The process of forming an ordered aggregate by a set of constituents or components spontaneously through their global energy minimization is called self assembly (Ali, 2005; Robinson, 2003). One of the recent demands and challenges in today’s world is the development of nano-circuits and devices. These circuits can be formed by nano-electronic components and the connectors used between these components. For these connectors nanowires may be used and with this increases the demand of fabricating nanowires using different techniques (Saif et al. 2003). The following section describes how self assembly is utilized to produce magnetic nanowires.
1. Self assembly of single crystal Ferromagnetic Iron (α-Fe) nanowires formed by decomposition:
The approach used to create self assembled nanowires of α-Fe is by the deposition of appropriate perovskite which are the materials belonging to the chemical class are called calcium titanium oxide (Mohaddes et al. 2004).
The reaction that takes place during the process of decomposition of La0.5Sr0.5FeO3 is:
With this process an array of single crystalline α-Fe nanowires occurs spontaneously under reducing conditions. This array develops perpendicular to the substrate and traverses the overall thickness of the film.
The ferromagnetic α-Fe nanowires are planted in a paramagnetic LaSrFeO4 matrix. The diameter denoted by’d’ and the spacing denoted by ‘S’ of the nanowires are managed directly by deposition temperature. The diameter range is from d = 40-50 nm at Td = 840 °C to d = 3-5 nm at Td = 560 °C whereas the spacing is around S 3d. Uniaxial anisotropy is depicted by the nanowires with an easy axis parallel to their growth direction. The values of magnetization are near to that of bulk α-Fe and the large coercivity (Hc = 3400 Oe for d 20 nm) of these iron nanowires causes them suitable for high density data storage and other magnetic device applications.
In the recent past, the perpendicular ferromagnetic nanowires have become the centre of interest because of their use in various fields of advanced nanotechnology, especially the recording media with high density. Here the ideal medium having high density is the one having densely organized assembly of nanometer-scale Ferro magnets with high magnetization and suitable coercivity (Mohaddes et al. 2004) (Shiraki et al. 2003)
These structures can be shaped by either process direction or self enabling. The process directed technique is more controlled and definitive while the self enabled technique is more suitable and easy to reach from the point of view of scaling up, cost efficiency and implementation. Many examples are available for process-directed perpendicular nanowires such as those proposed by Li & Metzger (1997), Huysmans et al. (1988), Metzger et al. (2000), Sellmyer et al. (2001) and Martín et al. (2003) in which porous aluminum oxide films are used as a model and ferromagnetic metals such as Fe, Co and Ni are electrodeposited into the pores of the film. Even so, the effort put is to a lower extent during the self assembly of perpendicular ferromagnetic nanowires preferentially by themselves (Mohaddes et al. 2004).
2. Self assembly of nanowires on silicon substrates (without using the technique of lithography):
Here is an approach for the fabrication of nanowires by self assembly in which the location where they can be created is determined by the mechanical stresses. These mechanical stresses can be managed but to a partial extent. Therefore, this technique is expected to generate self assembled nanowires that are patterned.
In this technique, the width of wires is 50 nm or less but the lengths can be 10s of micro meters. Here the first step is to coat the Si wafer with PECVD SiO2. OH is present as an impurity in the oxide which leads to the compressive stress in the film. After the process of gradually heating and cooling at 525 C for 24 hrs, the output is a tensile stress in the film because of the break away of OH. This stress is enough to make the film crack (Saif et al. 2003) (Thurn & Cook, 2002). This crack extends till the bottom of the film until it reaches the Si substrate. The sharp wedge formed by the cross sectional geometry of this crack produces a nano dimensional opening to the Si substrate. This crack is half filled up with Nickel with the help of electro less deposition. The oxide present there is moved out by the process of wet etching. In this way, the crack of the oxide film acts as a mold to create the wire. The perceptual structure of the cracks is dependent on the kind of stress i.e., uniaxial or biaxial, strength of the interface between the substrate and the film, and the fracture strength of the film. Hence, the pattern of the crack can be managed by adjusting these parameters as shown in fig.1 (Saif et al. 2003).
In this technique, the points, lines and curves of the surface of the nanowire pattern and that of the crack are dependent on the type of stress brought forth during the process. For example, a uniaxial tensile strength gives a periodic array of cracks that are parallel while on the other hand the biaxial tensile strength can cause a pattern that is random in nature.
As demonstrated by (Saif et al. 2003), if we take a polyimide (PI) film which is 125 micro meter thick and deposit a thin layer of Al on it which is 400 nm. The system of Al-PI is caused to experience uniaxial tensile strength when the cracks formed by the Al are parallel to the direction of tensile loading.
While if this system is caused to experience a biaxial tensile strength by a bulge test Then the Al is found to get cracked in a random way.
In the first technique, a simple and different idea has been established to create self assembled perpendicular arrays of single crystal ferromagnetic Fe nanowires with the help of spontaneous phase decomposition of a complex oxide. While the second technique, represents a method which fabricates nanowires on Silicon substrate but without the use of lithography.