γ-Fe${}_2$O${}_3$/SiO${}_2$ core-shell nanoparticles with different shell thicknesses were prepared to elucidate the condition for superspin-glass (SSG) dynamics. As the shell thickness decreases, the contribution of interparticle dipolar interaction becomes apparent in the magnetic dynamics of nanoparticle assembly. The frequency dependence of peaks in ac-magnetic susceptibility in samples with strong interactions slows down, which is characterized as the emergence of a spin-glasslike phase. Aging in magnetization relaxation is found in a strongly interacting sample with an interparticle distance of $L⩽14$ nm but is scarce in a sample with $L$ $=$ 18 nm. Scaling analysis reveals an increase in superparamagnetic properties with an increase in $L$. Therefore the critical interparticle distance necessary for SSG transition is 15–18 nm with 11-nm $\gamma$-Fe${}_2$O${}_3$ nanoparticles. This corresponds to the ratio of interparticle-interaction energy to the magnetic-anisotropy energy $E_{\mathrm{dip}}$/$E_{\mathrm{a}}$ of 6–12%.

• Received 24 February 2011

DOI:https://doi.org/10.1103/PhysRevB.83.224423

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Published by the American Physical Society