The ferromagnetic properties of ultrathin films grown on non-magnetic substrates provide interesting new insights into the physics of magnetism. In this report we review experiments in the very low coverage regime (Θ < 2 atomic layers). The Fe monolayer on W plays an outstanding role, because it forms a ferromagnetic and thermodynamically stable monolayer. Ferromagnetic Fe monolayers on W can be prepared with a high degree of perfection. We therefore focus on ultrathin Fe films on W(110) and W(100) substrates. Experimental results for these in-plane magnetized films, prepared as close as possible to 2-dimensional structures of perfect translational symmetry, are compared with appropriate theoretical models, We also discuss experimental results for the perpendicularly magnetized Co monolayer on Cu(111). The symmetry of the magnetic anisotropy is found to play an important role for the understanding of the magnetic phase transition in 2 dimensions. A new aspect is provided by finite size effects resulting from the morphology of magnetic ultrathin films. Microscopy with atomic resolution allows a systematic approach to the understanding of these finite size effects. Starting from the well-known magnetic properties of the extended Fe monolayer on W(110), we focus on ultrathin Fe films on W(110) deviating from the nearly perfect structure.

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The classical concept of band structure tuning as used for semiconductors by partly replacing one atom by a chemical neighbor without altering the structure is applied examplarily to the half-metallic ferromagnetic Heusler compound Co2Cr1−xFexAl. Band structure calculations are presented for ordered and disordered compounds. We present experimental and theoretical results. The connection between specific site disorder and the band structure is shown explicitly with particular emphasis on the half-metallic properties. Experimentally observed deviations from the ideal Heusler structure and from the simple Slater-Pauling rule for the magnetization are discussed in close relation to theoretical models. It has been found that the orbital magnetic moment and hence the spin-orbit coupling is important for the understanding of the half-metallicity. Experimental techniques which can be used to determine the electronic properties are described.

The influence of 30 keV He + ion irradiation on structural, electronic and magnetic properties of Co2MnSi thin films with B2 order was investigated. It was found, that irradiation with light ions can improve the local chemical order. This provokes changes of the electronic structure and elementspecific magnetization towards the bulk properties of the well-ordered Co2MnSi Heusler compound with L21 structure. [5,6,7,8,9], and an exceptionally high TMR effect of 570 % at 2 K has been reported for a MTJ structure with both electrodes consisting of Co 2 MnSi [7]. In the latter case, the spin polarization estimated by Jullière's formula was reported to be 89 % and 83 % for the bottom and top electrode, respectively. At room temperature (RT), however, the TMR effect is largely reduced. Moreover, spin polarization, experimentally observed on single crystalline Co 2 MnSi films at RT, remains with 12 % [10] far below the theoretically predicted 100 %. Strongly supported by ab initio calculations [11,12], partial chemical disorder is assumed to be one of the possible reasons for this discrepancy.Annealing at high temperatures, typically in the range of 400-500• C, is a conventional way to reduce the chemical disorder in the deposited Co 2 MnSi films. However, high temperature annealing often leads to interdiffusion and local changes of the stoichiometry in Heusler compounds [13]. For FePt thin layers, however, it has been demonstrated that the degree of chemical order can alternatively be controlled by post-growth irradiation with He + ions [14,15]. In both completely disordered and partially L1 0 ordered FePt films an enhancement of long range order was found after 130 keV He + ion irradiation at moderate processing temperatures. The post-growth irradiation process thus improves local order leaving the large-scale elemental distribution intact. Moreover, the initial crystallographic structure is maintained due to the absence of extended collision cascades. In view of the successful results for FePt and other binary systems [16,17], the question arises whether the light-ion irradiation technique is also applicable for the improvement of chemical order in Co 2 MnSi, representing, as a ternary compound, a more complex system.In this work, we investigate the effect of 30 keV He ions. The information about the ordering properties is obtained from X-ray diffraction (XRD), X-ray absorption and circular magnetic dichroism (XAS/XMCD), and photoemission spectroscopy at high energies (HAXPES). A (001)-oriented Co 2 MnSi layer of 30 nm thickness was grown on a Cr-buffered MgO(001) substrate by means of inductively coupled plasma (ICP) assisted magnetron sputtering. The chemical composition of the deposited film was nearly stoichiometric (Co: 48.9 %; Mn: 24.7 %; Si: 26.4 %). Annealing at 350• C followed the deposition of the Co 2 MnSi layer. Subsequently, a 1.3 nm Al capping layer was deposited to prevent oxidation of the Co 2 MnSi film. To ensure equal initial conditions for all irradiation experiments a single 1 in 2 Co 2 MnSi samp...

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