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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

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.

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.

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.

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.

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.

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 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.

Low-energy muon spin rotation and SQUID magnetization measurements were performed on proton-irradiated and non-irradiated highly oriented pyrolytic graphite samples. The samples were found to be ferromagnetic above and below room temperature and to include a substantial temperature-dependent surface contribution. Assuming uniformity, the thickness of the magnetic surface layer was estimated to be 13(2) nm. The discovered surface magnetism is intrinsic and not due to irradiation.

Complex impedance spectroscopy data in the frequency range 16Hz < f < 3 MHz at room temperature were acquired on pure ZnO single crystal and thin film. The measured impedance of the ZnO samples shows large changes with time after exposure to or covering them from visible light. At fixed times Cole-Cole-diagrams indicate the presence of a single relaxation process. A simple analysis of the impedance data allows us to obtain two main relaxation times. The behavior for both, ZnO crystal and thin film, is similar but the thin film shows shorter relaxation times. The analysis indicates the existence of two different photo-active defects with activation energies between ~0.8 eV and ~1.1 eV., 11 pages, 9 figures. Solid state communications, in press

Defect induced magnetic order is a new phenomenon in material science that refers to the triggering and manipulation of magnetic order and magnetic moments in nominally non-magnetic materials by lattice defects and/or non-magnetic add atoms. A noticeable example of this effect is the magnetic order at room temperature produced by proton irradiation of graphite. In this work we have managed to increase the ferromagnetic signal by cooling the graphite samples down to 110 K during proton irradiation, diminishing in this way annealing effects. SQUID measurements of the magnetization show a fluence dependent Curie temperature. The longitudinal magnetoresistance shows an irreversible behavior similar to that found in ferromagnetic films indicating spin/domain reorientation effects. The observed magnetoresistance effects and Curie temperatures above room temperature are promising facts that may lead to useful carbon-based devices in the near future.

Zn 90 Mn 7.5 Cu 2.5 O 100 films have been prepared on a-plane sapphire substrates by pulsed laser deposition. Paramagnetism mainly caused by Mn 2+ ions was observed in the films from room temperature down to 2 K. Magnetotransport properties (magnetoresistance (MR) and Hall effect) were studied from 5 K to 290 K up to a field of 6 T. Negative MR was observed at temperature above 100 K. Low field positive MR and high field negative MR was observed at 5 K. Clear anomalous Hall effect with a kink at low field was observed below 20 K, indicating that there exist two different scattering mechanisms for the Mn 2+ and Cu 2+ ions.

We have studied the field and temperature dependence of the magnetic moment of single crystalline sapphire substrates with different surface orientations. All the substrates show a ferromagnetic behavior that partially changes after surface cleaning. The amount of magnetic impurities in the substrates was determined by particle induced X-ray emission. The overall analysis of the data indicates that the magnetic impurities very likely contribute to the measured ferromagnetic behavior but does not rule out completely intrinsic contributions. Our work stresses the necessity to use other than bulk characterization methods for the study of weak ferromagnetic signals of thin films grown on oxide substrates.

Ferromagnetic, semi-insulating Mn-alloyed ZnO films with a Curie temperature above 375 K have been grown by pulsed laser deposition on c-plane sapphire substrates. Antiferromagnetic coupling is revealed by temperature-dependent magnetization measurements. The antiferromagnetic coupling would be compatible with the observed weak ferromagnetism by assuming that the magnetic moments order antiferromagnetically but nonparallel (canted). We find a clear correlation between coercivity and mosaicity of the ferromagnetic Mn-alloyed ZnO films and explain it on the basis of a coercivity mechanism known from soft magnetic materials.

Carbon–sulfur films were grown by pulsed laser deposition at room temperature using different graphite–sulfur mixtures as targets. The structure of the films was characterized by X-ray diffraction and transmission electron microscopy. The composition and the chemical bonds were analyzed by Rutherford-backscattering spectroscopy, X-ray photoelectron spectroscopy and energy dispersive X-ray analysis. The films were composed of amorphous carbon with sp 2 -, sp 3 - and S–C–C–S bonds and textured graphite on the top of the film. The thin graphite layer on top of the carbon–sulfur films is highly oriented, comparable to highly oriented pyrolytic graphite, and free of sulfur in the graphite lattice. The lateral size of the oriented graphite grains in the films was up to 8 μm. Magnetic measurements reveal that the films prepared under the conditions of our study show neither magnetic ordering nor superconductivity in the studied temperature range T > 2 K.

In this study, ferromagnetic microstructures in highly oriented pyrolytic graphite and superparamagnetic spots in polyimide foils were created by 2.25 MeV proton microbeam irradiation and characterized using atomic and magnetic force microscopy. For this purpose, graphite samples were irradiated with cross-like patterns of 15 μm × 15 μm size using ion fluences in the range of (0.003–2.5) × 1018 cm−2. The irradiated crosses showed strong magnetic signals and a complex domain structure in the magnetic images depending on the geometrical dimensions of the crosses. Furthermore, polyimide foils were irradiated with microspots and fluences in the range of (0.016–3.1) × 1019 cm−2. Magnetic force microscopy shows very strong phase shifts in these irradiated areas.

Recently, magnetic order in highly oriented pyrolytic graphite (HOPG) induced by proton broad- and microbeam irradiation was discovered. Theoretical models propose that hydrogen could play a major role in the magnetism mechanism. We analysed the hydrogen distribution of pristine as well as irradiated HOPG samples, which were implanted to micrometer-sized spots as well as extended areas with various doses of 2.25 MeV protons at the Leipzig microprobe LIPSION. For this we used the sensitive 3D hydrogen microscopy system at the Munich microprobe SNAKE. The background hydrogen level in pristine HOPG is determined to be less than 0.3 at-ppm. About 4.8e15 H-atoms/cm^2 are observed in the near-surface region (4 um depth resolution). The depth profiles of the implants show hydrogen located within a confined peak at the end of range, in agreement with SRIM Monte Carlo simulations, and no evidence of diffusion broadening along the c-axis. At sample with microspots, up to 40 at-% of the implanted hydrogen is not detected, providing support for lateral hydrogen diffusion., accepted for publication in Nucl. Instr. and Meth

We present Hall effect measurements on highly oriented pyrolytic graphite that indicate the occurrence of the integer quantum-Hall-effect. The evidence is given by the observation of regular plateau-like structures in the field dependence of the transverse conductivity obtained in van der Pauw configuration. Measurements with the Corbino-disk configuration support this result and indicate that the quasi-linear and non-saturating longitudinal magnetoresistance in graphite is governed by the Hall effect in agreement with a recent theoretical model for disordered semiconductors., 3 figures, to be published in Solid State Communication (2006)

Room-temperature ferromagnetic Zn1−x(TM)xO semiconductor thin films have been grown by pulsed laser deposition (PLD) on c -plane sapphire substrates and the effect of the substitution of Zn2+ ions by transition metal ions (Mn2+ or Ti2+) has been investigated. In contrast to ZnTiO thin films, the grain size of the ZnMnO thin films can be controlled in a wide range by the PLD growth conditions. The magnetic properties of the Zn1−x(TM)xO films have been investigated by a superconducting quantum interference device (SQUID). For semi-insulating ZnMnO films with an optimized grain size, we observed a weak ferromagnetism for temperatures up to 400 K [M. Diaconu, H. Schmidt, H. Hochmuth, M. Lorenz, G. Benndorf, J. Lenzner, D. Spemann, A. Setzer, K.-W. Nielsen, P. Esquinazi, M. Grundmann, Thin Solid Films 486 (2005) 117–121], while the ZnTiO films were only paramagnetic or superparamagnetic. This is expected with respect to the micromagnetic model.

The incorporation of 3d transition metals into ZnO can produce ferromagnetism above room temperature. For Mn-doped ZnO films grown at low temperatures around 500 °C by pulsed laser deposition, the ferromagnetic behavior is correlated to their textured structure. For optimized films showing homogeneous magnetic domain formation in magnetic force microscopy, the saturation magnetization and coercive field strength amount to 0.013 emu/g and 234 Oe at 300 K, respectively. Compared to undoped ZnO films, the luminescence of Mn-doped ZnO films is strongly quenched. However, luminescence can be observed around antiferromagnetic MnO or MnO2 clusters that crystallize during the growth process on the film surface.

An epitaxial magnetite (Fe 3 O 4 ) film has been deposited by the pulsed laser deposition (PLD) technique on a bicrystal substrate. Magnetoresistance measurements on patterned stripes (width=20 μm) across the bicrystal grain boundary have not revealed a distinguishable grain boundary contribution. Post-annealing of the film has a strong influence on the relaxation degree of the film and on the density of antiphase boundaries. Magnetotransport measurements of the micrometer-patterned annealed film show significantly enlarged high-field magnetoresistance values at low temperatures.

In this study ferromagnetic microstructures in highly oriented pyrolytic graphite (HOPG) were created by 2.25 MeV proton microbeam irradiation and characterized using AFM/MFM and SQUID measurements. For this purpose, spots of 1.8 × 1.8 μm2 size as well as cross-like patterns of 15 μm × 15 μm size were irradiated with different ion doses. Furthermore, arrays of 600 μm × 600 μm size containing 100 × 100 micro-spots were produced with ion doses of ∼0.3 nC/μm2. As calculated by SRIM2003 simulations, the corresponding defect densities in the near surface region are between 3 × 1018 cm−3 and 1 × 1021 cm−3 for the range of ion doses used. The irradiated crosses showed strong magnetic signals and a complex domain structure in the MFM images. SQUID measurements on a HOPG sample successively irradiated with micro-spots arrays showed that the irradiation increased the magnetic moment of the sample up to ∼0.6 × 10−5 emu and the remanent magnetization Mr(B = 0) up to 4.5 × 10−4 emu/g. However, annealing effects during irradiation have to be carefully taken into account as they can result in a complete loss of the magnetic ordering in the irradiated areas.

The interface between magnetite ( Fe 3 O 4 ) and Nb-doped SrTiO 3 shows typical characteristics of a Schottky barrier. Current–voltage characteristics depend on the applied magnetic field. Within a simple model the spin polarization of the magnetite layers was derived.

The pulsed laser deposition technique has been used to produce magnetite (Fe 3 O 4 ) films with thickness ranging from 26 to 320 nm grown on MgAl 2 O 4 (1 0 0) substrates. Microstructural characterisation of the films has revealed enlarged grains with increasing thickness, i.e. deposition time. Magnetisation hysteresis loops have been measured at different temperatures in the range 25–300 K for three samples: two films with thickness of 26 and 320 nm deposited on MgAl 2 O 4 (1 0 0) and, for comparison, a 30 nm thick film deposited on MgO (1 0 0). The evolution of the remanence and the coercivity with temperature shows significant differences between the thinner films and the thicker one which are explained in terms of the effect of the interfacial strain as well as the influence of the deposition time on the grain sizes and the variation in the density of antiphase boundaries. All the studied films exhibit a minimum of the coercivity at about 130 K, the temperature at which the magnetocrystalline anisotropy constant K 1 vanishes for the bulk material.

The critical current density and flux-creep activation energy of YBa 2 C 3 O 7 rings were determined from ac-susceptibility measurements. A novel approach was developed theoretically and experimentally to derive the flux-creep exponent from higher susceptibility harmonics. An alternative method using a scaling approach yielded similar sample-to-sample variations, but qualitatively different values for the flux-creep exponent. This is discussed considering the limits of validity of the logarithmic approximation to the flux-line activation energy.

Longitudinal ( κ xx ) and transverse (Hall) ( κ xy ) measurements of the thermal transport in YBa 2 Cu 3 O 7 crystals for magnetic fields parallel and perpendicular to the c -axis at different angles with respect to the heat current direction show: (1) the temperature dependence of the inelastic scattering rate of quasiparticles (QP) follows ∼ T 4 at 0.2 T / T c ⩽1, independently of the method used to estimate the electronic contribution; (2) for parallel fields, fourfold and twofold angular patterns were obtained for κ xx and κ xy at T T the fourfold pattern vanishes and transforms into a one-fold oscillation; (3) two processes, i.e. Andreev scattering and Doppler shift, appear to determine the scattering of the QP by vortices. The overall results support a d-wave symmetry for the superconducting order parameter up to T c .

The magnetoresistance of step edges in magnetite films was systematically studied. An enhancement of the magnetoresistance by the introduction of step edges was observed, especially in the high-field regime. This was modelled by spin-disorder scattering. The analysis revealed magnetic cluster formation at the step edges.

In this study ferromagnetic microstructures were created in highly oriented pyrolitic graphite (HOPG) by proton microbeam irradiation. For this purpose, spots of 1, 2 × 2 and 3.5 × 3.5 μm 2 were irradiated with ion fluences ranging from 3.1 × 10 16 to 4.7 × 10 19 cm −2 using 2.25 MeV proton and 1.5 MeV helium ion microbeams. As calculated by SRIM2003 simulations, the corresponding defect densities in the near surface region are between 3 × 10 19 and 4 × 10 22 cm −3 for the proton irradiation. The irradiated spots, which were characterized with atomic force microscopy (AFM) and magnetic force microscopy (MFM), show a clear swelling of the HOPG crystal proportional to the ion fluence. Strong magnetic force gradients were found even for the lowest proton fluences. Contrary to the topography, the magnetic force gradient changes after the application of a magnetic field. This rules out that the magnetic signals arise from topographical changes. Therefore, the MFM measurements reveal the existence of ferromagnetic domains in localized, disordered HOPG regions. On the contrary, helium ion irradiation of HOPG leads to much weaker magnetic signals only, which indicates that hydrogen plays a significant role in the formation of the magnetic moments and ordering. Very recently, the existence of ferromagnetism in ion beam irradiated HOPG was confirmed by SQUID measurements.

Fe 3 O 4 films of different thickness were epitaxially grown on single crystal MgO and MgAl 2 O 4 substrates and studied by X-ray absorption, X-ray photoemission spectroscopy and X-ray fluorescence (XF). All samples were prepared by pulsed laser deposition (PLD) and some of them were covered by in situ prepared thin BaTiO 3 protecting layer. The photoemission (VG ESCALAB 220i-XL) and absorption (total electron yield mode) measurements on epitaxial Fe 3 O 4 films demonstrate the important role of the protecting layer to prevent the further oxidation of Fe 3 O 4 to Fe 2 O 3 in the case of thin films (2.5 nm range). The resonant XF (X-ray monochromator XES 300 using synchrotron radiation at the U41-PGM beam-line at BESSY II) measurements at excitation energy 710 eV (Fe L 3 absorption edge) demonstrate the effect of the Fe 3d–O 2p hybridization in the iron oxide layers. Information about local partial densities of states was obtained for the oxidized Fe 3 O 4 film from a comparative analysis of the Fe L and O K XF and the valence-band photoelectron spectra.

The recently found ferromagnetic signals in pure graphite and in polymerised fullerenes are received mainly with scepticism by most of the scientific community. Actually, before those results were published, there were already several reports claiming an unusually large magnetization in pyrolytic carbon and oxidised fullerenes, without attracting the attention of the community. This scepticism may be well founded since not always a careful and systematic impurity study was provided to quantify the influence of ferromagnetic impurities. In this article and after a brief review of the existing data on magnetic carbon, we present and discuss recently obtained results on the ferromagnetism (or ferrimagnetism) induced by proton irradiation on pure pyrolytic graphite, which strengthen the importance of hydrogen in the formation of the magnetic ordering. The overall results indicate that room-temperature ferromagnetism in carbon-based structures containing only p- and s-electrons is a reality.

In order to study a possible magnetic proximity effect in magnetite–carbon structures, we have performed magnetization measurements of graphite–magnetite composites with different mass ratios as well as the measurement of the magnetoresistance of one of them and of the magnetization of a magnetite–carbon bilayer. The overall results do not indicate the induction of bulk ferromagnetism in graphite and disordered carbon structures through their contact with magnetite.

With a multiterminal technique we have studied the behavior of the in-plane voltage in high-Tc superconducting YBa2Cu3O7 films as a function of temperature T, magnetic field B and distance from the input current electrodes. Regions with different superconducting transition temperatures were obtained by selective sputtering between gold electrodes. The non-local voltages, measured outside of the direct current path, show an excess signal near the critical temperature Tc(B) which resembles that measured in different superconductors and attributed partially to charge imbalance effects, the presence of phase-slip centers and vortex–antivortex-interactions. We have determined the decay length of the non-local voltages as a function of field and temperature. We compared and analyzed the excess voltage and decay length with a current distribution model and an equivalent circuit of resistances. This model accounts for all the measured behavior of the non-local voltages and of the decay length. We discuss also estimates of the decay length based on charge imbalance and vortex correlations models.

In this study a C60 polymer has been characterized for the first time with respect to impurity content and ferromagnetic properties by laterally resolved particle induced X-ray emission (PIXE) and magnetic force microscopy (MFM) in order to prove the existence of intrinsic ferromagnetism in this material. In the sample studied the main ferromagnetic impurity found was iron with an average concentration of 175 ± 16 μg/g within the sample volume probed by the ion beam. However, the Fe distribution is very inhomogeneous and characterized by micrometer-large impurity grains of almost pure iron surrounded by an almost pure carbon matrix. With MFM, the ferromagnetic properties have been investigated both in pure and contaminated regions of the sample as determined by PIXE. We found that ∼30% of the area of pure regions (concentration of magnetic impurities

A C60 polymer has been characterized for the first time with respect to impurity content and ferromagnetic properties by laterally resolved particle induced X-ray emission (PIXE), superconducting quantum interference device (SQUID) and magnetic force microscopy (MFM) in order to detect intrinsic ferromagnetic domains. In parts of the pure regions (concentration of magnetic impurities

The phototransformation of bulk C60 and laser- and electron-beam treatment of C60 films in air changes their magnetic properties. Nonlinear magnetization is observed only for samples irradiated in the presence of oxygen, while, in the case of pressure-polymerized C60, oxygen adversely affects the magnetic properties. The contrasting roles of oxygen in these processes are discussed. Magnetic force microscopy shows that laser- and electron-beam irradiation of fullerene films produces magnetic images which are highly correlated with the topographic images.

We have studied the intensity dependence of the internal friction Q−1 and of the relative change of sound velocity Δv/v using the vibrating reed technique at very low temperatures (0.1 mK

Thermomagnetic flux-jump instabilities in type-II superconductors reduce the absolute irreversible magnetization relative to the isothermal critical state value at low enough magnetic fields. The recovering of the isothermal critical state with increasing field leads to the second magnetization peak (SMP) in the magnetization curve M(H) as has been unambiguously demonstrated for Nb films. The low-field SMP takes place in many other conventional and unconventional superconductors, and is often attributed to a critical current enhancement associated with a phase transition(s) in the vortex matter. The analysis of the most recent experimental results given in this communication provides clear evidence that the restoration of the isothermal critical state is responsible for the SMP in both conventional and unconventional superconductors and casts doubts about the interpretation based on phase transition(s) of the vortex matter.

With the vibrating reed technique we have studied the magnetoelastic behavior and the dynamical response of amorphous and crystalline magnets in the ferromagnetic and re-entrant spin glass states. The measurements have been performed as a function of magnetic field, temperature and angle between the sample’s main area and the applied field. The resonance frequency of the reed is influenced by a static contribution (pole effect), a dynamic contribution with a large effect due to the pinning of magnetic domains and magnetostrictive effects. When the field is applied perpendicular to the main area of the sample, a large decrease of the resonance frequency is observed which is rather independent of the material characteristics and is not related to a change of the Young modulus. A field misalignment of a few degrees strongly affects the field dependence of the resonance frequency. We briefly discuss the influence of the re-entrant spin glass state on the magnetoelastic behavior and the characteristics of the internal friction of vibrating ferromagnets in a magnetic field.

Magnetization hysteresis loop $M(H)$ measurements performed on a single crystalline Ba$_{0.63}$K$_{0.37}$BiO$_3$ superconducting thick film reveal pronounced sample geometry dependence of the "second magnetization peak" (SMP), i.e. a maximum in the width of $M(H)$ occurring at the field $H_{\rm SMP}(T)$. In particular, it is found that the SMP vanishes decreasing the film dimension. We argue that the observed sample geometry dependence of the SMP cannot be accounted for by models which assume a vortex pinning enhancement as the origin of the SMP. Our results can be understood considering the thermomagnetic instability effect and/or non-uniform current distribution at $H < H_{\rm SMP}$ in large enough samples.

Torque magnetometry was used to investigate the magnetic anisotropy of epitaxial Fe 3 O 4 films of thicknesses 420 and 40 nm grown by pulsed laser deposition on (0 0 1) MgO substrates. Torque measurements at 130 K in the (1 1 0) plane allow us to evaluate the influence of shape and stress anisotropy. As in bulk material field-cooled measurements performed at T =5 K show that the magnetic anisotropy is strongly influenced by the direction of the magnetic field applied above the Verwey temperature. In contrast to bulk magnetite we find smaller effective contributions of the magneto-crystalline anisotropy. In zero-field-cooled films our measurements suggest a tendency to a preferential out-of-plane orientation of the magnetic easy axis, but the dominating shape anisotropy favours an in-plane orientation of the magnetization.

The resonance frequency of vibrating ferromagnetic reeds in a homogeneous magnetic field can be substantially modified by intrinsic and extrinsic field-related contributions. Searching for the physical reasons of the field-induced resonance frequency change and to study the influence of the spin glass state on it, we have measured the low-temperature magnetoelastic behavior and the dynamical response of vibrating amorphous and polycrystalline ferromagnetic ribbons. We show that the magnetoelastic properties depend strongly on the direction of the applied magnetic field. The influence of the re-entrant spin glass transition on these properties is discussed. We present clear experimental evidence that for applied fields perpendicular to the main area of the samples the behavior of ferromagnetic reeds is rather independent of the material composition and magnetic state, exhibiting a large decrease of the resonance frequency. This effect can be very well explained with a model based on the dynamical response of the reed and the magnetomechanical pole effect within a domain rotation model and is not related to magnetoelasticity.

We have studied the magnetic properties of the barium hexaferrite series BaFe 12−2 x Zn x Ti x O 19 for 0⩽ x ⩽2.0, by means of magnetization (SQUID), neutron diffraction and Mossbauer measurements. For x ⩽0.8 we observe an increase of the low-temperature net magnetization relatively to the unsubstituted sample. For larger substitution, our studies indicate a deviation of the Fe spins from the collinear structure which leads to a decrease of the net magnetization. We have investigated to what extent and accuracy the molecular field approximation describes the observed magnetic properties for the x =0 and 0.4 samples taking into account the exchange integrals between different magnetic sublattices.

Magnetite films were deposited on Nb-doped SrTiO 3 substrates by pulsed laser deposition. The current–voltage characteristics were measured perpendicular to the interface. Strongly nonlinear, asymmetric current–voltage curves were observed. The resistance shows a crossover from insulating to metallic behavior for increasing voltages. The data can be neatly scaled using a scaling approach based on a quantum critical transition theory. This shows that a scaling analysis is clearly insufficient to conclude on the existence of a bulk-like metal/superconductor–insulator transition.

We have investigated the crystallization behavior of the organic glass former o-terphenyl (OTP) as a function of time and temperature by X-ray diffraction. Unstable crystalline structures, which appear and disappear alternately in time, were observed in the amorphous phase at the temperature T =258 K. These unstable crystalline structures differ from the thermodynamically stable structure of crystalline OTP and may be correlated to density fluctuations. At temperatures above 258 K a normal crystal growth was found. This nucleation shows an Arrhenius-like behavior with an activation energy of 160 kJ/mol.

The effect of magnetic fields on the sensitivity of a type E thermocouple made from thin wires of chromel and constantan has been measured from 5 to 115 K in magnetic fields up to 9 T. In general, the field-induced change in sensitivity depends non-monotonically on the field. Above ∼7.4 K the sensitivity changes between 2% and −6% but increases sharply below this temperature even in weak magnetic fields of the order of 0.1 T. Magnetization measurements of the thermocouple elements indicate a ferromagnetic ordering in chromel and constantan at T ≈53 K and ≈19 K, respectively. The magnetization data for constantan suggest a re-entrant spin-glass transition at T ∼10 K.

We have measured the resistance of commercial thick-film chip resistors (Dale Type RC550, nominal resistance R = 500 Ω ) as a function of temperature (8 mK ≤ T ≤ 10 K ) and magnetic field B ≤ 7 T (for T ≥ 0.3 K ) at different orientations between input current, main area and magnetic field. At 30 mK ≤ T K the temperature dependence of the resistance follows R = A exp ( B T 1 4 ) in agreement with published data for a 1 kΩ thick-film chip resistance. In contrast to previously published studies of similar resistors we measured a resistance that increases approximately linearly with field at T K and quadratically at higher temperatures. The sensitivity to magnetic fields decreases from (0.8 ± 0.1)% T −1 at 0.3 K to 0.1% T −1 above 2 K for B = 1 T .

The heat release Q has been measured in high temperature superconductors during and after a change of the external magnetic field μ 0 H a ≤ 0.5 T applied parallel to the c -axis at 0.1 K ≤ T ≤ 0.45 K. Two Y123 and one Dy123 twinned single crystals were investigated. An analysis of the heat release based on the Bean model and on a model for thin superconductors in perpendicular geometry recently proposed by Brandt et al. has been carried out. During field sweep we measured Q ∝ H a 2 d H a /d t . This dependence can be understood within the Bean model for longitudinal geometry. The critical current densities obtained with this method are in good agreement with those from magnetization measurements. We also present measurements, and a first quantitative analysis, of dissipation due to the time relaxation of the flux line lattice after a field sweep. For the heat release a t −1 dependence has been found which corresponds to a logarithmic time dependence of the magnetization. The normalized relaxation rate we obtained is in agreement with literature values.