Structure versus Magnetism in Magnetic Nanoparticles

Abstract

[eng] From the fundamental point of view, NPs formed by MFe2O4 with (M= Co, Fe) are ideal system models to study the new magnetic phenomena associated with the so-called particle-like behaviour, which emerges from the size reduction towards the nanometre scale and contrasts with the well-established magnetic properties of their bulk-counterparts. It is well known that most of the particle-like behaviour and in general the large variability of the magnetic properties observed in this kind of nanomaterials are related to structural features of the NPs rather than being originated from intrinsic finite-size or surface effects, at least for NPs bigger than a few nanometers. These structural features, such as crystallographic defects, polycrystalline nature of the NPs, lack of crystallinity at the particle surface, etc., have strong influence on their magnetic properties and can be modified at will through the synthesis method. Therefore, whenever this particle-like behaviour is unwanted for applications with highly demanding requirements, the choice of a suitable synthesis method is of key importance to obtain NPs of high-crystalline quality. On the contrary, particle-like behaviour controlled by the crystalline nature of the NPs could be useful to tailor their magnetic properties for specific applications. Among the common synthesis methods, high-temperature decomposition of metal-organic precursors results the best alternative due to the remarkable final properties of the obtained NPs, such as narrow size distribution, high crystallinity and relatively simple tuning of their size and shape. So this will be the chemical route chosen in this work to study the capabilities of this synthesis method to control the final properties of the NPs through their nanostructure. In addition, to get a deeper insight in the magnetic and structural properties of those materials and to shed light on relevant issues that are still under discussion (dynamic response, magnetic frustration or inter¬particle interactions) it could be useful to combine experimental techniques enabling the characterization of the system from macroscopic scales towards single-particle structures. Within this framework, we present this work that is divided into three main parts. First, it is studied the effect of the concentration of two common reactants, involved in the thermal decomposition method, on the final properties of magnetic NPs based on iron oxides aiming at optimizing the synthesis procedure and getting a good control of the structure of the final product. In the second part, those NPs obtained in the former way are applied to demonstrate the crucial role of the nanostructure on the physical properties of nanoparticulate systems; specially, the strong interplay existent between structure and both magnetic frustration and interparticle interactions. Finally, in the third part, MFM experiments with an external applied magnetic field have been performed to directly observe the reversal of the magnetization of isolated particles and the dynamic behaviour of small aggregates.