Abstract
\ensuremath{\gamma}-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 $L\ensuremath{\leqslant}14$ 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 $\ensuremath{\gamma}$-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%.
Highlights
Single-domain magnetic nanoparticles called superspins have attracted a great deal of attention for the last few decades as promising materials for ultrahigh density-magnetic recording, e.g., patterned media.1 In such an application single-domain magnetic islands, i.e., magnetic dipoles or superspins, are used as recording bits, and individually responding magnetic bits are required
Where the flipping time τ of the magnetic moment of a nanoparticle is governed by its anisotropy energy Ea and a given temperature T, where τ 0 is on the order of 10−9–10−12 s and kB is the Boltzmann constant
We investigated the magnetic behavior of γ -Fe2O3/SiO2 NCs with 11-nm diameter that were coated with SiO2 to control the interparticle distance based on the thickness
Summary
Single-domain magnetic nanoparticles called superspins have attracted a great deal of attention for the last few decades as promising materials for ultrahigh density-magnetic recording, e.g., patterned media. In such an application single-domain magnetic islands, i.e., magnetic dipoles or superspins, are used as recording bits, and individually responding magnetic bits are required. Single-domain magnetic nanoparticles called superspins have attracted a great deal of attention for the last few decades as promising materials for ultrahigh density-magnetic recording, e.g., patterned media.. It is difficult to satisfy this requirement in the dense magnetic-nanoparticle system because the dipolar interaction becomes very strong as the particle concentration increases. With decreasing temperature, the magnetic moment of a nanoparticle is fixed along its easy axis as the anisotropy-energy barrier cannot be thermally exceeded. Because of this blocking phenomenon the superparamagnetic magnetization exhibits a peak at a certain temperature Tpeak, below which it decreases as the temperature decreases.
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