In optoelectronic devices based on quantum dot arrays, thin nanolayers of gold are preferred as stable metal contacts and for connecting recombination centers. The optimal morphology requirements are uniform arrays with precisely controlled positions and sizes over a large area with long range ordering since this strongly affects device performance. To understand the development of gold layer nanomorphology, the detailed mechanism of structure formation are probed with time-resolved grazing incidence small-angle X-ray scattering (GISAXS) during gold sputter deposition. Gold is sputtered on a CdSe quantum dot array with a characteristic quantum dot spacing of ≈7 nm. In the initial stages of gold nanostructure growth, a preferential deposition of gold on top of quantum dots occurs. Thus, the quantum dots act as nucleation sites for gold growth. In later stages, the gold nanoparticles surrounding the quantum dots undergo a coarsening to form a complete layer comprised of gold-dot clusters. Next, growth proceeds dominantly via vertical growth of gold on these gold-dot clusters to form an gold capping layer. In this capping layer, a shift of the cluster boundaries due to ripening is found. Thus, a templating of gold on a CdSe quantum dot array is feasible at low gold coverage.

Polarized neutron reflectivity ͑PNR͒ and magnetometry studies have been performed on the granular multilayer ͓Co 80 Fe 20 ͑1.3 nm͒ /Al 2 O 3 ͑3 nm͔͒ 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.

Magnetic multilayer structures of Co/Cu prepared by dc magnetron sputtering are studied with respect to changing number of bilayers (N) for different thicknesses of the Cu spacer layer corresponding to different coupling conditions according to the oscillatory interlayer exchange coupling. X-ray reflectivity and diffuse scattering show that the multilayers become smoother with increasing N. The growth exponent of the roughness is found to be lower for a multilayer than for a single-layer film of similar thickness. The roughness of subsequent interfaces along the stack is conformal, and the lateral correlation does not change with the period number, but depends on the thickness of the spacer layers. The improved layer structure for larger N increases the antiferromagnetic coupling fraction as inferred from magneto-optic Kerr effect measurements and thereby increases the giant magnetoresistance (GMR) ratio up to 35% for N = 10. Thus, the first few bilayers do not contribute to the GMR but act as a buffer to improve the growth conditions for the following bilayers. The first about five bilayers can be replaced by a bottom Co layer of equivalent thickness which also improves the layer structure for a subsequently deposited lower number of bilayers without much loss in the GMR ratio. This smoothening effect due to the increasing of the thickness of the bottom-most layer is related to the simultaneously decreasing grain size.

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We have studied the magnetization reversal process in continuous: ͓Co/ CoO͔ 20 and separated: ͓Co/ CoO / Au͔ 20 exchange-biased polycrystalline multilayers ͑MLs͒. For continuous ML, reversal proceeds sequentially starting with the bottom ͑top͒ Co layer for increasing ͑decreasing͒ field. Each Co layer remagnetizes symmetrically for both field branches in a nonuniform mode similarly as we have observed earlier for ͓IrMn/ CoFe͔ 3/10 MLs ͓Phys. Rev. B. 70, 224410 ͑2004͔͒. By polarized neutron reflectivity, we observe increasing exchange bias field strengths down the stack. However, usual asymmetric reversal is observed for the separated ML. We explain the different magnetization behavior within a simple and general model. The increased anisotropy energy for continuous ML is responsible for the nonuniform symmetric reversal as the angular dependencies for reversal are guided by the relative strengths of exchange, anisotropy, and Zeeman energies. Asymmetric hysteresis loops due to a different magnetization reversal process in different branches of the hysteresis loop are common 2-6 in exchange biased systems. Neutron scattering under grazing incidence with polarization analysis has been proven decisive for identification of the reversal mechanism. Two mechanisms can be distinguished: uniform magnetization reversal by magnetization rotation 3-6 and nonuniform magnetization reversal by domain nucleation and growth. Magnetization rotation is identified by a significant increase of the specular reflectivities in the spin-flip ͑SF͒ channels ͑R +− and R −+ ͒, which correspond to in-plane magnetization components perpendicular to the guiding field H a applied collinear to H FC . Reversal by domain nucleation and propagation ͑nonuniform magnetization reversal͒ does not provide enhanced SF intensities because the magnetization is always collinear to H a . Reversal mechanisms are observed for the Co/ CoO bilayer systems, 5,6 where the domain wall motion occurs for the decreasing ͑positive to negative͒ and magnetization rotation for the increasing ͑negative to positive͒ field sweeping direction of H a for the hysteresis loop with respect to negative direction of H FC . This behavior is just opposite to that reported in Ref. 4. Theoretically the interpretation of the magnetization reversal was discussed in Ref. 7 where it was shown that depending on , the angle between H FC and the AF anisotropy axis, the reversal mode is either by coherent rotation for both loop branches or asymmetric with a nonuniform reversal for the decreasing branch.Very recently, Paul et al. 8 have shown symmetric and sequential reversal for polycrystalline ͓Ir 20 Mn 80 ͑6.0 nm͒ /Co 80 Fe 20 ͑3.0 nm͔͒ 10 multilayers. Here sequential refers to a process, where different layers reverse their magnetization at different field strengths one after another. This reversal mode-symmetric, and nonuniformcorresponds to the situation = 0, considered unlikely to occur in experiments.7 Interestingly, however, the samples were multilayers ͑MLs͒ unlike the bilayer specimen...

The magnetization reversal in ͓Ir 20 Mn 80 /Co 80 Fe 20 ͔ 10 exchange-biased multilayers is studied by specular reflection and off-specular scattering of polarized neutrons. The reversal proceeds sequentially starting with the bottom (top) CoFe layer for decreasing (increasing) field due to the evolution of the grain size along the stack. Each CoFe layer remagnetizes symmetrically for both field branches in a nonuniform mode. Concomitant in-plane magnetization fluctuations revealed by off-specular spin-flip scattering indicate a more complex reversal mechanism than hitherto considered.

Topologically stabilized spin configurations like helices in the form of planar domain walls (DWs) or vortex-like structures with magnetic functionalities are more often a theoretical prediction rather than experimental realization. In this paper we report on the exchange coupling and helical phase characteristics within Dy-Fe multilayers. The magnetic hysteresis loops with temperature show an exchange bias field of around 1.0 kOe at 10 K. Polarized neutron reflectivity reveal (i) ferrimagnetic alignment of the layers at low fields forming twisted magnetic helices and a more complicated but stable continuous helical arrangement at higher fields (ii) direct evidence of helices in the form of planar 2π-DWs within both layers of Fe and Dy. The helices within the Fe layers are topologically stabilized by the reasonably strong induced in-plane magnetocrystalline anisotropy of Dy and the exchange coupling at the Fe-Dy interfaces. The helices in Dy are plausibly reminiscent of the helical ordering at higher temperatures induced by the field history and interfacial strain. Stability of the helical order even at large fields have resulted in an effective modulation of the periodicity of the spin-density like waves and subsequent increase in storage energy. This opens broad perspectives for future scientific and technological applications in increasing the energy density for systems in the field of all-spin-based engineering which has the potential for energy-storing elements on nanometer length scales.

We report on the carrier-induced ferromagnetism in Ge͑Mn,Fe͒ magnetic semiconductor insulating-type thin-film structures prepared using sequential deposition at T g = 520 K with subsequent annealing at T g . In the resulting films Mn and Fe are diffused in the Ge matrix without compromising the epitaxial structure. The anomalous Hall effect serves as a manifestation of the carrier-induced magnetism, with p-type conductivity and the Curie temperature T C = 209 K. The additional doping with Fe stabilizes epitaxial growth and carrier-mediated magnetism at levels of magnetic doping exceeding 10%. We conclude that indirect ferromagnetic exchange is mediated by localized holes with concentration n ϳ 10 20 cm −3 and mobility ϳ 10 cm 2 / ͑V s͒.