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The stability of the stored information in recording media depends on theanisotropy energy Ea = KeffV of nanoparticles (NPs) of volume V or diameter D.Therefore, the knowledge of how the effective magnetic anisotropy Keff varieswith D for a given system is important for technological applications. In arecent paper [Appl. Phys. Lett. 110 (22), 222409 (2017)], the variation of Keffversus D in NPs of maghemite ({\gamma}-Fe2O3) was best described by the Eq.:Keff = Kb + (6KS/D) +Ksh{[1-(2d/D)]-3 -1}, where Kb, KS, and Ksh are theanisotropy constants of spins in the core, surface layer, and a shell ofthickness d, respectively. This core-shell-surface layer (CSSL) model is anextension of the often used core-surface layer (CSL) model described by Keff =Kb + (6KS/D) [Phys. Rev. Lett. 72, 282 (1994)]. The additional term involvingKsh was found to be necessary to fit the data for smaller NPs of {\gamma}-Fe2O3with D < 5 nm. Here we check the validity of the CSSL model for metallicmagnetic NPs of Co, Ni, Fe and magnetite using Keff vs. D data from publishedliterature. Care was taken in selecting data only for those NPs for which theeffects of interparticle interactions has been taken into account indetermining Keff. The importance of the new CSSL model is that it describeswell the Keff vs. D variation for all particles sizes whereas the core-surfacelayer model often fails for smaller particles with the notable exception of FeNPs. The verification of the CSSL model for metallic NPs of Co, Ni, andmagnetite along with NPs of NiO and {\gamma}-Fe2O3 validates its generalapplicability.

Size dependence of the effective magnetic anisotropy in Co, Ni, Fe, and magnetite nanoparticles: Testing the core-shell-surface-layer (CSSL) model