Functional modification in ultrathin films: from metastable magnets to molecular materials

Ultrathin films have become an area at the frontier of materials science due to their novel properties and new applications. In particular, ultrathin magnetic films and ultrathin organic films are drawing more and more attention. The former is associated with a series of new phenomena such as giant magntoresistence and oscillatory exchange coupling which enable fabrication of new devices and applications, while the interest in the later is ignited by the promise prospect of molecular optoelectronics and molecular electronics. To make a full use of these films, it is important to learn how to manipulate their growth and how to tailor their properties. The aim of this work was to explore methods efficient for functional modification of these films. (Fe, Ni) bilayers with different individual thickness and different deposition sequences have been investigated experimentally with special attention to the temperature dependence of their magnetic parameters. It is unequivocally found that in (Fe, Ni) bilayers the spin reorientation transition shifts to larger Ni film thicknesses compared with Ni/Cu(100) films. This result is explained by an enhanced demagnetization field when the two magnetic layers with unequal magnetization are put together and possibly by a Fe/Ni in-plane interface anisotropy. The non-monotonic temperature dependence of the coercivity observed in the bilayers strongly suggests the existence of an in-plane anisotropy at the Fe/Ni interface. Magnetic live layers of Fe have been found at the Fe/Ni interface. The magnetic structure of the surface layer of 9-ML Fe on Ni on Cu(100) is closely related to the thickness of the underlying Ni film. A magnetic live layer with Curie temperature around 230 K is observed when the thickness of Ni layer is chosen to be 10 ML, while it is absent when the Ni thickness increases to 15 ML. The structural relaxation of the Ni layers with thickness is thought to be responsible for the observations. These results provide a further evidence for a sensitive correlation between the structure and magnetism in fcc Fe. The single domain state is not stable for the (Fe, Ni) bilayer with small perpendicular anisotropy. With the withdrawal of the external field, the single domain state gradually relaxes to a multi-domain state, resulting in a decay of remanent magnetization with time. The relaxation behavior disappears when the perpendicular anisotropy is large. An exchange biasing in ultrathin Fe films on Ni/Cu(100) has been observed when the (2´1) and (4´1) phase coexist. This biasing could be attributed to the coexistence of AFM and FM domains as observed previously in granular systems. However, with all the direct experimental result objecting the existence of a AFM phase at the studied temperature, a new model is proposed to account for this unusual biased coupling phenomenon. It is based on the assumption that the strong biquadratic exchange coupling results in an orthogonal coupling between the (2´1) Fe domains and their underlying Ni layer. By measuring and analyzing the hysteresis loops of Ni/Cu(100) films with several thicknesses at different temperatures, the magnetization reversal mechanisms have also been investigated for Ni/Cu(100) films with perpendicular magnetic anisotropy. The magnetization reversal procedure consists of the nucleation of the reversed domains and the following motion of the domain wall. Which process is the dominant mechanism depends on the temperature and the film thickness. For thick films with large perpendicular anisotropy, the nucleation field, which is correlated to the anisotropy, is greater than the pinning field of most pinning centers. The magnetization reversal is dominated by the nucleation and the reversal procedure can be described by the nucleation followed by the viscous motion of the wall without obstacles, resulting in a high squareness in the shape of the hysteresis loops. With decreasing film thickness, the effective anisotropy field becomes comparable with the effective pinning field. Then the role of temperature becomes important. At high temperature, the motion of the domain wall is thermally assisted and thus easy, so the nucleation of the reversed domain is still the dominant process. At low temperature the pinning effect of the wall motion is visible. Therefore the shape of the hysteresis loop changes from rectangular at high temperature to inclined and round at low temperature. In this case the adsorbed residual gases have a pronounced effect on the reversal procedure acting as pinning centers. The high stability of the magnetic properties of Ni/Cu(100) films with perpendicular anisotropy upon multiple magnetization reversals has been measured and is confirmed by a theoretical analysis. AFM and x-ray diffraction have been used to investigate the growth behavior of perylene films on a (111)-oriented polycrystalline Au substrate, as well as on the same substrate but additionally coated with a self assembled monolayer (SAM) of 1-Octadecanethiol molecules. It has been found that the perylene molecules have a smaller diffusion coefficient on the SAM than on the Au surface. An additional self-assembled monolayer of 1-Octadecanethiol molecules on an Au-substrate greatly modifies the properties of the subsequently vacuum deposited perylene films. The grain size becomes smaller and a strong c-axis texture is introduced. Both factors tend to reduce the roughness of the perylene films. These effects are attributed to the changes in the perylene molecule-substrate interaction by the additional self assembled monolayer. To further reveal the unique growth behavior of molecular crystals, perylene have been evaporated on a oil film. The growth speed along the crystallographic [100] axis is found to be dramatically different from that along its opposite [-100]. The ratio of two growth speed is around 2.3, and does not show an obvious dependence on supersaturation. This asymmetric growth is believed to has a steric origin, which is special for organic crystals. An organic molecule, which is the unit building block of a organic crystal, consists of a number of atoms and shows a finite size and specific shape. These features complicate the growth behavior of organic crystals. For the growth of perylene crystals on an oil film, the steric environment for a perylene molecule to be incorporate to a crystal is different for [100] and [-100], resulting in a difference in the growth speed. Interesting growth patterns have also been observed for perylene crystals grown on a oil film. Though it is preliminary, our study indicates that the stress field at the growth interface and especially around a step has significant effects on the growth behavior.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2004

Author dc:creator

• Liu, Xiangdong

Contributors dc:contributor

• Wuttig, Matthias

• info:eu-repo/classification/ddc/530
• Eisen
• Nickel
• Doppelschicht
• Magnetische Eigenschaft
• Temperaturabhängigkeit
• Organischer Halbleiter
• Perylen
• Schichtwachstum
• Physik
• Magetic Films
• Organic Films
• Growth
• Morphology

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-121240%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:59457