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Submitted on: 29 June 2004.
Revised on: 04 October 2004.
Accepted on: 25 November 2004.
___________________JOURNAL OF PHYSICS D-APPLIED PHYSICS
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Submitted on: 17 October 2006.
Revised on: 19 November 2006.
Accepted on: 01 January 2007.
___________________JOURNAL OF PHYSICS D-APPLIED PHYSICS