24 research outputs found
Microwave magnetoplasmon absorption by a 2DEG stripe
Microwave absorption by a high mobility 2DEG has been investigated
experimentally using sensitive Electron Paramagnetic Resonance 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.Comment: to be published in International Journal of Modern Physics
Frequency quenching of microwave induced resistance oscillations in a high mobility two-dimensional electron gas
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.Comment: final version: total 13 pages including 6 figures (6 pages in journal
format
The influence of structural short-range order on the phase diagrams of diluted FCC magnet with arbitrary spin and modified RKKY interaction
A diluted FCC magnet with modified long-range RKKY interaction and arbitrary
Ising spin S is considered within two-sublattice model. In the molecular field
approximation the Gibbs free-energy is derived, from which all magnetic
thermodynamic properties can be self-consistently obtained. In particular, the
phase diagrams are studied for different magnetic ion and free-charge
concentration, the atomic short-range-order (Warren-Cowley) parameter being
taken into account.Comment: 12 pages, 6 figures, revtex4 fil
Anomalous behavior of spin wave resonances in Ga_{1-x}Mn_{x}As thin films
We report ferromagnetic and spin wave resonance absorption measurements on
high quality epitaxially grown Ga_{1-x}Mn_{x}As thin films. We find that these
films exhibit robust ferromagnetic long-range order, based on the fact that up
to seven resonances are detected at low temperatures, and the resonance
structure survives to temperatures close to the ferromagnetic transition. On
the other hand, we observe a spin wave dispersion which is linear in mode
number, in qualitative contrast with the quadratic dispersion expected for
homogeneous samples. We perform a detailed numerical analysis of the
experimental data and provide analytical calculations to demonstrate that such
a linear dispersion is incompatible with uniform magnetic parameters. Our
theoretical analysis of the ferromagnetic resonance data, combined with the
knowledge that strain-induced anisotropy is definitely present in these films,
suggests that a spatially dependent magnetic anisotropy is the most likely
reason behind the anomalous behavior observed.Comment: 9 pages, including 6 figure
Ferromagnetic semiconductors
The current status and prospects of research on ferromagnetism in
semiconductors are reviewed. The question of the origin of ferromagnetism in
europium chalcogenides, chromium spinels and, particularly, in diluted magnetic
semiconductors is addressed. The nature of electronic states derived from 3d of
magnetic impurities is discussed in some details. Results of a quantitative
comparison between experimental and theoretical results, notably for Mn-based
III-V and II-VI compounds, are presented. This comparison demonstrates that the
current theory of the exchange interactions mediated by holes in the valence
band describes correctly the values of Curie temperatures T_C magnetic
anisotropy, domain structure, and magnetic circular dichroism. On this basis,
chemical trends are examined and show to lead to the prediction of
semiconductor systems with T_C that may exceed room temperature, an expectation
that are being confirmed by recent findings. Results for materials containing
magnetic ions other than Mn are also presented emphasizing that the double
exchange involving hoping through d states may operate in those systems.Comment: 18 pages, 8 figures; special issue of Semicon. Sci. Technol. on
semiconductor spintronic
Ferromagnetic, ferrimagnetic and spin-wave resonances in GaMnAs layers
Molecular beam epitaxy grown Gal(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 Mn2+ ions, for very
diluted alloy, via (ii) paramagnetic spectrum, where the fine
and hyperfine structures are averaged by a long range Mn2+-Mn2+
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 Mn2+
spins
Magnetic order in semiconducting, ferromagnetic Ga1-xMnxAs
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
Magnetic order in GaMnAs layers
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