Magnetic Properties of Nanowires guided by Carbon Nanotubes

The physical properties of one-dimension al (1 D) nanostructure s of magnetic materials are presently the subject of intensive research, taking into account the considerable attention they have recently received and the few cases reported. [1-4] Much of the early work was concerned with exploratory issues, such as establishing an easy axis for typical preparation conditions and the essential involvement of shape anisotropy, as opposed to magnetocrystalline anisotropy. More recently, attention has shifted towards the understanding of magnetization processes and related applications. Particularly interesting problems are the magnetic hysteresis of the wires and the time dependence of the magnetic reversal. Thus, magnetic nanowires have provided a highly successful test ground for understanding the microscopic mechanisms that determine macroscopically important parameters in the different applications where they can be used. [5] On the other hand, these building blocks, as in the case of spherical nanoparticles, are at the border between the solid and molecular state displaying the novel effects that can now be exploited. Therefore, it becomes imperative to take into account the fact that the properties of materials composed of magnetic nanostructures are a result of both the intrinsic properties of the small building blocks and the interactions in between. [6] This chapter is not meant as a survey of th e present state and future developments of magnetic nanowires and since only two examples are considered, is far from being complete. The purpose of this chapter is three fold: a) an introductory level overview about magnetic colloids, the basic physics in the magnetism at the nanoscale; in terms of superparamagnetism, the concept of magnet ic anisotropy and the dynamics of these systems. We have emphasized the dominant role of the surface effects on the intrinsic properties at the nanoscale and the competition with the interactions in the case of assemblies, leading to a characteristic magnetic behavior termed as spin-glass. Additionally, a brief introduction referred to carbon nanotubes (CNTs) is included. b) Characteristi c examples of magnetic nanowires whose morphology was achieved by taking advantage of CNTs and exploiting wet-chemistry methods, and c) a complete analysis of the magnetic behavior displayed in both examples.