Fig. 1 : Synthetic antiferromagnet, composed of two ferromagnetic layers separated by a thin layer of ruthenium. In absence of magnetic field, the magnetization of the two ferromagnetic layers are antiparallel coupled, thus the surrounding magnetic field is null. The application of a magnetic field will slightly break this antiparallel alignment, allowing to act on the particle.
Spintec Laboratory has recently developed a new method for preparing magnetic nanoparticles for biomedical applications. This method takes advantages from the know-how of the laboratory in the field of spintronics, and allows a control of the size, shape, and composition of the particles, which presents performances higher than those obtained by chemical route.
The use of magnetic nanoparticles, functionalized and grafted on targeted objects, offers extremely promising prospects in the biomedical field. For example : biological species sorting by the application of a magnetic field, targeted drug delivery towards a tumor, guiding axons growth aiming at cellular tissue repairing…
The most common magnetic nanoparticles, spheres of magnetite Fe3O4, are fabricated by chemical route. The use of these superparamagnetic nanomaterials allows to avoid their agglomeration in solution (see insert). This approach allows to obtain large quantities at relatively low costs. However, it is impossible to choose with precision the shape and size of the particles and the dimension dispersions are important.
The approach developed at Spintec allows to avoid these problems and to address high added value applications, which couldn’t be envisaged with the traditional nanoparticles. In this approach, the particles are realized on pre-structured substrates: a resist layer is deposited on the substrate and patterned in a plot network with shapes and sizes determined by photolithography or nanoimprint lithography techniques. The nanoparticles are at this step realized by magnetic deposition on the plots. They can be then functionalized for the aimed application and released in solution by lift-off (resist dissolution). A second step of functionalization is then possible. In addition to the possibility to choose the shape and size of the particles with precision, this method allows to realize complex magnetic stacks. Thus it is possible to fabricate synthetic antiferromagnets (see insert), « actuable » with magnetic fields 100 times lower than those required for magnetite.
Fig. 2 : approche "top-down" développée à Spintec : particules anti-ferromagnétiques de taille 1x1x0.12µ, nanofils magnétorésistifs multicouches de 100nm de diamètre, nanoparticules de formes complexes et nano-nageurs.
Thanks to this know-how, the laboratory explores at present in the framework of three PhD thesis several innovative applications.
Superparamagnetism and synthetic antiferromagnet
Under the action of a magnetic excitation H, a material acquires a magnetization M proportional to the excitation. The ratio between the magnetization and excitation values is called the susceptibility of the material. A positive susceptibility corresponds to a paramagnetic material, a negative susceptibility corresponds to a diamagnetic material. The ferromagnetic materials have the property of keeping their magnetization after the excitation is switched off. This results from the coupling between the spins which tend to align themselves parallel to each other, creating a global magnetization. When a ferromagnet size is sufficiently small, the energy which should maintain the magnetization fixed on the anisotropy axis becomes low in comparison with the thermal perturbations, so that the magnetization loses its stability, and takes arbitrary directions, appearing null in average over the observation duration at human scale. This phenomenon is called superparamagnetism. Thus superparamagnetic nanoparticles don’t tend to agglomerate in solution in absence of solicitation, but can be magnetized and actuated by a magnetic field. The applied field should be in the order of the Tesla.
The technique proposed by the laboratory consists in using particles composed of a synthetic antiferromagnet, constituted of two ferromagnetic layers, whose magnetizations are coupled antiparallel to each other. This stack mimics the superparmagnetic behavior: the global magnetization is null, but the susceptibility and magnetization values are much higher than those of superparamagnetic particles. It is possible to control the particles agglomeration through the choice of susceptibility. The required applied magnetic field is only 4 à 30 mT. An applied magnetic field of only some mT is enough to exert important magnetic forces.
Last update : 04/29 2014 (1030)