The effect of the magnetically dead layer on the magnetization and the magnetic anisotropy of the dextran-coated magnetite nanoparticles

We present a study on the magnetic behavior of dextran-coated magnetite nanoparticles (DM NPs) with sizes between 3 and 19 nm, synthesized by hydrothermal-assisted co-precipitation method. The decrease saturation magnetization ($M_s$) decreasing particle size has been modeled assuming existence spin-disordered layer at surface, which is magnetically dead. Based this core-shell model taking into account weight contribution non-magnetic coating (dextran) to whole magnetization, dead thickness ($t$) $M_s$ cores in our samples were estimated be $t = 6.8~\mathrm{\AA}$ $M_s 98.8~\mathrm{emu/g}$, respectively. data analyzed using law approach saturation, indicating an increase effective anisotropy ($K_{eff}$) as expected from increased surface/volume ratio small MNPs. obtained $K_{eff}$ values successfully including extra dipolar interactions due formation chain-like clusters surface ($K_s$) was about $K_s 1.04\times10^5~\mathrm{J/m^3}$. Our method provides simple accurate way obtain core surface-disordered MNPs, relevant parameter required for modeling many applications.