Magnetic Susceptibility Measurements at Ultra-low Temperatures

We report the design and operation of a device for ac magnetic susceptibility measurements that can operate down to 1 mK. The device, a modification of the standard mutual inductance bridge, is designed with detailed consideration of the thermalization and optimization of each element. First, in order to reduce local heating, the primary coil is made with superconducting wire. Second, a low-temperature transformer which is thermally anchored to the mixing chamber of a dilution refrigerator, is used to match the output of the secondary coil to a high-sensitivity bridge detector. The careful thermal anchoring of the secondary coil and the matching transformer is required to reduce the overall noise temperature and maximize sensitivity. The sample is immersed in liquid (3)He to minimize the Kapitza thermal resistance. The magnetic susceptibility of several magnetic compounds, such as the well-known spin gap compound NiCl(2)-4SC(NH(2))(2) and other powdered samples, have been successfully measured to temperatures well below 10 mK.

TRANSPORT IN A BILAYER SYSTEM AT HIGH LANDAU FILLING FACTOR

We have studied Shubnikov de Haas oscillations and the quantum Hall effect in GaAs-double well structures in tilted magnetic fields. We found strong magnetoresistance oscillations as a function of an in-plane magnetic field B(parallel to) at nu = 4N + 3 and nu = 4N + 1 filling factors. At low perpendicular magnetic field B(perpendicular to), the amplitude of the conventional Shubnikov-de Haas (SdH) oscillations also exhibits B(parallel to)-periodic dependence at fixed values of B(perpendicular to). We interpret the observed oscillations as a manifestation of the interference between cyclotron orbits in different quantum wells.

MAGNETO-OPTICAL ABSORPTION AND PHOTOMAGNETISM IN EUROPIUM CHALCOGENIDES

The absorption threshold in EuTe and EuSe was investigated as a function of applied magnetic field in the Faraday geometry. A well-resolved doublet of sharp dichroic lines was observed when the magnetic field induced ferromagnetic alignment of the spins in the crystal lattice. In contrast, at zero magnetic field only a broad and featureless absorption onset is seen. These results are fully explained in terms of a model of electronic transitions between localized states at the Eu lattice site and a tight-binding conduction band, which incorporates the formation of spin domains. Based on this model, predictions are made concerning the possibility of inducing magnetization of the spin lattices by illuminating the material with circularly polarized light.

QUANTUM HALL FERROMAGNET IN A DOUBLE WELL WITH VANISHING g-FACTOR.

We have studied the quantum Hall effect in Al(x)Ga(1-x)As-double well structure with vanishing g-factor. We determined the density-magnetic field n(s) - B diagrams for the longitudinal resistance R(xx). In spite of the fact that the n(s) - B diagram for conventional GaAs double wells shows a striking similarity with the theory, we observed the strong difference between these diagrams for double wells with vanishing g-factor. We argue that the electron-electron interaction is responsible for unusual behavior of the Landau levels in such a system.

MAGNETORESISTANCE OSCILLATIONS IN DOUBLE QUANTUM WELLS UNDER MICROWAVE IRRADIATION

We report in detail oscillatory magnetoresistance in double quantum wells under microwave irradiation. The experimental investigation contains measurements of frequency, power and temperature dependence. In theory, the observed interference oscillations are explained in terms of the influence of subband coupling on the frequency-dependent photoinduced part of the electron distribution function. Thus, the magnetoresistance shows the interference of magneto-intersubband and conventional microwave induced resistance oscillations.

VALLEY SPLITTING AND g-FACTOR IN AlAs QUANTUM WELLS

Here we present the results of magneto resistance measurements in tilted magnetic field and compare them with calculations. The comparison between calculated and measured spectra for the case of perpendicular fields enable us to estimate the dependence of the valley splitting as a function of the magnetic field and the total Lande g-factor (which is assumed to be independent of the magnetic field). Since both the exchange contribution to the Zeeman splitting as well as the valley splitting are properties associated with the 2D quantum confinement, they depend only on the perpendicular component of the magnetic field, while the bare Zeeman splitting depends on the total magnetic field. This information aided by the comparison between experimental and calculated gray scale maps permits to obtain separately the values of the exchange and the bare contribution to the g-factor.

MAGNETIC FIELD INDUCED NEAR-BAND-GAP OPTICAL PROPERTIES IN EuTe LAYERS

The magnetic response of the near-band-edge optical properties is studied in EuTe layers. In several magneto-optical experiments, the absorption and emission are described as well as the related Stokes shift. Specifically, we present the first experimental report of the photoluminescence excitation (PLE) spectrum in Faraday configuration. The PLE spectra shows to be related with the absorption spectra through the observation of resonance between the excitation light and the zero-field band-gap. A new emission line appears at 1.6 eV at a moderate magnetic field in the photoluminescence (PL) spectra. Furthermore, we examine the absorption and PL red-shift induced by the magnetic field in the light of the d-f exchange interaction energy involved in these processes. Whereas the absorption red-shift shows a quadratic dependence on the field, the PL red-shift shows a linear dependence which is explained by spin relaxation of the excited state.

Electrical transport in disordered and ordered magnetic domains under pressures and magnetic fields

Magnetic M( T, H, P) and electrical transport.( T, H, P) measurements in a strong spin-lattice-charge coupled La(0.7)Ca(0.3)MnO(3) system have been conducted. The application of H and P leads to the formation of different magnetic domain structures in the vicinity and below the polaronic-to-ferromagnetic transition temperature. The charge mobility is more sensitive to the variation of the spatial wave function overlap between Mn(3+) eg and O(2-) 2p orbitals due to the applied compacting pressure rather than the relative spin orientation between neighbouring Mn ions when the magnetic field is applied. In spite of the presence of different magnetic domain structures due to the sample history, the effect of magnetic field and pressure is less pronounced at lower temperatures on electrical transport properties.

The effect of additional Si and SiGe layers on the confinement potential of GeMn diluted ferromagnetic semiconductor

In this work we analyze the spin-polarized charge density distribution in the GeMn diluted ferromagnetic semiconductors (DFS). The calculations are performed within a self-consistent k.p method, in which the exchange correlation effects in the local density approximation, as well as the strain effects due to the lattice mismatch, are taken into account. Our findings show that the extra confinement potential provided by the barriers and the variation of the Mn content in the DFS are responsible for a separation between the different spin charge densities, giving rise to higher mobility spin-polarized currents or high ferromagnetism transition temperatures systems. (c) 2008 Elsevier B.V. All rights reserved.

Electronic, vibrational and related properties of group IV metal oxides by ab initio calculations

We present our theoretical results for the structural, electronic, vibrational and optical properties of MO(2) (M = Sn, Zr, Hf and Ti) obtained by first-principles calculations. Relativistic effects are demonstrated to be important for a realistic description of the detailed structure of the electronic frequency-dependent dielectric function, as well as of the carrier effective masses. Based on our results, we found that the main contribution of the high values calculated for the oxides dielectric constants arises from the vibrational properties of these oxides, and the vibrational static dielectric constant values diminish with increasing pressure. (c) 2008 Elsevier B.V. All rights reserved.

Interparticle Interactions Effects on the Magnetic Order in Surface of Fe(3)O(4) Nanoparticles

We report interparticle interactions effects on the magnetic structure of the surface region in Fe(3)O(4) nanoparticles. For that, we have studied a desirable system composed by Fe(3)O(4) nanoparticles with (d) = 9.3 nm and a narrow size distribution. These particles present an interesting morphology constituted by a crystalline core and a broad (similar to 50% vol.) disordered superficial shell. Two samples were prepared with distinct concentrations of the particles: weakly-interacting particles dispersed in a polymer and strongly-dipolar-interacting particles in a powder sample. M(H, T) measurements clearly show that strong dipolar interparticle interaction modifies the magnetic structure of the structurally disordered superficial shell. Consequently, we have observed drastically distinct thermal behaviours of magnetization and susceptibility comparing weakly- and strongly-interacting samples for the temperature range 2 K < T < 300 K. We have also observed a temperature-field dependence of the hysteresis loops of the dispersed sample that is not observed in the hysteresis loops of the powder one.

Luminescence studies on nitride quaternary alloys double quantum wells

We present theoretical photoluminescence (PL) spectra of undoped and p-doped Al(x)In(1-xy)Ga(y)N/Al(X)In(1) (X) (Y)Ga(Y)N double quantum wells (DQWs). The calculations were performed within the k.p method by means of solving a full eight-band Kane Hamiltonian together with the Poisson equation in a plane wave representation, including exchange-correlation effects within the local density approximation. Strain effects due to the lattice mismatch are also taken into account. We show the calculated PL spectra, analyzing the blue and red-shifts in energy as one varies the spike and the well widths, as well as the acceptor doping concentration. We found a transition between a regime of isolated quantum wells and that of interacting DQWs. Since there are few studies of optical properties of quantum wells based on nitride quaternary alloys, the results reported here will provide guidelines for the interpretation of forthcoming experiments. (C) 2008 Elsevier B.V. All rights reserved.

Infrared emissions, visible up-conversion, thermoluminescence and defect centres in Er(3)Al(5)O(12) phosphor obtained by solution combustion reaction

The Er(3)Al(5)O(12) phosphor powders were prepared using the solution combustion method. Formation and homogeneity of the Er(3)Al(5)O(12) phosphor powders have been verified by X-ray diffraction and energy-dispersive X-ray analysis respectively. The frequency up-conversion from Er(3)Al(5)O(12) phosphor powder corresponding to the (2)H(9/2) -> (4)I(15/2), (2)H(11/2) -> (4)I(15/2), (4)S(3/2) -> (4)I(15/2), (4)F(9/2) -> (4)I(15/2) and the infrared emission (IR) due to the (4)I(13/2) -> (4)I(15/2) transitions lying at similar to 410, similar to 524, similar to 556, 645-680 nm and at similar to 1.53 mu m respectively upon excitation with a Ti-Sapphire pulsed/CW laser have been reported. The mechanism responsible for the frequency up-conversion and IR emission is discussed in detail. Defect centres induced by radiation were studied using the techniques of thermoluminescence and electron spin resonance. A single glow peak at 430A degrees C is observed and the thermoluminescence results show the presence of a defect center which decays at high temperature. Electron spin resonance studies indicate a center characterized by a g-factor equal to 2.0056 and it is observed that this center is not related to the thermoluminescence peak. A negligibly small concentration of cation and anion vacancies appears to be present in the phosphor in accordance with the earlier theoretical predictions.

Infrared emission and defect centres in Er and Yb codoped Y(3)Al(5)O(12) phosphors

YAG phosphor powders doped/codoped with Er(3+)/(Er(3+) + Yb(3+)) have been synthesised by using the solution combustion method. The effect of direct pumping into the (4)I(11/2) level under 980 nm excitation of doped/codoped Er(3+)/Yb(3+)-Er(3+) in Y(3)Al(5)O(12) (YAG) phosphor responsible for an infrared (IR) emission peaking at similar to 1.53 mu m corresponding to the (4)I(13/2)->(4)I(15/2) transition has been studied. YAG exhibits three thermally-stimulated luminescence (TSL) peaks at around 140A degrees C, 210A degrees C and 445A degrees C. Electron spin resonance (ESR) studies were carried out to identify the centres responsible for the TSL peaks. The room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0176 is identified as O(-) ion, while centre II with an isotropic g-factor 2.0020 is assigned to an F(+) centre (singly ionised oxygen vacancy). An additional defect centre is observed during thermal-annealing experiments and this centre (assigned to F(+) centre) seems to originate from an F-centre (oxygen vacancy with two electrons) and these two centres appear to correlate with the observed high-temperature TSL peak in YAG phosphor.

Infrared luminescence, thermoluminescence and defect centres in Er and Yb co-doped ZnAl(2)O(4) phosphor

Er and Yb co-doped ZnAl(2)O(4) phosphors were prepared by solution combustion synthesis and the identification of Er and Yb were done by energy-dispersive X-ray analysis (EDX) studies. A luminescence at 1.5 mu m, due to the (4)I(13/2) ->(4)I(15/2) transition, has been studied in the NIR region in Er and Yb co-doped ZnAl(2)O(4) phosphors upon 980 nm CW pumping. Er-doped ZnAl(2)O(4) exhibits two thermally stimulated luminescence (TSL) peaks around 174A degrees C and 483A degrees C, while Yb co-doped ZnAl(2)O(4) exhibits TSL peaks around 170A degrees C and 423A degrees C. Electron spin resonance (ESR) studies were carried out to identify defect centres responsible for TSL peaks observed in the phosphors. Room temperature ESR spectrum appears to be a superposition of two distinct centres. These centres are assigned to an O(-) ion and F(+) centre. O(-) ion appears to correlate with the 174A degrees C TSL peak and F(+) centre appears to relate with the high temperature TSL peak at 483A degrees C in ZnAl(2)O(4):Er phosphor.