The Boundary Between Volume and Surface-Driven Magnetic Properties in Spinel Iron Oxide Nanoparticles

Abstract

Despite modern preparation techniques offer the opportunity to tailor the composition, size, and shape of magnetic nanoparticles, understanding and hence controlling the magnetic properties of such entities remains a challenging task, due to the complex interplay between the volume-related properties and the phenomena occurring at the particle's surface. The present work investigates spinel iron oxide nanoparticles as a model system to quantitatively analyze the crossover between the bulk and the surface-dominated magnetic regimes. The magnetic properties of ensembles of nanoparticles with an average size in the range of 5-13 nm are compared. The role of surface anisotropy and the effect of oleic acid, one of the most common and versatile organic coatings, are discussed. The structural and morphological properties are investigated by X-ray diffraction and transmission electron microscopy. The size dependence of the surface contribution to the effective particle anisotropy and the magnetic structure are analyzed by magnetization measurements and in-field Mössbauer spectrometry. The structural data combined with magnetometry and Mössbauer spectrometry analysis are used to shed light on this complex scenario revealing a crossover between volume and surface-driven properties in the range of 5-7 nm.

Keywords: Exchange bias; Ferrites; Nanomagnetism; Nanoparticles; Surface properties.

Fig. 1

TEM images of MAG1 (a), MAG2 (b), MAG3 (c), and MAG4 (d), with inset high-resolution images of single particles representative of the morphology and crystallinity of the samples. The corresponding particles’ size distributions are reported in panels e, f, g, and h for MAG1, MAG2, MAG3, and MAG4, respectively. The frequency counts of the measured size are represented as blue spheres, while the continuous red line describes the log-normal fit to the data

Fig. 2

ZFC (empty circles) and FC (full circles) curves for samples MAG1 (a), MAG2 (b), MAG3 (c), and MAG4 (d), measured with an applied field of 2.5 mT

Fig. 3

a A magnification at low field of the M versus H curves measured at 5 K for all samples with b the full range of measurement. The size dependence of coercive field (µ₀H_C), anisotropy field (µ₀H_a), and the high-field susceptibility (dM/dµ₀H) measured at 5 T are reported in panels c, d, and e, respectively, with lines used as a guide to the eye

Fig. 4

a Low-field details of the ΔM-plots at 5 K. The full range of the measurements is reported in the inset. b The dipolar interaction energy and c the ΔM-plots peaks’ intensity is plotted as a function of the average particle’s volume, with lines used as a guide to the eye

Fig. 5

Mössbauer spectra measured at 10 K under an applied field of 8 T for samples MAG1 (a), MAG2 (b), MAG3 (c), and MAG4 (d). A sum of two sextets, one for the tetrahedral (red line) and one for the octahedral (blue line) component of the spectra, have been fitted to the experimental data (circles). A black line describes the total fit to the experimental data (grey circles)

Fig. 6

Low-field magnification of M(H) loops recorded at 5 K after cooling the sample MAG1 from 300 K in zero field (black line and circles) and in 1 T (red line and triangles). In the inset, the extended field range of measurement