers can be switched independently by an external magnetic field, according to their corresponding coercivities.
Rights: All rights reserved
Access: open access

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