This review focuses on the synthesis, protection, functionalization, and application of magnetic nanoparticles, as well as the magnetic properties of nanostructured systems. Substantial progress in the size and shape control of magnetic nanoparticles has been made by developing methods such as co-precipitation, thermal decomposition and/or reduction, micelle synthesis, and hydrothermal synthesis. A major challenge still is protection against corrosion, and therefore suitable protection strategies will be emphasized, for example, surfactant/polymer coating, silica coating and carbon coating of magnetic nanoparticles or embedding them in a matrix/support. Properly protected magnetic nanoparticles can be used as building blocks for the fabrication of various functional systems, and their application in catalysis and biotechnology will be briefly reviewed. Finally, some future trends and perspectives in these research areas will be outlined.

Antiferromagnetic Co3O4 nanowires with an average diameter of about 8 nm have been synthesized by using the nanocasting route. The nanowires were characterized by powder X-ray diffraction, transmission electron microscopy, and a superconducting quantum interference device magnetometer. The magnetic measurements show a unidirectional exchange anisotropy accompanied by an enhancement of the field-cooled coercivity at low temperature. These effects suggest the presence of an exchange interaction between the antiferromagnetic core and the surface spins. A strong dependence of the exchange field on the magnitude of the cooling field and a training effect have been observed. The dependence of exchange bias field on temperature reveals the important role played by the antiferromagnetic core in the exchange coupling.

Dieser Aufsatz beschreibt die Synthese, Stabilisierung, Funktionalisierung und Anwendung magnetischer Nanopartikel sowie die magnetischen Eigenschaften nanostrukturierter Systeme. Fortschritte hinsichtlich der Größen‐ und Formkontrolle magnetischer Nanopartikel wurden durch Methoden wie Mitfällung, thermische Zersetzung und/oder Reduktion, micellare Synthesestrategien und Hydrothermalsynthese erzielt. Entscheidend ist immer noch der Schutz gegen Korrosion, weshalb ein Schwerpunkt auf geeignete Schutzstrategien gelegt wird – z. B. Tensid/Polymer‐, Silicat‐ und Kohlenstoff‐Beschichtung magnetischer Nanopartikel sowie ihre Einbettung in eine Matrix oder ein Trägermaterial. Geschützte magnetische Nanopartikel können als Bausteine für verschiedene funktionale Systeme verwendet werden, und ihre Anwendung in Katalyse und Biotechnologie wird ebenfalls kurz diskutiert. Zum Schluss werden einige Trends auf diesen Forschungsgebieten umrissen und Ausblicke auf zukünftige Entwicklungen gegeben.

We report here for the first time on the synthesis and characterization of ordered mesoporous two-line ferrihydrite. Ordered mesoporous ferrihydrite has been prepared via the nanocasting route. Two types of ordered mesoporous silica, two-dimensional hexagonal SBA-15 and three-dimensional cubic KIT-6, were employed as hard templates. The magnetic behavior of ferrihydrite replicas with an average diameter of about 7 nm was investigated by direct current magnetometry. The temperature dependence of magnetization shows a superparamagnetic transition around 70 K. Field-induced changes in the low-field behavior of the magnetization were observed below 30 K. The results are explained assuming a spin-glass-like state of the surface spins. The 2D hexagonal two-line ferrihydrite showed very large coercivity up to 1.6 kOe at 5 K.