Tuning dipolar magnetic interactions by controlling individual silica coating of iron oxide nanoparticles

doi:10.1016/j.jmmm.2017.11.099

Journal of Magnetism and Magnetic Materials

Volume 451, 1 April 2018, Pages 688-696

https://doi.org/10.1016/j.jmmm.2017.11.099 Get rights and content

Highlights

•
Iron oxides nanoparticles prepared via thermal decomposition of organic precursors.
•
A silica shell of tunable thickness was added via inverse microemulsion method.
•
Core-shell structures are used to study controlled magnetic dipolar interactions.
•
Magnetic features are analyzed as a function of interparticle geometrical distance.
•
An interacting to non-interacting transition is identified.

Abstract

Single and fixed size core, core–shell nanoparticles of iron oxides coated with a silica layer of tunable thickness were prepared by chemical routes, aiming to generate a frame of study of magnetic nanoparticles with controlled dipolar interactions. The batch of iron oxides nanoparticles of 4.5 nm radii, were employed as cores for all the coated samples. The latter was obtained via thermal decomposition of organic precursors, resulting on nanoparticles covered with an organic layer that was subsequently used to promote the ligand exchange in the inverse microemulsion process, employed to coat each nanoparticle with silica. The amount of precursor and times of reaction was varied to obtain different silica shell thicknesses, ranging from 0.5 nm to 19 nm. The formation of the desired structures was corroborated by TEM and SAXS measurements, the core single-phase spinel structure was confirmed by XRD, and superparamagnetic features with gradual change related to dipolar interaction effects were obtained by the study of the applied field and temperature dependence of the magnetization. To illustrate that dipolar interactions are consistently controlled, the main magnetic properties are presented and analyzed as a function of center to center minimum distance between the magnetic cores.

Introduction

Magnetic nanoparticles have been a very active field of study for several decades owing to their multiple applications in several technologies. Since the initial formulations from the fundamental point of view [1], the physics beneath magnetism at the nanoscale still remains an incomplete puzzle [2].

Studies of superparamagnetic nanoparticles generally report the magnetic behavior either as a “weakly interacting system” or a “strongly interacting system”, yet many attempts are being made to obtain in the laboratory an ideal enough system to be used as a tool to quantify and fully understand the role of dipolar interactions between nanoparticles and to test some of the existing models that describe them [2].

Being able to differentiate the effects of interactions from the effects of size, shape and anisotropy easy axis distributions in real samples is not an easy task, and represents a big challenge from the very moment of the synthesis route selection. In this regard, chemical synthesis routes have become a good way to obtain uniform and nearly monodisperse samples with good crystallinity, with phases and size control. Among them, thermal decomposition of organic precursors has become a popular route, since it yields some of the best reported samples, providing an easy control of the parameters, and the advantage of an organic coating on the nanoparticles surface that facilitates further functionalization.

Beyond good quality magnetic nanoparticles, a framework to unravel the interplay between their intrinsic properties and the interactions between them can be provided by a non-magnetic isolating media like polymers or proteins [3], [4], [5], [6], [7], [8], [9], with limitations on concentration and homogeneity control. In order to overcome this constraint, the coating of each nanoparticle has been a good approach, and amorphous silica provides a chemically and mechanically resistant shell with several advantages for applications related to the widely studied chemistry of the SiO₂ and its intrinsic properties like biocompatibility [10], [11], [12], [13], [14], [15], [16], [17], [18].

Several advantages for morphologically controlled homogenous core-shell systems have been addressed from the applications point of view [11], [12], [19], but from the magnetism fundamental point of view, only a few establish a direct relationship between the geometrically generated interparticle distance and the magnetic features [7], [14], potentially useful to clarify the interspacing that defines the nature and magnitude of interactions in a given system. To this aim a profound structural characterization is indispensable and techniques like SAXS arises as a statistically strong characterization method. No reports of systematic SAXS studies were found by the authors for this synthetic route, as the one presented here.

In this work iron oxides nanoparticles (IONPs) were synthesized via thermal decomposition and systematically coated with silica (SiO₂) via the inverse microemulsion method, increasing the thickness of the shell with the amounts of silica precursor. A detailed structural and morphological characterization is presented here and related to the main magnetic features.

Section snippets

Experimental

As schematized in Fig. 1, IONPs were prepared by thermal decomposition of an iron organometallic precursor [20], and then coated with SiO₂ via a reverse microemulsion method, following a procedure to obtain nanoparticles with a single iron oxide core and different silica shell thicknesses [21].

Chemicals: Iron(III) chloride hexahydrate (FeCl₃·6 H₂O, 97%), Sodium hydroxide (NaOH) and ammonium hydroxide (28%) were purchased from Anedra. 1-octadecene, oleic acid, tetraethyl orthosilicate (TEOS) and

Results and discussion

XRD was employed to identify the crystalline phases and to perform a first estimation of the crystallite size of P1 nanoparticles. The XRD pattern shown in Fig. 2 exhibits a major broad peak near 20°, commonly ignored and attributed to oleic acid and other organic compounds residual from the synthesis [23]. The peak positions and its relative intensities displayed for the 2θ range between 28° and 80° match well with the pure spinel structure of space group Fd3m of the iron oxide, expected to be

Magnetic measurements

In order to correlate the changes in the magnetic properties with the cores interspacing exclusively, all the magnetic measurements were carried on the silica coated IONPs samples after drying the ethanol, to obtain a nanoparticle arrangement like the one shown in Fig. 5. Here, the magnetic response of the samples is presented and discussed as a function of the magnetic cores interspacing, tuned by the silica shell thickness deduced by TEM and SAXS. The interspacing length among magnetic cores,

Conclusions

A systematical synthesis of samples with fixed magnetic cores and silica shells of different thicknesses was carried out, employing typical chemical synthesis routes with gradual and controlled variation of synthesis parameters. SAXS studies provide a statistically rich survey of the structural conformation of the samples, confirming the single core, core-shell structure observed by TEM. Dried samples constitute systems of randomly distributed magnetic particles separated by the shells

Acknowledgments

This work benefited from SasView software, originally developed by the DANSE project under NSF award DMR-0520547. Authors would like to acknowledge CONICET (Argentina) for financial support. We thank the TEM facilities of the Brazilian Nanotechnology National Laboratory (LNNano) at Centro Nacional de Pesquisa em Energia e Materiais (CNPEM). SAXS spectra were recorded at the SAXS1 beam line of the Brazilian Synchrotron Light Laboratory (LNLS) at CNPEM under proposals 20150093 and 17727. Dr. E.

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Research articlesTuning dipolar magnetic interactions by controlling individual silica coating of iron oxide nanoparticles

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Magnetic measurements

Conclusions

Acknowledgments

J. Magn. Magn. Mater.

Eur. Polym. J.

J. Phys. Chem. Solids

Mater. Sci. Eng., A

Mater. Chem. Phys.

Compos. Sci. Technol.

J. Magn. Magn. Mater.

Superparamagnetism and other magnetic features in granular materials: a review on ideal and real systems

J. Nanosci. Nanotechnol.

Quasi-static magnetic measurements to predict specific absorption rates in magnetic fluid hyperthermia experiments

J. Appl. Phys.

Magnetic properties study of iron-oxide nanoparticles/PVA ferrogels with potential biomedical applications

J. Nanoparticle Res.

Evidence for magnetic interactions among magnetite nanoparticles dispersed in photoreticulated PEGDA-600 matrix

J. Nanoparticle Res.

Spacing-dependent dipolar interactions in dendronized magnetic iron oxide nanoparticle 2D arrays and powders

Nanoscale

Effect of dipolar interaction observed in iron-based nanoparticles

Phys. Rev. B – Condens. Matter Mater. Phys.

Effect of interparticle interactions and size dispersion in magnetic nanoparticle assemblies: A static and dynamic study

Appl. Phys. Lett.

Controlled close-packing of ferrimagnetic nanoparticles: an assessment of the role of interparticle superexchange versus dipolar interactions

J. Phys. Chem. C

Nanoparticle architectures templated by SiO2/Fe2O3 nanocomposites

Chem. Mater.

Achieving a noninteracting magnetic nanoparticle system through direct control of interparticle spacing

Appl. Phys. Lett.

Superspin glass originating from dipolar interaction with controlled interparticle distance among γ-Fe2O3 nanoparticles with silica shells

Phys. Rev. B

Determination of the critical interspacing for the noninteracting magnetic nanoparticle system

Appl. Phys. Lett.

Quantification of dipolar interactions in Fe3–xO4 nanoparticles

J. Phys. Chem. C

SiO2 coating effects in the magnetic anisotropy of Fe3-xO4 nanoparticles suitable for bio-applications

Nanotechnology

Research articles
Tuning dipolar magnetic interactions by controlling individual silica coating of iron oxide nanoparticles

Nanoparticle architectures templated by SiO₂/Fe₂O₃ nanocomposites

Quantification of dipolar interactions in Fe₃–xO₄ nanoparticles

SiO₂ coating effects in the magnetic anisotropy of Fe3-xO4 nanoparticles suitable for bio-applications