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The Skyrmion Hall Effect (SkHE) observed in ferromagnetic and ferrimagnetic (FI) skyrmions traveling due to a spin-polarized current can be a problematic issue when it comes to technological applications. By investigating the properties of FI skyrmions in racetracks through computational simulations, we have described the nature of their movement based on the relative values of the exchange, Dzyaloshinskii–Moriya, and anisotropy coupling constants. Beyond that, using a design strategy, a magnetic channel-like nano-device is proposed in which a spin-polarized current protocol is created to successfully control the channel on which the skyrmion will travel without the adverse SkHE. Additionally, a simple adjustment in the current strength can modify the skyrmion position sideways between different parallel channels in the nanostripe. [ABSTRACT FROM AUTHOR]

We investigate skyrmion configurations and dynamics in antiferromagnetic thin disks. It is shown that the skyrmion acquires oscillatory dynamics with well-defined amplitude and frequency, which may be controlled on demand by the spin-polarized current. Such a dynamics is also robust in the sense that an interface between two half-disks cannot change the dynamics appreciably whenever the exchange costs of these half-disks are not very different. Indeed, the skyrmion keeps its oscillatory motion despite crossing this interface but changes the shape of its core region in order to preserve its total energy. However, if exchange costs of the half-disks are quite distinct, the skyrmion is trapped in that half with less cost and cannot trespass the interface barrier. [ABSTRACT FROM AUTHOR]

In this study, we investigate the effects of the director reorientation on the spectral properties of a one-dimensional magnetophotonic crystal constituted by a sequence of magnetic and nematic layers, which exhibits a magneto-optical defect as the central layer. Using the Eidner-Oldano treatment to solve the Berreman equation, we numerically determine how the Faraday rotation and transmission spectrum of the photonic systems are affected by a field-induced reorientation in the nematic layers. Our results reveal that the transmittance spectrum of the magnetophotonic system is strongly sensitive to the director profile when the applied voltage is raised above the Freedericksz threshold. The director reorientation leads to the suppression of the typical wavelength mismatch between the Faraday rotation and light localization phenomena in one-dimensional photonic structures based on materials with different types of birefringence. Considering the polarization configurations of the propagation eigenmodes for a birefringent medium, the main mechanism behind mismatch suppression is analyzed, emphasizing the effects associated with the field-induced nematic reorientation. Our results show that the electric control of wavelength mismatch in magnetophotonic systems may be used as an efficient mechanism to develop a new class of optical switches and logic gate devices. [ABSTRACT FROM AUTHOR]

We present a study of the influence of gallium vacancy (VGa) point defects on the ferromagnetic properties of GaN:Mn and GaN:Mn,O micro- and nanostructures. Results demonstrate that the generation of these point defects enhances the ferromagnetic signal of GaN:Mn microstructures, while incorporation of oxygen as an impurity inhibits this property. XPS measurements revealed that Mn impurities in ferromagnetic GaN:Mn samples mainly exhibit a valence state of 2+. Cathodoluminescence (CL) spectra from Mn-doped GaN samples displayed emissions centered at about 1.97 eV, attributed to transitions between the 4T1-6A1 states of the Mn2+ d orbitals, and emissions centered at 2.45 and 2.9 eV, associated with the presence of VGa. CL measurements also revealed a blue shift of the GaN band-edge emission generated by the expansion of the wurtzite lattice due to Mn incorporation, which was confirmed by XRD measurements. These latter measurements also revealed an amorphization of GaN:Mn due to the incorporation of oxygen as impurities. The GaN:Mn samples were synthesized by thermal evaporation of GaN and MnCO3 powders onto Ni0.8Cr0.2/Si(100) in a horizontal furnace operated at low vacuum. The residual air inside the system was used as a source of oxygen during the synthesis of Mn and O co-doped GaN nanostructures. Mn and O impurities were incorporated into the nanostructures at different concentrations by varying the growth temperature. Energy Dispersive Spectroscopy, XRD, and XPS measurements confirmed that the obtained samples predominantly consisted of GaN. [ABSTRACT FROM AUTHOR]

We study the magnetic excitations of a square lattice spin ice recently produced in an artificial form as an array of nanoscale magnets. Our analysis, based on the dipolar interaction between the nanomagnetic islands, correctly reproduces the ground state observed experimentally. In addition, we find magnetic monopolelike excitations effectively interacting by means of the usual Coulombic plus a linear confining potential, the latter being related to a stringlike excitation binding the monopoles pairs, which indicates that the fractionalization of magnetic dipoles may not be so easy in two dimensions. These findings contrast this material with the three-dimensional analog, where such monopoles experience only the Coulombic interaction. We discuss, however, two entropic effects that affect the monopole interactions. First, the string configurational entropy may lose the string tension and then free magnetic monopoles should also be found in lower dimensional spin ices; second, in contrast to the string configurational entropy, an entropically driven Coulomb force, which increases with temperature, has the opposite effect of confining the magnetic defects. [ABSTRACT FROM AUTHOR]

Manganite with a double perovskite structure is an attractive material because of its interesting magnetoelectric and dielectric responses. In particular, colossal dielectric constant (CDC) behavior has been observed in La2CoMnO6 (LCMO) at radio frequencies and at room temperature. In this paper, we used infrared-reflectivity spectroscopy to study a LCMO ceramic obtained through a modified Pechini's method to determine the phonon contribution to the intrinsic dielectric response of the system and to investigate the CDC origin. The analysis of the main polar modes and of the obtained phonon parameters indicate that the CDC effect of LCMO is of pure extrinsic origin. In addition, we estimated the dielectric constant and the quality factor of the material in the microwave region to be ϵ's ~ 16 and Qu × f ~ 124 THz, which verifies that LCMO is appropriate for application in microwave devices and circuitry. [ABSTRACT FROM AUTHOR]

Manganite with a double perovskite structure is an attractive material because of its interesting magnetoelectric and dielectric responses. In particular, colossal dielectric constant (CDC) behavior has been observed in La2CoMnO6 (LCMO) at radio frequencies and at room temperature. In this paper, we used infrared-reflectivity spectroscopy to study a LCMO ceramic obtained through a modified Pechini's method to determine the phonon contribution to the intrinsic dielectric response of the system and to investigate the CDC origin. The analysis of the main polar modes and of the obtained phonon parameters indicate that the CDC effect of LCMO is of pure extrinsic origin. In addition, we estimated the dielectric constant and the quality factor of the material in the microwave region to be ϵ's ~ 16 and Qu × f ~ 124 THz, which verifies that LCMO is appropriate for application in microwave devices and circuitry. [ABSTRACT FROM AUTHOR]

The magnetoimpedance effect is a versatile tool to investigate ferromagnetic materials, revealing aspects on the fundamental physics associated to magnetization dynamics, broadband magnetic properties, important issues for current and emerging technological applications for magnetic sensors, as well as insights on ferromagnetic resonance effect at saturated and even unsaturated samples. Here, we perform a theoretical and experimental investigation of the magnetoimpedance effect for the thin film geometry at the high frequency range. We calculate the longitudinal magnetoimpedance for single layered, multilayered, or exchange biased systems from an approach that considers a magnetic permeability model for planar geometry and the appropriate magnetic free energy density for each structure. From numerical calculations and experimental results found in literature, we analyze the magnetoimpedance behavior and discuss the main features and advantages of each structure. To test the robustness of the approach, we directly compare theoretical results with experimental magnetoimpedance measurements obtained at the range of high frequencies for an exchange biased multilayered film. Thus, we provide experimental evidence to confirm the validity of the theoretical approach employed to describe the magnetoimpedance in ferromagnetic films, revealed by the good agreement between numerical calculations and experimental results. [ABSTRACT FROM AUTHOR]

Mercury iodide (HgI2) nanocrystals (NCs) were successfully grown in a barium phosphate glass matrix synthesized by fusion. Growth control of HgI2 NCs was investigated by Atomic Force Microscopy (AFM), Optical Absorption (OA), Fluorescence (FL), and X-ray diffraction (XRD). AFM images reveal the formation of HgI2 nanocrystals in host glass matrix. HgI2 NCs growth was evidenced by an OA and FL band red-shift with increasing annealing time. XRD measurements revealed the β crystalline phase of the HgI2 nanocrystals. [ABSTRACT FROM AUTHOR]

We have investigated the magnetic properties of Ni nanoaggregates produced by ion implantation in ZnO single crystals. Several deviations from classical models usually adopted to describe the magnetic properties of nanoparticle systems were found. The strain between host and Ni nanoaggregates induces a magnetic anisotropy with a preferred direction. We show that these anisotropy effects can be misinterpreted as a ferromagnetic or antiferromagnetic coupling among the nanoaggregates similar to that of an oriented, interacting nanocrystal ensemble. [ABSTRACT FROM AUTHOR]

The structure and the magnetic properties of nanopowdered samples of La1/3Sr2/3FeO3-γ with average particles size d in the range of 67-367 nm prepared by a sol-gel method were investigated in detail. The samples were characterized by X-ray diffraction, scanning electron microscopy, specific heat, Mössbauer spectroscopy, ac susceptibility, and magnetization measurements. Exchange bias with vertical magnetization shift was found in all samples. Charge ordering and antiferromagnetism were observed close to 200K for large particles (d ≥ 304 nm) samples, while for particles with intermediated and smaller values (d ≤ 156 nm) a cluster-glass like behaviour and a short range charge ordering were seen near 115K and 200 K, respectively. The spin-glass like and exchange bias behaviour in nanopowdered samples of La1/3Sr2/3FeO3-γ are associated to compact Fe3+ antiferromagnetic (AF) clusters caused by an oxygen deficiency, which was found to be higher in the samples with the smallest average particles sizes. The effect of exchange bias and vertical magnetization shifts are explained by a simple model involving the interaction of one AF phase with a canted AF phase. [ABSTRACT FROM AUTHOR]

We have investigated the temperature-dependent Raman-active phonons and the magnetic properties of Gd(Co1/2Mn1/2)O3 perovskite ceramics in the temperature range from 40 K to 300 K. The samples crystallized in an orthorhombic distorted simple perovskite, whose symmetry belongs to the Pnma space group. The data reveal spin-phonon coupling near the ferromagnetic transition occurring at around 120 K. The correlation of the Raman and magnetization data suggests that the structural order influences the magnitude of the spin-phonon coupling. [ABSTRACT FROM AUTHOR]

Diluted magnetic semiconductor nanocrystals, Pb1-xMnxS (x = 0.003, 0.005, and 0.010), of approximately 6 nm were successfully grown in a glass matrix by a combination of fusion and thermal annealing. X-ray diffraction measurements reveal that the as-produced nanocrystals (NC), of group IV-VI, are single-phase, nanosized and crystallized in the rock salt structure with replacement of Pb2+-ions by Mn2+-ions. Magnetic force microscopy images also confirmed the high quality of the Pb1-xMnxS NC samples, showing a uniform distribution of total magnetic moments in the nanocrystals. The observation of characteristic hyperfine structures from electron paramagnetic resonance measurements provided evidence for Mn2+ incorporation within the PbS dot structure. Moreover, low temperature magnetization and susceptibility data showed that most of the magnetic ions hosted by the nanocomposite samples are in a paramagnetic state. [ABSTRACT FROM AUTHOR]

Determining phase transitions has always been a great challenge in material science due to their important fundamental and technological aspects. Recently, iron-based perovskites (RFeO3), exhibiting phase transitions at high temperatures, have attracted much interest for their functional properties at room temperature, such as multiferroicity (BiFeO3) and ultrafast spin dynamics (TmFeO3). In this family of materials, Samarium orthoferrite (SmFeO3) is a weak ferromagnet, ordering at high temperatures and exhibiting an intrinsic spin reorientation transition above room temperature, which is 'hidden' in macroscopic magnetization measurements in polycrystalline samples. In the present article, we show that the related magnetic high temperature phase transitions can be studied through their dielectric functions by spectroscopic ellipsometry in situ and without any need for an applied external magnetic field. The presence of this intrinsic spin reorientation transition is demonstrated for textured SmFeO3 films and we have determined a critical exponent of β = 0.45 ± 0.01 for the magnetic phase transition, coherently from optical, magneto-optical, and structural investigations. [ABSTRACT FROM AUTHOR]

The importance of interface effects for organic devices has long been recognized, but getting detailed knowledge of the extent of such effects remains a major challenge because of the difficulty in distinguishing from bulk effects. This paper addresses the interface effects on the emission efficiency of poly(p-phenylene vinylene) (PPV), by producing layer-by-layer (LBL) films of PPV alternated with dodecylbenzenesulfonate. Films with thickness varying from ∼15 to 225 nm had the structural defects controlled empirically by converting the films at two temperatures, 110 and 230 °C, while the optical properties were characterized by using optical absorption, photoluminescence (PL), and photoluminescence excitation spectra. Blueshifts in the absorption and PL spectra for LBL films with less than 25 bilayers (<40-50 nm) pointed to a larger number of PPV segments with low conjugation degree, regardless of the conversion temperature. For these thin films, the mean free-path for diffusion of photoexcited carriers decreased, and energy transfer may have been hampered owing to the low mobility of the excited carriers. The emission efficiency was then found to depend on the concentration of structural defects, i.e., on the conversion temperature. For thick films with more than 25 bilayers, on the other hand, the PL signal did not depend on the PPV conversion temperature. We also checked that the interface effects were not caused by waveguiding properties of the excited light. Overall, the electronic states at the interface were more localized, and this applied to film thickness of up to 40-50 nm. Because this is a typical film thickness in devices, the implication from the findings here is that interface phenomena should be a primary concern for the design of any organic device. [ABSTRACT FROM AUTHOR]

The Stokes shift detected in PbS nanocrystals is analyzed by optical absorption (OA), photoluminescence (PL) and atomic force microscopy (AFM). The samples were synthesized by fusion method over two different glass substrates, and identical size PbS dot structures with radius 4.1 nm grown showed PL and OA peaks separated by 0.15 and 0.10 eV. The origin of these large Stokes shifts are analyzed whether they are attributed to the nonradiative recombination associated to impurity states in prepared doped samples or to radiative transitions between exciton states induced by electron-hole (e-h) exchange Coulomb interaction. [ABSTRACT FROM AUTHOR]

A model for describing structural pointlike defects in nanoscaled ferromagnetic materials is presented. Its details are explicitly developed whenever it interacts with a vortexlike state comprised in a thin nanodisk. Among others, our model yields results for the vortex equilibrium position under the influence of several defects along with an external magnetic field that is shown to be in good qualitative agreement with experiments. We also discuss how such defects may affect the vortex motion, such as its gyrotropic oscillation and dynamical polarization reversal. [ABSTRACT FROM AUTHOR]

In the framework of models that assume planar domain wall formed at the antiferromagnetic part of the interface of exchange-biased bilayers, one cannot distinguish between the cases of high or low ratios between the coupling and the antiferromagnet’s anisotropy fields by using hysteresis loop measurement, ferromagnetic resonance, anisotropic magnetoresistance, or ac susceptibility techniques applied on one and the same sample. The analysis of the experimental data obtained on a series of FeMn/Co films indicated that once the biasing is established the variation in the coercivity with the FeMn layer thickness could be essential for solving this problem. If the coercivity decreases with the thickness then the interlayer exchange coupling is the parameter that varies while the domain-wall energy of the antiferromagnet remains practically constant. [ABSTRACT FROM AUTHOR]

We consider a nanodisk possessing two coupled materials with different ferromagnetic exchange constants. The common borderline of the two media crosses at the disk center, dividing the system exactly in two similar half-disks. The vortex core motion crossing the interface is investigated using a two-dimensional model, which mimics a very thin, real material with such a line defect. The main result of this study is that, depending on the magnetic coupling which connects the media, the vortex core can be dramatically and repeatedly flipped up to down by the interface. This phenomenon produces a burstlike emission of spin waves each time the switching process takes place. [ABSTRACT FROM AUTHOR]

A gamma-ray telescope mission concept [gamma ray imager (GRI)] based on Laue focusing techniques has been proposed in reply to the European Space Agency call for mission ideas within the framework of the next decade planning (Cosmic Vision 2015–2025). In order to optimize the design of a focal plane for this satellite mission, a CdZnTe detector prototype has been tested at the European Synchrotron Radiation Facility under an ∼100% polarized gamma-ray beam. The spectroscopic, imaging, and timing performances were studied and in particular its potential as a polarimeter was evaluated. Polarization has been recognized as being a very important observational parameter in high energy astrophysics (>100 keV) and therefore this capability has been specifically included as part of the GRI mission proposal. The prototype detector tested was a 5 mm thick CdZnTe array with an 11×11 active pixel matrix (pixel area of 2.5×2.5 mm2). The detector was irradiated by a monochromatic linearly polarized beam with a spot diameter of about 0.5 mm over the energy range between 150 and 750 keV. Polarimetric Q factors of 0.35 and double event relative detection efficiency of 20% were obtained. Further measurements were performed with a copper Laue monochromator crystal placed between the beam and the detector prototype. In this configuration we have demonstrated that a polarized beam does not change its polarization level and direction after undergoing a small angle (<1°) Laue diffraction inside a crystal. [ABSTRACT FROM AUTHOR]

Defects introduced in ferromagnetic nanodisks may deeply affect the structure and dynamics of stable vortexlike magnetization. Here, analytical techniques are used for studying, among other dynamical aspects, how a small cylindrical cavity modifies the oscillatory modes of the vortex. For instance, we have realized that if the vortex is nucleated out from the hole, its gyrotropic frequencies are shifted below. Modifications become even more pronounced when the vortex core is partially or completely captured by the hole. In these cases, the gyrovector can be partially or completely suppressed, so that the associated frequencies increase considerably, say, from some times to several powers. Possible relevance of our results for understanding other aspects of vortex dynamics in the presence of cavities and/or structural defects are also discussed. [ABSTRACT FROM AUTHOR]

In this work, very high magnetoimpedance measured at frequencies up to 1.8 GHz in nanostructured Fe73.5Cu1Nb3Si13.5B9/Cu films is reported. Magnetoimpedance ratios of 280% and 50% were observed in multilayered and sandwiched samples, respectively. For the last one, the magnetoimpedance ratio exhibited a nearly constant value in the frequency range of 10–200 MHz. The results are discussed in terms of the anisotropies present in all samples and by considering the connection of magnetoimpedance effects with ferromagnetic resonance phenomena. [ABSTRACT FROM AUTHOR]

Field-dependent transverse permeability characterization of amorphous thin films with nominal composition Fe73.5Cu1Nb3Si13.5B9 was performed for frequencies in the range of 100 kHz–1.8 GHz. Dynamic and static magnetic properties were investigated in films with thickness in the range from 21 to 5000 nm. Samples with thicknesses below 85 nm exhibit a well-defined in-plane uniaxial anisotropy and uniform ferromagnetic resonance modes. Samples thicker than 85 nm were found to be magnetically isotropic in the plane, with complex magnetic dynamics depicted by several ferromagnetic resonance modes detected at relatively low fields. The results are discussed in terms of the stress contribution to the magnetic anisotropy of the samples. [ABSTRACT FROM AUTHOR]

We study CoFe/Al2O3 multilayers, varying from granular to continuous structure with CoFe nominal thickness (10 Å &les;t &les;18 Å). Structural percolation takes place at t =tc&assymp; 18 Å, changing from activated to metallic conductance. A sharp anomaly in ac coercivity of magneto-optical Kerr effect and in magnetization M by superconducting quantum intefferometer device is found at t =t∗ &assymp; 17 Å, interpreted as transition from superferromagnetism (by dipolar coupled CoFe clusters) to exchange ferromagnetism. Onset at t=t∗ of anisotropic magnetoresistance (AMR) is attributed to skew scattering of bandlike electrons within giant fractal clusters. These structures may cause the observed Barkhausen noises in M and AMR at t&assymp;t∗, disappearing at t>t∗. [ABSTRACT FROM AUTHOR]

Impedance spectra (100 kHz≤f≤1.8 GHz) were measured at different magnetic fields, and magnetoimpedance and magnetization measurements were performed in as-produced and Joule-heated Co[sub 70.4]Fe[sub 4.6]Si[sub 10]B[sub 15] glass-covered amorphous microwires. From the magnetization curves and ferromagnetic resonance features obtained from the impedance spectra, it was possible to propose a domain structure in the as-produced microwires, as well as to follow its evolution with the annealing current. The inner core and outer shell domain structure were observed. The outer shell domains evolve from a circumferential, in the as-cast sample, to a helical structure in the case of the annealed samples. On the other hand, the inner core evolves from a domain structure typical of a radial anisotropy to a longitudinal one. This domain structure evolution is explained in terms of the combined effects of the stress, promoted by the annealing temperature, and the magnetic field caused by the annealing current. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Semi-insulating GaAs samples present N-shaped negative differential conductivity under high-electric fields. This behavior can be associated with two physical processes: Impact ionization (generation) and field-enhanced trapping (recombination), both of which involve trapped and free electrons. We have analyzed the j(E) characteristic curves of a GaAs sample rich in As antisite defects at different conditions of temperature and illumination. The fitting was carried out using an analytical expression for j(E) based on the competition between the above-mentioned processes. Our analysis permits us to identify the temperature and illumination ranges in which those processes are relevant. The best fittings were obtained for measurements between 150 and 200 K and using an infrared photon flux of the order of 10[sup 11] photons/cm² s. [ABSTRACT FROM AUTHOR]

The La0.8Sr0.2MnO3 compound is synthesized in fiber single crystalline form by the Laser Heated Pedestal Growth method. Results indicate that the Curie temperature increases with pressure while the low temperature transition from the orthorhombic to the rhombhoedral structures decreases as pressure increases.

The La0.8Sr0.2MnO3 compound presents a ferromagnetic paramagnetic transition around room temperature to which a reasonably high magnetocaloric effect is associated, turning this material of interest for application in magnetic refrigeration. We synthesized this compound in fiber single crystalline form by the Laser Heated Pedestal Growth method. The sample was characterized by x-ray diffraction and magnetic measurements as a single phase and with the required magnetic properties. We measured the magnetic properties and the magnetocaloric effect under hydrostatic pressure for pressures up to 6 kbar as a function of temperature. Our results indicate that the Curie temperature increases with pressure while the low temperature transition from the orthorhombic to the rhombhoedral structures decreases as pressure increases. This is in close agreement with the literature. Measurement of the magnetocaloric effect at the high temperature transition indicates that the peak of the effect follows the trend of the Curie temperature, but its maximum value remains almost constant as a function of pressure. [ABSTRACT FROM AUTHOR]

In this work, results on the giant magnetoimpedance of glass-covered amorphous microwires with nominal composition CO[sub 70.4]Fe[sub 4.6]Si[sub 15]B[sub 10] are presented. The impedance Z = R + iX has been investigated as function of frequency (1 MHz-1.8 GHz) and magnetic field (up to ±400 Oe), using a HP4396B impedance analyzer and an appropriate coaxial microwave cavity. The effects of the thermal treatments (Joule heating, from 1 up to 50 mA for 10 min) for anisotropy induction and/or to induce recrystallization have been investigated. [ABSTRACT FROM AUTHOR]