  Experimental results and a model are presented to explain the observed
unusual enhancement of the effective magnetic anisotropy Keff with decreasing
particle size D from 15 nm to 2.5 nm in {\gamma}-Fe2O3 nanoparticles (NPs). The
samples include oleic acid-coated NPs with D = 2.5, 3.4, 6.3 and 7.0 nm
investigated here, with the results on other sizes taken from literature. Keff
is determined from the analysis of the frequency dependence of the blocking
temperature TB after considering the effects of interparticle interactions on
TB. The data of Keff vs. D is fit to the derived core-shell-based relation:
Keff = Kb + (6KS/D) +Ksh{[1-(2d/D)]^(-3) -1}, with Kb = 1.9 x10^5 ergs/cm^3 as
the bulk-like contribution of the core, KS = 0.035 ergs/cm^2 as the
contribution of the surface layer, and Ksh = 1.057 x10^4 ergs/cm^3 as the
contributions of spins in the shell of thickness d = 1.1 nm. This equation
represents an extension of the often-used Eq.: Keff = Kb + (6KS/D).
Significance of this new result is discussed.
