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In this paper, we discuss several strategies to tailor magnetic properties related to the magnetic anisotropy energy, such as the blocking temperature or the low temperature coercivity, of magnetic nanoparticles or materials made with magnetic nanoparticles. We describe a first approach that consists in synthesizing and dispersing bi and tri-magnetic core-shell nanoparticles that include a core made of a material with a weak anisotropy energy density and a shell made with a material with a large anisotropy energy density. This approach is a promising route to tune the blocking temperature of low temperature coercivity of a particle without altering its magnetization and with a good control of its size. Additionally, we also explore another route for the control of the shape of the hysteresis loop of material made with magnetic nanoparticles that consists in the simple mixture of magnetically soft and hard magnetic nanoparticles to create binary mixtures. In this case, it is the mixing ratio that allows one to adjust the properties of the final material.

A dynamic probing of magnetic liquids is performed experimentally, using a static magnetic field modulated by another smaller field, normal and alternating. The optical magneto-birefringence under these crossed magnetic fields is recorded as a function of the frequency for different field intensities and different sizes of the magnetic nanoparticles. A general reduced behavior is found for the in-phase and the out-of-phase optical response which is well-described by a simple mechanical model. Depending on the value Hani of the anisotropy field of the nanoparticles, we can distinguish two different high magnetic field regimes: - a rigid dipole regime (large anisotropy energy with respect to kBT) for cobalt ferrite nanoparticles with a relaxation time inversely proportional to the field intensity HC(HC Hani).

We present here new experimental results about the waves at the horizontal free surface of a magnetic fluid submitted to a normal magnetic field. The waves are generated by a small modulation at frequency of the vertical field . Using a shadowgraph method, we are able to measure the wavevector k of the 2D waves for a given value of and . The dispersion relation of the surface waves is established experimentally. On the other hand, we propose a theoretical derivation of the dispersion equation which includes a more complete treatment of the magnetic term than the previous works. Finally, we conclude that a linear and inviscid analysis is sufficient to fit well the experimental data, except in the vicinity of the critical field where a surface instability occurs.

Nanometric maghemite and cobalt ferrite particles are chemically synthesized. The process produces particles polydisperse in size. The positive charges of their surface allow one to disperse them in aqueous acidic solutions and to obtain dispersions stabilized through electrostatic repulsions. Increasing acid concentration (in the range 0.1 to 0.5 mol.L−1), interparticles repulsions are screened and phase transitions are induced. Using this phenomenon, we describe a two-step size sorting process, in order to get significant amounts of nanometric monosized particles (with diameters monitored between typically 6 and 13 nm). As the surface of the latter is not modified by the size sorting process, usual procedures are used to disperse them in several aqueous or oily media.