Magneto-optical properties of diluted magnetic semiconductor quantum dots

The magnetoexciton polaron (MP) is investigated theoretically in a diluted magnetic semiconductor quantum dot (QD), with the Coulomb interaction and the sp-d exchange interaction included. The MP energy decreases rapidly with increasing magnetic field at low magnetic field and saturates at high magnetic field for small QDs, and the dependences of the MP energy on magnetic field are quite different for different QD radii due to the different carrier-induced magnetic fields B-MP. The competition between the sp-d exchange interaction and the band gap shrinkage results in there being a maximum exhibited by the MP energy With increasing temperature. Our numerical results are in good agreement with experiment (Maksimov A A, Bacher G, MacDonald A, Kulakovskii V D, Forchel A, Becker C R, Landwehr G and Molenkamp L W 2000 Phys. Rev. B 62 R7767).

Electronic structure of diluted magnetic semiconductor superlattices: In-plane magnetic field effect

The electronic structure of diluted magnetic semiconductor (DMS) superlattices under an in-plane magnetic field is studied within the framework of the effective-mass theory; the strain effect is also included in the calculation. The numerical results show that an increase of the in-plane magnetic field renders the DMS superlattice from the direct band-gap system to the indirect band-gap system, and spatially separates the electron and the hole by changing the type-I band alignment to a type-II band alignment. The optical transition probability changes from type I to type II and back to type I like at large magnetic field. This phenomenon arises from the interplay among the superlattice potential profile, the external magnetic field, and the sp-d exchange interaction between the carriers and the magnetic ions. The shear strain induces a strong coupling of the light- and heavy-hole states and a transition of the hole ground states from "light"-hole to "heavy"-hole-like states.

Longitudinal spin transport in diluted magnetic semiconductor superlattices: The effect of the giant Zeeman splitting

Longitudinal spin transport in diluted magnetic semiconductor superlattices is investigated theoretically. The longitudinal magnetoconductivity (MC) in such systems exhibits an oscillating behavior as function of an external magnetic field. In the weak magnetic-field region the giant Zeeman splitting plays a dominant role that leads to a large negative magnetoconductivity. In the strong magnetic-field region the MC exhibits deep dips with increasing magnetic field. The oscillating behavior is attributed to the interplay between the discrete Landau levels and the Fermi surface. The decrease of the MC at low magnetic field is caused by the s-d exchange interaction between the electron in the conduction band and the magnetic ions. The spin polarization increases rapidly with increasing magnetic field and the longitudinal current becomes spin polarized in strong magnetic field. The effect of spin-disorder scattering on MC is estimated numerically for low magnetic fields and found to be neglectible for our system.

Oscillating magnetoresistance in diluted magnetic semiconductor barrier structures

Ballistic spin polarized transport through diluted magnetic semiconductor single and double barrier structures is investigated theoretically using a two-component model. The tunneling magnetoresistance (TMR) of the system exhibits oscillating behavior when the magnetic field is varied. An interesting beat pattern in the TMR and spin polarization is found for different nonmagnetic semiconductor/diluted magnetic semiconductor double barrier structures which arises from an interplay between the spin-up and spin-down electron channels which are split by the s-d exchange interaction.

Temperature and power-density-dependent inter-shell energy states in InAs/GaAs quantum dots

We have investigated the evolution of exciton state filling in InAs/GaAs quantum dot (QD) structures as a function of the excitation power density by using rnicro-photoluminescence spectroscopy at different temperatures. In addition to the emission bands of exciton recombination corresponding to the atom-like S, P and D, etc. shells of QDs, it was observed that some extra states V between the S and P shells, and D' between the P and D shells appear in the spectra with increasing number of excitons occupying the QDs at a certain temperature. The emergence of these inter-shell excitonic levels is power density and temperature dependent, which is an experimental demonstration of strong exciton-exciton exchange interaction, state hybridization, and coupling of a multi-exciton system in QDs. (c) 2006 Elsevier B.V. All rights reserved.

Magnetic Properties of FePt Nanoparticles Prepared by a Micellar Method

FePt nanoparticles with average size of 9 nm were synthesized using a diblock polymer micellar method combined with plasma treatment. To prevent from oxidation under ambient conditions, immediately after plasma treatment, the FePt nanoparticle arrays were in situ transferred into the film-growth chamber where they were covered by an SiO2 overlayer. A nearly complete transformation of L1(0) FePt was achieved for samples annealed at temperatures above 700 A degrees C. The well control on the FePt stoichiometry and avoidance from surface oxidation largely enhanced the coercivity, and a value as high as 10 kOe was obtained in this study. An evaluation of magnetic interactions was made using the so-called isothermal remanence (IRM) and dc-demagnetization (DCD) remanence curves and Kelly-Henkel plots (Delta M measurement). The Delta M measurement reveals that the resultant FePt nanoparticles exhibit a rather weak interparticle dipolar coupling, and the absence of interparticle exchange interaction suggests no significant particle agglomeration occurred during the post-annealing. Additionally, a slight parallel magnetic anisotropy was also observed. The results indicate the micellar method has a high potential in preparing FePt nanoparticle arrays used for ultrahigh density recording media.

PHOTOLUMINESCENCE INVESTIGATIONS OF CD1-XMNXTE UNDER HYDROSTATIC-PRESSURE

The photoluminescence of Cd1-xMnxTe with x=0.25, 0.40, and 0.60 is investigated at 77 K and different pressures. The pressure coefficients of the photoluminescence bands Cd0.75Mn0.25Te and Cd0.6Mn0.4Te are found to be positive and the magnitudes are about 8 X 10(-3) eV/kbar, which is in good agreement with the pressure coefficients of the interband transition. The pressure coefficient of the photoluminescence bands for Cd0.4Mn0.6Te is found to be -6 X 10(-3) eV/kbar, which is quite different from the pressure coefficient of the interband transition. The possible transition mechanism is discussed in terms of group theory and crystal field theory.

First-principles investigation of the electronic structure and magnetism of eskolaite

The electronic structure and magnetism of eskolaite are studied by using first-principles calculations where the on-site Coulomb interaction and the exchange interaction are taken into account and the LSDA+U method is used.The calculated energies of magnetic configurations are very well fitted by the Heisenberg Hamiltonian with interactions in five neighbour shells; interaction with two nearest neighbours is found to be dominant. The Neel temperature is calculated in the spin-3/2 pair-cluster approximation. It is found that the measurements are in good agreement with for the values of U and J that are close to those obtained within the constrained occupation method.The band gap is of the Mott-Hubbard type.

Solitary magnon excitations in a one-dimensional antiferromagnet with Dzyaloshinsky-Moriya interactions

We investigate solitary excitations in a model of a one-dimensional antiferromagnet including a single-ion anisotropy and a Dzyaloshinsky-Moriya antisymmetric exchange interaction term. We employ the Holstein-Primakoff transformation, the coherent state ansatz and the time variational principle. We obtain two partial differential equations of motion by using the method of multiple scales and applying perturbation theory. By so doing, we show that the motion of the coherent amplitude must satisfy the nonlinear Schrodinger equation. We give the single-soliton solution.

Structures and physical properties of n=3 Ruddlesden-Popper compounds Ca4Mn3-xNbxO10 (0 <= x <= 0.2)

The Ruddlesden-Popper series of compounds Ca4Mn3-xNbxO10(x = 0-0.2) have been prepared by solid-state methods. Structural, magnetic, electrical, and magnetoresistive studies were performed on the compounds. Nb doping caused increases in both unit cell volume and octahedral distortion. The magnetization measurements indicated that the doped samples displayed ferromagnetism-like behavior, which could be explained by the double-exchange interaction between Mn4+ and Mn3+ induced by the charge-compensation effect.

An investigation of low-field magnetoresistance in the double perovskites Sr2Fe1-xZnxMoO6, x=0, 0.05, 0.15 and 0.25

The electrical, magnetic and transport properties of Zn doped polycrystalline samples of Sr2Fe1-xZnxMoO6 ( x = 0, 0.05, 0.15 and 0.25) with the double perovskite structure have been investigated. The subtle replacement of Fe3+ ions by Zn2+ ions facilitates the formation of a more ordered structure, while further substitution leads to disordered structure because of the presence of a striped phase. Analysis of the x-ray powder diffraction patterns based on Rietveld analysis indicates that the replacement of Fe3+ by Zn2+ ions favours the formation of Mo6+ ions. The spin-glass behaviour can be explained on the basis of the competition between the antiferromagnetic superexchange and the ferromagnetic double-exchange interaction. The low-field magnetoresistance was moderately enhanced at x = 0.05, and its origin was found to be the competition between the decrease of the concentration of the itinerant electrons and the weaker antiferromagnetic superexchange in the antiphase boundaries. An almost linear negative magnetoresistance in moderate field has been observed for x = 0.25. A possible double-exchange mechanism is proposed for elucidating the observations; it also suggests a coexistence of (Fe3+, Mo5+) and (Zn2+, Mo6+) valence pairs.

Mieszanie stanów magnetycznych w skończonych układach spinowych opisywanych modelem Heisenberga

Wydział Fizyki: Zakład Fizyki Komputerowej

Reynolds Number Effects on Fully-Developed Pulsed Jets Impinging on Flat Surfaces

A systematic study of the effect of the Reynolds number on the fluid dynamics and turbulence statistics of pulsed jets impinging on a flat surface is presented. It has been suggested that the influence of the Reynolds number may be somewhat different for a jet subjected to pulsation when compared to an equivalent steady jet. A comparative study of both steady and pulsating jets is presented for a Reynolds number range from Re = 4;730 to Re = 10;000. All the other factors that affect the flowfield are kept constant, which are H/d = 3, St = 0.25, and d = 30.5 mm. It was found that for the range of the Reynolds numbers tested, pulsation results in a shortening of the jet core, the centerline axial velocity component declines more rapidly, and higher values of the radial velocity component for r/d > 0.75are observed. As the Reynolds number increases, the jet spreads more rapidly, the turbulent kinetic energy and nondimensional turbulent fluctuations decrease, and the flowfield near the impinging surface changes drastically, which is evident with the development of a turbulent momentum exchange interaction away from the wall for r/d > 1.5.

Magnetic properties of copper hexadecaphthalocyanine (F16CuPc) thin films and powders

The structural and magnetic properties of F16CuPc thin films and powder, including x-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometry, and theoretical modelling of exchange interactions are reported. Analysis of XRD from films, with thickness ranging between 100 and 160 nm, deposited onto Kapton and a perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) interlayer shows that the stacking angle (defined in the text) of the film is independent of the thickness, but that the texture is modified by both film thickness and substrate chemistry. The SQUID measurements suggest that all samples are paramagnetic, a result that is confirmed by our theoretical modelling including density functional theory calculations of one-dimensional molecular chains and Green's function perturbation theory calculations for a molecular dimer. By investigating theoretically a range of different geometries, we predict that the maximum possible exchange interaction between F16CuPc molecules is twice as large as that in unfluorinated copper-phthalocyanine (CuPc). This difference arises from the smaller intermolecular spacing in F16CuPc. Our density functional theory calculation for isolated F16CuPc molecule also shows that the energy levels of Kohn-Sham orbitals are rigidly shifted similar to 1 eV lower in F16CuPc compared to CuPc without a significant modification of the intramolecular spin physics, and that therefore the two molecules provide a suitable platform for independently varying magnetism and charge transport. 

New Building Blocks for Dual-Property Molecule-Based Magnets

Two classes of compounds have been prepared and characterized as building blocks for chiral magnets and ferromagnetic conductors. In the fIrst project, the organic framework of a pentadentate, (N302) macro cycle has been synthetically modifIed to introduce phenyl substituents into its organic framework and the synthesis of four new [Fe(In(N302)(CN)2] complexes (I) - (IV) is presented. [Molecular diagram availble in pdf] This work represents the fIrst structural and magnetic studies of a family of spin crossover macrocycles that comprise of both structural and stereo-isomers. Magnetic susceptibility and Mossbauer data for the R,R-complex (I) is consistent with both a thermal and a light induced spin crossover transition. The X-ray data supports a change in geometry accompanying the thermal spin transition, from a high spin (HS) 7 -coordinate complex at room temperature to a low spin (LS) 5-coordinate complex at 100 K. The crystal structure ofthe racemic complex (III) reveals a HS, 7-coordinate complex at 200 K that undergoes no signifIcant structural changes on cooling. In contrast, the magnetic - susceptibility and Mossbauer data collected on a powder sample of the racemic complex are consistent with a LS complex. Finally, the meso complex (IV) was prepared and its structure and magnetic properties are consistent with a 5-coordinate LS complex that remains low spin, but undergoes conformational changes on cooling in solution. The chiral [Fe(H)(N302)(CN)2] macro cycle (I), together with its Mn(H) and Fe(H) derivatives have also been exploited as building blocks for the self-assembly of chiral magnets. In the second project, a synthetic route for the preparation of tetrathiafulvalene (TTF) donors covalently attached to a diisopropyl verdazyl radical via a cross conjugated pyridyl linker IS presented. Following this strategy, four new TTF-py- (diisopropyl)verdazyl radicals have been prepared and characterized (V) - (VIII) . [Molecular diagram available in pdf] The first (2:1) charge transfer complex ofa TTF-py-(diisopropyl)verdazyl radical donor and a TCNQ acceptor has been prepared and structurally characterized. The crystal packing shows that the donor and acceptor molecules are organized in a mixed stacking arrangement consistent with its insulating behaviour. EPR and magnetic susceptibility data support intramolecular ferromagnetic interactions between the TTF and the verdazyl radicals and antiferromagnetic interactions between TTF donors within a stack. In an attempt to increase the intramolecular exchange interaction between the two radicals, a TTF-x-(diisopropyl)verdazyl radical (IX) was prepared, where the two radicals are connected ia a conjugated divinylene linker. The neutral radical donors stack in a more favourable head-to-head arrangement but the bulky isopropyl groups prevent the donor radicals from stacking close enough together to facilitate good orbital overlap. [Molecular diagram available in pdf].