Nuclear magnetic resonance study of the magnetic behavior of ultrafine Co clusters in zeolite NaY

59Co spin‐echo nuclear magnetic resonance (NMR) experiments have been carried out at 1.3 and 4.2 K, for external magnetic fields up to 9 kOe, in order to study 6–10 A, single‐domain Co clusters isolated inside zeolite NaY supercages. The magnetic behavior is characterized by a superparamagnetic blocking temperature, relaxation time behavior for increasing and decreasing fields, and a dependence on the magnetic history. The observed NMR spectra are broad with structural features characteristic of both fcc‐ and hcp‐like short‐range order. Unlike bulk materials, the application of fields up to 9 kOe causes a dramatic increase in the signal intensity. The results are discussed in terms of the particle‐size distribution and crystalline anisotropy, and their relationship to the temperature and field dependence of the superparamagnetic/ferromagnetic behavior.