Synthesis and structural,magnetic characterization of Zn(Mn)O diluted magnetic semiconductor

The bulk samples with nominal composition Zn_1−x Mn_x O [x = 5% and 7%] were synthesized at 930 °C by Standard Solid State Reaction method. The structural analysis reveals the single phase nature. The Topography study indicates the distribution of the particles. Magnetic property was affirmed by Vibrating Sample Magnetometer, Zn_1−x Mn_x O (with x = 5%), low concentration of dopant shows good ferromagnetism compared to high concentration in Zn_1−x Mn_x O (with x = 7%).     

Photomaganetization in diluted magnetic semiconductors

Starting from a many–body Hamiltonian for a system of photogenerated electrons and holes, spin-split by magnetic ions in diluted magnetic semiconductors, we derive, presumably for the first time, an expression for the photomagnetization as a function of the photon power, frequency, excitonic interaction and the magnetic ion concentration. Damping of nonequilibrium carriers and spin excitons is considered phenomenologically. Our results agree qualitatively with some of the systematics of the photomagnetization observed in Hg _1?x Mn _x Te.     

Magnetic properties of diluted magnetic semiconductors

A review will be given of the magnetic characteristics of diluted magnetic semiconductors and the relation with the driving exchange mechanisms. II–VI as well as IV–VI compounds will be considered. The relevance of the long-range interaction and the role of the carrier concentration will be emphasized.     

Theory of diluted magnetic semiconductor ferromagnetism

We present a theory of carrier-induced ferromagnetism in diluted magnetic semiconductors ( III1-xMnxV) which allows for arbitrary itinerant-carrier spin polarization and dynamic correlations. Both ingredients are essential in identifying the system's elementary excitations and describing their properties. We find a branch of collective modes, in addition to the spin waves and Stoner continuum which occur in metallic ferromagnets, and predict that the low-temperature spin stiffness is independent of the strength of the exchange coupling between magnetic ions and itinerant carriers. We discuss the temperature dependence of the magnetization and the heat capacity.     

Peculiar magnetic aftereffect of highly diluted frozen magnetic fluids

A very stably dispersed magnetic fluid (mother MF) and its 1000-times diluted solution were independently zero-field-cooled from room temperature to 5 K followed by application of a magnetic field of 2.86 MA/m for 300 s. After the field was removed (t=0)$(t=0)$, its residual magnetization M was measured as a function of time t for 80 000 s. After measurement, the MF sample was heated to room temperature, and the experiment was repeated after cooling to 5 K and again applying and removing the 2.86 MA/m field. We performed the same experiment several times, and obtained a different M vs t curve each time. With each cycle, the average M increased and the M vs t curve converged to a universal curve. In the initial few cycles, the value of M is very small, fluctuates and surprisingly increases with t in some time region. These characteristics are common in both the mother MF and diluted MF. We consequently propose the following physical model. When the MF is cooled, the isolated surfactant molecules in the solvent trigger the generation of magnetic colloid micelles. In other words, there occurs a phase transition from the magnetic colloids’ monodispersed phase to a micelle phase. The magnetic dipoles of the micelle's colloids make a closed magnetic flux loop. That is the origin of the anomalously small value of the residual magnetization in the early cycles. After a certain time elapses the micelles spontaneously break due to their residual stress, and a finite magnetic moment of the individual micelle develops. Consequently, M increases with t during this period.     

Modelling of experimentally measured Q-switched pulsations in InGaAs/GaAs diode lasers

A numerical calculation of a passively Q-switched two-section ridge-waveguide InGaAs/GaAs diode laser is presented in this study. The author has modelled the output power-current (L-I) variation under cw conditions, time evolution of photon numbers under transient conditions and Q-switched pulses of the device for various reverse bias voltages to the absorber section. Resulting simulations of the L-I characterisation and Q-switched pulsations are compared quantitatively with experimental results. Simulated Q-switched pulse profiles have been obtained in the absence and presence of noise. In both cases, proposed model shows that a tail occurring at the end of the Q-switched pulse is eliminated at −7.5 V reverse bias voltage, which is confirmed by experiment. As a result, experimentally obtained tail-free and single peak picosecond Q-switched pulses with peak powers of ∼1 W and durations of typically tens of picoseconds are also demonstrated theoretically. Simulations show consistency with the experimental data.     

Tilted magnetic field effect on the subbands of a GaAlAs diode with a GaAs quantum well

We have calculated the subbands in the GaAs quantum well at the n-side of the junction in a Ga_1−xAl_xAs diode. We show that the density of carriers confined in the quantum well increases by the increasing magnetic field strength but also decreases depending on the magnetic length. We have observed oscillatory behavior of the density of carriers due to the in-plane magnetic field.     

Griffiths phase in diluted magnetic semiconductors

We study the effects of disorder in the vicinity of the ferromagnetic transition in a diluted magnetic semiconductor in the strongly localized regime. We derive an effective polaron Hamiltonian, which leads to the Griffiths phase above the ferromagnetic transition point. The Griffiths-McCoy effects yield nonperturbative contributions to the dynamic susceptibility. We explicitly derive the long-time susceptibility, which has a pseudoscaling form, with the dynamic critical exponent being expressed through the percolation indices.     

Bound-magnetic-polaron molecule in diluted magnetic semiconductors

We formulate a complete microscopic theory of a coupled pair of bound magnetic polarons, the bound-magnetic-polaron molecule (BMPM) in a diluted magnetic semiconductor by taking into account both the proper two-body nature of the impurity-electron wavefunction and within the general spin-rotation-invariant approach to the electronic states. Also, the model takes into account both the Heisenberg and the antiferromagnetic kinetic-exchange interactions, as well as the ferromagnetic coupling within the common spin BMPM cloud. In this manner, we correct, unify and extend the weakly interacting BMP pair models of Wolff-Bhatt-Durst (2002 Phys. Rev. B 65 235205) and the model of nonoverlapping polarons considered by Angelescu and Bhatt (2002 Phys. Rev. B 65 75211). The resulting BMPM Hamiltonian is solved within the continuum-medium and the effective-mass approximations for the donor case and the thermodynamics is derived. In our approach the thermodynamic fluctuations of magnetization of the spins within BMPM are taken as Gaussian. It appears that the fluctuations can strongly stabilize the spin-triplet state, which may constitute a precursor effect of a ferromagnetic ordering in a many-impurity system.     

Magnetic percolation in diluted magnetic semiconductors

We demonstrate that the magnetic properties of diluted magnetic semiconductors are dominated by short ranged interatomic exchange interactions that have a strong directional dependence. By combining first principles calculations of interatomic exchange interactions with a classical Heisenberg model and Monte Carlo simulations, we reproduce the observed critical temperatures of a broad range of diluted magnetic semiconductors. We also show that agreement between theory and experiment is obtained only when the magnetic atoms are randomly positioned. This suggests that the ordering of diluted magnetic semiconductors is heavily influenced by magnetic percolation, and that the measured critical temperatures should be very sensitive to details in the sample preparation, in agreement with observations.     

Polaron percolation in diluted magnetic semiconductors

We theoretically study the development of spontaneous magnetization in diluted magnetic semiconductors as arising from a percolation of bound magnetic polarons. Within the framework of a generalized percolation theory we derive analytic expressions for the Curie temperature and the magnetization in the limit of low carrier density, obtaining excellent quantitative agreement with Monte Carlo simulation results and good qualitative agreement with experimental results.     

Electron states in diluted magnetic semiconductors

One of the remarkable properties of the II–VI diluted magnetic semiconductor (DMS) quantum dot (QD) is the giant Zeeman splitting of the carrier states under application of a magnetic field. This splitting reveals strong exchange interaction between the magnetic ion moment and electronic spins in the QD. A theoretical study of the electron spectrum and of its relaxation to the ground state via the emission of a longitudinal optical (LO) phonon, in a CdSe/ZnMnSe self-assembled quantum dot, is proposed in this work. Numerical calculations showed that the strength of this interaction increases as a function of the magnetic field to become more than 30 meV and allows some level crossings. We have also shown that the electron is more localized in this DMS QD and its relaxation to the ground state via the emission of one LO phonon is allowed.     

Magnetic properties of co-doped SnO2 diluted magnetic semiconductors

Polycrystalline samples of Sn_1−xCo_xO₂ are prepared for x=0.02, 0.05 and 0.10. They all exhibit room temperature ferromagnetism with decrease in saturation magnetization with increase in doping. The magnetization data recorded at 85 K, 295 K and 400 K for x ≥ 0.05 could be analyzed in terms of bound magnetic polaron model and the typical polaron concentration at room temperature is found to be in the order of 10²¹ per m⁻³.     

The magnetic behaviour of diluted CrAl alloys

Magnetic susceptibility as a function of temperature of binary CrAl alloys containing 0.7,1.07,1.35,1.59, 1.79,1.98, 2.18, 2.84, 3.74 and 4.36 at %A1 is investigated. Aluminium, by alloying, acts on the Néel temperature of pure chromium giving rise to a complicated phase diagram. The bahaviour of the magnetic susceptibility as a function of temperature, in the ordered phase, for the samples containing 1.28, 2.18 and 2.84 at %A1 is found to be temperature independent. In the paramagnetic region, for the samples containing between 1.07 and 2.84 at %A1, it is found a decrease of the magnetic susceptibility as a function of temperature.     

稀磁半导体的制备与性质

报道了稀磁半导体的制备、性质与实验研究进展，介绍了稀磁半导体的应用及发展前景．