The effect of surface spin disorder on the magnetism of γ-Fe₂O₃nanoparticle dispersions

Abstract

The nanomagnetism of monodisperse 7 nm γ-Fe2O3 nanoparticles exhibits unique features due to a significant amount of surface spin disorder. To correctly characterize the superparamagnetism of a dilute dispersion requires including the effects of the magnetic anisotropy and a shell of disordered spins surrounding the ordered core. The nanoparticle shell’s disordered spin structure is exchange coupled to that of the ordered core. This enables an exchange bias loop shift, Hex, when the nanoparticle dispersion is field cooled. The surface spin disorder also leads to an unusual exponential-like decrease of the nanoparticle’s total saturation magnetization with increasing temperature.

In-field Mossbauer study of disordered surface spins in core/shell ferrite nanoparticles

“…A smooth variation with temperature is observed above 70 K, while a steep upturn occurs below that temperature. Such behavior has been observed for other nanoparticle systems, and it can be understood as a superposition of two independent mechanisms according to the following equation: 22,32 …”

“…The characteristic temperature T f is nearly size independent and its mean value is ͗T f ͘ =18Ϯ 5 K, similar to values found for ferrofluids based on Cu and Mn ferrite nanoparticles and for ␥-Fe 2 O 3 nanoparticle dispersions. 22,32 From the theoretical side, Monte Carlo simulations have yielded an exponential-like decrease in the surface contribution and Bloch-like variations of the core one with increasing temperature. 35 The nanoparticle magnetization behavior described by Eqs.…”

Magnetic nanoparticle temperature estimation

“…31,32 The magnetization follows a Bloch's law T 3/2 relationship impacted by several factors including the coating of the nanoparticles, the surface structure, and the chemical composition. 33 The general relationship is of the form…”

Interface charge transfer in polypyrrole coated perovskite manganite magnetic nanoparticles

“…It should be noticed that the same type of increase of M S has been reported for the oleic acid coating and PPy coating of magnetite nanoparticles, respectively. [5][6][7][8][9][10][11][12] It seems that the attachment of organic molecules to the surface of magnetic nanoparticles could induce a reduction of the surface spin disorder resulting in an increase of the saturation magnetization values.…”

“…4,5 By adjusting the coating layers, one can induce changes in the nanoparticles' surface spin disorder and interparticle interactions; this results in different values for the characteristic parameters like saturation magnetization, coercitivity, and magnetic anisotropy of the nanoparticles obtained by different procedures. [6][7][8][9][10][11][12] In this work, we report the synthesis and characterization of novel core-shell hybrid organic-inorganic nanostructures, consisting in the deposition of a conjugated polymer layer on the magnetic La 0.67 Sr 0.33 MnO 3 (LSMO) nanoparticle surfaces. We focus especially on the polymer nanoparticle interface-properties' relationship and optimization of the nanocomposites physicochemical characteristics to make them suitable for biotechnology.…”