1. Department of Physics, University of California, San Diego, La Jolla, California 92093, United States
2. Center for Memory and Recording Research, University of California, San Diego, La Jolla, California 92093-0401, United States
3. Department of Physics, Colorado State University, Fort Collins, Colorado 80523, United States
4. School of Advanced Materials Discovery, Colorado State University, Fort Collins, Colorado 80523, United States