Universal Behavior in Frequency- and Temperature- Dependent Hysteresis Loops of Compacted Cobalt-rich Magnetic Nanoparticles*

We use the polyol process to synthesize cobalt-rich spherical nanoparticles with average particle size 200 nm and average grain size 20 nm that were consolidated via plasma pressure compaction to greater than 90\% theoretical full density. Measurements of the minor hysteresis loops of the compacted magnetic nanoparticles over a wide range of frequencies (0.1 to 100 kHz) and temperatures (77$^{\circ}$ to 873$^{\circ}$ K) indicate universal behavior in the height, width, and area of the loops. Simulated hysteresis loops determined through numerical solutions of the N\'{e}el-Brown thermal after-effect show similar frequency and temperature dependence.