Magnetic properties of free cobalt clusters

We have studied the magnetic properties of cobalt clusters in a beam as functions of cluster size (N=20–200), magnetic field, and cluster temperature. The measured magnetic moments per atom increase with applied magnetic field and cluster size but decrease with increasing temperature, consistent with superparamagnetic behavior. Rapid orientation fluctuations of the true magnetic moments of these isolated particles yield the smaller, time-averaged, observed values. The true magnetic moments are (2.08±0.20)${\mathrm{\mu }}_{\mathit{B}}$ per atom and exceed the bulk value. Magnetically resolved isomers are seen for N=55–66.