Temperature dependence of the magnetic interactions taking place in monodisperse magnetite nanoparticles having different morphologies

Enrique Navarro,Puerto Morales,Federico Cebollada,Jesús M González,F Javier Palomares,Unai Urdiroz,Yurena Luengo,Sabino Veintemillas

doi:10.1063/9.0000089

Abstract

We report on the temperature dependence of the interactions present in single crystal magnetite nanoparticles having octahedral and spherical morphologies. From our results we conclude that the inter-particle interactions are, at all temperatures and in both octahedral and spherical nanoparticles, demagnetizing in nature. These interactions are not describable in terms of a mean field but local and linked to the poles present at the surfaces of the particles and particles clusters. In both samples, the peak on the field dependence of the interactions has an associated maximum that decreases in magnitude with an increase of the measuring temperature. Also, that peak gets narrower when the temperature is increased. The high order multipolar moments of the octahedral nanoparticles, originated by the fact that their morphology includes the presence of edges an dihedra, is detectable in the larger field range in which the interactions are observable in these samples in comparison with that corresponding to the spherical nanoparticles, exhibiting close-to-dipolar moments.

Highlights

The coercivity, at temperatures slightly above the human body euthermia and ac field frequencies ranging from the tens of kHz up to tens of MHz, constitutes, jointly with the saturation magnetization value, the two most important magnetic parameters determining the use of biocompatible nanoparticles (NPs) in the hyperthermia treatment of carcinomata.1,2 Whereas the saturation magnetization is mostly dependent on the NPs chemical composition,3–5 their coercive force is an extrinsic quantity6 varying with effective anisotropy that can include contributions depending on size and morphology, nature and concentration of dopants, built-in stresses, thermally activated demagnetization and nanoparticles agglomeration and interactions.7 This complex origin of the coercivity of the NPs used in hyperthermia defines the playground for the optimization of these materials from the magnetic standpoint.In Ref. 8, the authors have evidenced how the thermally activated demagnetization processes taking place in magnetite NPs play a major role in deteriorating the dc coercivity values measured at temperatures above 150 K
We report on the temperature dependence of the interactions present in single crystal magnetite nanoparticles having octahedral and spherical morphologies
The high order multipolar moments of the octahedral nanoparticles, originated by the fact that their morphology includes the presence of edges an dihedra, is detectable in the larger field range in which the interactions are observable in these samples in comparison with that corresponding to the spherical nanoparticles, exhibiting close-to-dipolar moments

Summary

Introduction

The coercivity, at temperatures slightly above the human body euthermia and ac field frequencies ranging from the tens of kHz up to tens of MHz, constitutes, jointly with the saturation magnetization value, the two most important magnetic parameters determining the use of biocompatible nanoparticles (NPs) in the hyperthermia treatment of carcinomata.1,2 Whereas the saturation magnetization is mostly dependent on the NPs chemical composition,3–5 their coercive force is an extrinsic quantity6 varying with effective anisotropy that can include contributions depending on size and morphology, nature and concentration of dopants, built-in stresses, thermally activated demagnetization and nanoparticles agglomeration and interactions.7 This complex origin of the coercivity of the NPs used in hyperthermia defines the playground for the optimization of these materials from the magnetic standpoint.In Ref. 8, the authors have evidenced how the thermally activated demagnetization processes taking place in magnetite NPs play a major role in deteriorating the dc coercivity values measured at temperatures above 150 K.

Results

Conclusion