[en] Metamagnets have been playing an important role as model systems in the physics of phase transitions for 30 years. Especially the isolating substances FeCl₂ and FeBr₂ have attracted much interest both in experimental and theoretical physics. In this thesis we focus on experimental investigations of Fe_1-xMg_xBr₂ with x = 0 and 0.05. By SQUID-magnetometry, SQUID-susceptometry, elastic neutron scattering and calorimetry we studied the magnetic phase boundaries and especially the recently found new phase transition at H=H1(T) (Aruga Katori et al. 1996). Also we arrived to a physical understanding of the non-critical fluctuations at H=H-(T) in terms of a smeared 2-dimensional phase transition of the spin down sublattice

[en] Magnetization relaxation measurements show that aging occurs in a discontinuous metal-insulator multilayer (DMIM) [Co₈₀Fe₂₀(0.9 nm)/Al₂O₃(3 nm)]₁₀ below the spin glass transition temperature, T_g∼44 K. Furthermore, the DMIM system memorizes the structure of a quasi-equilibrium state reached after an intermittent stop-and-wait protocol during cooling. These results unambiguously corroborate the collective nature of the low temperature superspin dynamics

[en] Magnetization hysteresis and AC susceptibility measurements were performed on the discontinuous metal-insulator multilayer [Co₈₀Fe₂₀(1.4 nm)/Al₂O₃(3 nm)]₁₀. CoFe forms ferromagnetic single-domain particles being embedded in the Al₂O₃ matrix. Due to strong dipolar inter-particle interactions a 'superferromagnetic' correlation of particle moments was previously stated. The dynamical magnetic properties of the superferromagnet can be understood in the framework of domain wall motion in an impure ferromagnet. Kinetic simulations are employed, which are based on field and temperature-dependent domain wall velocities. The magnetization hysteresis as well as the AC susceptibility, χ'-iχ'', are calculated and compared to our experimental results and yield a good agreement. The analysis of the Cole-Cole plots, χ'' vs. χ', is particularly interesting. They are shown to serve as fingerprints for the different dynamic behavior as claimed previously

[en] Discontinuous metal-insulator multilayers [Co₈₀Fe₂₀(t)/Al₂O₃(3 nm)]₁₀ are prepared by ion-beam sputtering and investigated by SQUID magnetometry and AC susceptometry. For 1.0≤t≤1.6 nm, two coexisting subsystems of dipolar coupled nanoparticles are encountered. While a spin-glass-like freezing particulate fraction dominates at t=1.0 nm, a percolating dipolar ferromagnetic fraction becomes increasingly important at higher nominal thickness of t

[en] We have used the unique spatial sensitivity of polarized neutron and soft x-ray beams in reflection geometry to measure the depth dependence of magnetization across the interface between a ferromagnet and antiferromagnet. The new uncompensated magnetization near the interface responds to applied field, while the uncompensated spins in the antiferromagnetic bulk are pinned, thus providing a means to establish exchange bias

[en] Relaxation and temperature cycles of thermoremanent magnetization, M^TRM, in the superspin-glass phase of [Co₈₀Fe₂₀(0.9 nm)/Al₂O₃(3 nm)]₁₀ have been investigated. The relaxation of M^TRM exhibits ageing phenomena. In negative temperature cycles for temperature steps larger than 1 K the magnetic state is retrieved (memory effect) on returning to the measurement temperature. This property is independent of the application of a field step during intermediate cooling. In positive temperature cycles the relaxation is suppressed after temporary heating. The observations are discussed in the light of both the droplet and the hierarchical picture. (author)

[en] Polarized neutron reflectivity (PNR) and magnetometry studies have been performed on the metal-insulator multilayer [Co₈₀Fe₂₀(1.6 nm)/Al₂O₃(3 nm)]₉ which exhibits dominant dipolar coupling between the ferromagnetic layers. Our PNR measurements at the coercive field reveal a novel and unexpected magnetization state of the sample, exhibiting an oscillating magnetization depth profile from CoFe layer to CoFe layer with a period of five bilayers along the multilayer stack. With the help of micromagnetic simulations we demonstrate that competition between long- and short-ranged dipolar interactions apparently gives rise to this unprecedented phenomenon

[en] We prepared monolayers of iron oxide nanoparticles via self-assembly on a bare silicon wafer and on a vanadium film sputter deposited onto a plane sapphire substrate. The magnetic configuration of nanoparticles in such a dense assembly was investigated by polarized neutron reflectivity. A theoretical model fit shows that the magnetic moments of nanoparticles form quasi domain-like configurations at remanence. This is attributed to the dipolar coupling amongst the nanoparticles. (paper)

[en] We report on the fabrication and characterization of iron oxide nanoparticle thin film superlattices. The formation into different film morphologies is controlled by tuning the particle plus solvent-to-substrate interaction. It turns out that the wetting vs dewetting properties of the solvent before the self-assembly process during solvent evaporation plays a major role in determining the resulting film morphology. In addition to layerwise growth three-dimensional mesocrystalline growth is also evidenced. The understanding of the mechanisms ruling nanoparticle self-assembly represents an important step towards the fabrication of novel materials with tailored optical, magnetic or electrical transport properties. (papers)

[en] Polarized neutron reflectivity (PNR) and magnetometry studies have been performed on the granular multilayer [Co₈₀Fe₂₀(1.3 nm)/Al₂O₃(3 nm)]₁₀. Due to strong interparticle interactions, a collective superferromagnetic state is encountered. Cole-Cole plots drawn from the complex ac susceptibility are measured as functions of frequency, temperature, and field amplitudes that hint at the relaxation, creep, sliding, and switching regimes of pinned domain walls that are in close agreement with results obtained from simulations. Very slow switching with exponential relaxation under near-coercive fields is confirmed by PNR measurements. The complete absence of spin-flip scattering confirms that the magnetization reversal is achieved merely by domain nucleation and growth