Ferromagnetic nanostructures by laser manipulation

Lithography based on laser focusing of a beam of neutral iron atoms shows great promise for creating nanomagnetic structures. Laser focusing is a relatively new area, where successful experiments have been performed with, e.g., chromium atoms. Iron is perhaps one of the most difficult elements for this kind of experiment. Nevertheless, it is the ferromagnetic element most suitable for laser focusing. The production of well isolated sub–10 nm structures of iron requires two improvements to conventional laser focusing: chromatic aberration is reduced by using a supersonic beam of Fe atoms, whereas spherical aberration is reduced by implementing an additional mechanical grating placed just upstream of the standing light wave. In this thesis a description of the nanostructure project, preliminary results, and future plans are presented. The road to the fabrication of Fe nanostructures is full of challenges, and some of them have already been met. One of the most pronounced results is the development of a supersonic Fe beam source. Since these kind of beam sources are commercially not available, considerable effort has been put into the development of such a tool. The resulting source, as shown in Chapter 3, has demonstrated excellent performance. An Fe beam intensity in the range of 1015 to 1016 atoms/s/sr can be produced, which gives an ultimate deposition rate of the Fe structures in the range of 10 nm/h. Furthermore, the axial beam temperature is exceptionally low for atomic metal beams. The beam speed ratio, defined as the ratio of the mean axial velocity to the velocity spread, has been measured to be 11 for the Fe beam. As a result, chromatic aberration of the standing light wave (nanolenses) that focus the atoms to nanostructures, which up till now has been one of the limiting factors for the size of the structures in similar experiments, will not play a significant role anymore. Another result is the development of a UV laser system for laser manipulation of the atomic Fe beam, as described in Chapter 4. The requirements are a relatively high UV laser power of 500 mW at 372 nm, and a frequency stabilizing within 3 ?? 10-9 and locked to the Fe resonance frequency. These requirements have been met. Using cavity-enhanced second harmonic generation of a laser beam of 2 W at 744 nm a power of more than 500 mW has been obtained at 372 nm. Polarization spectroscopy applied to a hollow Fe cathode discharge has been used to obtain an error signal with a width of 40 MHz. Using this signal to lock the laser, a frequency stabilizing of better than 1 MHz results on the fundamental frequency, or 2 MHz on the second harmonic beam at 372 nm. In similar experiments performed so far the standing light wave is used as a thick lens. However, calculations presented in Chapter 5 show that using the standing wave in the thin lens regime, the focused structures are smaller by a factor of 3 at similar light field parameters. Channeling of atoms in a standing wave to produce small structures has also been investigated, since channeling is much more tolerant for errors in substrate placement and laser instability. However, this process requires a relatively high laser power of up to 1000 mW, which is not available. The resulting structure widths are then in the range of 10 to 30 nm, albeit with excellent contrast. Different models have been used for the calculations, and their validity has been checked. As a result, a thorough understanding of the process of laser focusing has been achieved. The next step that has to be taken experimentally, is laser cooling of the Fe beam. A well collimated beam of less than 200 ??rad angular divergence is necessary in order to obtain sub- 10 nm structures. Since the Fe beam is already "collimated axially" by using the supersonic Fe beam source, the transverse collimation is the main limiting factor to the focused Fe structure width. Numerical simulations presented in Chapter 6 have shown that a collimation of 100 ??rad can be achieved using an acceptable number of scattered photons. We are now at the verge of having the first laser cooled atomic Fe beam. Experimental results on laser cooling are to be expected soon. It is clear that several challenges are still present on the road to the fabrication of Fe nanostructures. However, the most important steps have already been taken, i.e., the development of a supersonic Fe beam source and UV laser system.