The structural and transport properties of GaAs/Mn/GaAs/InxGa1−xAs/GaAs quantum wells (x≈0.2) with Mn δ-layer (4–10 at. %), separated from the well by a GaAs spacer, have been studied. The hole mobility in the investigated structures has exceeded the values known for magnetic III-V heterostructures by two orders of magnitude. For structures with the conductivity of the metal type, we have succeeded to observe at low temperatures Shubnikov–de Haas oscillations just confirming the two dimensionality (2D) of the hole energy spectrum. Exactly those 2D holes promote the ferromagnetic ordering of the Mn layer. That has been proven by (i) observing maxima (at 25–40 K) in temperature dependencies of the resistance, which positions agree with calculated values of Curie temperatures (for structures with the indirect interaction of Mn atoms via 2D holes), and (ii) revealing the negative spin-dependent magnetoresistance (NMR) as well as the anomalous Hall effect (AHE), which values are also in good agreement with calculations relating to ferromagnetic 2D III-V systems. As for the structures with the insulator type of the conductivity, their NMR and AHE features evidence the phase separation—the sample fragmentation with the formation of mesoscopic ferromagnetic areas separated by paramagnetic strata of the high tunnel conductivity.
This paper deals with the extension of the well known Stoner–Wohlfarth (SW) model widely used to compare magnetic properties of real single-domain particle systems with its ideal predictions. The model is often discussed in connection with nanomagnetism. The extension of this successful SW model is gained by combining it with Néel’s ideas concerning the dynamical behavior and relaxation of the magnetization in such systems. We present the derivation of a universal relaxation equation which holds for the populations of the SW energy levels defined by the SW model. By solving this differential equation with properly chosen initial conditions, a number of magnetization phenomena observed experimentally versus temperature, time, and external magnetic fields can be understood and described quantitatively. So, hysteresis loops, including those in high-frequency external magnetic fields, can be calculated within this model as a function of temperature, and demagnetization curves for arbitrary heating rates in different external magnetic fields can be simulated. In contrast to the difficulties encountered when treating the experimental data within more general stochastic models based on the Landau–Lifshitz–Gilbert equation, one can easily fit to a first approximation a wide set of data taken from the same sample within the extended SW model. The well known Henkel and Thamm–Hesse plots are reviewed and it is shown that by using these for plotting experimental data deviations from the ideal SW behavior and influences caused by relaxation can be detected. The plot recently proposed by Michele–Hesse–Bremers is shown not to be sensitive to relaxation influences and therefore reveals only the particle–particle interaction.
We present a way to analyze the chemical composition of periodical multilayer structures using the simultaneous analysis of grazing incidence hard X-Ray reflectivity (GIXR) and normal incidence extreme ultraviolet reflectance (EUVR). This allows to combine the high sensitivity of GIXR data to layer and interface thicknesses with the sensitivity of EUVR to the layer densities and atomic compositions. This method was applied to the reconstruction of the layered structure of a LaN/B multilayer mirror with 3.5 nm periodicity. We have compared profiles obtained by simultaneous EUVR and GIXR and GIXR-only data analysis, both reconstructed profiles result in a similar description of the layered structure. However, the simultaneous analysis of both EUVR and GIXR by a single algorithm lead to a ∼ 2x increased accuracy of the reconstructed layered model, or a more narrow range of solutions, as compared to the GIXR analysis only. It also explains the inherent difficulty of accurately predicting EUV reflectivity from a GIXR-only analysis.
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