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 Å, 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.