We use our measurements of the magnetization and the magnetoresistance for very small superparamagnetic particles of Co to obtain the low-temperature value of the magnetic anisotropy energy density, C≊3×${10}^8$ erg/${\mathrm{cm}}^3$. This is nearly two orders of magnitude larger than the corresponding value for C for bulk Co. The enormous enhancement of C for very small particles of Co is consistent with results previously reported for very small particles of Fe and of FeNi. © 1996 The American Physical Society.

• Received 2 August 1995

DOI:https://doi.org/10.1103/PhysRevB.53.32