Ultrafine particles of Ni${\mathrm{Fe}}_2$${\mathrm{O}}_4$ coated with organic molecules show a behavior consistent with extremely strong pinning of the ferrite surface spins. The low-temperature magnetization is only ∼75% saturated in fields of 200 kOe, and Mössbauer spectra show that the spins remain at large angles with respect to the direction of an applied field of 68.5 kOe. Uncoated ultrafine Ni${\mathrm{Fe}}_2$${\mathrm{O}}_4$ particles do not show this behavior.

• Received 19 December 1974

DOI:https://doi.org/10.1103/PhysRevLett.34.594