We report on the experimental observation of the quantum oscillations in microwave magnetoabsorption of a high-mobility two-dimensional electron gas induced by Landau quantization. Using original resonance-cavity technique, we observe two kinds of oscillations in the magnetoabsorption originating from inter-Landau-level and intra-Landau-level transitions. The experimental observations are in full accordance with theoretical predictions. Presented theory also explains why similar quantum oscillations are not observed in transmission and reflection experiments on high-mobility structures despite of very strong effect of microwaves on the dc resistance in the same samples.

Microwave (MW) absorption by a high mobility 2DEG has been investigated experimentally using sensitive Electron Paramagnetic Resonance (EPR) cavity technique. It is found that MW absorption spectra are chiefly governed by confined magnetoplasmon excitations in a 2DEG stripe. Spectra of the 2D magnetoplasmons are studied as a function of magnetic field, MW frequency and carrier density. The electron concentration is tuned by illumination and monitored using optical photoluminescence technique.

A frequency tuneable (40 – 60 GHz) EPR spectrometer was used for direct measurements of microwave power absorption of a high mobility two‐dimensional electron gas (2DEG) in GaAs/GaAlAs heterojunctions. The spectra reflect a relatively broad absorption band (due to cyclotron resonance absorption), with superimposed sharp features attributed to confined plasmon modes. The influence of the sample size on the sequence of the observed plasmon modes is investigated.

The frequency dependence of microwave-induced resistance oscillations (MIROs) has been studied experimentally in high-mobility electron GaAs/AlGaAs structures to explore the limits at which these oscillations can be observed. It is found that in dc transport experiments at frequencies above 120 GHz, MIROs start to quench, while above 230 GHz, they completely disappear. The results will need to be understood theoretically but are qualitatively discussed within a model in which forced electronic charge oscillations (plasmons) play an intermediate role in the interaction process between the radiation and the single-particle electron excitations between Landau levels.

In this paper, we study the behavior of a high mobility two dimensional electron gas under microwave irradiation by means of magneto-photoluminescence (PL) and absorption measurements. The high mobility sample investigated is a 15nm wide GaAs/AlGaAs quantum well with an electron concentration between 1-2×1011 cm -2, tunable by visible-light illumination. Structures in the microwave absorption at 40-60GHz are identified as geometrically confined magneto-plasmons.

The results of magnetic resonance investigation of GaMnAs samples prove that metal to insulator transition is accompanied by changing magnetic order from ferro- to ferrimagnetic. The observation of spin wave resonance (SAIR) in ferrimagnetic samples let us to evaluate the range of exchange interaction, the range of spin correlations and the magnitude of local fluctuations. The observed range of exchange interaction is related to the length of hole localization. Dispersion dependence of spin waves (SW) in ferrimagnetic Ga1-xMnxAs qualitatively differs from the dispersion dependence of SW in classical ferromagnets. That difference is also explained. We relate the origin of uniaxial anisotropy field to the layer inhomogeneity.

Magnetic resonance studies allow us to distinguish paramagnetic, ferromagnetic and ferrimagnetic phases in Ga1−xMnxAs. The transition from ferromagnet to ferrimagnet is correlated with a metal to insulator transition. The analysis of spin wave resonance spectra, which occur in the ferrimagnetic phase, allows us to estimate the magnitude and the distance dependence of exchange coupling. The experimentally evaluated long range of exchange causes an effective averaging of the fluctuation of exchange interactions. As a consequence, in the semimetallic phase both spin subsystems coherently precess forming the ferrimagnetic structure. In the insulating phase, fluctuations of the local exchange field lead to a fast decoherence of the carrier spins and only the localized Mn spins form the ferromagnetic moment.

We compare the results of electrically detected magnetic resonance in a 2D electron gas in Si/SiGe quantum wells with transport and magnetic resonance measurements on ferromagnetic Ga1-xMnxAs. The results lead us to the conclusion that observation of electrically detected magnetic resonance is possible only in the case of a slow spin relaxation, where the microwave resonant absorption leads to a noticeable change of spin magnetization.

Monocrystalline thin layers of (Eu,Gd)Te, n-type ferromagnetic semiconductor, were grown by molecular beam epitaxy technique on BaF2 (111) substrates. Reflection high-energy electron diffraction, X-ray diffraction, and atomic force microscopy characterization proved epitaxial mode of growth and high crystal quality of the layers. Magnetic susceptibility and magnetic resonance measurements showed that in (Eu,Gd)Te layers ferromagnetic transition takes place at about 13 K. Electrical characterization carried out by the Hall effect and resistivity measurements revealed very high electron concentration of 10(20) cm(-3) and sharp maximum of resistivity at transition temperature.

Different types of magnetic resonance observed in Ga1-xMnxAs reflect three different magnetic phases: para-, ferro-, and ferrimagnetic. Ferromagnet is characterized by single isotropic resonance line. A complex spectrum in ferrimagnet can be described by g factor equal to 1.44 and a sum of an axial and cubic anisotropy field. The axial field is by an order of magnitude greater than the cubic one. The complex structure of ferrimagnetic resonance is attributed to spin-wave resonance. Quantitative analysis of the dispersion of spin wave shows that the range of exchange coupling is very long, of the order of 25 nm, while spin-wave stiffness and the total exchange field are very small. The exchange field as evaluated from spin wave is by two orders of magnitude smaller than the Zener field corresponding to the critical temperature.

MBE grown Ga 1− x Mn x As layers were investigated by means of magnetic resonance techniques. Two phases can be distinguished: an almost isotropic ferromagnetic phase in insulating layers and an anisotropic ferromagnetic phase in the metallic Ga 1− x Mn x As. Under a strong magnetic field the field-induced insulator-to-metal transition is accompanied by the change from the ferromagnetic to the ferrimagnetic phase.

Molecular beam epitaxy grown Ga 1 - x Mn x As layers were investigated by means of magnetic resonances. With an increase in Mn concentration, x, the spectrum changes from the (i) paramagnetic one, with resolved fine and hyperfine structures, typical of S = 5/2 spin of substitutional Mn 2 + ions, for very diluted alloy, via (ii) paramagnetic spectrum, where the fine and hyperfine structures are averaged by a long range Mn 2 + -Mn 2 + exchange coupling, (iii) single, isotropic line of ferromagnetic resonance. Insulator to metal transition is accompanied with occurrence of (iv) a very complex spectrum of the ferrimagnetic resonance, accompanied with the well-resolved spin wave resonance. Reentrance to insulator phase for the most condensed alloys is accompanied with the reentrance to (v) ferromagnetic phase. The data confirm that the effective mass holes transfer the exchange interaction between localized Mu 2 + spins.

Electron paramagnetic resonance (EPR) study of MBE grown Mn doped GaAs is presented. The resolved fine structure allows us to evaluate the crystal field parameters of the spin Hamiltonian. The obtained cubic constant is a= -14.1 10^-4/cm. The axial field parameter, D, increases with Mn concentration, x, i.e., with the strain of (Ga,Mn)As layers. Extrapolation of D shows that the single ion anisotropy is the important contribution to the magnetic anisotropy which is observed in ferromagnetic layers with greater Mn concentrations. The analysis of the EPR linewidth shows that native defects of the concentration of 5 10^19/cm^3, but not the Mn ions, are the main origin of crystal field fluctuations.