Mikromagnetische Strukturen in ultradünnen Kobaltschichten

The micromagnetic structure of ultrathin cobalt films was studied under UHV conditions by means of a Scanning Electron Microscope with Polarization Analysis (SEMPA). The microscopic domain and domain wall structure was determined for Co/Cu(100)- and Co/Cu(1113)-films in the thickness range of 3 - 15 monolayers. In addition, the existence of domains and the structure of domain walls in ultrathin films has been investigated theoretically. After deposition, Co/Cu(100)- as well as Co/Cu(11113)-films exhibit a single domain state with edge domains only at the sample boundaries. This behavior can be well explained by taking into consideration that the magnetization for all those films is within the surface plane. Thus, a homogeneous magnetization state does not create any magnetic field, except at theboundaries, and therefore represents the lowest energy state. This simple picture remains valid for films with in-plane magnetization, even if the effect of film roughness during growth is taken into account. In contrast, the single domain state for films with out-of-plane magnetization is destabilized by an inhomogeneous, rough film structure during growth. Ultrathin Co/Cu(100)- as well as Co/Cu(1113)-films can be demagnetized by a magnetic AC-field of decreasing strength. In this demagnetized state, all ultrathin films investigated are characterized by a rnultidomain structure with domain sizes between several 10 and 100 pm and very irregular boundaries. The domain walls are not straight and have no preferential orientation with regard to the magnetization direction, in contrast to bulk or thick film samples. Thus, the magnetostatic energy is of minor importance for domain walls in ultrathin films because this energy term is generally the origin of a referential domain wall orientation. Due to the uniaxial magnetocrystalline anisotropy in the film plane, only 180°- domain walls are present in Co/Cu(1113)-films. The micromagnetic structure of Co/Cu(100)- films which have a fourfold in-plane magnetocrystalline anisotropy is dominated by 90 $^\circ$- walls, although 180$^\circ$-walls are also present. The experimental investigations for Co/Cu(100)-films show that within the domain wall the magnetization rotates in the film plane, i. e. the domain wall exhibits a Neel type structure. Domain wall structures and energies for Neel type walls in ultrathin films were calculated, using a 1-dimensional model. In 180$^\circ$-domain walls, for uniaxial as well as fourfold magnetocrystalline anisotropy, the magnetostatic energy contribution is significantly higher than in comparable 90$^\circ$-walls. Therefore, 90$^\circ$-walls are energetically more favorable than 180°-walls in materials with fourfold magnetocrystalline anisotropy. This is confirmed by the experimentally observed dominance of 90$^\circ$-walls in demagnetized Co/Cu(100)-films. The magnetostatic energy contribution increases with the film thickness for all Néel wall types and alters the wall structure significantly. It sharpens the wall in its center region andalso creates long tails in the outer segments. Long domain wall tails into the adjacent domains were found for 1800-walls in Co/Cu(100)-films. This type of wall also exhibits a 2-dimensional magnetization structure depending on the surrounding domain configuration