R. Aquino

Surface spin freezing of ferrite nanoparticles evidenced by magnetization measurements

Magnetization and in-field Mossbauer measurements were performed on copper ferrite nanoparticles with average sizes ranging from 3.5 to 10.4nm. Our results show that the nanoparticles are well-crystallized single domains with a magnetically disordered surface shell. A sharp increase in the saturation magnetization at low temperatures, in addition to the usual modified Bloch behavior, was observed for the smallest particles. This jump in magnetization curves seems to be related to the freezing of the surface spins below a temperature of about 45K.

Low temperature experimental investigation of finite-size and surface effects in CuFe 2 O 4 nanoparticles of ferrofluids

Abstract Size sorted magnetic nanoparticles based on copper ferrite have been chemically synthesized. Their magnetic properties have been investigated at low temperature using Mossbauer spectroscopy and magnetisation measurements. The thermal variation of the magnetization is strongly affected by finite size and surface effects. Indeed, the Mossbauer results show that the structure of the nanoparticles is made of a monodomain ordered core and a surface shell of disordered spins. Introduction Magnetic ferrite nanoparticles have been studied for the last decades from both scientific and technological point of view [1]. They are largely used as components in recording tape, flexible disc recording media, magnetic fluids as well as biomedical material. These applications require the knowledge of the properties of nanostructured systems and how bulk properties are modified as the crystal size decreases to the nanometric range [2]. As an example, in γ-Fe 2

Rare earth doped maghemite EDL-MF: a perspective for nanoradiotherapy?

We report on electric double layered magnetic fluids based on samarium-doped maghemite nanoparticles. The nanostructures chemical composition is carefully checked and X-ray diffraction patterns provide both their mean size and a structural characterization. Magnetization results are presented. Since these particles can become radioactive after neutron activation, they could therefore represent a new perspective for biomedical applications.

Influence of the spatial confinement at nanoscale on the structural surface charging in magnetic nanocolloids.

In this work we focus on the surface charging properties of core shell ferrite nanoparticles dispersed in water, namely magnetic nanocolloids. This structural charge results from the Brönsted acid-base behavior of the particles surface sites and is achieved through hydrolysis reactions. It can be modeled by considering identical charged sites behaving as weak diprotic acids. Then, electrochemical techniques could be implemented to study the acid-base equilibrium between the particle surface and the colloid bulk solution. Simultaneous potentio-conductimetric titrations are therefore performed to determine the thermodynamical constants of the p H-dependent reactions and to obtain the p H variations of the surface charge density. The results reveal that the saturation value of the structural charge strongly depends on the nanoparticle mean size. For large particles, the surface tends to be fully ionized whereas for smaller particles the saturated structural charge decreases drastically. This surface charge reduction is attributed to the existence in smaller particles of metallic surface sites, which cannot be accessible to the proton charge. The existence of such dead sites would be related to hydroxo-bonded sites with very low acidity combined with a quantum size effect, which would affect the charging/discharging process at the surface of the semiconductor ferrite quantum dot.Graphical abstract

In-field Mossbauer study of disordered surface spins in core/shell ferrite nanoparticles

Magnetization and Mossbauer spectroscopy measurements are performed at low temperature under high field, on nanoparticles with a nickel ferrite core and a maghemite shell. These nanoparticles present finite size and surface effects, together with exchange anisotropy. High field magnetization brings the evidences of a monodomain ordered core and surface spins freezing in disorder at low temperature. Mossbauer spectra at 4.2 K present an extra contribution from the disordered surface which is field dependent. Field and size dependences of this latter show a progressive spin alignment along the ferrite core which is size dependent. The weak surface pinning condition of the nanoparticles confirms that the spin disorder is localized in the external shell. The underfield decrease in the mean canting angle in the superficial shell is then directly related to the unidirectional exchange anisotropy through the interface between the ordered core and the disordered shell. The obtained anisotropy field HEa scales as ...

Size dependence of the surface charge density in EDL-MF

Abstract We determine the surface charge density of electric double layered magnetic fluids based on manganese ferrite nanoparticles of two different sizes using simultaneous potentiometric–conductimetric titrations. The saturation superficial density of charge is reduced for smaller particles.

Electrodic reduction of core–shell ferrite magnetic nanoparticles

We present square-wave voltammetry (SWV) and controlled potential coulometry (bulk electrolysis) experiments performed on maghemite and ferrite nanoparticles of core–shell-type MFe2O4@γ-Fe2O3 (where M = Mn2+, Co2+, Cu2+ or Zn2+), dispersed in aqueous media. The potential position of the iron(III) reduction peak identified in SWV measurements does not depend on the core chemical composition. No variations with size and polydispersity are detected. The shift of the reduction peak observed in coordinated configurations is related to the iron oxidizing strength. The electrolysis performed at a controlled potential of NPs indicates that only a fraction of iron(III) is reduced. Using the individual net charge deduced from the controlled potential coulometry measurements, we determine the thickness of the surface shell, which is electrolyzed.

Exchange bias properties and surface spins freezing in ferrite nanoparticles of magnetic nanocolloids

We investigate magnetic nanocolloids based on 3.3 nm sized particles of manganese ferrite. The observation of shifted hysteresis loops after cooling the sample in high flelds indicates the existence of a coupling between the ferrimagnetic ordered core and the disordered surface layer. The exchange bias fleld, determined from the fleld ofiset from the origin, decreases as the cooling fleld increases. Zero fleld cooling measurements of the thermal dependence of the high fleld magnetization allow separating two contributions. One is associated to the well ordered core and the other is related to surface spins frozen in a disordered spin glass like structure. As the applied fleld increases the freezing temperature decreases from 23 K to 12 K.