Search

Several diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of $\lesssim 80~$ppm, the so-called C- or P1-centres, were irradiated with electrons at 10 MeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below $\sim 30$~K, after a decrease of $\lesssim 40~$ppm in the concentration of C-centres produced by the electron irradiation. This result indicates that magnetic C-centres are at the origin of the large hysteretic behavior found recently in nitrogen-doped diamond crystals. This is remarkable because of the relatively low density of C-centres, stressing the extraordinary role of the C-centres in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered., 15 pages, 10 figures

We report on a spin filter effect that emerges at potential barriers between a ≃10 nm thick magnetic surface layer and nonmagnetic regions prepared at the surface of Li-doped ZnO microwires by low-...

We have studied the magnetization of a recently synthesized Cu9S5 compound and found two phase transitions around room temperature. The phase transitions in the crystalline structure, chara...

In order to study substrate influence in Mg ferrite properties, thin films weresimultaneously grown by DC magnetron sputtering on different substrates (MgO(100) and SrTiO3 (100)). In both cases samples show high values of saturationmagnetization and Curie temperatures above room temperature. While in thecase of the sample grown on MgO the hysteresis loops indicate the existenceof more than one ferrimagnetic component and lower value of remanence tosaturation ratio, the deposited one on SrTiO3 is mono-component and has arectangular-shape magnetic loop at all temperatures below 300 K. The differenceis attributed to different microstructures due to the misfit strain caused by thedifferent lattice constants between substrates and ferrite. The electric transport and photoconductivity properties have been investigated on both samples. Thin film growth on STO presents an increase around 40% of the photoconductance in the near UV range. The photocurrent shows two clear onsets that coincide with the indirect and direct band gaps of STO indicating the important role of this substrate in the generation of photo carriers. Fil: Salcedo Rodriguez, Karen Lizeth. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina Fil: Bridoux, German. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentina Fil: Heluani, S. P.. Universidad Nacional de Tucumán. Laboratorio de Física del Sólido; Argentina Fil: Pasquevich, Gustavo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina Fil: Esquinazi, P. D.. University of Leipzig; Alemania Fil: RodrÍguez Torres, Claudia Elena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina

Tunneling atomic force microscopy (TUNA) was used at ambient conditions to measure the current-voltage ($I$-$V$) characteristics at clean surfaces of highly oriented graphite samples with Bernal and rhombohedral stacking orders. The characteristic curves measured on Bernal-stacked graphite surfaces can be understood with an ordinary self-consistent semiconductor modeling and quantum mechanical tunneling current derivations. We show that the absence of a voltage region without measurable current in the $I$-$V$ spectra is not a proof of the lack of an energy band gap. It can be induced by a surface band bending due to a finite contact potential between tip and sample surface. Taking this into account in the model, we succeed to obtain a quantitative agreement between simulated and measured tunnel spectra for band gaps $(12 \ldots 37)$\,meV, in agreement to those extracted from the exponential temperature decrease of the longitudinal resistance measured in graphite samples with Bernal stacking order. In contrast, the surface of relatively thick graphite samples with rhombohedral stacking reveals the existence of a maximum in the first derivative $dI/dV$, a behavior compatible with the existence of a flat band. The characteristics of this maximum are comparable to those obtained at low temperatures with similar techniques., 12 pages, 10 figures

Although ferromagnets are found in all kinds of technological applications, only few substances are known to be intrinsically ferromagnetic at room temperature. In the past twenty years, a plethora of new artificial ferromagnetic materials have been found by introducing defects into non-magnetic host materials. In contrast to the intrinsic ferromagnetic materials, they offer an outstanding degree of material engineering freedom, provided one finds a type of defect to functionalize every possible host material to add magnetism to its intrinsic properties. Still, one controversial question remains: Are these materials really technologically relevant ferromagnets? To answer this question, in this work the emergence of a ferromagnetic phase upon ion irradiation is systematically investigated both theoretically and experimentally. Quantitative predictions are validated against experimental data from the literature of SiC hosts irradiated with high energy Ne ions and own experiments on low energy Ar ion irradiation of TiO$_2$ hosts. In the high energy regime, a bulk magnetic phase emerges, which is limited by host lattice amorphization, whereas at low ion energies an ultrathin magnetic layer forms at the surface and evolves into full magnetic percolation. Lowering the ion energy, the magnetic layer thickness reduces down to a bilayer, where a perpendicular magnetic anisotropy appears due to magnetic surface states., 38 pages, 21 figures

Cryogenic vector magnet systems make it possible to study the anisotropic magnetic properties of materials without mechanically rotating the sample but by electrically tilting and turning the magnetic field. Vector magnetic fields generated inside superconducting vector magnets are generally measured with three Hall sensors. These three probes must be calibrated over a range of temperatures, and the temperature-dependent calibrations cannot be easily carried out inside an already magnetized superconducting magnet because of remaining magnetic fields. A single magnetometer based on an ensemble of nitrogen vacancy (NV) centers in diamond is proposed to overcome these limitations. The quenching of the photoluminescence intensity emitted by NV centers can determine the field in the remanent state of the solenoids and allows an easy and fast canceling of the residual magnetic field. Once the field is reset to zero, the calibration of this magnetometer can be performed in situ by a single measurement of an optically detected magnetic resonance spectrum. Thereby, these magnetometers do not require any additional temperature-dependent calibrations outside the magnet and offer the possibility to measure vector magnetic fields in three dimensions with a single sensor. Its axial alignment is given by the crystal structure of the diamond host, which increases the accuracy of the field orientation measured with this sensor, compared to the classical arrangement of three Hall sensors. It is foreseeable that the magnetometer described here has the potential to be applied in various fields in the future, such as the characterization of ferromagnetic core solenoids or other magnetic arrangements.

Author(s): Stiller, M; N'Diaye, AT; Ohldag, H; Barzola-Quiquia, J; Esquinazi, PD; Amelal, T; Bundesmann, C; Spemann, D; Trautmann, M; Chasse, A; Hamed, HB; Adeagbo, WA; Hergert, W | Abstract: This work focuses on the generation of ferromagnetism at the surface of anatase TiO2 films by low-energy ion irradiation. Controlled Ar+-ion irradiation resulted in a thin (∼10) nm ferromagnetic surface layer. The intrinsic origin and robustness of the magnetic order has been characterized by x-ray magnetic circular dichroism at room temperature revealing that a Ti band is spin-polarized. These results, together with density functional theory calculations, indicate that Ti vacancy-interstitial pairs are responsible for the magnetic order. Superconducting quantum interference device measurements show the existence of a perpendicular magnetic anisotropy and a low remanent magnetization. Magnetic force microscopy reveals that this low remanence is due to oppositely aligned magnetic domains with magnetization vectors normal to the main surface. The weak domain-wall pinning, the magnetic anisotropy, together with the simplicity of the preparation method, open up interesting possibilities for future applications. As an example, single domain patterns of ∼1μm width and several μm length can be easily prepared.

We have studied the resistance, magnetoresistance and Hall effect of AlCu2Mn Heusler alloy thin films prepared by flash evaporation on substrates cooled at 4He liquid temperature. The as-prepared samples were amorphous and were annealed stepwise to induce the transformation to the crystalline phase. The amorphous phase is metastable up to above room temperature and the transition to the crystalline phase was observed by means of resistance measurements. Using transmission electron microscopy, we have determined the structure factor S ( K ) and the pair correlation function g ( r ) , both results indicate that amorphous AlCu2Mn is an electronic stabilized phase. The X-ray diffraction of the crystallized film shows peaks corresponding to the well ordered L 2 1 phase. The resistance shows a negative temperature coefficient in both phases. The magnetoresistance (MR) is negative in both phases, yet larger in the crystalline state compared to the amorphous one. The magnetic properties were studied further by anomalous Hall effect measurements, which were present in both phases. In the amorphous state, the anomalous Hall effect disappears at temperatures below 175 K and is present up to above room temperature in the case of crystalline AlCu2Mn.

In the last 43 years several hints were reported suggesting the existence of granular superconductivity above room temperature in different graphite-based systems. In this paper, some of the results are reviewed, giving special attention to those obtained in water and n-heptane treated graphite powders, commercial and natural bulk graphite samples with different characteristics as well as transmission electron microscope lamellae. The overall results indicate that superconducting regions exist and are localized at certain internal interfaces of the graphite structure. The existence of the rhombohedral graphite phase in all samples with superconducting-like properties suggests its interfaces with the Bernal phase as a possible origin for the high-temperature superconductivity, as theoretical calculations predict. High precision electrical resistance and magnetization measurements were used to identify a transition at $$T_\mathrm{c} \gtrsim 350~$$ K. To check for the existence of true zero resistance paths in the samples we used local magnetic measurements, which results support the existence of superconducting regions at such high temperatures.

Amorphous thin films were produced embedded in a diamond matrix by high energy He+-ion irradiation. The films were subsequently annealed at different temperatures up to graphitization and characterized by spectroscopic ellipsometry, Raman spectroscopy and electrical transport measurements. Changes of the optical constants depending on the annealing temperature were observed and using the optical effective-medium-approximation-model (EMA), assuming stochastic packing of hard spheres and a grain size estimate from the Raman results, an average distance between the spheres of about 6 A was found. This distance can be correlated with the localization length parameter of the variable range hopping transport mechanism obtained from resistance measurements.

Zinc oxide (ZnO) thin films were grown by pulsed layer deposition under an N2 atmosphere at low pressures on a- and r-plane sapphire substrates. Structural studies using X-ray diffraction confirmed that all films had a wurtzite phase. ZnO thin films on a- and r-plane sapphire have grown with orientations along the [0002] and [110] directions, respectively. Room temperature photoluminescence measurements indicate that the presence of native point defects (interstitial zinc, oxygen vacancies, oxygen antisites and zinc vacancies) is more preponderant for ZnO thin films grown on the r-plane sapphire substrate than the sample grown on the a-plane sapphire substrate. Room temperature impedance spectroscopy measurements were performed in an alternating current frequency range from 40 to 105 Hz in the dark and under normal light. An unusual positive photoresistance effect is observed at frequencies above 100 kHz, which we suggest to be due to intrinsic defects present in the ZnO thin films. Furthermore, an analysis of the optical time response revealed that the film grown on the r-plane sapphire substrate responds faster (characteristic relaxation times for τ1, τ2 and τ3 of 0.05, 0.26 and 6.00 min, respectively) than the film grown on the a-plane sapphire substrate (characteristic relaxation times for τ1, τ2 and τ3 of 0.10, 0.73 and 4.02 min, respectively).

We present a systematic study of the magnetic properties of semiconducting ZnFe$_2$O$_4$ thin films fabricated by pulsed laser deposition at low and high oxygen partial pressure and annealed in oxygen and argon atmosphere, respectively. The magnetic response is enhanced by annealing the films at 250$^{\circ}$C and diminished at annealing temperatures above 300$^{\circ}$C. The initial increase is attributed to the formation of oxygen vacancies after argon treatment, evident by the increase in the low energy absorption at $\sim$ 0.9 eV involving Fe$^{2+}$ cations. The weakened magnetic response is related to a decline in disorder with a cation redistribution toward a normal spinel configuration. The structural renormalization is consistent with the decrease and increase in oscillator strength of respective electronic transitions involving tetrahedrally (at $\sim$ 3.5 eV) and octahedrally (at $\sim$ 5.7 eV) coordinated Fe$^{3+}$ cations., Comment: 5 pages, 4 figures, 1 table

The formation of self-assembled structures is of great interest in the field of ferroelectric (FE)–ferromagnetic (FM) oxide interfaces with novel functionalities driven by the combination of strain relaxation and diffusion/segregation processes occurring during epitaxial growth of Mn-based heterostructures. In epitaxial bilayers and multilayers of La0.67Sr0.33MnO3 (LSMO)/BaTiO3 (BTO) on (001) SrTiO3, using the grazing-incidence small-angle X-ray scattering technique, self-assembled in-plane structural ordering with a repeated sequence of the bilayers has been found. This ordering has important magnetic consequences, as the materials show characteristics of a superparamagnetic type of behavior even with an increased number of bilayers. Transmission electron microscopy images reveal strain due to lattice mismatch between BTO and LSMO. This strain is greatly enhanced with the number of BTO and LSMO repetitions in a multilayer as an interdiffused columnar structure is formed. Electron energy loss spectra indicate a variation in oxygen environment from one monolayer to another within one LSMO layer. Reflectivity measurements with polarized neutrons prove that the LSMO layers are grown with sufficient periodicity but have a strongly reduced magnetic moment. This reduction is plausibly associated with interfacial strain and varying oxygen deficiencies within the layers or symmetry breaking effects which can turn the LSMO layer almost antiferromagnetic.

Recently published structural analysis and galvanomagnetic studies of a large number of different bulk and mesoscopic graphite samples of high quality and purity reveal that the common picture assuming graphite samples as a semimetal with a homogeneous carrier density of conduction electrons is misleading. These new studies indicate that the main electrical conduction path occurs within 2D interfaces embedded in semiconducting Bernal and/or rhombohedral stacking regions. This new knowledge incites us to revise experimentally and theoretically the diamagnetism of graphite samples. We found that the c-axis susceptibility of highly pure oriented graphite samples is not really constant, but can vary several tens of percent for bulk samples with thickness t ≳ 30 μm, whereas by a much larger factor for samples with a smaller thickness. The observed decrease of the susceptibility with sample thickness qualitatively resembles the one reported for the electrical conductivity and indicates that the main part of the c-axis diamagnetic signal is not intrinsic to the ideal graphite structure, but it is due to the highly conducting 2D interfaces. The interpretation of the main diamagnetic signal of graphite agrees with the reported description of its galvanomagnetic properties and provides a hint to understand some magnetic peculiarities of thin graphite samples.

Band-edge photoluminescence of zinc oxide materials reveals a phonon-assisted ultraviolet emission enhanced by Na doping. We report the synthesis of ZnO and Na-doped ZnO, the changes in structure and optical properties as a function of doping concentration and synthesis temperature. The obtained samples are nanoneedles, microparticles, and microwires. The structural effect of Na doping is studied by X-ray diffraction and Raman spectroscopy. Room temperature photoluminescence reveals a contribution of band bending at the surface of the nanostructures, decreasing with Na concentration. Franz–Keldysh effect predicts strong electric field localized in the depletion region and appears to be the cause of red-shift in UV band emission of nanoneedles in comparison to microwires. Our results indicate that Na doping introduces an acceptor level between 150 and 200 meV above the valence band. Fil: Straube, Benjamin. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Sólido; Argentina. Instituto de Física del Noroeste Argentino, INFINOA (CONICET-UNT); Argentina Fil: Bridoux, German. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Sólido; Argentina. Instituto de Física del Noroeste Argentino, INFINOA (CONICET-UNT); Argentina Fil: Zapata, María Cecilia. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Sólido; Argentina Fil: Ferreyra, Jorge Mario. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Sólido; Argentina Fil: Villafuerte, Manuel Jose. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Sólido; Argentina. Instituto de Física del Noroeste Argentino, INFINOA (CONICET-UNT); Argentina Fil: Simonelli, Gabriela. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Sólido; Argentina. Instituto de Física del Noroeste Argentino, INFINOA (CONICET-UNT); Argentina Fil: Ezquinazi, Pablo. Universidad de Leipzig; Alemania Fil: Rodríguez Torres, Claudia. Universidad Nacional de La Plata; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Pérez, Silvia Inés. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Sólido; Argentina

We investigated the influence of thickness reduction on the transport properties of graphite microflakes. Using oxygen plasma etching we decreased the thickness of highly oriented pyrolytic graphite (HOPG) microflakes from $\sim 100$~nm to $\sim 20$~nm systematically. Keeping current and voltage electrodes intact, the electrical resistance $R(T)$, the magnetoresistance (MR) and Raman spectra were measured in every individual sample and after each etching step of a few nm. The results show that $R(T)$ and MR can increase or decrease with the sample thickness in a non-systematic way. The results indicate that HOPG samples are inhomogeneous materials, in agreement with scanning transmission electron microscopy images and X-ray diffraction data. Our results further indicate that the quantum oscillations in the MR are not an intrinsic property of the ideal graphite structure but their origin is related to internal conducting interfaces., 20 figures, 28 pages, submitted to Carbon

Temperature and field dependent measurements of the electrical resistance of different natural graphite samples, suggest the existence of superconductivity at room temperature in some regions of the samples. To verify whether dissipationless electrical currents are responsible for the trapped magnetic flux inferred from electrical resistance measurements, we localized them using magnetic force microscopy on a natural graphite sample in remanent state after applying a magnetic field. The obtained evidence indicates that at room temperature a permanent current flows at the border of the trapped flux region. The current path vanishes at the same transition temperature $T_c\approx370$~K as the one obtained from electrical resistance measurements on the same sample. This sudden decrease of the phase is different from what is expected for a ferromagnetic material. Time dependent measurements of the signal show the typical behavior of flux creep of a permanent current flowing in a superconductor. The overall results support the existence of room-temperature superconductivity at certain regions in the graphite structure and indicate that magnetic force microscopy is suitable to localize them. Magnetic coupling is excluded as origin of the observed phase signal., Comment: 10 pages, 9 Figures

Nanomaterials based on ZnO have been used to build glucose sensors due to its high isoelectric point, which is important when a protein like Glucose Oxidase (GOx) is attached to a surface. It also creates a biologically friendly environment to preserve the activity of the enzyme. In this work we study the electrical transport properties of ZnO thin films (TFs) and single crystals (SC) in contact with different solutions by using impedance spectroscopy. We have found that the composition of the liquid, by means of the charge of the ions, produces strong changes in the transport properties of the TF. The enzyme GOx and phosphate buffer solutions have the major effect in the conduction through the films, which can be explained by the entrapment of carriers at the grain boundaries of the TFs. These results can help to design a new concept in glucose biosensing. Fil: Sappia, Luciano David. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Investigaciones Biológicas. Universidad Nacional de Tucumán. Instituto Superior de Investigaciones Biológicas; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Bioingeniería. Laboratorio de Medios e Interfases; Argentina Fil: Trujillo, Ricardo Matias. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Investigaciones Biológicas. Universidad Nacional de Tucumán. Instituto Superior de Investigaciones Biológicas; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Bioingeniería. Laboratorio de Medios e Interfases; Argentina Fil: Lorite, Israel. University of Leipzig; Alemania Fil: Madrid, Rossana Elena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Investigaciones Biológicas. Universidad Nacional de Tucumán. Instituto Superior de Investigaciones Biológicas; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Bioingeniería. Laboratorio de Medios e Interfases; Argentina Fil: Tirado, Monica Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Departamento de Nanomateriales y Propiedades Dieléctricas; Argentina Fil: Comedi, David Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Física. Laboratorio de Física del Sólido; Argentina Fil: Esquinazi, Pablo. University of Leipzig; Alemania

The effect of an external magnetic field B on the martensite phase transformation temperature T 0 was studied for single crystalline Fe72Pd28 thin films. From in-plane and out-of-plane aligned magnetic field dependent resistance measurements under various fields up to 9 T, T 0 was determined. The relation between T 0 and B is explained quantitatively by the Clausius–Clapeyron equation. The calculated value of 0.62 K/T is close to the values of the linear fits of 0.79 K/T and 0.76 K/T obtained from measurements with B aligned in-plane and out-of-plane.

We report a large circular photogalvanic effect (CPGE) in a 2DEG created at the interface of a semiconductor/insulator homojunction at the $(10\overline{1}0)$ surface of a Li-doped ZnO microwire by low energy proton implantation. We show that the CPGE originates from the Rashba spin-orbit interaction at the interface. Our sample arrangement allows tuning the spin-orbit interaction strength by manipulating the electron spin orientation via an external magnetic field. The results of the present work obtained at 305 K indicate the experimental realization of a persistent spin helix in ZnO.

The electrical transport properties of anodically grown TiO2 nanotubes was investigated. Amorphous nanotubes were anodically grown on titanium foil and transformed through annealing into the anatase phase. Amorphous and polycrystalline single nanotubes were isolated and contacted for measurements of the electrical resistance. Non-linear current-voltage characteristics were explained using the fluctuation induced tunneling conduction model. A clear enhancement of the conductance was induced in an insulating anatase nanotube through low-energy Ar/H ion irradiation. Confocal Raman spectroscopy shows that the annealed samples were in anatase phase and a blueshift due to phonon confinement was observed., Comment: 7 pages, 5 figures

Fe layers with thicknesses between 5 and 100 nm were sputtered on mesoporous nanostructured anatase TiO2templates. The morphology of these hybrid films was probed with grazing-incidence small-angle X-ray scattering and X-ray reflectivity, complemented with magnetic measurements. Three different stages of growth were found, which are characterized by different correlation lengths for each stage. The magnetic behavior correlates with the different growth regimes. At very small thicknesses the TiO2template is coated and a porous Fe film results, with in-plane and out-of-plane magnetization components. With increasing thickness, agglomeration of Fe occurs and the magnetization gradually turns mostly in plane. At large thicknesses, the iron grows independently of the template and the magnetization is predominantly in plane with a bulk-like characteristic.

We propose an explanation for the appearance of superconductivity at the interfaces of graphite with Bernal stacking order. A network of line defects with flat bands appears at the interfaces between two slightly twisted graphite structures. Due to the flat band the probability to find high temperature superconductivity at these quasi one-dimensional corridors is strongly enhanced. When the network of superconducting lines is dense it becomes effectively two-dimensional. The model provides an explanation for several reports on the observation of superconductivity up to room temperature in different oriented graphite samples, graphite powders as well as graphite-composite samples published in the past., 4 pages, two figures, JETP Letters (September 2014, in press)

Heterostructures with competing magnetic interactions are often exploited for their tailored new functionalities. Exchange bias is one such outcome of interfacial coupling across ferromagnetic-antiferromagnetic, multiferroic-ferromagnetic, two antiferromagnetic, or antiferromagnetic and paramagnetic interfaces. Apart from the usual horizontal shift of the hysteresis loop (exchange bias shift), a small `vertical shift´ of the hysteresis loops along the magnetization axis has also been seen, but it was always relatively small. Recently, an unusually large `vertical shift´ in epitaxial bilayer heterostructures comprising ferromagnetic La0.7Sr0.3MnO3 and multiferroic orthorhombic YMnO3 layers was reported. Here, using polarized neutron reflectometry, the magnetic proximity effect in such bilayers has been investigated. A detailed magnetic depth profile at the interface, elucidating the intrinsic nature of the vertical shift in such heterostructures, is reported. Further corroboration of this observation has been made by means of first-principles calculations, and the structural and electronic properties of YMnO3/La0.7Sr0.3MnO3 heterostructures are studied. Although in the bulk, the ground state of YMnO3 is an E-type antiferromagnet, the YMnO3/La0.7Sr0.3MnO3 heterostructure stabilizes the ferromagnetic phase in YMnO3 in the interface region. It is found that, in the hypothetical ferromagnetic phase of bulk YMnO3, the polarization is suppressed, and owing to a large difference between the lattice constants in the ab plane a strong magnetocrystalline anisotropy is present. This anisotropy produces a high coercivity of the unusual ferromagnetic YMnO3 phase at the interface, which is responsible for the large vertical shift observed in experiment. Fil: Paul, Amitesh. Technische Universitat Munchen; Alemania Fil: Zandalazini, Carlos Ivan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química (i); Argentina. University of Leipzig; Alemania Fil: Esquinazi, Pablo. University Of Leipzig; Alemania Fil: Autieri, Carmine. Uppsala University. Department Of Physics And Astronomy; Suecia Fil: Sanyal, Biplab. Uppsala University. Department Of Physics And Astronomy; Suecia Fil: Korelis, Panagiotis. Paul Scherrer Institut; Suiza Fil: Böni, Peter. Technische Universitat Munchen; Alemania

In this work we studied the changes of the electrical and optical properties after hydrogen plasma treatment of polycrystalline ZnO thin films grown under different atmosphere conditions. The obtained results show that the gas used during the growth process plays an important role in the way hydrogen is incorporated in the films. The hydrogen doping can produce radiative and non-radiative defects that reduce the UV emission in ZnO films grown in oxygen atmosphere but it passivates defects created when the films are grown in nitrogen atmosphere. Impedance spectroscopy measurements show that these effects are related to regions where hydrogen is mostly located, either at the grain cores or boundaries. We discuss how hydrogen strongly influences the initial semiconducting behavior of the ZnO thin films.

Nanocrystalline graphite thin films were synthesized at atmospheric pressure by pyrolysis of toluene without a catalyst and using an aerosol assisted chemical vapor deposition method. The film thickness is controllable by the process time and pyrolysis temperature. Micro-Raman spectroscopy reveals that the films have nanocrystalline graphite structure. Energy dispersive X-ray spectroscopy confirms the high purity of the carbon films, indicating that all properties measured are only due to the graphitized carbon thin film. Electronic transport properties such as temperature dependent resistance, Hall effect and magnetoresistance show the typical behavior of low-dimensional carbon based materials.

Millimeter size high quality crystals of CuGaS2 were grown by chemical vapor transport. The highly ordered chalcopyrite structure is confirmed by X-ray diffraction and Raman spectroscopy. According to energy dispersive X-ray spectroscopy the composition of the crystals is very close to the formula CuGaS2. Room temperature photoluminescence measurements indicate the presence of an emission peak at about 2.36 eV that can be related to a donor–acceptor pair transition. The electrical resistance as a function of temperature is very well described by the Mott variable range hopping mechanism. Room temperature complex impedance spectroscopy measurements were performed in the alternating current frequency range from 40 to 107 Hz in the dark and under normal light. According to the impedance spectroscopy data the experimental results can be well described by two circuits in series, corresponding to bulk and grain boundary contributions. An unusual positive photoresistance effect is observed in the frequency range between 3 and 30 kHz, which we suggest to be due to intrinsic defects present in the CuGaS2 crystal.

Placing several electrodes at the edge of a micrometer-size Sri Lankan natural graphite sample at distances comparable to the size of the internal crystalline regions, we found record values for the change of the resistance with magnetic field. At low temperatures and at $B \sim 21$T the magnetoresistance (MR) reaches $\sim 10^7$%. The MR values exceed by far all earlier reported ones for graphite and they are comparable or even larger (at $T > 50$K) than the largest reported in solids including the Weyl semimetals. The origin of this large MR lies in the existence of highly conducting 2D interfaces aligned parallel to the graphene planes.

We have studied the magnetoresistance (MR) of hydrogen plasma-treated pure ZnO wires of tens of micrometer diameter at different temperatures. A negative MR of 1% at 8 T applied field is measured for all wires at 4 K, independent of the temperature (300 K … 773 K) used during the hydrogen treatment. However, a positive MR develops, the higher the treatment temperature. The MR can be explained with a semiempirical model taking into account local magnetic moments and the s–d exchange interaction. These results together with field anisotropy in the MR indicate the appearance of magnetic order due to the hydrogen treatment in agreement with recently published reports on the influence of hydrogen in bulk ZnO single crystals. Hydrogen doping may provide a way to trigger defect-induced magnetism in small oxide structures.

Epitaxial zinc ferrite thin films are grown on (001) SrTiO3 substrates by pulsed laser deposition. The magnetic anisotropy of the films is examined by superconducting quantum interference device magnetometry and ferromagnetic resonance, with various in-plane and out-of-plane orientations of the magnetic field. The volume magnetization of the films is found to be around 55 kA/m. The existence of a cubic magnetic anisotropy is shown, with the easy axes oriented along the [111] crystallographic directions. The films show ferrimagnetic properties with a Curie temperature higher than 350 K. The nature of a strong “easy-plane”-type bimodal anisotropy in the films is discussed.

The electric noise can be an important limitation for applications of conducting elements of size in the nanometer range. The intrinsic electrical noise of prospective materials for opto-spintronics applications like ZnO has not been characterized yet. In this study we have investigated the conductivity fluctuations in 10~nm thick current paths produced by proton implantation of ZnO microwires at room temperature. The voltage noise under a constant dc current bias in undoped as well as in Li-doped microwires is characterized by $1/f^a$ power spectra with $a \sim 1$. The noise intensity scales with the square of the bias current pointing out to bias-independent resistivity fluctuations as a source of the observed noise. The normalized power spectral density appears inversely proportional to the number of carriers in the probed sample volume, in agreement with the phenomenological Hooge law. For the proton-implanted ZnO microwire and at 1~Hz we obtain a normalized power spectral density as low as $\sim 10^{-11}~$Hz$^{-1}$., 10 pages,3 figures to be published in Nanotechnology (2016)

The electrical, in-plane resistance as a function of temperature $R(T)$ of bulk and mesoscopic thin graphite flakes obtained from the same batch was investigated. Samples thicker than $\sim 30$ nm show metalliclike contribution in a temperature range that increases with the sample thickness, whereas a semiconductinglike behavior was observed for thinner samples. The temperature dependence of the in-plane resistance of all measured samples and several others from literature can be very well explained between 2 K and 1100 K assuming three contributions in parallel: a metalliclike conducting path at the interfaces between crystalline regions, composed of two semiconducting phases, i.e. Bernal and rhombohedral stacking. From the fits of $R(T)$ we obtain a semiconducting energy gap of $110 \pm 20$meV for the rhombohedral and $38\pm 8 $meV for the Bernal phase. The presence of these crystalline phases was confirmed by x-ray diffraction measurements. We review similar experimental data from literature of the last 33 years and two more theoretical models used to fit $R(T)$., 11 pages, 10 figures

This chapter reviews the experimental evidence obtained for the existence of embedded interfaces between crystalline regions with Bernal and/or rhombohedral stacking order in usual graphite samples, and their relationship with the observed metallic and superconducting behavior.

This chapter reviews the experimental evidence obtained for the existence of magnetic order triggered by defects and/or non-magnetic ions in graphite with very low magnetic impurities concentrations. We demonstrate how and where this magnetically ordered state is produced in graphite bulk, thin flakes and powders in a reproducible manner. The experimental evidence obtained in the last 12 years leaves no doubt that this ferro- or ferrimagnetism is intrinsic of the graphite structure containing certain defects and not related to magnetic impurities. The main aim of this chapter is to provide the reader the key experimental facts and characterisation methods necessary to arrive at such a conclusion. We restrict the discussion to results obtained from two main magnetic characterisation methods, namely magnetization and transport measurements. In addition, we describe in Sect. 3.2 the main characterization method for trace element analysis to obtain the magnetic and non-magnetic impurities concentration. The chapter is concluded with an overview of further, independently obtained evidence for magnetic order in graphitic materials from literature.

An emerging branch of electronics, the optospintronics, would be highly boosted if the control of magnetic order by light is implemented in magnetic semiconductors nanostructures being compatible with the actual technology. Here we show that the ferromagnetic magnetization of low Fe-doped ZnO nanowires prepared by carbothermal process is enhanced under illumination up to temperatures slightly below room temperature. This enhancement is related to the existence of an oxygen vacancy V$_{\rm O}$ in the neighbouring of an antiferromagnetic superexchange Fe$^{3+}$-Fe$^{3+}$ pair. Under illumination the V$_{\rm O}$ is ionized to V$_{\rm O}^+$ giving an electron to a close Fe$^{3+}$ ion from the antiferromagnetic pair. This light excited electron transition allows the transition of Fe$^{3+}$ to Fe$^{2+}$ forming stable ferromagnetic double exchange pairs, increasing the total magnetization. The results here presented indicate an efficient way to influence the magnetic properties of ZnO based nanostructures by light illumination at high temperatures., 4 pages with 4 figures, to be published in Appl. Phys. Lett. (2016)

Trying to dope graphite flakes we found that the magnetization of pure, several tens of micrometers grain size graphite powder and after a simple treatment with pure water shows clear and reproducible granular superconducting behavior with a critical temperature above 300K. The observed magnetic characteristics as a function of temperature, magnetic field and time, provide evidence for weakly coupled grains through Josephson interaction, revealing the existence of superconducting vortices., Comment: 19 pages, 5 figures; http://onlinelibrary.wiley.com/doi/10.1002/adma.201202219/abstract (2012)

Substrate-attached and freestanding single crystalline Fe70Pd30 ferromagnetic shape memory alloy membranes, which were synthesized by molecular beam epitaxy on MgO (001) and later released from their substrates, are characterized with respect to their structural, thermal and magnetic properties. Residing in the two-phase region of austenite and the correct martensite phase with face centered tetragonal (fct) structure at room temperature, they reveal martensite transition with little hysteresis at 326 K and 320 K, respectively. Comparing substrate-attached with freestanding films, which show fundamentally different magnetic fingerprints, it is proposed that domain structure is capable of posing a bias on the austenite fct-martensite phase transition by favoring martensite variants with their easy axis aligned along the field – just as the substrate constitutes a mechanical constraint on the transition. If confirmed, this would suggest thermo-magnetic actuation as an alternative where only moderate magnetic fields are feasible, but moderate temperature changes are possible.

HoMnO 3 films were grown on pure and Nb-doped SrTiO 3 (001) substrates by pulsed laser deposition. The films grew epitaxially with the c -axis along the substrate normal. Varying the deposition temperature between 650 and 850 °C did not significantly affect the structural and magnetic properties of the films, whereas growth in oxygen partial pressures below 0.01 mbar lead to a degradation of the structural properties. Some of the films had a ferromagnetic-like magnetic phase transition at about 45 K, probably related to Mn 3 O 4 precipitates; this magnetic response was isotropic. The Ho sublattice was found to be paramagnetic down to 5 K, but showing a pronounced anisotropy with the c -axis being the hard axis. The films showed a distinct dielectric anomaly at 16 K that depended on voltage and slightly on frequency in the range between 1 kHz and 1 MHz. The magnetoelectric effect was large with an in-plane field of 8 T suppressing the dielectric anomaly completely.

We have studied the spin transport on a 30-nm thick and several micrometer long oriented graphite flake using a spin-valve configuration with four ferromagnetic Co electrodes of different widths and several $$\upmu\hbox{m}$$ separation. A 5-nm thin Pt layer has been introduced in between the ferromagnetic Co injector/detector and the graphite surface. In spite of the conductivity mismatch problem, efficient electrical spin injection and detection in graphite has been achieved. The magnetoresistance in the local and half-local electrodes shows clear maxima with symmetry around zero field. The spin transport can be detected up to 150 K.

We have measured the magnetization of bulk samples of highly oriented pyrolytic graphite (HOPG) at magnetic fields applied parallel and perpendicular to the graphene layers. Within experimental error the intrinsic ferromagnetic signals of the samples show similar magnetic moments at saturation for the two magnetic field directions, in contrast to recently published data (J. Cervenka et al., Nat. Phys. 5 (2009) 840). To check that the SQUID device provides correctly the small ferromagnetic signals obtained after subtracting the 100 times larger diamagnetic background, we have prepared a sample with a superconducting Pb-film deposited on one of the HOPG surfaces. We show that the field dependence of the measured magnetic moment and after the background subtraction is highly reliable even in the sub- μ emu range providing the real magnetic properties of the embedded small ferromagnetic and superconducting signals.

Point-contact Andreev reflection and magnetoresistance measurements were done using amorphous tungsten carbide (WCx) superconductors. Superconducting tips as well as microwires were grown directly under a focused Ga+-ion beam (FIB) on pre-patterned samples. Using e-beam lithography, the electrical contacts were prepared later using a special geometry. Current–voltage measurements as a function of temperature and magnetic field clearly showed the signatures of Andreev reflection. We observed anomalies in the differential conductivity at voltages above the energy gap values. These anomalies can be well understood as due to a weak-link formation with narrow band properties contributing in parallel at the interfaces of the contacts. We also observed Andreev oscillations as a function of magnetic field similar to those found recently in specially prepared normal–superconducting–normal nanostructures and multigraphene samples. In some of the structures, we were able to produce interfaces in which clear granular behavior in a certain temperature region was observed, such as, for example, an anomalous field hysteresis loop compatible with the existence of granular superconductivity, similar to that found also in thin mesoscopic graphite samples.

We have studied the magnetoresistance (MR) of thin highly oriented pyrolytic graphite mesoscopic samples without and with micro-constrictions of different widths between the voltage electrodes. The MR for fields parallel to the c-axis shows an anomalous hysteresis loop compatible with the behavior expected for granular superconductors. The smaller the constriction width, the larger is the anomalous hysteresis and the higher the temperature for its observation. Our results support the existence of granular superconductivity probably embedded at interfaces between crystalline graphite regions.

We have studied the influence of intrinsic and extrinsic defects in the magnetic and electrical transport properties of Co-doped ZnO thin films. X ray absorption measurements show that Co substitute Zn in the ZnO structure and it is in the 2+ oxidation state. Magnetization (M) measurements show that doped samples are mainly paramagnetic. From M vs. H loops measured at 5 K we found that the values of the orbital L and spin S numbers are between 1 and 1.3 for L and S = 3/2, in agreement with the representative values for isolated Co 2+. The obtained negative values of the Curie-Weiss temperatures indicate the existence of antiferromagnetic interactions between transition metal atoms., Comment: To be published in Journal of Materials Science