Magnetostatic interactions in arrays of nanostructures

The physics of nanomagnetic elements has become one of the main areas of research in the last few decades. Their magnetic characterization is fundamental in order to understand the magnetic reversal mechanisms and to control the magnetic switching precisely in view of device applications. In such small elements, the magnetic behaviour is strongly determined by the interplay of magnetic anisotropy with the geometric shape. Moreover, in closed packed arrays, the effects of the inter-element interactions become important and reveal often unresolved issues regarding the strength of these couplings and their dipolar or exchange mediated origin. In this thesis three different cases have been discussed about the mechanisms of interactions among the magnetic elements of very densely packed systems. The experimental section opens with the study of the magnetization reversal process in magnetostatically interacting permalloy (an alloy of nickel and iron) nanowires, using the vectorial magneto-optical Kerr effect magnetometry. The measured in-plane magnetization components parallel and perpendicular to the applied field show a transition from coherent rotation to inhomogeneous reversal mode over and above a determined value of the wires thickness. A strong dependence on wires thickness is also observed in the analysis of effects of dipolar interactions. Successively, the motion and pinning of domain walls have been investigated in vertical and horizontal chains of permalloy triangular microrings and compared with the isolated rings case. Using longitudinal and diffracted magnetooptic Kerr effects, magnetic force microscopy and micromagnetic simulations, the field dependence of the spin structure into the ring has been determined. This investigation has allowed to observe how the dipolar inter-element interaction changes qualitatively the manner in which reversal occurs. Considering that the magnetic domain walls displacement can also be controlled by using magnetic fields, such structures could be viewed as a preliminary approach for the development of magnetic logic applications. Finally, static and dynamical properties have been studied in patterned arrays of pseudo spin-valves in which a layer of permalloy has been coupled with a layer of cobalt, through an interspacer of non-magnetic copper. Magnetic hysteresis loops have been measured by longitudinal magneto-optic Kerr effect and by X-ray resonant magnetic scattering, whereas the dynamical properties have been investigated using the Brillouin light scattering. The results show a complex magnetization reversal process determined by the interplay between the interlayer dipolar interaction and the different reversal nucleation fields in the two ferromagnetic layers. Reducing the thickness of the copper spacer, an interlayer magnetic exchange coupling between the permalloy and cobalt layers emerges, in addition to the dipolar interaction, introducing further possibilities for manipulating the magnetic behaviour of the system.