Magnetic and electrical transport properties of nanoparticle / thin film composites and random magnetic network systems

Ziel dieser Arbeit ist die Charakterisierung von magnetischen, nanostrukturierten System, welche mit Hilfe von Selbstorganisationsmethoden hergestellt wurden. Im ersten Teil wird das Kopplungsverhalten von Kompositsystem, bestehend aus einem magnetischen dünnen Film und magnetischen Nanopartikeln (NP) untersucht. Bei kleiner Variation des NP-Typs erhält man entweder eine sehr starke Kopplung oder ein nahezu entkoppeltes Verhalten. Ferner zeigt sich eine Verringerung der Blocking Temperatur aufgrund der magnetischen Kopplung. Im zweiten Teil wurden mittels Selbstorganisation zufällig geordnete magnetische Netzwerke hergestellt. Magneto-Transportmessungen zeigen einen deutlichen Einfluss der Herstellungsmethode auf die elektrischen Eigenschaften und führen zu einem charakteristischen Signal in Messungen des anisotropen Magnetowiderstandes (AMR). Zusätzlich wurde untersucht, ob diese Systeme einen Perkolationsübergang von elektrisch leitendem zu isolierendem Verhalten zeigen

Magnetic field-dependent spin structures of nanocrystalline holmium

The results are reported of magnetic field-dependent neutron diffraction experiments on polycrystalline inert-gas condensed holmium with a nanometre crystallite size (D = 33 nm). At T = 50 K, no evidence is found for the existence of helifan(3/2) or helifan(2) structures for the nanocrystalline sample, in contrast with results reported in the literature for the single crystal. Instead, when the applied field H is increased, the helix pattern transforms progressively, most likely into a fan structure. It is the component of H which acts on the basal-plane spins of a given nanocrystallite that drives the disappearance of the helix; for nanocrystalline Ho, this field is about 1.3 T, and it is related to a characteristic kink in the virgin magnetization curve. For a coarse-grained Ho sample, concomitant with the destruction of the helix phase, the emergence of an unusual angular anisotropy (streak pattern) and the appearance of novel spin structures are observed

Magnetic coupling mechanisms in particle/thin film composite systems

Magnetic γ-Fe2O3 nanoparticles with a mean diameter of 20 nm and size distribution of 7% were chemically synthesized and spin-coated on top of a Si-substrate. As a result, the particles self-assembled into a monolayer with hexagonal close-packed order. Subsequently, the nanoparticle array was coated with a Co layer of 20 nm thickness. The magnetic properties of this composite nanoparticle/thin film system were investigated by magnetometry and related to high-resolution transmission electron microscopy studies. Herein three systems were compared: i.e. a reference sample with only the particle monolayer, a composite system where the particle array was ion-milled prior to the deposition of a thin Co film on top, and a similar composite system but without ion-milling. The nanoparticle array showed a collective super-spin behavior due to dipolar interparticle coupling. In the composite system, we observed a decoupling into two nanoparticle subsystems. In the ion-milled system, the nanoparticle layer served as a magnetic flux guide as observed by magnetic force microscopy. Moreover, an exchange bias effect was found, which is likely to be due to oxygen exchange between the iron oxide and the Co layer, and thus forming of an antiferromagnetic CoO layer at the γ-Fe2O3/Co interface

Magnetic field dependent small-angle neutron scattering on a Co nanorod array: evidence for intraparticle spin misalignment

The structural and magnetic properties of a cobalt nanorod array have been studied by means of magnetic field dependent small-angle neutron scattering (SANS). Measurement of the unpolarized SANS cross section d[Sigma]/d[Omega] of the saturated sample in the two scattering geometries where the applied magnetic field H is either perpendicular or parallel to the wavevector ki of the incoming neutron beam allows one to separate nuclear from magnetic SANS, without employing the usual sector-averaging procedure. The analysis of the SANS data in the saturated state provides structural parameters (rod radius and centre-to-centre distance) that are in good agreement with results from electron microscopy. Between saturation and the coercive field, a strong field dependence of d[Sigma]/d[Omega] is observed (in both geometries), which cannot be explained using the conventional expression of the magnetic SANS cross section of magnetic nanoparticles in a homogeneous nonmagnetic matrix. The origin of the strong field dependence of d[Sigma]/d[Omega] is believed to be related to intradomain spin misalignment, due to magnetocrystalline and magnetoelastic anisotropies and magnetostatic stray fields