DC magnetization studies of nano- and micro-particles of bilayered manganite LaSr2Mn2O7 ,

M.H. Ehsani, M.E. Ghazi, P. Kameli, F.S. Razavi

Systematic studies of the magnetic properties of LaSr2Mn2O7 as a function of crystalline grain size provide information on how the crystalline grain size affects the magnetic and charge ordering in this compound. The half-doped bilayered manganite LaSr2Mn2O7(x= 0.5) in its bulk form has the CE-type anti- ferromagnetic (CE-AFM) charge ordering phase transition. In this work, we prepare LaSr2Mn2O7ceramic samples using the Pechini sol–gel method in order to produce different grain sizes, and the effects of different crystalline grain sizes between 200 and 1000 nm on the magnetic properties of the compound, obtained by a SQUID magnetometer, are investigated. The DC magnetization (DCM) measurements for all samples indicate that the crystalline grain size has no considerable effect on the charge ordering transi- tion temperature,Tco (just a few degrees shift to lower temperatures as the grain sizes become bigger). The width of the peak observed in these measurements (transition to the charge ordering phase) becomes broader, and susceptibility measurement at the zero field cooling (ZFC) and filed cooling (FC) increases as the grain size becomes systematically smaller. The results obtained from the magnetic hysteresis curves confirm the ground state phase of the system is anti-ferromagnetic and the Arrott plots obtained manifest existence of the first and second order magnetic phase transition in all samples. In addition, in the sample with a grain size of 200 nm, enhancement of the magnetic properties, which is accompa- nied with the formation of FM phase on the surface of grain or particle, is observed

Characterization of the spin-1/2 linear-chain ferromagnet CuAs2O4 ,

Caslin, K.; Kremer, R. K.; Razavi, F. S.; et al.

The magnetic and lattice properties of the S = 1/2 quantum-spin-chain ferromagnet CuAs2O4, mineral name trippkeite, were investigated. The crystal structure of CuAs2O4 is characterized by the presence of corrugated CuO2 ribbon chains. Measurements of the magnetic susceptibility, heat capacity, electron paramagnetic resonance, and Raman spectroscopy were performed. Our experiments conclusively show that a ferromagnetic transition occurs at similar to 7.4 K. Ab initio DFT calculations reveal dominant ferromagnetic nearest-neighbor and weaker antiferromagnetic next-nearest-neighbor spin exchange interactions along the ribbon chains. The ratio of J(nn)/J(nnn) is near -4, placing CuAs2O4 in close proximity to a quantum critical point in the J(nn)-J(nnn) phase diagram. TMRG simulations used to analyze the magnetic susceptibility confirm this ratio. Single-crystal magnetization measurements indicate that a magnetic anisotropy forces the Cu2+ spins to lie in an easy plane perpendicular to the c axis. An analysis of the field-and temperature-dependent magnetization by modified Arrott plots reveals a 3d-XY critical behavior. Lattice perturbations induced by quasihydrostatic pressure and temperature were mapped via magnetization and Raman spectroscopy.

Tunable magnetic and magnetocaloric properties of La0.6Sr0.4MnO3 nanoparticles ,

M. H. Ehsani1, P. Kameli, M. E. Ghazi, F. S. Razavi and M. Taheri

We report the phase stabilization and properties of BiVO3 (BVO) thin films, grown on (001) SrTiO3 and LaAlO3, using the pulsed laser deposition technique. Bi and V are in 3+ oxidation states as measured by using x-ray photoelectrons spectroscopy. BVO exhibits a Curie-Weiss paramagnetic behaviour and about -26 K Weiss temperature. This demonstrates the presence of a strong correlation effect due to the spin fluctuation. Additionally, these films exhibit a semiconducting behaviour owing to the thermally activated conduction process. A plausible explanation of the observed properties is presented by comparing with the closely related LaVO3 and other orthovanadates.

Influence of Sm-doping on the structural, magnetic, and electrical properties of La0.8-xSmxSr0.2MnO3 (0 < x < 0.45) manganites ,

Ehsani, M. H.; Kameli, P.; Razavi, F. S.; et al.

Structural, magnetic, and electrical properties of the La0.8-xSmxSr0.2MnO3 (0 ? x ? 0.45) manganites prepared by a solid-state reaction technique was studied systematically. It was found that with increase in the Sm content, the crystal structure transformed from rhombohedral (x < 0.3 samples) to orthorhombic (x > 0.3 samples). The ac magnetic susceptibility measurements show that all samples undergo a transition from paramagnetic (PM) to ferromagnetic (FM) phase at the Curie temperature, TC, which decreases from 296 K down to 165 K with increase in the Sm doping level from x = 0 to x = 0.45. In addition, the glassy state exists in the x = 0.15–0.45 samples, which is stronger in higher doped compounds (x = 0.30 and x = 0.45). This behavior indicates that the substitution of Sm weakens the double exchange (DE) process. The field dependence of magnetization for the samples shows a soft FM nature with a small hysteresis loop and a low coercive field, Hc, for the doped samples. The irreversibility in the magnetization for increasing and decreasing the applied field is due to the glassy behavior observed in highly doped samples. The temperature dependence of resistivity, ?(T), measurement indicates that by increasing the Sm doping level, the metal–insulator transition temperature decreases, and the heavily doped samples become insulators. The metallic region of the ?(T) curve for the x = 0–0.10 samples was fitted with the model of electron–electron and electron–magnon scattering, while the insulating region was fitted with the small polaron hopping (SPH) at T > ?D/2 (?D, Debye temperature) and the variable range hopping (VRH) models at T < ?D/2.

A Study of Structural and Physical Properties of Heavily Co-doped LaSr2Mn2O7 Bi-layered Manganite ,

Ehsani, M. H.; Kameli, P.; Ghazi, M. E.; Sing M. P.; and Razavi F. S.,

The Co-doped bilayered LaSr2Mn2O7 manganite at low Co concentrations (0-0.15) was synthesized by the sol-gel process. The X-ray diffraction (XRD) technique confirms phase formation for all the samples under investigation. The results of ac magnetic susceptibility measurements indicate the effect of Co doping on the magnetic ordering phases. The indications of charge ordering (CO) transitions were observed in all the prepared compounds. The CO magnetic phase transition temperature was observed systematically shift to lower temperatures as the Co concentration increases. There was an anomalous oscillating magnetic behavior in all samples with a few peaks before the CO temperature in the paramagnetic (PM) region so that with an increasing Co doping, the number of peaks and amplitude were decreased. Also, the ac susceptibility measurements were performed in the presence of an applied dc magnetic field to further study of this oscillating behavior.

Effect of Nd doping on the magnetic properties of charge-ordered Bi0.6-x Nd (x) Ca0.4MnO3 (0.0a parts per thousand currency signxa parts per thousand currency sign0.6) perovskite manganite ,

Yadav K.; Singh, M. P.; Singh, H. K.; Varma G. and Razavi F. S.

We have studied structure, magnetic and transport properties of polycrystalline Bi0.6-x Nd (x) Ca0.4MnO3 (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6). Substitution of Nd at Bi sites induces a strong interplay between the magnetic and charge ordering. The charge-ordering temperature (T (CO)) decreases with increasing x. Further, the antiferromagnetic ordering temperature (T (N)) increases sharply at both extremes and remains nearly constant for x=0.2-0.4. At T < T (N) a transition to a metamagnetic glass-like state is also seen. Nd doping also leads to enhancement in the magnetic moment and a concomitant decrease in resistivity up to x=0.3 and then an increase in resistivity up to x=0.5. Furthermore, Nd doping promotes an antiferromagnetic to ferromagnetic type fluctuation in the materials at room temperature, as evidenced by the change in the value of the paramagnetic Curie temperature. We find that the local lattice distortion induced by the size mismatch between the A-site cations and the 6s(2) character of Bi3+ lone pair electrons explains the observed peculiarity in magnetic and transport properties of Nd-doped Bi0.6Ca0.4MnO3.

Structural, electrical, and magnetic properties of Pb2-xLaxCrO5 (0 <= x <= 0.15) ,

Indovski, Biljana; Singh, M. P.; Razavi, F. S.

We report structural, electrical, and magnetic properties of a parent and La-doped polycrystalline Pb2-xLaxCrO5. The X-ray study suggests that La-doping alters the lattice parameters of Pb2CrO5 and the solubility limit of La is 7.5%. Temperature dependent resistivity data exhibit that both doped and parent materials are semiconducting in nature. Furthermore, La-doping induces electrical conduction and significantly reduces the electrical bandgap of Pb2CrO5. A lowest bandgap of 1.16 eV is observed in the samples containing 7.5% of La. Magnetic measurements reveal that Pb2CrO5 is characterized by a weak Curie-Weiss type paramagnetic behavior with the effective magnetic moment of 0.17 mu(B). Also, La-doping has induced a ferromagnetic behavior with a Curie temperature of 293 K. Observed physical properties are explained based on the possible oxygen vacancy and multiple oxidation states of Cr induced by La-doping. (C) 2012 Elsevier B.V. All rights reserved.

Multifunctional nanoparticles for rapid bacterial capture, detection, and decontamination ,

Chen, Longyan; Razavi, Fereidoon S.; Mumin, Abdul; et al.

Fluorescent magnetic nanoparticles (FMNPs) with a core-shell structure are synthesized through a one-pot chemical method followed by the bioconjugation of gentamycin (Gm). The average diameter of the FMNPs is estimated to be 65 +/- 8 nm. The results of transmission electron microscopy (TEM), X-ray absorption near edge structure spectroscopy (XANES), and fluorospectrometry indicate that the FMNPs consist of a Fe3O4 core and a fluorescent silica (SiO2) shell. The FMNPs show typical superparamagnetic properties with a blocking temperature (T-B) of 120 K. We demonstrate that gentamicin (Gm)-bioconjugated FMNPs can capture gram-negative bacteria, i.e. E. coli, (1 x 10(7) CFU mL(-1) from 10 mL of solution) within 20 min. TEM micrographs clearly show that the Gm-FMNPs disrupt the cell wall of E. coli prior to the lysis of E. coli as the interaction time (t) increases; whereas FMNPs without Gm are inert towards E. coli. In addition, the Gm-FMNPs are able to detect diluted E. coli cells at a concentration as low as 1 x 10(3) CFU mL(-1), which is revealed by a slight red-shift in fluorescent emissions from 517 nm to 528 nm along with a dramatic decrease in intensity. The Gm-conjugated FMNPs can be a multifunctional platform for simultaneous rapid capture, sensitive detection, and decontamination of bacteria.

Far-IR excitations in Cd2Re2O7 in the normal and superconducting states. ,

Hajialamdari, M.; Razavi, F. S.; Crandles, D. A.; et al

Fluorescent magnetic nanoparticles (FMNPs) with a core-shell structure are synthesized through a one-pot chemical method followed by the bioconjugation of gentamycin (Gm). The average diameter of the FMNPs is estimated to be 65 +/- 8 nm. The results of transmission electron microscopy (TEM), X-ray absorption near edge structure spectroscopy (XANES), and fluorospectrometry indicate that the FMNPs consist of a Fe3O4 core and a fluorescent silica (SiO2) shell. The FMNPs show typical superparamagnetic properties with a blocking temperature (T-B) of 120 K. We demonstrate that gentamicin (Gm)-bioconjugated FMNPs can capture gram-negative bacteria, i.e. E. coli, (1 x 10(7) CFU mL(-1) from 10 mL of solution) within 20 min. TEM micrographs clearly show that the Gm-FMNPs disrupt the cell wall of E. coli prior to the lysis of E. coli as the interaction time (t) increases; whereas FMNPs without Gm are inert towards E. coli. In addition, the Gm-FMNPs are able to detect diluted E. coli cells at a concentration as low as 1 x 10(3) CFU mL(-1), which is revealed by a slight red-shift in fluorescent emissions from 517 nm to 528 nm along with a dramatic decrease in intensity. The Gm-conjugated FMNPs can be a multifunctional platform for simultaneous rapid capture, sensitive detection, and decontamination of bacteria.

Effect of Cu doping and oxygen-annealing on the magnetic properties of Nd0.5Sr0.5Mn1-xCuxO3-delta (x=0.0, 0.01, 0.03, 0.05 and 0.10) ,

Yadav, Kamlesh; Singh, M. P.; Razavi, F. S.; et al.

The polycrystalline samples with nominal compositions Nd0.5Sr0.5Mn1-xCuxO3 (x = 0.0, 0.01, 0.03, 0.05 and 0.10) have been synthesized with the aim to study the change in the structural, magnetic and electrical properties due to substitution of Cu for Mn and oxygen-annealing. The temperature dependence magnetization measurement shows a charge ordering transition (T-CO) at 240 K in Nd0.5Sr0.5MnO3 sample. The Neel temperature (T-N) increases with increasing the Cu doping content but changes induced by even 0.1 Cu doping are quite small. As compared to the air sintered samples, higher values of TN are observed for the oxygen annealed samples. Furthermore, oxygen annealing enhances the magnetization of the samples. It is also found that T-C increases and T-N decreases with increase in the applied magnetic-field. The value of resistivity decreases with the increasing Cu content from x = 0.0 to x = 0.03 and afterwards it increases with increasing value of x up to 0.10 for both air and oxygen sintered samples. It is also found that oxygen annealed samples exhibit higher resistivity than the air sintered samples. In the present paper the results are discussed according to the change of magnetic exchange interaction caused by Cu-doping. It is also found that the amount of Mn4+ appears to be the main variable which influences the physical properties. (C) 2012 Elsevier B.V. All rights reserved

Magnetic and charge ordering properties of Bi0.6-xEuxCa0.4MnO3 (0.0 <= x <= 0.6) ,

Yadav, Kamlesh; Singh, M. P.; Razavi, F. S.; et al.

We have studied structure, magnetic and transport properties of polycrystalline Bi0.6-xEuxCa0.4MnO3 (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) perovskite manganites. Magnetic measurements show that the charge-ordering temperature (T-CO) decreases with increasing x up to x=0.4 and then slightly increases with further increasing x up to x=0.6. Further, the antiferromagnetic (AFM) ordering temperature (T-N) decreases with increasing x. At T

Energy loss of protons in SrTiO3 studied by medium energy ion scattering ,

Dedyulin, S. N.; Singh, M. P.; Razavi, F. S.; et al.

The energy loss of medium energy protons (55-170 keV/amu) was studied for a thin SrTiO3 film on Si. The thickness of the film and the structure of the SrTiO3/Si interface was determined by the combination of X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS) and medium energy ion scattering (MEIS). These film parameters, together with energy losses extracted from MEIS spectra, were used to calculate stopping cross sections of SrTiO3 by an iterative procedure. In comparison with Andersen and Ziegler values, the data are systematically lower over the whole energy range. (C) 2012 Elsevier By. All rights reserved.

Effect of A-site cation size on magnetic and charge-ordering properties of Ln(0.5)Sr(0.5)Mn(0.9)Cu(0.1)O(3) (Ln = La, Pr, Nd, or Ho) ,

Yadav, Kamlesh; Singh, M. P.; Razavi, F. S.; et al.

The structural, magnetic, and electrical properties of Ln(0.5)Sr(0.5)Mn(0.9)Cu(0.1)O(3) (Ln = La, Pr, Nd, or Ho) perovskite manganites have been investigated to explore the influence of A-site cation radius (< r(A)>) and the A-site cation size-disorder (sigma(2)) on the various interdependent phenomena such as ferromagnetism (FM), phase separation (PS), and charge ordering (CO). The temperature dependence magnetization (M-T) curve of La-based sample shows four distinct points at similar to 269 K, 255 K, 200 K, and 148 K corresponding to strong FM, cluster glass (CG), weak FM, and charged ordered antiferromagnetic (COAFM) transitions, respectively. Our investigation shows that Neel temperatures (T-N) increases, whereas Curie (T-c) and irreversibility temperatures (T-irr) decrease with decreasing < r(A)>, i.e., with increasing sigma(2). Furthermore, the value of the magnetization decreases and resistivity increases with decreasing < r(A)>. All samples exhibit insulating behavior in the temperature range 77-300 K and above 110K the electronic conduction mechanism has been described within the framework of the variable range hopping (VRH) model. (c) 2012 Elsevier B.V. All rights reserved.

Impact of substrate on magnetic phase coexistence in bicritical Sm0.53Sr0.47MnO3 thin films ,

Srivastava, MK; Singh, MP ; Siwach, PK ; Kaur, A ; Razavi, FS ; Singh, HK

Sm0.53Sr0.47MnO3 thin films were deposited on single crystal LaAlO3 (LAO/(001)) and SrTiO3 (STO/(001)) substrates by DC magnetron sputtering. The theta-2 theta and omega-2 theta scans show that these films have very good crystallinity and the films on LAO and STO are under compressive and tensile strain, respectively. The films on LAO and STO substrates show ferromagnetic (insulator-metal) transition at T-C similar to 126 K (at T-IM similar to 128 K) and 120 K (T-IM similar to 117 K), respectively. The magnetic state at T < T-C is akin to cluster glass, which is formed by the presence of charge ordered-antiferromagnetic clusters in the ferromagnetic matrix. Huge drop in the resistivity at T-IM and the associated hysteresis with respect to cooling and warming cycles reveal the bicritical and the first order nature of phase transition, which is also confirmed by the Banerjee criterion. The differences and similarities in the functional properties of films are explained in terms of substrate modified magnetic phase coexistence. (C) 2011 Elsevier Ltd. All rights reserve

Low field anisotropic colossal magnetoresistance in Sm0.53Sr0.47MnO3 thin films ,

Srivastava, MK ; Singh, MP ; Siwach, PK; Kaur, A; Razavi, FS; Singh, HK

Sm0.53Sr0.47MnO3 (SSMO) thin films (thicknesses similar to 200 nm) were deposited by on-axis dc magnetron sputtering on the single crystal LSAT (001) substrates. These films are oriented along the out of plane c-direction. The ferromagnetic and insulator-metal transition occurs at T-C similar to 96 and T-IM similar to 91 K, respectively. The magnetization easy axis is observed to lie in the plane of the film while the magnetic hard axis is found to be along the normal to this. The magnetotransport of the SSMO films, which was measured as a function of angle (theta) between the magnetic field (H) and plane of the film, shows colossal anisotropy. Magnetoresistance (MR) decreases drastically as theta increases from 0 degrees (H//easy axis) to 90 degrees (H//hard axis). The out-of-plane anisotropic MR is as high as 88% at H = 3.6 kOe and 78 K. The colossal anisotropy has been explained in terms of the magnetic anisotropies at play and the magnetic domain motion in applied magnetic field. (C) 2011 American Institute of Physics.

Structural, and Magnetic Properties of ZnO:Co Nanoparticles Prepared by a Thermal Treatment of Ball Milled Precursor ,

Sarsari, IA ; Salamati, H ; Kameli, P ; Razavi, FS

In this study, ZnO nanoparticles with different cobalt concentration were prepared by a simple and rapid method. This method is based on a short time solid state milling and calcinations of zinc acetate, cobalt acetate, and citric acid powders. The samples were characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared (FTIR), photoluminescence, and UV-vis. spectroscopy. It was shown that a very low substitution of Co (less than 1% of molecular weight) has little effect on the lattice parameters of ZnO and significantly decreases the band gap ( E g ) value of the synthesized ZnO:Co nanoparticles. Calculation based on the XRD data shows that the average crystallite sizes of ZnO particles are nearly 18 nm. Photoluminescence spectroscopy shows that many defects such as interstitial zinc, zinc vacancy, and exciton recombination are responsible for the observed optical properties. Magnetization measurements which were performed by using a superconducting quantum interference device (SQUID) magnetometer determine the paramagnetic behavior for all samples due to the absence of oxygen vacancy.

Effect of PbZr0.52Ti0.48O3 thin layer on structure, electronic and magnetic properties of La0.65Sr0.35MnO3 and La0.65Ca0.30MnO3 thin-films ,

Hezareh, T; Razavi, FS ; Kremer, RK ; Habermeier, HU ; Lebedev, OI ; Kirilenko, D ; Van Tendeloo, G

Epitaxial thin film heterostructures of high dielectric PbZr1-xTixO3 (PZT) and La(1-x)A(x)MnO(3) (A-divalent alkaline earth metals such as Sr (LSMO) and Ca (LCMO)) were grown on SrTiO3 substrates and their structure, temperature dependence of electrical resistivity, and magnetization were investigated as a function of the thickness of the LSMO(LCMO) layer. The microstructures of the samples were analyzed by TEM. By applying an electric field across the PZT layer, we applied a ferrodistortive pressure on the manganite layer and studied the correlations between lattice distortion and electric transport and magnetic properties of the CMR materials. (C) 2011 American Institute of Physics.

One-pot synthesis and characterization of rhodamine derivative-loaded magnetic core-shell nanoparticles ,

Zhang, J ; Li, JX ; Razavi, FS ; Mumin, AM

A new method to produce elaborate nanostructure with magnetic and fluorescent properties in one entity is reported in this article. Magnetite (Fe3O4) coated with fluorescent silica (SiO2) shell was produced through the one-pot reaction, in which one reactor was utilized to realize the synthesis of superparamagnetic core of Fe3O4, the formation of SiO2 coating through the condensation and polymerization of tetraethylorthosilicate (TEOS), and the encapsulation of tetramethyl rhodamine isothiocyanate-dextran (TRITC-dextran) within silica shell. Transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were carried out to investigate the core-shell structure. The magnetic core of the core-shell nanoparticles is 60 +/- A 10 nm in diameter. The thickness of the fluorescent SiO2 shell is estimated at 15 +/- A 5 nm. In addition, the fluorescent signal of the SiO2 shell has been detected by the laser confocal scanning microscopy (LCSM) with emission wavelength (lambda(em)) at 566 nm. In addition, the magnetic properties of TRITC-dextran loaded silica-coating iron oxide nanoparticles (Fe3O4@SiO2 NPs) were studied. The hysteresis loop of the core-shell NPs measured at room temperature shows that the saturation magnetization (M (s)) is not reached even at the field of 70 kOe (7T). Meanwhile, the very low coercivity (H (c)) and remanent magnetization (M (r)) are 0.375 kOe and 6.6 emu/g, respectively, at room temperature. It indicates that the core-shell particles have the superparamagnetic properties. The measured blocking temperature (T (B)) of the TRITC-dextran loaded Fe3O4@SiO2 NPs is about 122.5 K. It is expected that the multifunctional core-shell nanoparticles can be used in bio-imaging.

Effect of vanadium deficiency on properties of polycrystalline LaVO3,

Gharetape SJ, Singh MP, Razavi FS, et al.

We report the influence of V deficiency on structure, transport, and magnetic properties of polycrystalline LaVO3. Up to 10% V-deficient samples were synthesized using standard solid state chemistry route. Structural and spectroscopic studies show that it intricately modifies lattice parameters and oxidation states of V. Further temperature dependent resistivity data reveal that V deficiency induces an enhancement in the resistivity and activation energy. The paramagnetic moment of these samples depends on net V composition while all samples exhibit a paramagnetic-to-antiferromagnetic transition at about 140 K. These distinct properties are understood owing to the presence of multiple oxidations states of V, predominantly V3+ and V4+, in nonstoichiometric LaVO3. (C) 2011 American Institute of Physics.

Magnetic and Structural Properties of Ni/NiO Nanoparticles Prepared using Nickel Acetate and Polyvinyl Acetate Precursor ,

Ahmadwand H., Salamati, H., Kameli P., Razavi F. S.

Ni/NiO nanoparticles were synthesized using a mixture of nickel acetate and polyvinyl acetate as precursors, followed by heating between 350 to 450 C. The prepared samples were investigated using XRD, TEM and SQUID magnetometry. Depending on the heating temperature, the average sizes of Ni and NiO crystallites were found to be in the ranges of 25 to 37 nm and 10 to 26 nm, respectively. The analysis of XRD patterns shows that the volume percentage of Ni can be easily controlled by the heating temperature. Magnetization measurements show that the samples consist of ferromagnetic moments which are blocked at low temperatures and a disordered NiO component which causes spin-glass like behavior.

A search for defect related ferromagnetism in SrTiO3 ,

Crandles D. A. , DesRoches B., Razavi F. S. ,

Abstract: Room temperature ferromagnetic hysteresis is observed in commercial SrTiO3 substrates purchased from a variety of suppliers. It is shown that the ferromagnetic signal comes from the unpolished surfaces. Surface impurity phases cannot be detected using either x-ray diffraction or energy dispersive x-ray spectra on the unpolished surfaces. However, a possible correlation between surface disorder (x-ray diffraction peak linewidth) and ferromagnetism is observed. Ar ion bombardment (10-90 keV) can be used to produce surface layer disorder but is not found to induce ferromagnetism. Annealing of the substrates at temperatures ranging from 600 to 1100 degrees C is found to alter the hysteresis curves differently depending on whether the annealing is performed in air or in vacuum. Identical annealing behavior is observed if the substrates are artificially spiked with iron. This suggests that the ferromagnetic hysteresis of as purchased SrTiO3 could be due to Fe contamination of the unpolished surfaces. In addition, it is shown that no ferromagnetism is observed in samples that contain 10(19)-10(21) cm(-3) of oxygen vacancies if all the faces are polished. (C) 2010 American Institute of Physics.

JOURNAL OF APPLIED PHYSICS Volume: 108 Issue: 5 Article Number: 053908 Published: SEP 1 2010

Persistent supercurrents in ring-shaped Bi2Sr2CaCu2Ox single crystal

Ma RC , Mansour AI , Egilmez M , Winterfield CE , Fan I , Chow KH , Jung J , Prabhakaran D , Razavi F

Abstract: A superconducting ring could be used to simulate the supercurrent conduction, and its interaction with magnetic vortices in a superconducting solenoid, allowing one to investigate the nature of the vortex structure and its pinning in the presence of the persistent supercurrent. The dissipation of the persistent supercurrent has been studied in a ring-shaped high purity single crystal of Bi2Sr2CaCu2Ox in order to obtain the information about the exponent mu, a parameter in the scaling relation between the effective energy barrier against vortex motion U-eff and the persistent current density J. The measurements of the persistent supercurrent decay show a transition from a strongly nonlogarithmic to a logarithmic decay regime with an increasing temperature. In response to a small increase in the concentration of oxygen vacancies mu decreases in the logarithmic decay regime but remains almost constant in the nonlogarithmic one. (C) 2010 American Institute of Physics.


Evidence for random networks of diodes in thin films of LaVO3 on SrTiO3 substrates

Razavi FS , Gharetape SJ , Crandles DA , Christiani G , Kremer RK , Habermeier HU

Highly conducting interfaces of transition metal oxide heterolayers could provide a promising access to transition metal oxide based electronic devices similar to those based on semiconductor heterostructures. Recently, metallic conductivity has been reported at the LaAlO3/SrTiO3 heterointerface. Here we report the observation of diodelike behavior and the formation of a random diode network in thin films of LaVO3 deposited on as-polished SrTiO3 substrates. Depending on films annealing conditions we observed linear or nonlinear I-V characteristic below 50 K.

APPLIED PHYSICS LETTERS , 96:042110 (2010).

Spontaneous Stoichiometry Change in Single Crystals of Superconducting (Ba1-x K (x) )Fe2As2 Grown by a Rapid-Heating Sn-Flux Method

Reuvekamp P , Razavi FS , Hoch C , Kim JS , Kremer RK 1, Simon A

To prevent the loss of K in growing single crystals of Ba1-x K (x) Fe2As2 we developed a rapid-heating Sn-flux method. Large single crystals with the optimal superconducting transition temperature T (C)a parts per thousand 38 K were obtained and their structural, chemical and superconducting properties were investigated. Additionally, the effect of post-growth annealing on these crystals at different temperatures was examined. Scanning electron microscopy microprobe studies on a crystal with the composition goal of Ba0.25K0.75Fe2As2 revealed a well defined separation of two phases with compositions that are suggestive of rational ratios of the K and Ba content.


Preparation and Magnetic Properties of Iron(3+) Spin-Crossover Complexes Bearing a Thiophene Substituent: Toward Multifunctional Metallopolymers

The synthesis of a new 3-ethynylthienyl-substituted QsaIH ligand (QsalH is the short form for N-(8-quinolyl)salicylaldimine) (ThEAsalH 3), and the preparation, electronic, and magnetic properties of three homoleptic and cationic iron(3+) complexes containing this ligand with PF6- 4, SCN- 5, and ClO4- 6 counteranions are reported. In all three complexes a spin-crossover is observed in the solid state by variable temperature magnetic susceptibility measurements and Mossbauer spectroscopy, indicating that the synthetic modification of the QsalH ligand has not significantly altered the electronics at the metal center. This includes the observation of a very rare S = 5/2 to 312 spin-crossover in a non-porphyrin iron(3+) complex 5. The molecular structure and magnetic properties of an unusual iron(2+) complex 7 generated by reduction of complex 6 serendipitously during a recrystallization attempt in aerobic acetone solution is also reported: Complexes 4-6 feature iron(3+) reduction and oxidation of the thiophene ring at potentials of approximately -0.7 and +1.2 V (vs Fc), respectively.


Kramers-Kronig-constrained variational dielectric fitting and the reflectance of a thin film on a substrate

A test was made of the ability of Kramers-Kronig-constrained variational dielectric fitting to extract the optical conductivity of a thin film from reflectance data containing structure due to both thin film and substrate. The reflectance of a series of well-characterized thin filius of SrRuxMg1-xO3 and SrRuxO3 with a variety of thicknesses (approximate to 56-300nm) and dc resistivities (approximate to 250-2200 mu Omega cm) was measured. The low frequency values of the extracted optical conductivities agree with the dc measurements. however, removal of features due to the substrate improves with increasing film thickness. (c) 2008 Optical Society of America

APPLIED OPTICS, 47, 4205, 2008.

Pressure effects on the superconducting transition in nH-CaAlSi

: We present a combined experimental and theoretical study of the effects of pressure on T-c of the hexagonal layered superconductors nH-CaAlSi (n=1, 5, and 6), where nH denotes the different stacking variants that were recently discovered. Experimentally, the pressure dependence of T-c has been investigated by measuring the magnetic susceptibility of single crystals up to 10 kbars. In contrast to previous results on polycrystalline samples, single crystals with different stacking sequences display different pressure dependences of T-c. 1H-CaAlSi shows a decrease in T-c with pressure, whereas 5H- and 6H-CaAlSi exhibit an increase in T-c with pressure. Ab initio calculations for 1H-, 5H-, and 6H-CaAlSi reveal that an ultrasoft phonon branch associated with out-of-plane vibrations of the Al-Si layers softens with pressure, leading to a structural instability at high pressures. For 1H-CaAlSi, the softening is not sufficient to cause an increase in T-c, which is consistent with the present experiments but adverse to previous reports. For 5H and 6H, the softening provides the mechanism to understand the observed increase in T-c with pressure. Calculations for hypothetical 2H and 3H stacking variants reveal qualitative and quantitative differences

PHYSICAL REVIEW B ,77 144502, 2008.

A novel bis tridentate bipyridine carboxamide ligand and its complexation to copper(II): Synthesis, structure, and magnetism

: A new bis tridentate ligand 2,2'-bipyridine-3,3'-[2-pyridinecarboxamide] H2L1 Which can bind transition metal ions has been synthesized via the condensation of 3,3'-diamino-2,2'-bipyridine together with 2-pyridine carbonyl chloride. Two copper(II) coordination compounds have been prepared and characterized: [Cu-2(L-1)(hfac)(2)]center dot 3CH(3)CN center dot H2O (1) and [Cu-2(L-1)Cl-2]center dot CH3CN (2). The single-crystal X-ray structures reveal that complex 1 crystallizes in the triclinic space group P (1) over bar, with the unit cell parameters a = 12.7185(6) angstrom, b = 17.3792(9) angstrom, c = 19.4696(8) angstrom, alpha = 110.827(2)degrees, beta = 99.890(3)degrees, gamma = 97.966(3)degrees, V = 3868.3(3) angstrom(3), Z = 4, R = 0.0321 and R-W = 0.0826. Complex 2 crystallizes in the monoclinic space group P-21/n with the unit cell parameters a = 12.8622(12) angstrom, b = 9.6100(10) angstrom, c = 19.897(2) angstrom, beta = 102.027(3)degrees, V = 2405.3(4) angstrom(3), Z = 4, R = 0.0409 and R-W = 0.1005. In both complexes the ligand is in the dianionic form and coordinates the divalent Cull ions via one amido and two pyridine nitrogen donor atoms. In 1, the coordination geometry around both Cull ions is best described as distorted trigonal bipyramidal where the remaining two coordination sites are satisfied by hexafluoroacetylaceton ate counterions. In 2 both Cull ions adopt a (4 + 1) distorted square pyramidal geometry. One copper forms a longer apical bond to an adjacent carbonyl oxygen atom, whereas the second copper is chelated to a neighboring Cu-Cl chloride ion to afford a mu-Cl-bridged dimerized [Cu-2(L-1)Cl-2](2) complex. The magnetic susceptibility data for 1 (2-270 K), reveal the occurrence of weak antiferromagnetic interactions between the Cull ions. In contrast, variable-temperature magnetic susceptibility measurements for 2 reveal more complex magnetic properties, with the presence of a weak antiferromagnetic exchange (J = -10.1 K) between the copper ions in each dinuclear copper complex and a stronger ferromagnetic exchange interaction (J = 32.9 K) between the Cull ions of the Cu(mu-Cl)(2)Cu climeric bridging units.

INORGANIC CHEMISTRY , 46, 8560, 2007 .

Superconductivity in heavy alkaline-earth intercalated graphites

J. S. Kim, L. Boeri, J. R. O'Brien, F. S. Razavi, , R. K. Kremer

Abstract: We report the discovery of superconductivity below 1.65(6) K in Sr-intercalated graphite SrC6, by susceptibility and specific heat (C-p) measurements. In comparison with CaC6, we found that the anisotropy of the upper critical fields for SrC6 is much reduced. The C-p anomaly at T-c is smaller than the BCS prediction, indicating an anisotropic superconducting gap for SrC6 similar to CaC6. The significantly lower T-c of SrC6 as compared to CaC6 can be understood in terms of "negative" pressure effects, which decreases the electron-phonon coupling for both in-plane intercalant and the out-of-plane C phonon modes. We observed no superconductivity for BaC6 down to 0.3 K.

PHYSICAL REVIEW LETTERS 99 (2): Art. No. 027001 JUL 13 2007

Rational design of a covalently tethered dinuclear [Mn-II(N3O2)Cl(OH2)](2)(2+) macrocyclic building block: Synthesis, structure, and magnetic properties

J. Wang , B. Slater, A. Alberola , H. Stoeckli-Evans , F. S. Razavi , M. Pilkington

Abstract: A novel dimeric Mn-II complex {[Mn(N3O2)]Cl(OH2)}(2)2Cl (2) of a macrocyclic Schiff base ligand derived from the condensation of 2,2', 6,6'-tetraacetyl-4,4'-bipyridine with 3,6-dioxaoctane-1,8-diamine in the presence of a stoichiometric amount of MnII has been prepared and characterized. The X-ray analysis of 2 reveals that the two Mn ions assume a pentagonal-bipyramidal geometry, with the macrocycle occupying the pentagonal plane and the axial positions being filled by a halide ion and a H2O molecule. Magnetic susceptibility data (2-270 K) reveal the occurrence of weak antiferromagnetic interactions between covalently tethered Mn-II-Mn-II dimeric units.

INORGANIC CHEMISTRY 46 (12): 4763-4765 JUN 11 2007

Formation and thickness evolution of periodic twin domains in manganite films grown on SrTiO3(001) substrates

U. Gebhardt , N. Y. Kasper, A. Vigliante, P. Wochner , H. Dosch , F. S. Razavi , H_U. Habermeier

Abstract: We present an extended synchrotron x-ray scattering study of the structure of thin manganite films grown on SrTiO3(001) substrates and reveal a new kind of misfit strain relaxation process which exploits twinning to adjust lattice mismatch. We show that this relaxation mechanism emerges in thin films as one-dimensional twinning waves which freeze out into a twin domain pattern as the manganite film continues to grow. A quantitative microscopic model which uses a matrix formalism is able to reproduce all x-ray features and provides a detailed insight into this novel relaxation mechanism. We further demonstrate how this twin angle pattern affects the transport properties in these functional films.

PHYSICAL REVIEW LETTERS 98 (9): Art. No. 096101 MAR 2 2007

Effect of pressure on superconducting Ca-intercalated graphite CaC66

J. S. Kim , L. Boeri, R. K. Kremer, F. S. Razavi

Abstract: The effect of pressure on the superconducting transition temperature (T-c) of the Ca-intercalated graphite compound CaC6 has been investigated up to similar to 16 kbar. T-c is found to increase under pressure with a large relative ratio Delta T-c/T-c of approximate to 0.4%/kbar. Using first-principles calculations, we show that the positive effect of pressure on T-c can be explained within the scope of electron-phonon theory due to the presence of a soft phonon branch associated with in-plane vibrations of the Ca atoms. The implications of the present findings for the current debate about the superconducting mechanism in graphite intercalation compounds are discussed.

PHYSICAL REVIEW B 74 (21): Art. No. 214513 DEC 2006

Specific heat of the Ca-intercalated graphite superconductor CaC6

J. S. Kim , R. K. Kremer, L. Boeri, F. S. Razavi

Abstract: The superconducting state of Ca-intercalated graphite CaC6 has been investigated by specific heat measurements. The characteristic anomaly at the superconducting transition (T-c=11.4 K) indicates clearly the bulk nature of the superconductivity. The temperature and magnetic field dependence of the electronic specific heat are consistent with a fully gapped superconducting order parameter. The estimated electron-phonon coupling constant is lambda=0.70 +/- 0.04, suggesting that the relatively high T-c of CaC6 can be explained within the intermediate coupling BCS approach.

PHYSICAL REVIEW LETTERS 96 (21): Art. No. 217002 JUN 2 2006 .

Pressure induced phase transition in periodic microtwinned thin film of La0.88Sr0.1MnO3

F. S. Razavi, G. V. S. Rao, H. Jalili, H.-U. Habermeier,

Abstract: We have studied the resistivity of a hole-doped 95 nm thin film of La0.88Sr0.1MnO3 sample as a function of pressure up to 2.0 GPa. For pressures below 0.81 GPa, the sample showed that the resistivity decreased at all temperatures and the metal insulator transition temperature T-MI increased to higher temperatures similar to the pressure results observed previously in bulk samples and thin films of manganites. However, for pressures above 0.81 GPa, we observed a rapid increase in resistivity and a rapid decrease of T-MI as a function of pressure. The rate of change of T-MI with pressure is -133.5 K/GPa which is an order of magnitude larger than that of bulk samples and thin films of manganite with an opposite sign. We interpreted our results as a consequence of a phase change in the film by pressure from having strain induced periodic microtwinned regions below 0.81 GPa to a strained free region and formation of coherent twins above this pressure in 95 nm thin film of La0.88Sr0.1MnO3.

APPLIED PHYSICS LETTERS 88 (17): Art. No. 174103 APR 24 2006 .

The thickness dependence of the efffect of pressures on magnetic and electronic properties of thin films of La2/3Ca1/3MnO3

S. Jacob, T. Roch, F.S. Razavi, G.M. Gross, and H.-U. Habermeier.

The discovery of the colossal magnetoresistance and pressure effects introduced new questions concerning the complex interplay among lattice structure, magnetism, and transport in doped perovskite manganites. To observe the interplay between the effect of pressure and strain due to the substrate in the La2/3Ca1/3MnO3 compound, we produced epitaxial films with the thickness varied between 20 and 100 nm on SrTiO3 substrates, using the pulse laser deposition technique and under the same epitaxial growth conditions. We measured magnetic transition temperature T-C and resistivity rho, as a function of pressure up to 8 kbar. Using these results, the dependence of bond compression on pressure, and the lattice distortion induced by the substrate, are discussed. We also report a universal behavior from the plot of d ln T-C/dP and drho/dP versus T-C for these films, as well as previously published results on ceramic and thin films of this compound. (C) 2002 American Institute of Physics.

J. Appl. Phys., 91, 2232-5, 2002.

Effect of pressure on the superconductivity of MgB2

The superconducting transition temperature Tc of polycrystalline samples of MgB2 was studied under hydrostatic and quasi-hydrostatic pressures up to 1 and 12 GPa, respectively. The rate of Tc change with hydrostatic pressure, (dT(c)/dP), was found to be -1.18 K(GPa)(-1). Quasi-hydrostatic measurements yield a slightly higher value: (dT(c)/dP) = -1.2 K(GPa)(-1). Analysis of the electron-phonon coupling parameter lambda and its logarithmic volume derivative using the McMillan expression for Tc yields results that fall within the range of a conventional s-wave BCS superconductor with intermediate coupling, in agreement with several experimental and theoretical studies that have already been reported. Based on electronic structure calculations performed by us as well as those reported in the literature, it is concluded that the change in the coupling parameter lambda is dominated by the change of the average phonon frequency with pressure. feasible.

Physica C, 366, 73-9, 2002.

Unusual x-ray transport phenomena in La1-xSrxMnO3

An interesting memory effect occurs when La1-xSrxMnO3 (x~1/8) is repeatedly exposed to x rays. While the "dark" conductivity remains unaffected by the irradiation history, the conductivity is markedly enhanced upon exposure to x rays at low temperatures. Immediately after renewed exposure, it recovers the value attained at the end of the previous exposure. We provide a qualitative explanation of this unusual effect in terms of three distinct states with different orbital correlations.

Phys. Rev. B, 64, 100404, 2001.

Coupling between lattice distortions and magnetism in La0.9Sr0.1SMnO3 thin films

X-ray scattering experiments have demonstrated that for thin lms (100 Angstrom to 500 Angstrom) of La0.9Sr0.1SMnO3 epitaxially grown on SrTiO3 (100) substrates, the accommodation of the lattice mismatch between lm and substrate gives rise to a modulated structure. The non-bulk like structural properties of these thin lms are the origin of anomalies in the electronic and magnetic behavior. Studies of the lattice as a function of temperature clearly reveal a magnetostrictive effect close to the Curie temperature. For thicker lms the strain relief mechanism is replaced by formation of coherent microtwins and some of the bulk properties are recovered, but some differences are still observed.

Europhys Lett., 54, 619-25, 2001.

Studies of the effect of internal and hydrostatic pressure on the structure and transport properties of La2/3Cal/3MnO3 thin films

Lanthanum manganites belong to the perovskite type Mott insulators. They show a doping dependent drastic modification of conductivity including a metal-insulator transition associated with ferromagnetic ordering. We investigated La2/3Cal/3MnO3 thin films, a member of this class of material showing a colossal magnetoresistance. Application of internal and external pressure can be used to modify the electronic and magnetic properties of this material. It has been shown that, in La2/3Cal/3MnO3, as the band width for the itinerant e, electrons broadens, it causes an enhancement of the ferromagnetic coupling and consequently an increase of the metal-insulator transition T-MI, which is close to the ferromagnetic transition temperature T-MI. Thin films were deposited on SrTiO3 (STO) single crystals using the pulsed laser deposition technique. In this study we prepared films with different T-MI by choosing an appropriate depositing temperature, annealing condition and thickness range. We studied the effect of hydrostatic pressures on the in-situ annealed films in order to investigate the effect of external pressure on T-MI and resistivity as a function of annealing. Due to lattice mismatch between film and substrate we introduced controllable biaxial strain. The increse of lattice parameters and T-MI shows the relaxation process by varying the film thickness and by annealing thin layers. Our results are a contribution to the clarification of the interplay between the microscopic structure of Mn-O-Mn units with the macroscopic transport properties.

J. Alloy Compd., 317, 141-4, 2001.

Tailoring of epitaxial strain in doped lanthanummanganite thin films

We have grown single crystalline doped LaMnO3 thin films using the pulsed laser deposition technique and analysed the interrelation of epitaxial strain with transport properties. An unexpected possibility of tailoring the metal-insulator transition temperature, T-MI, has been developed and correlated with biaxially epitaxial strain. Whereas in the case of La2/3Cal/3MnO3 a reduction of T-MI is observed, we demonstrate an unexpected epitaxial strain-induced metal-insulator transition for La.88Sr.lMnO3 thin films. The results are explained by biaxial tensile and compressive stress of the films, respectively.

Physica C, 341, 777-8, 2000.

Thickness dependence of microstructure in LaCaMnO thin films

Introducing biaxial strain in doped lanthanum manganite thin films is an excellent tool to modify the electrical and transport properties systematically by controlled changes of the microstructure on an atomic scale. We deposited La2/3Cal/3MnO3 thin films onto SrTiO3 single-crystal substrates under reproducible conditions in an optimized pulsed laser deposition process. A thickness range of 40 to 500 nm was chosen for the manganite layers to investigate the variation of the average lattice parameters with thickness using X-ray diffraction. We re- investigated the samples after annealing to determine the structural aspects of the relaxation behavior of the layers. A second set of samples was prepared with an analogous in situ annealing process. AFM was applied to examine the morphology and grain size of the layers. Due to a thermally induced strain relaxation process the thicker films show a gradual approach of the out-of-plane lattice constant to the bulk ceramic value. Further relaxation could be achieved with an annealing at 900 degrees C for 1 h. Lattice parameter changes with varying thickness and annealing processes are correlated with transport and magnetic properties.

J. Magn. Magn. Mater., 211, 22-7, 2000.

Effects on strain on the properties of La0.67Ca0.33 thin films

The lattice mismatch of doped manganites and conventional oxide substrate materials offers an interesting opportunity to tailor the epitaxial strain of thin films systematically. This can be used to study the interrelation of the microscopic arrangement of the atoms and the magnetotransport properties. In our experiment La0.67Ca0.33MnO3 thin films with varying thickness (40-520 nm) were grown on SrTiO3 (0 0 1) single- crystal substrates using the pulsed laser deposition technique. The thickness-dependence epitaxial strain leads to a systematic modification of the characteristic parameters of the magnetization curve and the dependence of resistivity on temperature. We analyse the magnetic and transport data of as-grown films and compare them to results of measurements performed on identical films subjected to a Ih post-deposition annealing process at 900 degrees C in flowing oxygen.

J. Magn. Magn. Mater., 211, 41-6, 2000.

Correlation of microstructure and magnetotransport properties of epitaxially grown La2/3Ca1/3MnO3 thin films

Using the technique of pulsed laser deposition, we have studied the correlation of the film deposition conditions with the microstructure and magnetotransport properties of La-2/3 Ca1/3MnO3 thin films. Detailed TEM microstructure studies revealed the dependence of the local microstructure with the deposition temperature. The TEM studies reveal a distortion of the La2/3Ca1/3MnO3 lattice leading to a breakdown of the symmetry from orthorhombic to monoclinic. At high deposition temperatures (T < 800 degrees C), the substrate/film interface is perfectly coherent, suggesting that the film is strained due to the lattice mismatch. Effects of hydrostatic pressure on the resistivity of the films reveal an increase in temperature of the metal-insulator transition and a colossal change in resistivity qualitatively comparable to the magnetoresistance effect. The data are interpreted in terms of microstructure sensitive changes of the contraction and alignment of the Mn-O-Mn bonds.

J. Superconductivity., 12 (1), 285-8, 1999.

Magnetic and high pressure studies in the YPd2B2C system

The macroscopic properties of the superconducting phase YPd2B2C in the multiphase compound YPd5B3C0.3 have been investigated. The onset of superconductivity was observed at 22.6 K, zero resistance at 21.2 K, the lower critical field H-cl at 0 K was estimated to be H-cl(0) similar to 310 G and the compound was found to be an extreme type-II superconductor with the upper critical field in excess of 50 000 G below 15 K. At T=15 K, the Ginzburg-Landau parameter was determined to be kappa > 9 corresponding to a coherence length xi similar to 80 Angstrom and magnetic penetration depth of 800 Angstrom. The superconducting transition temperature T-c(P) was also measured under hydrostatically applied pressure. T-c(P) of YPd2B2C decreases linearly with dT(c)/dP similar to -9 x10(-2) K/kbar. The discussion of T-c(P) will focus on the influence pressure has on the phonon spectrum and the density of states near the Fermi level.

Physica C, 321, 108, 1999.

Specific heat of the ternary compounds UPd2Si2 and UNi2Si2

The specific heat of the single crystals of UPd2Si2 and UNi2Si2 has been studied using the step heating method for the temperature range between 2 and 250 K. Successive phase transitions at high temperatures were observed for both samples, which are consistent with previously reported magnetization and neutron scattering results. No evidence of any phase transition was observed at temperatures below 70 K, which has been previously reported. The Debye temperature and the electronic specific heat coefficient gamma of UPd2Si2 are found to be 117 K and 19 mJ/mol K2, respectively. In low temperatures between 2 and 4 K, an upturn in specific heat with T2 dependence was observed. Several possibilities for this upturn in the specific heat are discussed.

Phys. Rev. B, 58, 113-6, 1998.

Ultrasonic study of the spin-Peierls system CuGeO3 under pressure

We present the temperature dependence of the acoustic sound velocity for a CuGeO3 single crystal measured under pressure up to 10 kbar. Anomalous elastic behavior along the c-axis, observed around 100 K, is accounted for by the existence of a magnetoelastic coupling. We show that this coupling is weakly affected by pressure. Moreover, the temperature dependence of the sound velocity below the spin-Peierls phase transition is analyzed using a Landau-type model from which we obtain the temperature and pressure dependences of the order parameter. We find that the magnetic spin-Peierls energy gap increases with pressure at a rate of d Delta /dP=0.11+or-00.01 meV/kbar.

Phys. Rev. B, 58, 882-6, 1998.

Pressure-temperature phase diagram of UPd2Si2 and UNi2Si2

It is well established that the ternary intermetallic family of compounds UT2Si2 , where T denotes a transition metal, shows a rich variety of electronic and magnetic ground-state properties. In order to better understand the magnetic properties of these materials, we have investigated the behavior of the electrical resistivity of UNi22Si2 and UPd2Si2 as a function of temperature and pressure. In the case of UNi2Si2, the proposed pressure-temperature phase diagram is very similar to its magnetic-field-temperature phase diagram. The pressure-temperature phase diagram obtained for both compounds is also compared to predictions made using a mean-field Landau-type analysis.

Phys. Rev. B, 57< 5220-4, 1998.

Effect of pressure on the superconductivity of ZrRuP

The superconducting transition temperature of hexagonal and orthorhombic ZrRuP was studied under hydrostatic and quasi-hydrostatic pressures up to 7 GPa. It was found that the superconducting transition temperature decreases with the pressure slightly for both structures of this compound. The rate of Tc change with pressure is (dTc/dP) = -53mK (GPa)-1 for hexagonal structure and (dTc/dP) = -98 mK (GPa)-1 for orthorhombic structure. We calculated the logarithmic volume derivative of the electron-phonon coupling parameter using the McMillan equation. It was found that under pressure the change in the coupling parameter depends mainly on the change of the phonon frequency with the pressure.

Phys. C, 292, 79-82, 1997.

Magnetoresistance and the effect of superconducting fluctuations in YBa2Cu3O7/PrBa2Cu3O7 superlattices

The results of the magnetoresistance on two thin-film samples of YBa2Cu3O7/PrBa2Cu3O7 superlattices have been reported for the temperatures near and below the superconducting transition temperature. The thermally activated flux-creep motion theory was used to describe the magnetoresistance results at temperatures much lower than Tc. The existence of the superconducting fluctuation near Tc as well as the two-dimensional behavior of superlattices were studied by comparing the magnetoresistance results and Larkin's quasi-two-dimensional (2D) superconducting fluctuation theory. A good fit between experimental results and theory was obtained and contrary to previous results, no crossover between quasi-2D and 3D was observed.

Phys. Rev. B, 55, 6631-5, 1997.

Doping effects on the magnetic phase diagram of the spin-Peierls system CuGe1-xSixO3

The magnetic phase diagram of Si-doped (0.7 and 1.0%) CuGeO3 single crystals was obtained from an ultrasonic velocity measurement. The magnetic phase diagram is similar to what is generally found in pure spin-Peierls (SP) systems: a SP instability is identified although an additional antiferromagnetic phase is observed at the lowest |temperatures. The diagram is weakly anisotropic: a spin-flop (SF) phase is observed for the magnetic field aligned along the c axis and a hard axis along a is deduced from the angular dependence of the SF field. These results are complemented by superconducting quantum interference device magnetization measurements.

Phys. Rev. B, 52, R6971, 1995.

Spin fluctuations in metallic glasses Zr75(NixFe1-x)25 at ambient and higher pressures

The superconducting properties of metallic glasses Zr 75(NixFe1-x)25 (x=0, 0.25, 0.50, 0.75, 1.0) were studied both at ambient pressure and under quasi- hydrostatic pressure up to 8 GPa (80 kbar). To obtain information on the spin fluctuation parameter lambdaSF the following measurements were carried out: the dc magnetic susceptibility, the upper critical field at the ambient pressure superconducting transition temperature Tc/(P=1 bar), mass density, and room temperature resistivity. From the pressure dependence of the superconducting transition temperature Tc(P) in Cu25Zr75 glass, we estimate the volume derivative of ph/ in this alloy using the McMillan equation. This is used as a reference value for the volume derivative of lambdae-ph in the Zr75(NixFe1-x)25 alloys. Using the modified McMillan equation, which includes spin fluctuation effects and the above reference value for the volume derivative of the e-ph coupling parameter, we determine lambdae-ph, the electron-spin fluctuation coupling parameter lambdaSF and the rate of lambdaSF/d ln V) as a function of Fe concentration in these alloys. Both spin fluctuations and the rate of suppression of spin fluctuations with pressure (d ln lambdaSF/d ln V) are found to increase with higher Fe concentration in these alloys.

Phys. Rev. B, 52, 9674, 1995.

Compositional dependence of thin films of BiPbSrCaCuO on target to substrate distance

Using magnetron sputtering techniques, several thin films of superconducting BiPbSrCaCuO were fabricated by varying the distance between the substrate (single crystal of MgO with polished (100) plane) and the targets. During the deposition the gas pressure was kept constant at 0.3 mbar (1 mbar = 0.1 kPa) and the substrate temperature was kept at 700 degrees C. An energy-dispersive X-ray fluorescence was designed using a radioisotope source with a secondary target and a Si(Li) X-ray spectrometer and it was used to measure the atomic composition of the film quantitatively. It was found that the Ca concentration relative to Sr increases linearly as the distance between the substrate and the targets increases. However, both Cu and Bi show a more complex variation of concentration with distance. The X-ray diffraction results also indicated that the films are grown epitaxially along the C axis, which showed a semiconducting behavior with TC, zero below 60 K.

Can. J. Phys., 73, 35, 1995.

Tunneling studies in Bi2Sr2CaCu2O8+x

Planar tunnel junctions have been realized on liquid-etched Bi2Sr2CaCu2O8+x (BSCCO) single crystals with natural barriers and Pb, Pb-Bi, Au crystals with natural barriers and Pb, Pb-Bi, Au counterelectrodes. At T=4.2 K and I-V characteristics of these junctions often show multiple switches that disappear at the Tc of the BSCCO compound. The dV/dI vs. V curves show gaplike structures at about +or-22 mV and second features at about +or-40 mV. These are very similar to the double structure observed in YBa2Cu3O7 (YBCO) junctions. The background resistance above the gaplike features is flat in the majority of the junctions, while about 20% of the junctions show decreasing resistance with bias. For Pb and PbBi counterelectrodes, negligible leakage currents at low temperatures indicate good quality tunnel barriers, continuous and pin-hole free. The phonon structures of these materials indicate a single-step tunneling process. The behavior of the zero-bias resistance dip observed in some junctions is discussed.

IEEE Trans. Appl. Supercon., 3, 1-3, 1540, 1993.

Single crystal EPR and thermal expansion studies of the high-Tc superconductor Bi2Sr2CaCu2O8+delta

Tc superconductor Bi2Sr2CaCu2O8+delta X-band (approximately 9.4 GHz) EPR and thermal-expansion measurements have been performed on a single crystal of the high-Tc superconducting material Bi2Sr2CaCu2O8+delta in the temperature range 77- 295 K. Cu2+ EPR spectra due to at least four different Cu2+ centers located at about 0.3 T, were observed at room temperature. The EPR spectrum disappeared almost completely below the superconducting transition temperature 84 K. The temperature variation of the EPR line intensity indicated the occurrence of a second-order phase transition at 84 K. Thermal-expansion measurements confirmed the occurrence of this phase transition. The thermal-expansion coefficients in the phases of the crystal above and below 84 K have been estimated.

Solid State Commun., 81, No. 6, 1992.

The 'in-situ' preparation of Bi-2212 films using the pulsed laser deposition technique

Bi-Sr-Ca-Cu-O thin films were deposited on (100)-oriented MgO single crystals using the pulsed-laser deposition technique and targets of nominal composition Bi2Sr 2CaCu2O 8+delta (i.e.((Bi):(Sr):(Ca):(Cu)=)2:2:1:2) with the mass density larger than 90% of the theoretical value. To obtain an in-situ process of producing single-phase 2:2:1:2 films, the authors varied three parameters, namely the substrate temperature, the oxygen pressure and the distance between the target and substrate. The results of surface studies using scanning electron microscopy, energy-dispersive X-ray analysis, the superconducting transition temperature and the critical current density are discussed.

Mat. Sc. Eng., B13, 1992.

Preparation of thin films of BiPbSrCaCuO using the laser deposition technique

Using KrF-excimer laser and high mass density target of (Bi,Pb) 2Sr2Ca2Cu3O 10+ delta (Bi-2223), the authors have fabricated 'in-situ' thin films of this high Tc superconductor. By varying the substrate temperature, the oxygen pressure and the distance between target and substrate, they were able to optimize the 'in-situ' process for c-axis oriented thin films of Bi- compounds. As deposited films of Bi-2223 show Tc, zero at about 60 K and following annealing, they show Tc, zero at 105 K. These films were examined using the scanning electron microscope (SEM) and X-ray diffraction (XRD). The results of magnetic susceptibility, electrical resistivity and critical current density measurements will be discussed.

Phys. C, 180, No. 1-4, 81, 1991.

Pressure dependence of spin fluctuations in metallic glasses Ni25Zr75 and Fe100-xZrx(x=75, 80)

The superconducting transition temperature Tc of metallic glasses Fe20/Zr80/ Fe25/Zr75, and Cu25Zr75 were measured under quasihydrostatic pressure up to 10 GPa. The volume (pressure) dependence of the electron-phonon coupling parameter, lambdae-ph, for Cu25Zr75 was calculated using the McMillan equation. Using this volume dependence of lambdae-ph and the modified McMillan equation, which incorporates spin fluctuations, the authors determined the volume dependence of the spin-fluctuation parameter, lambdaSF, in Ni25Zr75 and Fe100-xZrx (x=80, 75). It was found that with increasing pressure spin fluctuations are suppressed at a faster rate in Fe100-xZrx as the Fe concentration is increased. The rate of suppression of spin fluctuations with pressure was also found to be higher in the Fe-Zr glasses than in Ni-Zr glasses of similar composition.

Phys. Rev. B, 43, 3649, 1991.

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