Proprietes magnetostatiques de reseaux de nanofils via les courbes de renversement du premier ordre

Nanostructured magnetic materials are increasingly integrated into various devices, given their versatility and the advance of fabrication techniques. Depending on their geometry, dipole interactions between the magnetic nano-entities can be very high and alter the magnetic behavior of the entire structure. Knowledge of the individual magnetostatic properties of the entities and of the interaction field acting on each is therefore essential if we are to achieve the specific overall magnetic properties required for devices.;In this sense, the first-order reversal curve (FORC) technique has been improved and used to characterize the magnetic behavior of various ferromagnetic nanowire arrays. All exhibited an axially preferential direction of magnetization and a high aspect ratio (diameter = 15 to 175 nm, length = 1 to 60 mum, interwire distance = 50 to 300 nm). The influence of composition and of geometrical dimensions was studied with nanowires of uniform composition of Ni, CoFeB, and CoFe, whereas the structure effect was studied by using nanowires with a multilayer structure, alternating nanodiscs of Ni and of Cu. The first-order reversal curve technique is based on a statistical analysis of the magnetic behavior of each magnetic entity in a system.;The results from the FORC method have been modified so that they graphically represent the whole reversal of the magnetization, reversible and irreversible. To do so, a reversibility indicator was defined from the initial susceptibility of the first-order reversal curves, whereas a calculation method suitable for the irreversible processes with a low coercivity has been developed. The values of FORC coercivity and global interaction field can be extracted from this graphical representation. In order to give a physical meaning to these values, a new interpretation model was specifically developed for the characterization of magnetic nanostructures.;The characterization of ferromagnetic nanowire arrays by the FORC technique yielded several results and hypotheses about their magnetic behavior. The results for uniform nanowires submitted to an applied field parallel to the axis of the nanowires are consistent with the hypothesis of a reversal of the nanowire magnetization by nucleation-propagation. With a perpendicular field, the reversal would rather be a coherent rotation toward the nanowire axis, in the case of uniform and relatively high diameter (175 nm). In the case of multilayer nanowire arrays, the reversibility indicator proved to adequately characterize the array anisotropy. When the field is applied along the axis of nanowires, the interaction field is proportional to the thickness of the nickel nanodiscs, whereas the magnetization reversal in the perpendicular direction is through a mixture of coherent and incoherent reversal. Finally, in all cases, the dipolar interaction field is not uniform within the array, neither in the plane, nor in length.