Open problems in the magnetic behavior of iron-oxide nanoparticles

Abstract

Nanotechnology has gained great relevance in the last decades, due to the great variety of applications in medicine, chemistry, physics and biology, among others. Iron oxide nanoparticles are particularly attractive in medicine, in the development of novel techniques for early diagnosis, noninvasive therapy and biochemical and physiological studies. In scientific research, the possibility of controlling their size and the particle-particle separation, allows these materials to be used as model systems for the study of magnetic properties. There are many examples, such as the physical phenomena arising from their finite size, the influence of dipolar interaction, the quantum tunneling, the giant magnetoresistance, ... just to mention some of them. The work developed in this thesis deals with some of these phenomena. The first chapter comprises a brief description of iron oxides and the magnetic properties of nanoparticles. In chapter 2, we provide a short introduction to the experimental techniques related with the work developed in this thesis. Then, we present two chapters devoted to antiferromagnetic nanoparticles. In chapter 3, we analyze the influence of chlorine content in the magnetic properties of akaganeite nanoparticles. Previous works report that intrinsic properties, such as the Neel temperature and the effective spin of this antiferromagnetic material, are greatly influenced by the amount of interstitial ions. Based on this idea, we analyze how the magnetic relaxation of the nanoparticles is affected by the amount of chlorine contained in the crystal structure. In chapter 4, we show that akaganeite nanoparticles posses a thermoinduced magnetic moment. Antiferromagnetic nanoparticles have a finite magnetic moment arising from the decompensation of atomic spins. In addition, they may exhibit a thermoinduced magnetic moment, due to their finite size, which has the unusual property of increasing with temperature. One of the main complications in the study of this phenomenon is that the magnetic properties of the bulk material are often unknown. To overcome this problem we have chosen akaganeite nanoparticles as a model system because akaganeite can be produced in bulk and therefore, its bulk magnetic properties can be determined in a rather straightforward manner. The following chapters are devoted to studies on ferrimagnetic maghemite nanoparticles. In chapter 5 we show that the saturation magnetization in this system decreases with the nanoparticle size. This decrease can be expressed in terms of bulk saturation magnetization, particle size and thickness of a magnetically disordered layer. The proposed equation is based on the so called core-shell model, which assumes that nanoparticles consist of a bulk-like ferrimagnetic core and a shell of disordered spins. The experimental determination of shell thickness is, in fact, not so straightforward, because there is a noticeable spreading in saturation magnetization values of samples prepared by different synthetic procedures. Therefore we have studied a representative number of nanocomposites, with an average particle size in the range from 1.5 to 15nm. We estimate a layer thickness of about 1 nm.