Fe on Cu(100) is a system which has been extensively studied over the last decades due to its complex and rich variety of structures and magnetic properties as well as for providing a prototypical system for magnetic ultrathin films. For 2-4 ML Fe grows as strained bcc with nano-martensitic structure, for 5-10 ML it grows as fcc (paramagnetic) film at room temperature with rare needle shaped bcc crystallites. Below room temperature, a (21) or (22)p4g reconstruction occurs with the so called 'magnetic live layer' and for thickness more than 10 ML, it grows as relaxed bcc. It has been shown that with Ar+ ion irradiation, 8 ML Fe film undergoes a structural change from fcc to bcc. This change was confirmed by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). A "Thermal Spike" model was proposed for the transformation process and writing a magnetic pattern was demonstrated by ion-beam projection lithography. In the current thesis, STM experiments will be presented, providing more insight into the surface magneto-optical Kerr effect (SMOKE) data previously presented by Rupp et al. for 8 ML Fe films with different ions. Best transformation results were obtained for heavier ions for all ion energies. Also the fcc-bcc transformation of 8 ML Fe film through a Au layer by Ar+ ion irradiation was investigated using STM. Films with thickness < 2 nm are too thin for some applications like magnetic flux guides, therefore thicker films are desirable. Thus, one aim of the present work is to show that we can extend the above mentioned work to thicker Fe films provided we use some additional material to support them. For this purpose we have used CO pressure during growth to stabilize the Fe film as previously suggested by Kirilyuk et al. Then Ar+ ion irradiation is used to induce a structural change in a 4 nm (22 ML) thick Fe film from fcc to bcc grown in CO pressure. Oxygen and carbon together contribute to the surface and bulk stability of fcc Fe phase respectively. STM images show the nucleation of bcc crystals which increase with the Ar+ ion dose and eventually result in complete transformation of the film from fcc to bcc.
Intermixing with the Cu substrate impedes the transformation. The best transformation results are obtained for 2 keV Ar+ energy. LEED also shows the transformed patterns. For the magnetic properties, SMOKE measurements confirm the transformation from paramagnetic to strongly ferromagnetic with an in-plane easy axis. We were also successful in stabilizing even thicker metastable fcc Fe films. The growth technique used for stabilizing 8 nm (44 ML) thick fcc Fe film is co-evaporation of Fe and Invar (Fe64 Ni36). The structural analysis was carried out using STM. The main factor affecting the stability limit of these thick fcc Fe films was found to be the concentration of Ni in the films. The most suitable Ni-concentration for stable fcc Fe films was found to be close to 15.1%. We have employed the technique of the ion-induced transformation to write small ferromagnetic patches by Ar+ irradiation through a gold coated SiN mask with regularly arranged 80-nm diameter holes, which was placed on top of the as-prepared fcc Fe films. Nanopatterning was performed on both 8-monolayer (ML) Fe films grown in ultrahigh vacuum as well as 22-ML films stabilized by dosing carbon monoxide during growth.
The structural transformation of these nano-patterned films was investigated using STM. In both 8 and 22-ML fcc Fe films, the bcc needles are found to protrude laterally out of the irradiated part of the sample, limiting the resolution of the technique to a few 10 nm. The magnetic behavior of the transformation areas was confirmed by magnetic force microscopy.