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.

Ultrasonic standing waves, excited in an (111) crystal through magnetoelastic coupling, are visualized using synchrotron radiation diffraction imaging (`topography'). This observation relies on the possibility at the ESRF of recording images with long sample-to-film propagation distances (up to 1.5 m) without substantial loss of resolution. The crystal acts as an x-ray focusing lens periodically in time, an effect that strongly depends both on the amplitude of the magnetic field excitation and on the propagation distance. The standing-wave-related contrast is hardly visible close to the sample, very sharp for a focusing distance (typically ) whereas beyond this position the intensity maxima broaden due to the defocusing effect. We propose a model which accounts for the presented results, allows the understanding of the details of the resonance patterns and gives access to the main characteristics of the vibration: amplitude, shape, polarization, wavelength and sound velocity. This effect could be used to simultaneously monochromatize and focus the x-ray beam simultaneously.

We report results of investigations of the structural and transport properties of GaAs/Ga1−xInxAs/GaAs quantum wells (QWs) having a 0.5–1.8 monolayer (ML) thick Mn layer, separated from the QW by a 3 nm thick spacer. The structure has hole mobility of about 2000 cm2 (V s)−1, being by several orders of magnitude higher than in known ferromagnetic two-dimensional (2D) structures. The analysis of the electro-physical properties of these systems is based on detailed study of their structure by means of high-resolution x-ray diffractometry and glancing-incidence reflection, which allow us to restore the depth profiles of the structural characteristics of the QWs and thin Mn-containing layers. These investigations show the absence of Mn atoms inside the QW. The quality of the structures was also characterized by photoluminescence spectra from the QWs. The transport properties reveal features inherent to ferromagnetic systems: a specific maximum in the temperature dependence of the resistance and the anomalous Hall effect (AHE) observed in samples with both ‘metallic’ and activated types of conductivity up to ∼100 K. AHE is most pronounced in the temperature range where the resistance maximum is observed. The results are discussed in terms of the interaction of 2D-holes and magnetic Mn ions in the presence of large-scale potential fluctuations related to the random distribution of Mn atoms. The AHE values are compared with calculations taking into account the ‘intrinsic’ mechanism in ferromagnetic systems.

Ultrasonic waves, excited in an FeBO3 crystal via magneto-elastic coupling, were visualized using synchrotron radiation diffraction imaging (`topography'). In particular, restricted volume images were used in order to distinguish the contribution of the various parts of a thin (50 µm) sample to the diffracted beam. The magnetic field dependence of the sound velocity was then exploited to tune the FeBO3 crystal at a resonance four times the ESRF single bunch frequency, allowing stroboscopic section imaging and the deconvolution of the time-integration effect. This set of experiments confirmed that the vibrating crystal model used to explain the focusing of x-rays in FeBO3 at certain resonances is correct.

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