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The characteristic size, shape and specific alignment of magnetite crystals synthesized by magnetotactic bacteria is a highly coordinated process with precise control over magnetosome vesicle formation, uptake and transport of Fe, and magnetite biomineralization. Magnetosome membranes along with some specific membrane proteins regulate crystal nucleation and morphology of magnetite. Several previous works have indicated that the morphology of mature magnetite crystals is largely unaffected by environmental conditions, though some recent studies have shown the possibility of manipulation of the biomineralization process. In this study we have examined the effects of high concentrations of Zinc and Nickel on the growth of Magnetospirillum magnetotacticum (MS-1) and the corresponding magnetosome formation. Using various characterizations it is shown that the growth of the bacterial cells, as well as the size, shape and magnetosome chain alignment is significantly influenced in the presence of high concentrations of Zn or Ni.

Diluted magnetic semiconductors (DMS) are among the most intensely investigated materials in recent times in view of their great application potential. Yet, they are also the most controversial because of the possibility of extrinsic effects attributable to dopant solubility and clustering, point defects, incorporation of unintentional impurities, etc. This has highlighted the central role of materials chemistry in rendering a specific microstate and property response. In this work, we provide a combined window of high-resolution scanning transmission electron microscopy and electron energy-loss spectrometry, X-ray absorption (XAS)/X-ray magnetic circular dichroism (XMCD), and magnetization measurements on epitaxial rutile Co x Ti 1-x O 2 (x = 0-0.06) system (the first discovered oxide-DMS, which continues to be controversial) grown at low temperature (400 °C) under different ambient atmospheres. The study brings out a mixed-state scenario of ferromagnetism involving intrinsic DMS (uniform dopant distribution at low dopant concentration) and coupled cluster magnetism, involving cobalt associations within the matrix at higher concentrations. We also show that by matrix valence control during growth, it is possible to realize a uniform embedded cluster state and the related coupled cluster magnetism.

Interfacial stress is thought to have significant effects on electrical and oxygen transport properties in thin films of importance in solid oxide fuel cell applications. We investigate how in-plane biaxial stress modifies the electronic structure of La2/3Ca1/3MnO3 and La1/2Sr1/2MnO3 thin films prepared by pulsed laser deposition on three different substrates to vary the in-plane stress from tensile to compressive. The electronic structure was probed by X-ray absorption spectroscopy of the Mn L2,3-edge to characterize the interfacial disruption in this region in an element-specific, site-specific manner. The compressive or tensile interfacial strain modifies the relative concentrations of La and Sr in the interfacial region in order to achieve a better lattice match to the contact material. This atomic migration generates an interfacial region dominated by a compound with a single valency for the transition metal ion, resulting in a severe barrier to oxygen and electron transport through this region.

Incorporating a dopant into a nanoparticle is a nontrivial proposition in view of the size dependent surface versus bulk energy considerations and the intrinsic proximity of the surface to the interior, which facilitates migration to the surface. If realized and controlled, however, it can open up new avenues to novel nanomaterials. Some previous studies have shown the dopability of nanosystems but only with specific surface functionalization. Here, we demonstrate the successful dopant incorporation via a new route of pulsed high energy electron induced synthesis. We choose a system Co:CdS (dilutely cobalt doped cadmium sulfide) in view of the well-known application-worthy properties of CdS and the potential possibility of its conversion to a diluted magnetic semiconductor of interest to spintronics. By using various techniques, we show that matrix incorporation and uniform distribution of cobalt are realized in CdS nanocrystals without the need for additional chemical or physical manipulation. Optical an...

La07Sr03MnO3 (LSMO) nanoparticles having average particle size of the order of 20 nm are synthesized by citrate-gel route and evaluated for their microwave response. The structural and magnetization studies establish phase pure nature of nanoparticles with the intended stoichiometry (Curie temperature ∼360 K). These nanoparticles exhibit a temperature increase from room temperature to 45 � C in 120 s when subjected to microwave exposure of 2.45 GHz. Considering hyperthermia as one of the possible application domain of such materials, the cytotoxicity studies on human skin carcinoma and human fibrosarcoma cell lines was done. These studies reveal the percentage cell viability with the exposure to these nanoparticles with a dose of 10 g/ml to be about 85% and 78% respectively. To study the effect of nanoparticles on normal cells, results were compared with primary mice fibroblast cells which show the corresponding cell viability of 94%.

The interplay of electronic charge and spin in many of the novel multicomponent oxides presents opportunities for new device concepts. In this paper we summarize three different approaches to making field-modulated devices. The first is the observation of a giant electroresistance (ER) effect in the colossal magnetoresistive manganites, where it is shown that in the same system two phenomena operate, colossal magnetoresistance (CMR) and colossal electroresistance (CER) and both processes are complementary. In the second example we show the modulation of the magnetic property of a diluted magnetic semiconductor by an electric field but the role of unidentified ferromagnetic entities cannot be ruled out at this stage. In the third and last case we show the example of the multiferroic materials where the magnetic properties are field dependent as well, but for totally different reasons.

Radiation Processing of Surfaces: Applications of Conversion Electron Mossbauer Spectroscopy

YBa2Cu3O7−δ thin films grown on a Nb-doped SrTiO3 substrate by a pulsed laser deposition method have been fully characterized by scanning transmission electron microscopy Z-contrast imaging and electron energy loss spectroscopy techniques. The Nb distribution was found to be uniform and unchanged across the interface, ensuring a high quality p–n junction heterointerface. We first observed the coexistence of 124 and 125 YBCO defect structure phases, appearing as planar defects in a YBCO thin film. Dispersive Y2O3 nanoparticles have also been observed in the thin film. The interaction of these defect structures and Y2O3 nanoparticles is thought to be beneficial for pinning flux through the entire film thickness.

The splash and spill effects caused by the impact of a ball dropped from a height into a granular medium held in a small open container are examined. Different granular media, namely rice, mustard seeds, cream of wheat and plastic beads are used. The quantity of spilled-over granular matter ( W , grams) is measured as a function of the ball-drop height and compared for different cases. Digital pictures of the splash process are also recorded. The quantity W is seen to vary approximately linearly with the energy of impact. Interestingly, a distinct upward jump is seen in the spilled quantity at specific impact energy in the case of mustard seeds, which have spherical shape and also exhibit some charging effects. Similar jump was also confirmed for the case of plastic beads with broadly similar properties. Although the parameters such as mass per grain and packing density for the case of mustard seeds are intermediate between those for rice and cream of wheat, the spill quantity for comparable impact energy is considerably higher in the former case. The possible reasons for this non-monotonicity of behavior are discussed in terms of the differences in grain shapes and properties. Experiments are also performed using plastic beads of the same type but with four different sizes to explore the dependence of spilled quantity on bead size. The container size dependence is also examined for various bead types. Interesting systematics are seen, which are discussed qualitatively.

Transport across a CrC2/Ag/YBCO interface was studied using a flip-chip configuration. The results were interpreted in the Andreev reflection scenario. It is shown that the surface spin-polarization of CrO2 film, even after exposing to air, remained close to 100% to the T c of YBa2Cu3O7− x , a temperature limited by this method.

The stability of a pile of steel balls formed in a thin cell is studied for different liquids and air. The dependence of the angle of repose (AOR) on the medium and the cell thickness is examined. The AOR is observed to increase considerably with a decrease in cell width. In a thin cell (width comparable to a few times the ball diameter) the AOR is seen to depend on liquid viscosity, in contrast to the case of thick cells. The viscosity dependence in thin cells is attributed to the boundary wall effects, presumably caused by the influence of viscosity on granular arching. The case of a pile formed in a thin cell in air stands out to be distinctly different as compared to the piles formed in liquids. Image analysis of sphere position distributions and intersphere cavities reveals some useful systematics. Various issues such as the surface roughness of the balls, possible air trapping in micro-cavities and related formation of liquid bridges, effects of energy of impact on pile equilibrium etc. are addressed in the analysis.

The oxygen content of La0.35Ca0.65MnOy was decreased by annealing the samples at high temperatures in vacuum with graphite powder nearby. Insulator metal (I-M) transition occurred upon dramatic decrease of oxygen content. Correspondingly the samples showed a large magnetoresistance (MR) and MR = C(M/M-s)(2) at 4.5 K up to relatively large M/M-s. The results were explained in terms of the variation of Mn3+/Mn4+ ratio and oxygen vacancy ordering induced percolation of the ferromagnetic regions. Spin-dependent hopping model can account for the variation of MR with M/M-s and the variation of C with the decrease of oxygen content.

Control over the crystallography, morphology, and spontaneous organization of ceramic crystals are important goals in advanced materials engineering with important application potential. Most studies hitherto have concentrated on static charged interfaces as templates for the controlled nucleation and growth of ceramic crystals. In this communication, we show that BaSO 4 crystals grown at a steadily expanding liquid-liquid interface populated by fatty acid molecules spontaneously organize themselves into highly linear superstructures over large length-scales. This experiment is realized in a radial Hele-Shaw cell where the liquid-liquid interfacial growth rate and consequently time scales such as arrival of surfactant molecules to the interface etc. may be readily modulated. Possible reasons for the regular assembly of barite crystals are discussed.

We have studied the physical properties of epitaxial La0.67−xPrxCa0.33MnO3(LPCMO) (x=0.13, 0.20, 0.27) thin films under different type and degree of substrate induced biaxial strain. Films with thickness of 300, 600, and 1000 A grown on crystalline NdGaO3 (negligible strain), LaAlO3 (compressive strain), and SrTiO3 (tensile strain) by pulsed laser deposition are examined. Our data suggest occurrence of metastable phase mixtures in these films, with the volume fractions controlled by strain. In particular, melting of the charge ordered phase is seen to occur in thin films on SrTiO3 at fields as low as 1 T, in the low temperature regime. The deviation of ρ(T) from the effective medium theory further indicates that LPCMO thin films cannot be described as simple metal–insulator mixtures.

Thin films of Sr2FeMoO6 have been deposited on single-crystalline and polycrystalline SrTiO3 substrates by pulsed laser deposition from a stoichiometric target. In order to obtain high-quality films, the deposition parameters were systematically optimized. The films grown under optimized conditions show properties comparable to those of bulk Sr2FeMoO6 single crystals. Surprisingly, polycrystalline films obtained by depositing on polycrystalline SrTiO3 substrates do not show any significant low-field magnetoresistance (MR), contrary to expectations for a spin-polarized material. However, after controlled annealing-induced alteration of the grain-boundary properties, a development of the low-field MR feature is clearly observed. The occurrence of the low-field MR is accompanied by nonlinearity in the current-voltage characteristics. These results are analyzed in the light of the prevalent understanding of the properties of this double perovskite material.

The oxygen content of La0.5Ca0.5MnOy was tuned by annealing the samples at high temperatures in flowing nitrogen with graphite powder nearby. The reduction of oxygen content has dramatic effect on the electrical transport and magnetic properties. The samples with y=2.983, 2.83, and 2.803 show an insulator-metal transition, and an unusual temperature and magnetic-field dependence of the magnetoresistance. The paramagnetic-ferromagnetic transition also shifts to lower temperatures and the antiferromagnetic transition at lower temperature is suppressed. The results are discussed in terms of the effect of oxygen vacancies on the various properties of La0.5Ca0.5MnOy. (C) 2002 American Institute of Physics.

Energetics and structural relaxations related to the surface complexes formed by mixed Si–Ge and C–C dimer adsorption on predefined adsorption sites on a (2×1) reconstructed Si (001) surface are investigated. Monte Carlo simulated annealing procedure is used in conjugation with Tersoff's semi-empirical potentials. The reliability check of the method is performed by comparing our results for the case of Si–Ge dimer adsorption with the results reported by using ab initio pseudo-potential calculations. The agreement is found to be good. For carbon dimer adsorption, the nucleation centers are found to be different from those for Si and Ge. It is seen that carbon has a tendency to get adsorbed at the dangling bond site, or to form a Si–C–C–Si chain like structure under specific conditions.

In this paper, the photon-energy dependence of the optical Cooper pair breaking rate (CPBR) was studied for YBa2Cu2.8Zn0.2O7−δ (YBCZO) thin films, and was compared to that in YBCO. In YBCZO the CPBR also shows strong photon-energy dependence, similar to that in YBCO, but with a change in the position and shape of the absorption feature. The results were discussed in terms of the resonance of Cooper pair breaking and the involvement of the insulating stripes. This study favours a physical scenario based on electronic phase separation in high-Tc superconductivity.

All-oxide devices consisting of Niobium-doped Strontium Titanate (Nb:STO)/Strontium Titanate (STO)/Lanthanum Strontium Cuprous Oxide (LSCO) heterostructures were fabricated and characterized electrically for their interface properties through capacitance-voltage (C-V) and current-voltage (I-V) techniques, in the context of electric field effect studies. The C-V studies establish the occurrence of charge modulation in the LSCO channel. Absence of hysteresis in the C-V characteristic when the voltage is retraced suggests the absence of mobile ions in the gate oxide and slow interface traps. This is further corroborated by the absence of drift in the C-V characteristic and shift in the flat band voltage (VFB) when the device is subjected to temperature-bias aging. The interface state density obtained from VFB is ∼1012/cm2. The uncompensated hole concentration in the LSCO channel calculated from the measured room temperature C-V data is ∼1020/cm3 and is in good agreement with the expected hole concentration in LSCO. Current-time and current-voltage plots are invariant with respect to the polarity of the applied voltage up to ∼5 V. This, in a structure with asymmetric interfaces, indicates that the electrical contacts to STO are non-blocking and the conduction through STO is bulk-limited in this voltage regime. Thickness dependent current and capacitance studies also corroborate the bulk-limited nature of conduction through the device in this voltage regime. However, I-V characteristic shows a rectifying nature beyond ∼8 V indicating that the mechanism in this voltage regime could be interface limited.

We report observations of the ferroelectric domain structure in epitaxial lead zirconate titanate (PZT) ferroelectric thin films using piezoresponse microscopy. EpitaxialPZT films with a nominal composition of PbZr0.2Ti0.8O3were deposited onto single crystal SrTiO3 substrates by pulsed laser deposition, with an epitaxial layer of La-Sr-Co-O as the bottom electrode. By manipulating thefilm thickness, a uniform 2-dimensional grid of 90° domains (a-domains, i.e., c-axis in the plane of the film) has been induced. Our studies show that the polarization direction in the film is substantially preferentially oriented and that nucleation of reverse 180° domains occurs preferentially at 90° domain interfaces. Polarization reversal occurs through the nucleation and subsequent growth of “semicircular/elliptical” reverse domains, which subsequently consume the entire region as a function of reversal time. The reversal is seen to fit a stretched exponential.

Frequency-dependent capacitance measurements and transport measurements tuned to reveal system impedance have been performed on epitaxial thin films of ${\mathrm{Pr}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}.$ These reflect strong dielectric response features correlated to the various ordering transition temperatures in the system, namely, \ensuremath{\sim}220 K (charge ordering), \ensuremath{\sim}130 K (antiferromagnetic ordering), and \ensuremath{\sim}117 K (spin canting). An equally strong feature is also seen at \ensuremath{\sim}125 K, which does not correspond to any reported transition. The enhanced dielectric response at the transitions is attributed to the nonequilibrium transport and slow carrier reorganization dynamics defined by polaronic hopping and magnetic frustration.

We present systematic studies of polycrystalline La 1-x Sr x MnO 3 where x is varied from 0.46 to 0.53 at 0.01 intervals. We have measured the magnetic resonance, ac susceptibility, dc magnetization, x-ray diffraction, four-probe resistivity, and optical reflectivity. The data seem to indicate that, at low T, the materials are intimate mixtures of ferromagnetic and antiferromagnetic microregions. For x>0.48 we observe anomalies pointing toward charge ordering on lowering T. An effective-medium theory is presented, and an appropriate phase diagram is proposed.

We have investigated the structure and electronic properties of ferrimagnetic double perovskites, {A_2FeReO_6} (A = Ca, Sr, Ba). The A = Ba phase is cubic (Fm3m) and metallic, while the A = Ca phase is monoclinic {P2_1 /n} and nonmetallic. Fe{^57 Mossbauer} spectroscopy shows that iron is present mainly in the high spin S = (5/2) {Fe^3+} state in the Ca compound, while it occurs in an intermediate state between high spin {Fe^2+ and Fe^3+} in the Ba compound. It is argued that a direct {Re t_2g-Re_ t2g} interaction is the main cause for the metallic character of the Ba compound; the smaller size of Ca and the monoclinic distortion (which lifts the degeneracy of {t_2g} states seem to disrupt the Re-Re interaction in the case of the Ca compound, making it nonmetallic for the same electron count.

We present a detailed study of synthesis by pulsed laser ablation and the magnetic characterization of nanosized powders of iron oxides and strontium ferrite. In the case of iron oxide, it is found that the particle formation and their growth take place in the gas phase before reaching the cold finger (used as substrate for condensation). However, in the case of strontium ferrite, the as-condensed material is amorphous, and requires annealing at high temperature to induce nanoparticle growth. Very high values of intrinsic coercive field (∼6665±10 Oe) are realized for the strontium ferrite powder having an average particle size of about 35 nm. The temperature variation of coercive field and remanence of the nanosized powders is found to be substantially different from those of the corresponding bulk materials.

La0.67Sr0.33MnO3 thin films were grown on LaAlO3 substrate in vacuum using pulsed laser deposition to investigate the effect of changing oxygen content. Transmission electron microscopy studies showed that the epitaxial (La0.67Sr0.33)2MnO4 phase with K2NiF4 structure formed unexpectedly as a matrix with a square-shaped nanometer-sized MnO phase distributed in a regular pattern throughout the whole film like self-assembled quantum dots. The MnO phase grew epitaxially from the LaAlO3 substrate to the top of the film with no outgrowth. High-resolution image simulation indicated that Sr ions take up only positions in every other La layer in the (La0.67Sr0.33)2MnO4 structure. Basing our theory on the composition and structure of the matrix phase, we propose that it is possibly electron-doped with a mixed valence of Mn2+/Mn3+ instead of the Mn3+/Mn4+ as in the hole-doped case.

Detailed measurements and analysis of hyperfine properties of iron-doped La 0.75 Ca 0.25 Mn 1-x Fe x O 3 are reported. In a previous paper [Phys. Rev. B 57, 7841 (1998)] it was shown that a very small increase of x from 0.0425 to 0.045 causes dramatic changes in the transport properties, whereas no changes are detected either in the structural phase or the Mossbauer data. In the present study a complete analysis of Mossbauer data registered in the temperature range of 300-4 K is presented. From 300 K down to 190 K the samples are in a paramagnetic state. Below 190 K a magnetic order begins to develop, although a paramagnetic contribution persists down to 77 K. This existence of paramagneticlike domains deep into the so-called ferromagnetic metallic regime provides evidence of the two-phase character of the metallic state in the mixed-valent manganite system. The quadrupole splitting of the major spectral component shows a jump at the para-ferro transition. This reflects lowering of the local point-charge symmetry in the remaining paramagneticlike phase in the ferromagnetic regime, presumably because of the changes in its shape function or its character, as the ferromagnetic clusters stabilize in space. Another small component of the spectrum, which appears to represent the grain-boundary region, exhibits ferromagnetic ordering at around 100 K and also a behavior opposite to that of the major fraction under external magnetic field.

We report on the structural, transport, and magnetic properties of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{x}$ thin films grown in vacuum by pulsed-laser deposition. The as-grown thin films have both the matrix ${\mathrm{La}}_{1.34}{\mathrm{Sr}}_{0.66}{\mathrm{MnO}}_{4}$ phase with ${\mathrm{K}}_{2}{\mathrm{NiF}}_{4}$ structure and an embedded MnO phase. The electrical transport and magnetic properties of the films are determined mainly by those of the matrix phase. By annealing, the as-grown thin films can be transformed into the normal ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_{3}$ single phase, which shows the expected colossal magnetoresistance effect. Based on the composition of the matrix phase, and the structural, electrical, and magnetic properties of the films, we propose that the matrix phase is possibly electron doped with a mixed valence of ${\mathrm{Mn}}^{2+}{/\mathrm{M}\mathrm{n}}^{3+}$ instead of the ${\mathrm{Mn}}^{3+}{/\mathrm{M}\mathrm{n}}^{4+}$ as in the hole-doped case.

This paper examines the possibility of enhancing the room temperature magnetoresistance at low applied magnetic fields in single layer La0.7 Ba0.3 MnO3 thin films. The influence of lattice mismatch strain, as well as the effect of different frequency regimes, on the magnetoresistance is explored. The effects of lattice mismatch strain are studied by measuring the magnetoresistance as a function of the La0.7 Ba0.3 MnO3 film thickness, oxygen annealing, and lattice matched buffer layers. We find that the release of the lattice mismatch strain improves the magnetoresistance at room temperature and at low magnetic fields. In fact, the highest magnetoresistance at room temperature (−1.7% at 500 Oe) has been found for the 1600 A as-grown La0.7 Ba0.3 MnO3 film, whereas the largest magnetoresistance (−1.9% at 500 Oe) is found at 309 K for the 1000 A La0.7 Ba0.3 MnO3 film annealed in flowing O2 for 1 h at 900°C. Finally, we find that the microwave magnetoresistance is the same as the dc magnetoresistance when the cavity corrections are applied. In the single layer La0.7 Ba0.3 MnO3 system, the low field magnetoresistance at room temperature is far from being technologically viable.

(110) oriented La1−δMn1−δO3 thin films with different oxygen content were grown on (001) LaAlO3 substrates by pulsed laser deposition. Samples prepared in higher oxygen partial pressures show a ferromagnetic transition around 200 K. The transport is thermally activated with a change in slope at the ferromagnetic transition. Samples prepared and annealed in vacuum show signatures of mixed ferromagnetic and antiferromagnetic phases, and are insulators. The pure antiferromagnetic phase (as expected and observed in bulk materials with optimum oxygen stoichiometry) was not obtained in our experiments, even in the strongly reduced films.

The influence of 250 MeV 107Ag ion irradiation on electrical transport and low frequency conduction noise properties of La0.75Ca0.25MnO3 (LCMO) thin films have been investigated. A systematic variation of metal–insulator transition temperature Tp and resistivity as a function of ion fluence has been observed. It has been noted that for the sample irradiated with 1011 ions/cm2, Tp increased by 10 K and the resistivity is decreased in the ferromagnetic metallic state from that of an unirradiated sample, whereas our noise results show a monotonic increase in the normalized noise Sv/V2, as a function of ion fluence. The observed modifications in the noise properties of the irradiated LCMO thin films are consistent with the general expectation concerning the effect of swift heavy ions irradiation induced lattice defects.

Oscillatory exchange coupling is observed in $\mathrm{TiN}/{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ superlattices over length scales comparable to metallic superlattices. However, the strength of the coupling is almost an order of magnitude stronger than that commonly observed in metallic superlattices. In addition, a unique positive magnetoresistance effect is also seen. These observations are discussed in terms of the carrier confinement effects caused by the half metallicity of the magnetic layers.

We report on three different experiments on high temperature superconducting (HTS) cuprates and colossal magnetoresistive (CMR) manganites, which clearly bring out some of the important similarities and differences between the two material systems. The experiments involve the measurement of temperature dependence of the mean squared displacement of Cu and Mn ions from their equilibrium site in the case of the cuprates and the manganites, respectively, and their correlation with the transport property. In both cases the key ions in the materials (Cu for HTS and Mn for CMR) exhibit vibration amplitudes larger than that of ions in simple Debye solids and clearly show discontinuities in the vibration amplitudes as a function of temperature close to the phase transition temperatures. These point to the unequivocal participation of phonons in the transport processes and possibly in the onset of the phase transitions (i.e. superconductivity and ferromagnetism). The second set of experiments, involves femtosecond optical excitation of micro-strip resistors made of cuprates or manganites, and the subsequent measurement of the changes in the impedance on a 20 ps time scale. In the case of the manganites one measures the time scales involved in the ionization and reformation of a Jahn-Teller polaron and also the decay times of magnon excitors. In the case of the cuprates one sees a highly efficient pair breaking process with a very sharp resonance, with a width of only 100 meV, which is indicative of the role of a large intermediate excitation in the mechanism of high temperature superconductivity. In the third experiment, spin-polarized electrons injected from a manganite electrode into a superconductor are observed to break pairs at a rate far larger than unpolarized electrons. This effect seems very orientation dependent for the case of YBCO, which may shed new light on the transport of quasi-particles at YBCO interfaces.

FET-type devices have been fabricated by using trilayers of Nd/sub 0.7/Sr/sub 0.3/MnO/sub 3/LaNiO/sub 3/ (NSMO) or LaNiO/sub 3/ (LNO) (gate)/LaAlO/sub 3/ (LAO) (barrier)/YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) (channel) in order to investigate effect of quasiparticle injection into YBCO. Here, NSMO and LNO were used as gate electrodes for injection of spin-polarized and spin-unpolarized quasiparticles into the superconducting channel, respectively. When injecting along the c-axis of YBCO, the critical current was suppressed with spin-polarized quasiparticles 30 times more efficiently than with spin-unpolarized quasiparticles. Differential current gain, defined as a differential change of the critical current to injection current change, has been achieved up to /spl sim/16 for c-axis YBCO. If the response time is limited by quasiparticle relaxation time of /spl sim/10 ps, the device may be useful for fast electronics. Preliminary high-speed measurements indicate that part of the critical current suppression may be caused by quasiparticle injection, not all by heating. When injecting along a-axis, no significant dependence on quasiparticle polarization was observed. Other superconductors such as Pr/sub 1.85/Ce/sub 0.15/CuO/sub 4/ (PCCO) and PbIn have been tested in similar devices for comparison.

We report the results of systematic studies of the effect of thin film deposition conditions, such as deposition temperature, oxygen pressure during deposition, etc., on the microstructural, magnetic, and microwave properties of pulsed laser deposited epitaxial thin films of barium ferrite on single crystal sapphire substrates. Ferromagnetic resonance (FMR) linewidths are very sensitive to the presence of defects and inhomogeneities and therefore change markedly with the variation of deposition parameters. After careful optimization of the deposition conditions, relatively narrow resonance lines were realized in these films. For further improvement in the film quality, these films were annealed at elevated temperatures in flowing oxygen. As a result of the high degree of epitaxy, good stoichiometry, and reduced concentration of defects, FMR linewidths as small as 37 Oe were obtained in films deposited at 920 °C and subsequently annealed at 1000 °C in an oxygen atmosphere.

Surface complexes involving silicon, germanium and carbon adatom on predefined adsorption sites of the 2×1 reconstructed Si(001) surface are investigated for their energetics and associated structural relaxations. Tersoff’s semi-empirical potential is used and the relaxations are obtained by employing the Monte Carlo simulated annealing technique. The results for the cases of Si, Ge and carbon adsorptions are compared. It is found that Si and Ge as adatoms on Si(001) 2×1 reconstructed surface behave in a similar fashion but the carbon adatom behaves in a markedly different way. Specifically, the carbon adatom induces a Si-C-Si chain configuration. It is also found that the adsorption sites between two dimer rows are the most favourable ones.

The evolution of viscous fingering morphology is examined for the case of a system of miscible fluids in an anisotropic radial Hele-Shaw cell. It is shown that dendritic morphologies similar to the kinetic and surface-tension morphology types coexist for this case. The critical role of the means of introducing anisotropy in the Hele-Shaw cell is established, and an explanation of the pattern behavior is offered on the basis of shape discontinuities of the individual elements of the lattice used to induce anisotropy. The ramifications of such an explanation are experimentally verified by demonstrating a clear difference in the morphology evolution in two halves of a single Hele-Shaw cell, one half of which contains square lattice elements, and the other half of which contains circular lattice elements.

The transient photoimpedance response of epitaxial YBa2Cu3O7−δ thin films has been studied using 100 fs Ti:Sapphire laser pulse excitation. Both temperature and photon energy dependence of the fast optical response signal (whose temperature dependence can be explained by a kinetic inductance model involving Cooper pair breaking) were studied. The pair breaking rate is strongly photon energy-dependent, with a resonance around 1.5 eV with a width of only 100 meV, which is very surprising given the strong electron correlation in this metallic system and may be explained in terms of the stripe phase model.

The influence of swift heavy ion (SHI) irradiation on transport properties of epitaxial thin films of La0.75Ca0.25MnO3 (LCMO) is studied. The films were irradiated with 90 MeV16O beams and 250 MeV107 Ag beams at different fluence values. In the case of 90 MeV16O ions LCMO specimens were irradiated to 1011-1014 ions/cm2 and for 250 MeV107Ag ions with 1010–1013 ions/cm2. A systematic variation in Curie temperature (T c) or resistivity peak temperature (T p) has been observed. It has been noted that for both types of ions the T p increased for the specimen irradiated at 1011 ions/cm2 fluence. Further increase of fluence decreased the T p value and at higher fluence (1014 ions/cm2 for 90 MeV16O and 1013 ions/cm2 for 250 MeV107 Ag ions) the specimens showed no metal-to-insulator transition even at low temperature down to 77K. They show completely semiconducting behavior with high resistance values. These results are discussed in terms of SHI irradiation-induced structural distrotion and strain on thin...

Epitaxial films of La0.7Ca0.3MnO3 have been irradiated with 90 MeV oxygen ions at different dose values ranging from 1011–1014 ions/cm2. The structural, magnetization, and magnetotransport properties have been studied as a function of the ion dose. It is found that the properties change gradually up to a dose of 1013 ions/cm2; however, drastic changes occur when the sample is irradiated at the higher dose of 1014 ions/cm2. Specifically, this sample exhibits a large, nearly temperature independent magnetoresistance in the low temperature regime. The Rutherford backscattering channeling data bring out the presence of defects in the irradiated films. The x-ray diffraction data, the temperature dependence of resistivity and magnetization, and the low temperature magnetic hysterisis data collectively indicate the presence of two different phases in the sample irradiated at 1014 ions/cm2. The surface morphology of this film, observed by atomic force microscopy, exhibits swelling, presumably due to subsurface cl...

Fe 3 O 4 /SrTiO 3 /La 0.7 Sr 0.3 MnO 3 (LSMO) epitaxial heterostructures were grown by pulsed laser deposition on LaAlO 3 (001) substrates. The high structural quality of the heterostructures was established by X-ray diffraction measurements. The vertical (current perpendicular to plane-CPP) transport in the heterostructure, without and with the magnetic field applied in and perpendicular to the film plane was studied as a function of temperature. The magnetization behavior was also examined. Application of the magnetic field in the film plane is found to yield a large positive magnetoresistance (MR) while a perpendicular field is found to lead to a negative MR at low fields followed by a large positive MR at higher fields. The observed MR features do not show any obvious correlation with the hysterisis behavior of the ferromagnetic bilayers. A possible explanation of these results is given in terms of the relative differences in the majority and minority spin bands of the two ferromagnetic layers and the field induced modifications of domain structures therein.

Pulsed laser excitation induced conductance changes in colossal magnetoresistance material ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ were studied on the picosecond time scale. A two-component signal was seen consisting of a fast positive transient associated with the paramagnetic insulating state and a slower negative signal associated with the ferromagnetic metallic state. The fast component corresponds to the photoionization of the Jahn-Teller small polaron. The slow component is explained in terms of the reduced carrier mobility due to photogenerated magnetic excitations.

Since the rejuvenation of interest in the rare earth manganites owing to their potential use as magnetoresistive sensors, there has been adequate research to arrive at some evaluation of the potent...

Epitaxial films of magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ have been grown on ${\mathrm{SrTiO}}_{3}$ (100) and sapphire (\ensuremath{\alpha}-${\mathrm{Al}}_{2}{\mathrm{O}}_{3},$ 0001) substrates by pulsed laser deposition, and they exhibit crystal orientations of [100] and [111], respectively. Films of both orientations show a clear Verwey transition near 120 K. The temperature dependence of magnetoresistance of the films is examined at fields up to 8.5 T applied parallel to the film plane. The [100] oriented films show a pronounced extremum in magnetoresistance (MR) at the Verwey transition temperature ${(T}_{v}),$ while no such feature is observed in the case of the [111] oriented films. However, the [111] oriented films exhibit a sudden change in the MR dependence on $T$ at ${T}_{v}$ and films of both orientations show a rapid increase of MR as the temperature is decreased below about 110 K. An attempt is made to analyze these data in terms of the known anisotropy in magnetostriction and optical magnon-phonon dispersions, and the small polaron band and hopping conductivity mechanisms.

The effect of laser power on the transient response of photoimpedance and the in situ sample temperature is studied for epitaxial YBa2Cu3O7−δ (YBCO) films at a wavelength of 810 nm using 100 fs laser pulses. The temperature dependences of the dc resistance and the amplitudes of the fast and slow photoresponse signals were measured simultaneously. For laser energy density of 20 μJ/cm2 per pulse (average power 22 mW), the average sample temperature is found to increase by about 1 K for 300 nm thick YBCO film with 0.5 mm thick LaAlO3 substrate as shown by the shift of resistance versus temperature curves. Calculations of time constant show that heat diffusion in LaAlO3 is the bottleneck for heat escape which causes the observed increase of the sample temperature. The amplitudes of both slow and fast signals show a peak in the temperature dependence curves near the superconducting transition temperature; and the peak temperature decreases, while the peak amplitude increases with laser power. At 82 K, the lase...

A distinctive characteristic of silver in oxygen incorporation of oxide thin films during pulsed laser ablation has been discovered. Optical emission spectroscopy studies of laser-induced plume of Ag-target indicates the presence of AgO species whose concentration increases with an increase in oxygen partial pressure. The formation of AgO in laser-plume has been found to be very useful for the realization of high temperature superconducting YBa2Cu3O7-delta (YBCO) and giant magnetoresistive La0.7MnO3-delta (LMO) thin films with dramatically superior quality if the target materials contained a small amount of silver. The improvement in the quality of these films is brought about by the supply of atomic oxygen to oxide lattices during their formation. This becomes possible due to the fact that Ag, after it is ablated with other constituent materials in the target, gets moderately oxidized in an oxygen atmosphere and the oxidized species dissociate back into Ag and nascent O at the substrate surface. The nascent oxygen is very highly reactive and is easily assimilated into the lattice of these compounds. (C) 1997 Elsevier Science S.A.

Thin Fe2O3 layers (44 and 55 nm) were deposited by pulsed excimer laser ablation on single‐crystal alumina (α‐Al2O3) substrates heated at 675 °C. The ion beam mixing of these α‐Fe2O3/α‐Al2O3 couples was carried out using 300 keV Kr3+ ions at a fluence of 2×1016 ions/cm2. The mixing effect was followed by Rutherford backscattering spectrometry (RBS), conversion electron Mossbauer spectroscopy, and grazing incidence x‐ray diffraction method. RBS spectra do not show any evidence of mixing at the interface, whereas the data obtained with the two other techniques display phases like oxygen deficient Fe3O4, Fe3−yAlyO4, and Fe1+xAl2−xO4. For this last phase, x is determined as being equal to about 0.5. It is shown that ion beam mixing is more efficient for the thinner Fe2O3 layer, in accordance with the projected range of the Kr3+ ions.

Enhanced electric field (E) induced modulation of the resistance of epitaxial thin films of N${\mathrm{d}}_{0.7}$S${\mathrm{r}}_{0.3}$Mn${\mathrm{O}}_{3}$ has been observed. The results show several remarkable features: first, field-direction-independent decrease of the resistance above the temperature at which the peak in resistivity occurs $({T}_{p})$; second, proportionality of the modulation to ${E}^{2}$; third, a reduction in the magnitude and field-direction-independent reversal of the sign of $\ensuremath{\Delta}R/R$ just below ${T}_{p}$; and fourth, a prompt time response at high temperatures and a slower response near ${T}_{p}$. These results are consistent with a model of a polarization-induced lattice distortion coupling with both spin and charge transport.

Pulsed-laser-deposited polycrystalline/amorphous films of mixed-valent manganites [La0.7Ca0.3MnO3 (LCMO), La0.5Sr0.5MnO3 (LSMO), La0.5Ba0.5MnO3 (LBMO), and (La0.6Pr0.4)0.67Ca0.33MnO3 (LPCMO)] grown at low temperature (450 °C) on single crystal (001) silicon substrate are evaluated for uncooled bolometric applications. It is shown that the temperature coefficient of resistance (TCR) and electrical noise (Sv) depend on the chemical composition. The optimum performance is found for LCMO with TCR of ∼7% K−1 and spectral noise ∼8.9×10−13 V2/Hz. In LBMO and LSMO the noise is much lower, but so is the TCR (1.5%–2% K−1). In LPCMO the TCR is high (∼10% K−1) and the noise is as well.