Magnetotransport properties in ferromagnetic metal-polymer granular films

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Magnetotransport properties in ferromagnetic metal-polymer granular films

 

Author: Lau, Wing-yin
Title: Magnetotransport properties in ferromagnetic metal-polymer granular films
Degree: M.Sc.
Year: 2005
Subject: Hong Kong Polytechnic University -- Dissertations
Ferromagnetism
Magnetoresistance
Granular materials
Department: Dept. of Applied Physics
Pages: v, 74 leaves : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1813838
URI: http://theses.lib.polyu.edu.hk/handle/200/4787
Abstract: Magnetoresistance (MR) has attracted attention in the last two decades since the discovery of giant magnetoresistance (GMR) in magnetic multilayers, such as Fe/Cr or Co/Cu. Tunneling magnetoresistance (TMR), the MR effect between ferromagnetic metals and insulators, has been studied at a later stage and is resulted from spin-dependent tunneling process. With an applied magnetic field, magnetization of ferromagnetic metals is aligned and a reduction in resistance is observed. Past studies on MR have been focused on the ferromagnetic (F) metal - nonferromagnetic (N) metal systems. For works on ferromagnetic (F) metal - insulator (I) systems, ceramics, such as SiO2, Al2O3, MgO, are widely used as the insulators. No work using polymer as the insulator has been done. In view of that, the objectives of this study are to investigate whether ferromagentic metal - polymer systems can demonstrate MR, the maximum value of MR attainable under our experimental setup and hence the development prospect of ferromagnetic metal - polymer systems. In the study, cobalt has been chosen as the ferromagnetic metal and polytetrafluoroethylene (PTFE or Teflon) the polymer material. Granular films of Co granules embedded in Teflon matrix have been fabricated by pulsed laser deposition (PLD) using Nd:YAG laser. This dissertation focuses on the fabrication and characterization of Co-Teflon granular films. The electronic transport and magnetotransport properties are studied by MR, hysteresis loops and resistance-temperature curves of the granular films. In all Co-Teflon granular films fabricated, negative MR has been observed. Resistance of granular films decreases with the applied magnetic field H, which aligns the magnetization of Co granules. Summarizing the prediction of granule size from MR versus H curves, hysteresis loops and resistance-temperature relation of Co-Teflon granular films, it is most probable that the size of Co granules distributes from superparamagnetic (a few nanometers), to tens and hundreds of nanometer, and to micrometer scale. The majority of Co granules probably lie in the middle region, i.e. tens and hundreds of nanometer. The maximum MR observed in the study in a magnetic field of 10kOe at room temperature is 4.12% and is considered reasonable. A greater MR value in Co-Teflon granular films is highly possible if the maximum applied field is higher and the Co particles are smaller and are of superparamagnetic scale.

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