The discovery of large, room temperature magnetoresistance (MR) in magnetic tunneljunctions in 1995 sparked great interest in these devices. Their potential applicationsinclude hard disk read head sensors and magnetic random access memory (MRAM).However, the fabrication of repeatable, high quality magnetic tunnel junctions is stillproblematic. This thesis investigates methods to improve and quantify the quality oftunnel junction fabrication.Superconductor-insulator-superconductor (SIS) and superconductor-insulatorferromagnet(SIF) tunnel junctions were used to develop the fabrication route, due to theease of identifying their faults. The effect on SIF device quality of interchanging the topand bottom electrodes was monitored. The relationship between the superconducting andnormal state characteristics of SIS junctions was investigated. Criteria were formulatedto identify devices in which tunneling is not the principal conduction mechanism innormal metal-insulator-normal metal junctions.Magnetic tunnel junctions (MTJs) were produced on the basis of the fabrication routedeveloped with SIS and SIF devices. MTJs in which tunneling is the principalconduction mechanism do not necessarily demonstrate high MR, due to effects such asmagnetic coupling between the electrodes and spin scattering. Transmission electronmicroscope images were used to study magnetic tunnel junction structure, revealing anamorphous barrier and crystalline electrodes.The decoration of pinholes and weak-links by copper electrodeposition was investigated.A new technique is presented to identify the number of copper deposits present in a thininsulating film. The effect of roughness, aluminium thickness and voltage on the numberof pinholes and weak-links per unit area was studied.High frequency testing of read heads at wafer level was performed with a networkanalyser. Design implications for read head geometry were investigated, independent ofmagnetic performance. This technique has great potential to aid the rapid development ofread and write heads whilst improving understanding of the system.
Materials Science and Metallurgy, University of Cambridge 2002
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