J. Depeyrot

Magnetic and optical properties of ionic ferrofluids based on nickel ferrite nanoparticles

New ionic ferrofluids containing NiFe2O4 nanoparticles of size ⩽10 nm are investigated. The crystalline structure of the particles is probed by transmission electron microscopy and x-ray scattering. Static magnetization and field-induced birefringence measurements are performed on three samples differing by particle volume fraction. Cross analyzing of the results of those two types of macroscopic tests completely rejects a simple single-domain particle model but readily supports the two-component scheme of a particle as consisting of a core with a uniform magnetization and a surface layer of comparable thickness stowed with a spin-glass-like arrangement.

NiFe2O4 nanoparticles in ferrofluids: evidence of spin disorder in the surface layer

We show that surface magnetic properties of NiFe2O4 nanoparticles constituting ionic ferrofluids can be investigated in macroscopic experiments. Cross-analysis of static magnetization and field-induced birefringence prove that the particles consist of a uniformly magnetized core and a spin-disordered surface layer of comparable thickness. r 2002 Elsevier Science B.V. All rights reserved.

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.

Optical properties of nickel ferrite ferrofluids

We investigate magneto-optical properties of chemically synthesized ionic ferrofluids based on nickel ferrite nanoparticles. These new ferrofluids with potential biological applications become birefringent under low magnetic fields. Both a static and a dynamic probing are here presented.

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

Size control of MnFe2O4 nanoparticles in electric double layered magnetic fluid synthesis

We propose a method based on the pH of the synthesis to control the nanoparticle size during the ferrofluid elaboration. The particle diameter is determined by means of X-ray diffraction experiments. The measured mean size depends on the type of buffer used during the coprecipitation process. The results therefore confirm that the nanoparticle size can be monitored by the hydroxide concentration and suggest to consider the induced interplay between nucleation and crystal growth.

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.

Dynamic optical probing of the magnetic anisotropy of nickel-ferrite nanoparticles

Field dependence of dynamic magneto-orientational birefringence in a ferrocolloid based on the nickel-ferrite nanoparticles is examined. The nanoparticles are electrostatically stabilized and suspended in glycerin at low-volume fractions Φ⩽0.75%. The colloids are tested under crossed magnetic fields: an alternating weak (probing) and a constant strong (bias) one. By comparison to a theoretical model of the birefringence relaxation, an evaluation of the mean nanoparticle magnetic anisotropy energy, Ea, is done. We get σ=Ea∕kT∼10, given that the particles are rather polydisperse: a log-normal distribution with exp[⟨lnd⟩]=6.5nm and the width s=0.55. With the parameter σ in such a range, the NiFe2O4 grains make a first example of a nanodisperse material (ferrofluid) whose anisotropy properties differ substantially from both magnetically soft (e.g., maghemite with σ 50) nanosubstances of the same grain size.

Electric Double Layered Magnetic Fluids (EDL-MF) based on spinel ferrite nanostructures [(M1-x+2Fex+3)]A [(Fe2-x+3 Mx+2)]BO4-2

This paper presents a review of Electric Double Layered Magnetic Fluids (EDL-MF) based on spinel ferrite type [(M1-x+2Fex+3)]A [(Fe2-x+3 Mx+2)]BO4-2, with M(II) = Mn, Co, Ni, Cu and Zn. The chemical synthesis of the nanoparticles using hydrothermal techniques and their complete peptization in an aqueous medium results in a ultra stable magnetic colloid, EDL-MF. The characterization of the particles was performed using X-ray diffraction and electronic microscopy techniques. The resulted sols were investigated by magnetic, magneto-optical measurements and magnetic resonance techniques.

The role of magnetic interactions in exchange bias properties of MnFe2O4@γ-Fe2O3 core/shell nanoparticles

Low-temperature magnetic properties of assemblies of 3.3?nm sized nanoparticles (NPs) based on a MnFe2O4 core protected by a maghemite shell are investigated. These NPs are obtained by a chemical core/shell method developed for the synthesis of the electrostatically stabilized ferrofluid colloidal dispersions that we probe here. They are model systems where the interparticle interaction is tuned by the NP volume fractions, ranging here between 0.4% and 13.9%. It has been shown that these NPs consist of a well-ordered ferrimagnetic core surrounded by a disordered spin glass-like surface layer and that they display uniaxial magnetic anisotropy. We compare the magnetic hysteresis loops of non-textured frozen dispersions (with magnetic anisotropy axis of NPs distributed at random) with those of a powder based on the same NPs. After cooling under field the hysteresis loops shift along the H axis, expressing the coupling between the spin-ordered cores and the disordered surface layers. The negative H-shift provides an evaluation for the exchange bias (EB) field. The EB field is optimum for a cooling field of the order of the anisotropy field. A comparison between frozen dispersions and disordered powder allows us to distinguish the influence of intra- and interparticle interactions on the EB. Interparticle collective effects dominate in the powder while an intraparticle EB, eventually hindered by dipolar interactions at large volume fraction, is observed in frozen dispersions.