1. Materials Science Division, Lawrence Livermore National Laboratory, Livermore, California 94551, USA
2. Department of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, Oregon 97331, USA
3. IBM/Macronix PCRAM Joint Project, Macronix Emerging Central Laboratory, Macronix International Co., Hsinchu, Taiwan
4. Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, D-14109 Berlin, Germany