[en] Textured nanocomposite magnets attract a lot of interest due to their potential of exhibiting higher energy products than those of currently existing rare earth-transition metal permanent magnets. In such nanostructured materials interaction domains are expected in contrast to classical magnetic domain structures that are commonly known from e.g. sintered magnets. Domain structures of this kind occur due to the magnetic coupling between grains whose sizes are about equal or smaller than the expected single-domain grain size. For hot-deformed samples made of melt-spun Nd₂Fe₁₄B-type ribbons the existence of interaction domains due to magnetostatic coupling has been reported. We choose this system to investigate the magnetic microstructure in dependence of a magnetic field along the initial magnetization curve. Images taken with Magnetic Force Microscopy are correlated to global hysteresis measurements. We show for the first time how interaction domains evolve in a bulk magnetic system from the thermally demagnetized state to a field of 6T. The growing/shrinking of the interaction domains happens from the domain border to the inside. Hardly any reversal within one domain is visible which affirms the collective nature of those domains. At high magnetic fields domains are left that are in the range of the grain size which implies a grain by grain switching.

[en] Magnetic thin films and multilayers display a fascinating variety of domain structures and magnetization processes due to competing interactions. For single layers one has to consider intra- and intergrain exchange interactions and magnetostatic interactions versus complex anisotropies. In multilayer systems, additional interlayer interactions (e.g. exchange bias, antiferromagnetic or ferromagnetic coupling) broaden the range of possible coupling scenarios and thus the diversity of the magnetic behavior. For a microscopic understanding of the magnetization processes we perform magnetic force microscopy (MFM), which offers a direct observation of the domain structure on a length scale down to 10 nm, and its evolution in a magnetic field. We present multilayers with perpendicular anisotropy and antiferromagnetic interlayer coupling of the type[(Co/Pt)_XCo/Ru]_N, which possess complex antiferromagnetically and ferromagnetically coupled domain structures, depending on temperature and magnetic history. As an example of a highly anisotropic single layer film with in-plane easy axis orientation, the magnetization process of a nanocrystalline epitaxial SmCo₅ film is investigated. Whereas the magnetizing process proceeds on a small length scale of 100 nm, in the demagnetizing process the switching occurs via a fast propagation of few interaction domain walls

[en] Highly coercive, epitaxial SmCo₅ films with unique in-plane alignment of the easy magnetization axis were prepared by pulsed laser deposition on Cr buffered MgO(110) single crystal substrates (μ0 H_c= 2 to 3 T). Despite the good orientation of neighboring grains, these films possess a very small scaled domain structure in the as prepared state. Magnetic force microscopy (MFM) studies have been performed in the remanent state on structured 10μm x 10μm elements after applying increasingly higher magnetic fields. The magnetization process proceeds slowly and without the formation of larger domains until fields exceed 1 T. Changes are most pronounced between 1.2 T and 1.6 T, i.e. about 0.6 H_c. Close to saturation, the element is almost fully magnetized, as seen from contrast free inner areas and charge build-up at the element edges, but for small isolated areas. We interpret this behavior as a pinning dominated magnetization process with a large pinning density. The defect distance is estimated from the size of the smallest observable isolated magnetic domains and is of the order of 50 nm for a film thickness of likewise value. This observation offers an explanation for the high coercivities of these well textured films and is of importance for the possible use in magnetic recording applications. There, a new concept of percolated media is based on granular, exchange coupled materials with high pinning density

[en] Successful cleaning and polishing of a set of YBa₂Cu₃O_7-δ(YBCO) thin films prepared by pulsed laser deposition (PLD) and chemical solution deposition (CSD) have been performed. The roughness of the films was reduced to a value of less than 5 nm, which opens a way to apply local surface sensitive techniques even on formerly very rough samples (some hundred nm peak-to-valley) such as CSD YBCO films. As one application flux lines of YBCO films were imaged with the omicron cryogenic SFM in MFM mode. The knowledge about geometry and distribution of artificial nanodefects in the interior of the film is crucial for further improvement of superconducting properties of these materials. The above mentioned polishing procedure has been further developed to prepare smooth low angle wedges of such samples. This offers the possibility to obtain depth dependent information with different surface sensitive scanning techniques. A high resolution electron backscattered diffraction image on the polished wedge of CSD YBCO sample reveals the homogeneous distribution of non superconducting BaHfO₃ nanoparticles in the whole volume of the film

No abstract available

[en] SmCo₅ thin films with a strong magnetic anisotropy are prepared epitaxially on Cr buffered MgO(110) and MgO(100) substrates either with a unique alignment of the c-axis throughout the film or with two perpendicular orientations of the easy axes in the film plane. For magnetic force microscopy (MFM) and magneto optical indicator film (MOIF)measurements, patterned elements were magnetized up to 4 T to adjust different remanent states and afterwards the domain structure was imaged. In the thermally demagnetized state magnetic contrast is visible on a length scale of 200 nm, which is about 4-5 times larger than the size of the individual grains. Thus, one has to describe the magnetic state by interaction domains originating from exchange or dipolar coupling of neighbouring grains. Upon applying an external field, the stray field contrast inside the element reduces and magnetic charges of opposite sign build up at the two edges of the squares perpendicular to the field direction. The domain structure resolved by MFM is compared with the quantitative stray field measurements obtained by MOIF

No abstract available

[en] The detailed study of the magnetization process is important for the understanding of the coupling mechanism of the recently developed AF-coupled [Co/Pt]/Ru multilayers. The field-dependent domain structure of a [(Co(4 A)/Pt(7 A))₈/Ru(9 A)]₁₈ multilayers was investigated by magnetic force microscopy (MFM). Measurements were performed using a DI Dimension 3100, upgraded for measurements with an external magnetic field applied perpendicular to the sample. In the zero field state, band domains with average domain width of 300 nm are observed. Upon increasing the external magnetic field, the domains first modify gradually, without changing the general configuration, until they transform into a bubble domain structures at 0.4 T, which is about 80% of the saturation field of the sample. An analysis of individual field dependent line scans represents the first attempt towards finding a quantitative correlation between domain structure and net magnetization

[en] Using template-assisted electrodeposition in partially Au-covered polycarbonate templates, we have successfully deposited equiatomic Co–Pt nanotubes with an external radius R of 100 nm, lengths L of 800–2000 nm and tube-wall thicknesses of 70 nm. By applying different characterization methods – vibrating-sample magnetometry and combined atomic and magnetic force microscopy – we have shown that in as-deposited face-centred cubic Co–Pt nanotubes the magnetization in the remanent state can be described with a vortex-type of closure. Such curling of the magnetization distribution has its origin in the minimization of the demagnetizing field that arises from the surface or volume charges. The demagnetization of single-vortex-type nanotubes proceeds gradually and exhibits only a modest hysteresis. Such nanotubes with a single vortex-type of magnetization distribution possess the unique property of emitting no stray fields in the remanent state, despite their elongated shape, and therefore represent perfect candidates for magnetoresistive random-access-memory devices and for biomedical applications such as targeted drug delivery

[en] Quantitative interpretation of magnetic force microscopy (MFM) data is a challenge, because the measured signal is a convolution between the magnetization of the tip and the stray field emanated by the sample. It was established theoretically that the field distribution just above the surface of the superconductor can be well approximated by the stray field of a magnetic monopole. The description of the MFM tip, however, needs a second approximation. The temperature-dependent vortex-distribution images on a NbN thin film were fitted using two different tip models. Firstly, the magnetic tip was assumed to be a monopole that leads to the simple monopole-monopole model for the tip-sample interaction force. Performing a 2D fitting of the data with this model, we extracted λ, Δ and the vortex pinning force. Secondly, a geometrical model was applied to calculate the tip-transfer-function of the MFM tip using the numerical BEM method.