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Static magnetization of immobilized, weakly interacting, superparamagnetic nanoparticles

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Abstract

The magnetization curve and initial susceptibility of immobilized superparamagnetic nanoparticles are studied using statistical–mechanical theory and Monte Carlo computer simulations. The nanoparticles are considered to be distributed randomly within an implicit solid matrix, but with the easy axes distributed according to particular textures: these are aligned parallel or perpendicular to an external magnetic field, or randomly distributed. The magnetic properties are calculated as functions of the magnetic crystallographic anisotropy barrier (measured with respect to the thermal energy by a parameter σ), and the Langevin susceptibility (related to the dipolar coupling constant and the volume fraction). It is shown that the initial susceptibility χ is independent of σ in the random case, an increasing function of σ in the parallel case, and a decreasing function of σ in the perpendicular case. Including particle–particle interactions enhances χ, and especially so in the parallel case. A first-order modified mean-field (MMF1) theory is accurate as compared to the simulation results, except in the parallel case with a large value of σ. These observations can be explained in terms of the range and strength of the (effective) interactions and correlations between particles, and the effects of the orientational degrees of freedom. The full magnetization curves show that a parallel texture enhances the magnetization, while a perpendicular texture suppresses it, with the effects growing with increasing σ. In the random case, while the initial response is independent of σ, the high-field magnetization decreases with increasing σ. These trends can be explained by the energy required to rotate the magnetic moments with respect to the easy axes.

Graphical abstract: Static magnetization of immobilized, weakly interacting, superparamagnetic nanoparticles

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Publication details

The article was received on 28 Aug 2019, accepted on 30 Oct 2019 and first published on 31 Oct 2019


Article type: Paper
DOI: 10.1039/C9NR07425B
Nanoscale, 2019, Advance Article
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    Static magnetization of immobilized, weakly interacting, superparamagnetic nanoparticles

    E. A. Elfimova, A. O. Ivanov and P. J. Camp, Nanoscale, 2019, Advance Article , DOI: 10.1039/C9NR07425B

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