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Hall-effect in semiconductors has wide applications for magnetic field sensing. Yet, a standard Hall sensor retains two problems: its linearity is affected by the non-uniformity of the current distribution; the sensitivity is bias-dependent, with linearity decreasing with increasing bias current. In order to improve the performance, we here propose a novel structure which realizes bias-free, photo-induced Hall sensors. The system consists of a semi-transparent metal Pt and a semiconductor Si or GaAs to form a Schottky contact. We systematically compared the photo-induced Schottky behaviors and Hall effects without net current flowing, depending on various magnetic fields, light intensities and wavelengths of Pt/GaAs and Pt/Si junctions. The electrical characteristics of the Schottky photo-diodes were fitted to obtain the barrier height as a function of light intensity. We show that the open-circuit Hall voltage of Pt/GaAs junction is orders of magnitude lower than that of Pt/Si, and the barrier height of GaAs is smaller. It should be attributed to the surface states in GaAs which block the carrier drifting. This work not only realizes the physical investigations of photo-induced Hall effects in Pt/GaAs and Pt/Si Schottky junctions, but also opens a new pathway for bias-free magnetic sensing with high linearity and sensitivity comparing to commercial Hall-sensors.

Because of its low-cost, silicon is the standard material for photovoltaic conversion. Yet, its band-edge absorption spectrum is narrower than the spectrum of the solar radiation, which reduces its conversion efficiency. In this paper, it is shown that the spectrum of absorbance of silicon can be extended to longer wavelengths by proximity to a two-dimensional (2D) semiconductor. Photo-induced Hall effect, together with standard absorption spectroscopy, was employed to estimate the increase of photo-conversion efficiency of a 2D-platinum-diselenide/intrinsic-silicon heterostructure. The system shows a significantly higher absorption in the infrared as compared to the single films. Angle resolved X-ray Photoelectron Spectroscopy (XPS) confirm that a change of the band structure occurs in the silicon substrate at the interface between the two semiconductors. The results are interpreted in the framework of band-gap narrowing due to hole-confinement in the Si, induced by electron-confinement in the 2D film. This allows us to claim that the increase of photo-conversion efficiency in the Pt/PtSe2/Si sample is due to an enhancement of the light absorbance of silicon near the interface. Possible application of the effect in photo-voltaic cells is discussed.

Novel hybrid architectures based on ZnO nanostructures supported on reduced graphene oxide films are reported by a facile two-step aqueous electrochemical approach. Graphene oxide (GO) obtained by oxidizing graphite with either H2SO4:H3PO4 mix or H2SO4 was subjected to electrochemical reduction (ErGO) by constant potential and sweeping potential methods and further employed as seeding layer for electrodeposition of ZnO nanorods. The results showed the GO reduction rate was markedly influenced by oxygen content while Infrared spectroscopy indicated the sweeping potential was effective in removing the carbonyl and alkoxy groups but also as more defect-inducing reduction approach than constant potential, according to the Raman measurements. The subsequent electroreduction of ErGO during the electrodeposition of ZnO resulted in interfacial interaction between the seed layer and ZnO nanorods. H2SO4:H3PO4 mix induced a less disrupted graphitic plane in GO which proved beneficial for the growth of higher density ZnO nanorods with improved crystalline quality. By controlling the chemical introduction/electroreduction conditions for oxygen groups in GO materials, the interfacial interaction with ZnO nanorods and their properties could be tailored in order to obtain high performance nanoplatforms.

The Hall effect in metals is too small to have practical applications. Instead, the same effect in semiconductors is the standard for magnetic field sensing. Yet, in semiconducting Hall-sensors, Joule heating severely compromises the linearity range. We here show that a Hall effect can be photo-induced in metals used for bias-free magnetic sensing. The system consists of a transparent metal that forms a Schottky contact to a semiconductor. Light reaching the interface results in the injection of charge from the space charge region. If a magnetic field is applied, a transverse, open-circuit voltage appears at the metal edges that is proportional to the field, as well as light intensity. The system shows sensitivities that are comparable to semiconducting Hall-sensors but no net current flows, therefore its performances are not affected by Joule heating.

sp–d exchange interaction was used to narrow the room-temperature band gap of zinc oxide films prepared by sol–gel method. Zinc oxide was doped with manganese ions by adding manganese chloride to the precursor and post-annealing in hydrogen. Films with different concentrations of manganese were prepared. Exchange interaction was established between the manganese ions in the lattice by introducing oxygen vacancies. The magnetic moment was found to increase with the concentration of manganese. For low concentrations of manganese, the band gap of the doped semiconductor was found to be wider than that of undoped ZnO films, in agreement with Vegard’s law. High concentrations of dopant resulted in a narrowing of the band gap. We ascribe the narrowing of the band gap to conduction band-edge Zeeman shifting. 12 at.% Mn was successfully substituted in ZnO films deposited by sol–gel method. Exchange interaction was established between the Mn ions through the mediation of oxygen vacancies. A narrowing of the band gap of ZnO was achieved by Zeeman shifting of the bottom of the conduction band.

The vortex state is one of the equilibrium configurations of soft magnetic materials and occurs in thin ferromagnetic square and disk-shaped elements of sub-micrometer size. The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position. This mode can be electrically excited by a spin-polarized current. When the gyrating vortex reaches a critical velocity, a switching of the vortex core can occur. We show here how to increase the vortex velocity to reduce the switching current. The vortex core can be switched back by reversing the current direction.

Spin transfer torque oscillators based on magnetic vortex precession can deliver output powers large enough for practical applications. In addition they do not require discrete electronic components, such as capacitors or inductors, to tune the oscillation frequency. At the current stage, this oscillation frequency is still too low and can be increased only by trading off the output power. Here, we present our experimental results on nanocontact oscillators, which are, amongst the spintronic oscillators, those that can deliver the largest power. Based on the analysis of our experimental results, we propose a new device geometry and numerically demonstrate that an increase of the frequency, up to a useful range for practical applications, can be obtained without trading off output power.

Multifunctional, Fe-doped indium tin oxide (ITO) nanostructured films were grown by pulsed laser deposition on sapphire substrates. The films showed low resistivity and n-type conductivity with a carrier density as high as $3\times 10^{21}~{\rm cm}^{-3}$ . A magnetic moment at room temperature larger than those of any iron oxides was measured in films with Fe doping as low as 2%. We ascribe it to the formation of bound polarons. A strong green photoemission at room temperature was observed, which we ascribe to quantum confinement effect. The coexistence of ferromagnetism and luminescence in the visible range makes our films of potential for magneto-optoelectronic applications.

Gold/iron oxide nanocomposites offer the promise of combining the unique traits of both gold and iron oxide nanomaterials to enable new applications. In this work, a simple and safe method has been established whereby the morphology of gold material in gold/iron oxide nanocomposites can be controlled to be nanowire or nanosphere by adjusting the initial molar ratio of citrate-coated iron oxide nanoparticles to auric acid. Gold/iron oxide nanowires and nanospheres were synthesized through this method. Their structural, optical, and magnetic properties were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), dynamic light scattering (DLS), ultraviolet-visible spectroscopy (UV-Vis), and vibrating sample magnetometry (VSM). These materials substantially maintained the optical properties of gold nanomaterials and magnetic features of iron oxide magnetic nanoparticles.

Extended Abstract Memristive switching in oxide semiconductors relies on the formation and disruption of conductive nano-filaments [1]. This effect is considered promising for the next generation of non-volatile memories. Yet, the switching event is a complicated electronic and ionic process, which may involve more than one mechanism. In order to elucidate the switching mechanism, advanced microscopy investigations proved challenging because of heavily dependence of the results on specimen preparation techniques. Important information can be obtained if switching leads to a macroscopic change of nonelectrical properties, for instance magnetic properties. In this respect, n-type Mn-ZnO and p-type NiO provide a unique testbeds since the magnetic properties of these oxides are strongly dependent on the distribution of oxygen vacancies [2]. We show that resistive switching in n-type ferromagnetic Mn-ZnO and p-type antiferromagnetic NiO coexists with a switching of the magnetic phase. Thin films of these oxides were sandwiched between two metallic electrodes and resistive switching was induced. Nano-devices were patterned out of the trilayers by resorting to electron beam lithography and physical etching. We found that a switching of the resistance corresponds to a switching of the magnetic phase in the film [3, 4]. By measuring the magnetic properties of the devices in the two resistive states, we can draw important conclusions on the underlying switching mechanism. For instance, in Mn-ZnO the effect is not filamentary type and occurs uniformly under the interface, whereas in NiO the effect is filamentary type. By measuring the change of magnetic properties we could exclude that switching was due to the formation of Ni-ion filaments across the device. We have demonstrated [4] that the switching is due to the formation and rapture of oxygen-vacancy filaments.

We have investigated the origin of the ferromagnetic ordering in undoped and Mn-doped ZnO thin films. A systematic comparison is presented between two sets of films grown by pulsed laser deposition from targets prepared under the same conditions. We show that ferromagnetic ordering in both compounds is directly related to the concentration of oxygen vacancies. The incorporation of Mn dramatically increases the magnetic moment in highly oxygen-deficient films. By analyzing the photoluminescence spectra and the structural properties of the films, we can exclude that magnetic ordering in our films originates from unintentional doping.

Magnetic vortex-based oscillators represent a unique opportunity to study the mutual spin-transfer torque coupling between two ferromagnetic layers in a spin-valve configuration. This will eventually lead to a complete understanding of this phenomenon and, potentially, to electronic devices of practical application. Mutual spin-transfer torque can lead to multiple-mode high-frequency dynamics in a spin-valve system. We here report our study on the emission response of an ohmic nanocontact to a spin-valve multilayer subject to perpendicularly applied magnetic field. In this configuration, three modes are accessible, two of which correspond to the precessional motion of a vortex in one of the two ferromagnetic layers with the other working as a static polarizer. At high currents, the third mode can be observed that is ascribed to the simultaneous precession of two vortices, one in each layer, with the other layer working as a dynamic polarizer. The simultaneous application of a field in the sample plane tends to suppress the mode that corresponds to the precessional motion of the vortex in the thinner ferromagnetic layer.

In an elliptical magnetic dot two vortices and a flower state configuration can be accommodated at remanence. We show that a two-fold degeneracy in the position of the flower state can be geometrically introduced and exploited for solid state data storage. Switching between two, energetically-equivalent, magnetic configurations is obtained by injecting a perpendicular electric current. Switching currents of the order of ~105 A/cm2 and high thermal stability are demonstrated by micromagnetic simulations. This system can be used as a bit-cell for magnetic solid state data storage. The low value of the switching current and the high thermal stability allow direct integration with the current complementary metal oxide semiconductor technology.

The magnetic and magnetotransport properties of Ni80Fe20/La0.7Sr0.3MnO3 (NiFe/LSMO) bilayers were investigated after bombarding the LSMO surface with low-energy Ar+ or O-2(+)/Ar+ ion beams before the growth of the top NiFe layer. A variety of magnetic properties are revealed, including an asymmetric two-stepped hysteresis loop with an exchange bias loop shift, and alternatively, a symmetric two-stepped hysteresis loop with an enhanced coercivity. Polarized neutron reflectometry measurements provide details of the magnetic depth profile and interface layer magnetism at different temperatures. The LSMO surface modifications determine a complex magnetic and electric NiFe/LSMO interface having a strong effect on the magnetoresistance of the bilayer. Surface engineering based on ion beam bombardment is presented as a promising technique for optimizing the electronic and magnetic properties of NiFe/LSMO junctions for future device applications. Published by AIP Publishing.

The studies of the effects of magnetic field on surface enhanced Raman scattering (SERS) have been so far limited to the case of ferromagnetic/noble-metal, core/shell nano-particles, where the influence was always found to be negative. In this work, we investigate the influence of magnetic field on a diluted magnetic semiconductor/metal SERS system. Guided by three dimensional finite-difference time-domain simulations, a high efficient SERS substrate was obtained by diluting Mn into Au-capped ZnO, which results in an increase of the dielectric constant and, therefore, an enhancement of Raman signals. More remarkably, an increase of intensities as well as a reduction of the relative standard deviation (RSD) of Raman signals have been observed as a function of the external magnetic strength. We ascribe these positive influences to magnetic-field induced nucleation of bound magnetic polarons in the Mn doped ZnO. The combination of diluted magnetic semiconductors and SERS may open a new avenue for future magneto-optical applications.

Hybrid nanostructure can inherit the physiochemical properties of its individual components to realize its multi-functionality. The coupling of plasmonic effect of gold nanoparticles with magnetic properties of iron oxide nanoparticles has shown great promise as bi-functional agents allowing simultaneous magnetic resonance imaging (MRI)/ computed tomography (CT) imaging and magnetic/photonic thermal therapy.[1, 2] However, since gold and iron oxide are two dissimilar materials, the precise morphology and structure control for the hybrid nanostructure remains a great challenge, and there are few published studies on the correlation between composites morphologies and the optical/magnetic properties. In this work, we aim to fabricate gold/iron oxide hybrid nano-structures using less toxic precursors, control the morphology growth of iron oxide component on gold nanoparticles surface, and study the effects on optical and magnetic properties of final products. Here, nearly monodisperse gold/iron oxide hybrid nanoparticles were fabricated through thermal decomposition method. (Fig. 1a-1f) Spherical gold nanoparticles were pre-synthesized and used as seeds for the reduction of iron precursor to produce hybrid nanostructures. Various morphologies of iron oxide grown on the gold nanospheres surfaces were realized, including nano-shell, nano-octahedron, nano-flower, and dumbbell-shaped end. (Fig. 1c-1f) Pure gold nanospheres and iron oxide nanospheres were synthesized for comparison. The morphology and structure of obtained products were characterized by using TEM, EDX, electron diffraction pattern, and SEM. Their optical and magnetic properties were studied using UV-Vis spectroscopy and VSM. The plasmonic property of gold nanoparticles was shown to be affected by the optical index of its environment, and its absorbance peak was right-shifted after iron oxide shell coating. (Fig. 2a) The gold/iron oxide dumbbell-shaped nanostructures displayed typical superparamagnetic behavior with no coercivity (Fig. 2b), while the coercivity of gold/iron oxide nano-octahedron was about 45 Oe (inset in Fig. 2b). The morphology-related blocking temperatures of the gold/iron oxide nano-structured samples were also studied through their ZFC/FC curves.

We report a large negative magnetoresistance in Manganese-substituted Zinc Oxide thin films. This anomalous effect was found to appear in oxygen-deficient films and to increase with the concentration of Manganese. By combining magnetoresistive measurements with magneto-photoluminescence, we demonstrate that the effect can be explained as the result of a magnetically induced transition from hopping to band conduction where the activation energy is caused by the sp-d exchange interaction.

We investigated the role of strain on the magnetic properties of La0.7Sr0.3MnO3 epitaxial films grown on substrates with different crystallographic orientation. The magnetic characterization confirms the large magnetostriction due to the tensile stress applied by the SrTiO3 substrates. The data demonstrate that the easy direction of magnetization is aligned to the elongated, in-plane axis both in (110)- and (001)-oriented films. The in-plane anisotropy of (110) films is interpreted in the frame of a simple structural model. We provide evidence of a spatial separation between a conducting ferromagnetic and an insulating paramagnetic phase at the Curie temperature. The possible effect of such phase separation on the coercitive field and on the magnetization versus temperature is discussed. (c) 2006 Elsevier B.V. All rights reserved.

Polymer-embedding of nano-sized indium tin oxide (SnO center dot In2O3, ITO) produces electrically conductive materials transparent to the visible light at filling factors higher than the percolation threshold. ITO powders are commercially available in an aggregated form and a disaggregation technique was required. Here, aggregated ITO nanoparticles were transformed to colloidal suspension by high-speed stirring. This finely dispersed ceramic suspension was stabilized by addition of poly(vinyl pyrrolidone) and the obtained colloidal system was cast on an optical-grade substrate (PET) to produce electrically conductive-transparent nanocomposite films. Preliminary mechanical and electrical characterization of these films showed good conductivity and interfacial properties.

The critical Josephson current (IC) in superconducting/ferromagnetic (S/F) multilayer-based junctions can be controlled by changing the relative directions of the magnetization in the F-layers. Recent experimental works [1, 2] show that an enhancement of IC is achieved in S/F weak links when the alternating F-layers are antiparallel aligned. We present preliminary experimental results concerning the dependence of IC on the relative orientation of the ferromagnetic layers in S/F1/I/F2/S tunnel junctions where the F-layers are obtained by changing the relative composition of NiCu alloys. The multilayers were grown by electron beam deposition, and processed by Focused Ion Beam lithography. The magnetic state of the devices was directly determined by measuring the current perpendicular to plane (CPP) magnetoresistance (MR) at high bias. IC was found to be larger when the F-layers are antiparallel aligned. The maximum change of IC corresponds to the maximum change of MR. The application of a magnetic field induces a transition in the shape of the currentvoltage curve that seems to suggest Coulomb blockade effect.

We present experimental results concerning both the fabrication and the characterization of superconducting tunnel junctions based on Nb/NiCu superconductor/ferromagnet (S/F) bilayers. Josephson junctions have been characterized down to T=1.4 K in terms of current-voltage I-V characteristics, Josephson critical current vs magnetic field, role of external magnetic field. Proximity parameters of the bilayer, such as the coherence length and the exchange field energy of the F metal, have been estimated by means of a numerical deconvolution of the I-V data the electronic density of states on both sides of the S/F bilayer. Results have been compared with theoretical predictions from a proximity model for S/F bilayers in the dirty limit in the framework of Usadel equations for the S and F layer, respectively.

Spin orbit coupling in heavy metals allows the conversion of unpolarized light into an open-circuit voltage. We experimentally prove that this photo-spin voltaic effect is due to photo-excitation of carriers in the proximized layer and can exist for light in the visible range. While carrying out the experiment, we discovered that, in closed-circuit conditions, the anisotropic magnetoresistance of the proximized metal is a function of the light intensity. We name this effect photo-magnetoresistance. A magneto-transport model is presented that describes the change in magnetoresistance as a function of the light intensity.

We present novel low-field magnetoresistive devices based on the ferromagnetic manganite La0.7Sr0.3MnO3 operating in the current in plane (CIP) configuration. In these planar spin-valve devices, a focused Ga+ beam is used to create pinning centers for magnetic domain walls. The spin-dependent scattering of polarized electrons at the domain walls (DW) is responsible for the magnetoresistance observed in the patterned tracks. The magneto-transport properties of these devices are interpreted within a model for DW magnetoresistance. Applications such as magnetic data storage can be envisaged for the structures we investigated. (C) 2006 Elsevier B.V. All rights reserved.

Band engineering of transparent semiconductors, such as TiO2, SnO2, has recently been attempted to become black colour in order to increase photocatalytic efficiency in the visible and near infrared ranges. Here, we investigated a previously-neglected property in black oxides by using the example of iron doped indium tin oxide nanoparticles: the photo-thermal effect in the visible and near-infrared ranges. We found a significant enhancement of the photo-thermal generation in the near-infrared biological window that is due to photon absorption, with no significant contribution from surface plasmon resonance. This allows independent magnetic functionalization of the particles by iron doping. These particles may be used for in-vivo cancer treatment where heat is photo-termally generated, while keeping the magnetism as a mean for remote manipulation through permanent magnets.

The magnetic properties of hematite (?-Fe2O3) nanoparticles synthesized by in situ thermal decomposition of an iron-mercaptide precursor in polymeric matrices are studied. In Polystyrene, nanoparticles with average size ~20 nm can be produced that show ferromagnetic behavior with no Morin transition for temperatures as low as 10 K. Poly(9-vinylcarbazole)-based nanocomposites, produced by the same synthesis process, show a different magnetic behavior with a clear Morin transition. The effect of the polymeric matrix on the structural and magnetic properties of dispersed hematite nanoparticles is discussed. It is concluded that, in heavily viscous medium, the ability to control the particle growth and distribution is compromised.

We demonstrate that the physical mechanism behind electroresistive switching in oxide Schottky systems is electroformation, as in insulating oxides. Negative resistance shown by the hysteretic current-voltage curves proves that impact ionization is at the origin of the switching. Analyses of the capacitance-voltage and conductance-voltage curves through a simple model show that an atomic rearrangement is involved in the process. Switching in these systems is a bulk effect, not strictly confined at the interface but at the charge space region., 4 pages, 3 figures, accepted in PRB

We present a detailed investigation of the electrical properties of epitaxial ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}∕\mathrm{Sr}{\mathrm{Ti}}_{0.98}{\mathrm{Nb}}_{0.02}{\mathrm{O}}_{3}$ Schottky junctions. A fabrication process that allows reduction of the junction dimensions to current electronic device size has been employed. A heavily doped semiconductor has been used as a substrate in order to suppress its series resistance. We show that, unlike standard semiconductors, high-quality oxide-based Schottky junctions maintain a highly rectifying behavior for doping concentration of the semiconductor larger than ${10}^{20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Moreover, the junctions show hysteretic current-voltage characteristics.

We present experimental results concerning both the fabrication and characterization of superconducting tunnel junctions containing superconductor/ferromagnet (S/F) bilayers made by niobium (S) and a weak ferromagnetic Ni0.50Cu0.50 alloy. Josephson junctions have been characterized down to T=1.4 K in terms of current-voltage I-V characteristics and Josephson critical current versus magnetic field. By means of a numerical deconvolution of the I-V data the electronic density of states on both sides of the S/F bilayer has been evaluated at low temperatures. Results have been compared with theoretical predictions from a proximity model for S/F bilayers in the dirty limit in the framework of Usadel equations for the S and F layers, respectively. The main physical parameters characterizing the proximity effect in the Nb/NiCu bilayer, such as the coherence length and the exchange field energy of the F metal, and the S/F interface parameters have been also estimated.