UNA IMAGEN EN LA NOCHE

4πa2_N32 Re ∞ X s=1 1 s3 δ2kz δ2_k x −1 2 _δk z δ −1 cos[2N ask0_z(µ)] (1.8) the δ2_k z δ2_k x −1 2

in the coupling equation will affect the interaction as the more curved the Fermi surface is then larger this term and the RKKY interaction will be stronger and the

N32 results in a slower decay of the exchange then would be experienced in 3D systems. By this mechanism is how the singularities (at necks and bellies) of the Fermi surface produce the strongest coupling effects. With a pseudomorphic Au capping layer over the RKKY interaction will occur along the [11¯2] direction for both the Pt and the Au where neighbouring wires interact with each other; and along the [111] direction when the wire interacts with itself through the capping layer. As can be see from figures 1.11 and 1.12, these directions will have significant contributions to the strength of the RKKY interactions due the singularities along these directions in the Fermi surfaces of the Au and Pt. As shown in table 1.2 the period of the RKKY interaction along the [111] direction in Au is 4.83 ML which means that a capping layer of 5 ML should maximise the strength of the RKKY interaction from the cap.

As shown above the indirect coupling is evident in many systems and is evident up to even room temperature, the full coupling effect in our system will be closely approximated by a superposition of the coupling mediated by the Pt and Au separately as shown in experiments [56].

1.9

Domain walls

Magnetic materials rarely form a uniformly magnetised state, they more generally form several regions each with their individually magnetically aligned, multiples of these regions exist in most magnetic solids. These regions are called domains, and they exist to lower the energy of the system. Where these domains meet the direction of the magnetic alignment changes, this regions where the change occurs is called the domain wall in a domain wall the spins in the wall are rotated for the direction of one domain to the other. The width of a domain wall is a balancing act between the energy required to have spins aligned along the hard axis of the system and the reduction in energy from there not being a abrupt change in the magnetisation direction. The width of the domain wall is given by [19]:

δw=π

r

A

K (1.9)

whereAis the exchange stiffness andK is the anisotropy constant,Ais proportional to the TC withA≈ kB_2aT₀C, the TC of a system can be easily seen to be dependent on the strength of

1.9. Domain walls Background

Figure 1.13: Strength of RKKY interaction for a smooth and rough Cu(001) [12].

the exchange interaction of the system (J), which means the the alteration of the exchange between the wires could change the width of the domain wall of the uncapped wires and their TC. In bulk Co the domain wall of the order of 15 nm, while in a Fe nanostripe system

on W the domain wall was measures to have an upper limit of 0.6±0.2 nm [65] which agrees with the Ising model which predicts atomically sharp domain walls.

So it can be seen that variation in J via changes in the indirect coupling in a system can alter the TC and the domain wall width. It is also worth noting that the strength of RKKY

2

Experimental background, theory and

details

In this chapter the theory behind the techniques, and the details of the various the ex- perimental sets up used, through out this work and are detailed, in separate sections, as appropriate.

2.1

Ultra High Vacuum (UHV)

As outlined previously the structures being studied here are grown on clean surfaces, a clean surface in this context means atomically clean. Clean surfaces are very reactive and the gas atoms that collide with the surface may absorb and stick to the surface, for a surface to remain clean for a useful period of time it has to be in a vacuum with a very low base pressure.

As shown in table 2.1 sample preparation has to be performed in UHV to maintain a clean Pt surface and uncontaminated wires. In this study samples were grown in a vacuum chamber with a base pressure of the 5×10−11 mbar.

Modern vacuum chambers are made from stainless steel, normally 300 series because of it non-magnetic properties, lower carbon content and resistance to corrosion. The machinable and weld ability of stainless steel means the vacuum chambers can be produced in any configuration, but in their design it is important to keep the surface area to a minimum. A detailed description of vacuum hardware is unsuited to this work and the reader is suggested to read a dedicated text on the topic for example [66].

2.2. Cleaning Experimental background, theory and details