Enhanced magnetic anisotropy energy density for superparamagnetic particles of cobalt

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.