1. Department of Materials Science, University of North Texas, Denton, TX 76230-5310, USA, Fax: ,
2. Department of Applied Physics and Advanced Technology, National Autonomous University of Mexico, Apartado postal 1–1010 Querétaro, Qro. 67000, Mexico
3. Institute of Materials Research, National Autonomous University of Mexico, Ciudad Universitaria, Mexico, D.F. 04510, Mexico
4. Institute of Electrical Research, Apartado postal 475, Curenavaca, Morelos 62000, Mexico
5. Center for Instrumentation, National Autonomous Universtiy of Mexico, Ciudad Universitaria, Mexico, D.F. 04510, Mexico