Temperature Dependence of the Magnetic Interactions Taking Place in Monodisperse Magnetite Nanoparticles Having Different Morphologies

Abstract

In a recent paper [1], the authors have evidenced how the thermally activated demagnetization processes taking place in magnetite nanoparticles (NPs) play a major role on deteriorating the dc coercivity values measured in these samples at temperatures above ca. 150 K. The mechanisms for that deterioration are the increase with the temperature of both the fluctuation field and the activation volume, the latter reaching at room temperature values of up more than 40 times the average particle volume value. We aim here at the quantitative evaluation of the inter-particles interactions underlying that phenomenology and, particularly, at the identification of the role played by the nanoparticles morphology. For that purpose, we have measured magnetite NPs prepared via oxidative precipitation by controlling their size through the nature of the base and nitrate salt and the ethanol content in the media, and their morphology (either octahedral or spherical) by changing the final base concentration [2]. Particle size, shape and crystal structure were determined by transmission electron microscopy (TEM), high-resolution electron microscopy (HRTEM), and X-rays diffraction (XRD), respectively. The magnetic properties of were measured by means of a vibrating sample magnetometer, working in the temperature range from 5 K up to 290 K by applying a maximum field of 9 T. Figure 1 displays the temperature dependence of the reduced saturation remanence (Mr/MS) clearly showing that in both morphologies, but to a larger extent in the spherical NPs, the measured isotropic powder exhibits reduced remanences below 0.5, which suggests the occurrence of demagnetizing interactions. The actual occurrence of that type of interactions was confirmed from the measurement (at 5 K, 125 K and 200 K) of the field dependencies of the isothermal and demagnetization remanences of the samples and the evaluation of the associated Henkel plots. The differences identified in those plots between the spherical and octahedral NPs are discussed in terms of the particle aggregation into close-flux structures.