Search

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.

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.

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.

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...

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.

The effect of 200 MeV Ag ion irradiation on the temperature and field dependence of critical current density (JC) of high quality MgB2 thin films is studied. Substantial increase in JC is observed over a certain field range for the film irradiated at a dose of 1012 ions/cm2. Our analysis suggests that columnar defects are not formed under irradiation conditions used in these studies, which correspond to an electronic energy loss of about 16 keV/nm. Defects clusters are likely to be responsible for the observed improvement in JC.

Transparent conducting films of SnO2 are grown on single-crystal R-Al2O3 substrates by the pulsed electron beam deposition (PED) technique, and their electrical, optical, and microstructural properties are compared with the films grown by pulsed-laser deposition (PLD). In PED, the pulsed electron beam used for ablation is generated by a channel-spark system with discharge voltage of 12 kV, current ∼1 kA, and pulse duration of ∼100 ns. The PED films are highly oriented, show good epitaxy, and are highly transparent with transmittance of ∼80% in the visible and IR range. The measured band gap is close to 3.9 eV. It is shown that the PED film properties compare well with those of PLD films, with the 119Sn Mossbauer spectroscopy showing comparable local environments in the two.

An all-perovskite ferroelectric field-effect transistor with a ferroelectric Pb(Zr0.2Ti0.8)O3 (PZT) gate and a colossal magnetoresistive La0.8Ca0.2MnO3 (LCMO) channel has been successfully fabricated by pulsed-laser deposition on Si. A clear and square channel resistivity hysteresis loop, commensurate with the ferroelectric hysteresis loop of PZT, is observed. A maximum modulation of 20% after an electric field poling of 1.5×105 V/cm, and 50% under a magnetic field of 1 T, are achieved near the metal-insulator transition temperature of the LCMO channel. A data retention time of at least one day is measured. The effects of electric and magnetic fields on the LCMO channel resistance are discussed within the framework of phase separation scenario.

Ferromagnetism is observed at and above room temperature in pulsed laser deposited epitaxial films of Co-doped Ti-based oxide perovskite (La1-xSrxTiO3-d). The system has the characteristics of an intrinsic diluted magnetic semiconductor (metal) at low concentrations (

The effect of simultaneous substitution of Nd and Ca in the antiferromagnetic insulator, LaMnO3, is studied by x-ray diffraction, magnetoresistance, and magnetization measurements. X-ray powder diffraction patterns of La1−2xNdxCaxMnO3 polycrystalline bulk samples for x=0.2, 0.33, and 0.4 confirm the single-phase formation of orthorhombic perovskite structure (space group Pbnm). The unit cell volume decreases with increasing Nd/Ca substitution. Changes in the temperature dependence of resistivity from semiconducting to metallic, and an increase of the ferromagnetic transition temperature, are observed in this series with increasing x. For x=0.2, 0.33, and 0.4, the metal-insulator transition temperature (Tp) is 72, 140, and 155 K, respectively, and the magnetoresistance (MR) observed in this system in 2 T applied magnetic field around Tp is 35%, 99% and 65%, respectively. The MR of 99% for the x=0.33 sample is promising for practical device applications where MR at low fields is of special interest. The mag...

Epitaxial thin films of ferromagnet/insulator/ferromagnet layers (La0.7Ca0.3MnO3/LaAlO3/La0.7Sr0.3MnO3) of different thickness were fabricated by pulsed laser deposition. Microstructural analysis shows island growth on the thicker film surface. dc magnetization study demonstrates the strong signature of domain wall freezing for the composite film. An interesting feature is a cusp/upturn in field-cooled curve in the composite film-1, much below TB, the blocking temperature. The low temperature cusp disappears in the composite film-2. We suggest that a quasistable charge ordering via spin localization at low temperature, especially for film-1, is the origin of this feature.

A commercial atomic force microscope/magnetic force microscope (MFM) was modified to cool magnetic samples down to around 100 K under a high vacuum while maintaining its routine imaging functionality. MFM images of a 120 nm thick La0.7Ca0.3MnO3 film on a LaAlO3 substrate at low temperature show the paramagnetic-to-ferromagnetic phase transition. Evolution of magnetic domains and magnetic ripples with decreasing temperature are also observed near the edge of a 20 nm thick patterned Co film on a Si substrate.

A prominent theme in inorganic materials research is the creation of uniformly flat thin films and heterostructures over large wafers, which can subsequently be lithographically processed into functional devices. This letter proposes an approach that will lead to thin film topographies that are directly counter to the above-mentioned philosophy. Recent years have witnessed considerable research activity in the area of self-assembly of materials, stimulated by observations of self-organized behavior in biological systems. We have fabricated uniform arrays of nonplanar surface features by a spontaneous assembly process involving the oxidation of simple metals, especially under constrained conditions on a variety of substrates, including glass and Si. In this letter we demonstrate the pervasiveness of this process through examples involving the oxidation of Pd, Cu, Fe, and In. The feature sizes can be controlled through the grain size and thickness of the starting metal thin film. Finally, we demonstrate how...

We report on the effects of using SrFe7Al5O19 as a buffer layer for growth of high-quality epitaxial barium-ferrite thin films on sapphire substrates. X-ray diffraction studies reveal that the buffer layer causes the interfacial strains in the barium-ferrite films to relax. As a result, the ferromagnetic resonance linewidth decreases even in the as-deposited case. However, the more striking result is the drastic reduction in the linewidth that occurs when the barium-ferrite film is deposited on the buffer layer and subsequently annealed at 1000 °C for 2 h, allowing the observation of a large number of spin-wave resonances (up to the 15th mode), indicating an improvement in both the surface and interface characteristics.

The effect of substrate-induced lattice strains on the structural and magnetic properties of epitaxial barium ferrite (BaFe12O19) thin films is studied. The 5000 A thick film shows a very narrow ferromagnetic resonance (FMR) line with a width of about 140 Oe. After appropriate annealing, the linewidth measured at 58 GHz decreases to 37 Oe, which to the best of our knowledge is about two times smaller than the best previously reported value for epitaxial thin films. FMR studies show that the presence of strains broadens resonance absorption.

The influence of 90 MeV 16O ion irradiation on the electric and magnetic properties of epitaxial thin films of La0.75Ca0.25MnO3 is studied. These films were deposited on LaAlO3(100) substrates by the pulsed excimer laser deposition technique. The thickness of each film was about 3000 A. The high quality of the films was ensured by x-ray diffraction and transport measurements. The films were irradiated with 90 MeV oxygen ions at different doses (1011–1014 ions/cm2). It is observed that at a low dose of 1011 ions/cm2 the resistivity peak temperature (Tp) and the Curie temperature (Tc) increase, however as the dose is increased further, the Tp and Tc decrease. At the higher dose of 1014 ions/cm2, the film becomes insulating and does not show any peak in the resistivity–temperature curve down to 60 K. These results are discussed in terms of the effects of ion induced compositional inhomogeneity and strain on the film properties.

Strontium hexaferrite nanoparticles have been successfully prepared from a sintered SrFe12O19 target, using pulsed excimer laser ablation and cold condensation technique. The as‐condensed powders do not exhibit the same magnetic properties as the target material. However, annealing at a high temperature of 850 °C for 3 h in air followed by fast cooling yields very high coercivity of 6665 Oe, exhibiting presence of single domain particles. Vibrating sample magnetometry (VSM), scanning electron microscopy (SEM), and x‐ray diffraction (XRD) techniques are used to study the magnetism, microstructure, and phase formation in the powders, respectively.

We performed x-ray absorption spectroscopy measurements at the cobalt L2,3 edge and the oxygen K edge of Co-doped SnO2 and Co-doped TiO2. Our measurements confirm that doped cobalt atoms are in the same local environment in both compounds. Furthermore, the results support the idea that cobalt atoms occupy substitutional cation sites. Additionally, the oxygen spectral shapes offer insight into a possible cause for the observed giant magnetic moment of cobalt atoms present in SnO2, but not in TiO2.

LaxLu1−xO3 thin films were deposited on 300 mm silicon wafers by physical vapor deposition and fabricated into field-effect transistors using a gate-first process flow. The films were characterized using transmission electron microscopy, Rutherford backscattering spectrometry, and synchrotron x-ray diffraction. The results show the films remain amorphous even at temperatures of 1000 °C. The dielectric properties of LaxLu1−xO3 (0.125≤x≤0.875) thin films were evaluated as a function of film composition. The amorphous LaxLu1−xO3 thin films have a dielectric constant (K) of 23 across the composition range. The inversion thickness (Tinv) of the LaxLu1−xO3 thin films was scaled to

Temperature and field dependence of magnetization of Dy-substituted Gd5Si2Ge2 compounds has been studied in the temperature range of 5–330K. The (DyxGd5−x)Si2Ge2 compounds with x=0.5, 1.5, 3.0, and 3.5 are found to order ferromagnetically with magnetic ordering temperature TC ranging between ∼270 and 80K. The Dy substitution in Gd5Si2Ge2 leads to a lowering of TC, an increase in coercivity, and also to a loss of the first order magnetic phase transition seen in the parent compound. The magnetic entropy change ΔSM associated with the magnetic transition has been computed using the magnetization-field isotherm data. The maximum magnetic entropy changes of ∼8.8, 13.9, and 6.9J∕kgK are obtained for x=0.5, 3.0, and 5 compounds, respectively, for a field change of 55kOe. The ΔSM is spread around TC over a few tens of Kelvin and this feature makes these materials promising for possible use in Ericsson-cycle magnetic refrigerator applications.

The use of CoFeB ferromagnetic electrodes in place of CoFe has been shown to significantly increase the tunneling magnetoresistance (TMR) of MgO based magnetic tunnel junctions (MTJs). By using soft x-ray scattering techniques, we show that the behavior of the magnetic moments located at the CoFe–MgO interface are drastically different from the rest of the CoFe film, whereas the magnetic response of the CoFeB–MgO interfacial moments is coherent with the film’s bulk. Our results support the view that the high TMR values observed in MgO based MTJs with CoFeB electrodes are due to the uniform magnetic response of the entire CoFeB electrode including the MgO interfacial moments.

We report on the search for ferromagnetism in undoped and cobalt-doped high-k dielectric HfO2 films. Over a broad range of growth conditions, we do not observe ferromagnetism in undoped HfO2 films. On the other hand, we do observe room temperature ferromagnetism in dilutely Co-doped HfO2 films, but the origin of the same appears extrinsic (a Co rich surface layer) at least for the regime of growth conditions explored.

We report on the growth of highly oriented HfO2 thin films of monoclinic phase stabilized by 5% Co doping on (001) yttrium-stabilized zirconia (YSZ) using pulsed-laser deposition at 700°C at an oxygen partial pressure of 10−4Torr. On the other hand, pure HfO2 of such quality did not grow on YSZ in wide range of growth parameters. Rutherford backscattering-ion channeling in this film showed a 24% minimum yield (χmin) indicating highly oriented film growth, while hardly any ion channeling was observed in the undoped sample. High-resolution transmission electron microscopy revealed a sharp interface, and no signature of Co clusters. Electron energy loss spectroscopy showed that Co is in the 2+ state. Attempts were also made to grow films on a (001) Si substrate, and the results showed a very low ion channeling yield (∼8%).

Sintered La0.7Sr0.3MnO3 ceramics were prepared by standard solid-state reaction route. Some of these pellets were again heated to 850 °C and quenched to room temperature. These samples were then studied for low-field magnetoresistance (LFMR) effects. Remarkably, upon quenching from high temperature the polycrystalline as well as the crystalline samples are seen to exhibit a dramatic change in LFMR with the emergence of a large positive MR. In the quenched samples a positive LFMR as high as 48% was noted in a field of 200 Oe. This positive MR effect was found to show time-dependent relaxation.

A mixture of a colossal magnetoresistance manganite (La5∕8Sr3∕8MnO3) and a multiferroic (LuMnO3), of potential interest for magnetoelectronics, was found to be bulk immiscible due to its structural∕chemical incompatibility. Here, we demonstrate that thin-film epitaxy offers a route to achieve continuous perovskite solid solutions of such a bulk-immiscible mixture, allowing studies of their physical properties. The films grow with (110)- and (001)-preferred orientation on SrTiO3(100) and LaAlO3(100), respectively. The details of structural, magnetotransport, and magnetization data are discussed within the context of metal-insulator phase coexistence. Epitaxial stabilization of chemically immiscible phases clearly widens the scope of research on manganites as well as other materials.

We have measured the temperature (T) dependencies of the dc resistances (Rdc) and the microwave loss (Rμw) in a variety of samples of (La1−xPrx)0.7Ca0.3MnO3 while varying x from 0 to 0.4. Whereas both the sets of data exhibit maxima, the ac loss peak is much flatter and, during cooling, appears at a much lower temperature than the peak temperature in Rdc. The discrepancy, which vanishes for x=0, increases with lowering tolerance factor (t) (or increasing x). Also Rμw vs T exhibits large thermal hysteresis for x=0.4 indicating that the transition is first order. Cooling in a magnetic field of 9 kOe causes an upward shift of about 20 K in the Rμw peak, in some of the x=0.4 films, yielding a large magnetoimpedance. Further, once these films are exposed to a magnetic field at low T, they fail to recover their virginal behavior on subsequent cooling from room T. These films could be brought to their original state by annealing at high T. The discrepancy between Rdc and Rμw implies that the system is inhomogene...